Send Orders for Reprints to [email protected] Recent Patents on Mechanical Engineering 2014, 7, 1-37 1 Research, Development and Recent Patents on Aerodynamic Surface Circulation Control - A Critical Review

Harijono Djojodihardjo* and Naveeyindren Thangarajah

Department of Aerospace Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor Darul Ehsan, Malaysia

Received: December 26, 2013; Accepted: January 17, 2014; Revised: January 22, 2014

Abstract: The ever increasing demand for better performance of , from the fundamental understand- ing of flight, requires enhanced circulation. Circulation Control and hence its enhancement can be achieved by surface blowing in the form of Coand jet and have always been referred to in the consideration of various flow control methods to enhance aerodynamic performance, along with continuous, synthetic and pulsed jets, compliant surface, vortex-cell, and the like. Coand jet has also been applied in the development of novel aircrafts for short , while another circu- lation enhancement technique known as Trapped Vortex Cavity (TVC) is currently being given significant considerations. It is with such motivation that salient features, progress and development of various techniques in circulation control are here identified, including some of the recent inventions that have been registered as patents in this area. The present work reviews the influence, effectiveness and configuration of surface blowing of Circulation Enhancement and Control of aerodynamic surfaces. The crux of the TVC active research is their stabilization, while Coand enhanced enhance- ment technique has, to a certain extent reached a stage that it can be easily implemented with advantage. Keywords: Aerodynamic surface blowing, circulation control, Coand effect, reduction, lift augmentation, trapped vortex cell, wall blowing.

I. INTRODUCTION could be considered as the first who introduced Circulation Control technology for early models of fixed aircraft Coand effect is experienced by almost everybody when which was referred to as “blown flaps” [1]. Interest in active pouring water out of a glass or bottle. In fact, such observa- blowing systems increased with the advent of the turbojet tion may well be the reason by a top research funding deci- engine, initially in Great Britain and France with a jet sion maker when rejecting a Coand related research by configuration. While the addition of energy near the surface commenting that “Coand effect is just Bernoulli principle of a lifting body can be used to increase lift, and thus circula- and there is nothing new about it”. In spite of such a remark, tion, by retarding separation, most of the it has been observed that Coand effect has been a subject of high lift applications are performed on specially designed great interest and innovations in the last 80 years since it was where the addition of high velocity air can be used to patented. Circulation control (CC) as a lift augmentation control the boundary layer and to virtually extend the camber device, which to a large extent capitalizes on Coand effect, and the . The definition of “Circulation Control” is is traditionally used on the main wing of an aircraft (Kweder strictly related to the circulation characteristics around any et al., [1]). A possible practical solution for the generation aerodynamic body and can be controlled or managed with and control of very high lift coefficients emerged with the many different control schemes such as airfoil shape and discovery of the jet flap, which is one of the first Circulation shape change, flaps, , blowing, suction, etc. (Jones et Control technologies in the aerodynamic re-synthesis of the al., [4]). Gad-el-Hak [5, 6] stipulated that the science of flow lifting and propulsive means; the entire propulsive means control can be traced back to Prandtl [7], who made a break- was introduced as a jet being ejected in the form of a thin, through in the science of fluid mechanics by introducing the full span sheet from the of the wing. Independ- boundary-layer theory and elucidated the physics of the ent discoveries were made in various countries like Ger- separation phenomena and the control of the boundary lay- many, USA, Great Britain and France. The first published ers. Gad-el-Hak [5] utilization of this scientific method for information was that contained in a paper written by Hage- flow tailoring can be regarded as marking the birth of the dorn and Ruden at Hanover in 1938 [2], who noticed an un- second era of flow control. The utilization of flow control accountable increase in lift at high blowing rates during in- has noticeably increased in the last decades due to the vestigations into on a flap [3], and growth of aircraft and propulsion technologies (Gad-el-Hak

[6]; Englar et al., [8, 9]; Jones and Englar [10]; Lan and *Address correspondence to this author at the Department of Aerospace Campbell, [11]; Liu, [12]; Mamou and Khalid, [13]; Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor Radespiel et al., [14]; Shojaefard et al., [15]; and Min et al., Darul Ehsan, Malaysia; Tel: +603-8946 6397; Fax: +603-8656 7125; [16]). These active control techniques can appreciably im- E-mail: [email protected]

1874-477X/14 $100.00+.00 © 2014 Bentham Science Publishers 2 Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1 Djojodihardjo and Thangarajah

Fig. (1). Various types of active and passive methods for controlling aerodynamic surface circulation (extended from C. Werner-Spatz et al. [18]). prove the performance of many aerodynamic components program. Circulation control (CC) technology initiatives such as , wings and bodies. Since the wing areas are highlight the necessity for enhanced high-lift systems to sized by the conditions, the wings may meet takeoff and landing objectives that are incorporated have areas approximately twice as large as required for effi- into transonic cruise configurations. Consequently, there is a cient cruise (Moore, [17]), as well as lower need to optimize the entire wing and propulsion system with which is much more susceptible to turbulence, thus reducing particular attention on aerodynamic system efficiency, en- the cruise efficiencies of Transport Aircrafts. Various meth- gine out condition safety, and large aerodynamic moments ods for enhancing aerodynamic surface control and wing control issues. circulation enhancement have consequently been introduced. With the imperative of green and environmental friendly These circulation enhancement methods can be classified as technologies, as well as reducing airport noise and pollution summarized in Fig. (1), adapted from R. Radespiel et al. [14] from aircraft, there has been an increasing interest in reduc- and Werner-Spatz et al. [18]. ing the noise emitted during takeoff and landing. The con- Kweder et al. [1] noted that four main benefits have been ventional high-lift systems, consisting of slats and slotted achieved with the use of an active circulation control method flaps, are a major contributor to noise. Active flow on fixed wing aircraft for aerodynamic moment enhance- control, like trailing edge blowing, produces a gapless high- ment. Circulation control requires very small movement, or lift device capable of generating the high lift coefficients even non-moving, control surfaces. Lift augmentation can be needed for takeoff and landing. Energy saving can be af- achieved independent of the airfoil angle-of-attack, and the forded by utilizing a small fraction of the cold engine flow jet turning angle is neither limited by physical jet exit angle (about 5%) for circulation control (CC) blowing. The bleed nor by flap deflection angle. In addition, the jet blowing air is pipelined from the engine to a slot directly upstream of momentum is very effective in producing high aerodynamic the flap and thus the flow over the flap can bear large ad- force augmentation. These benefits make circulation control verse pressure gradients without separation. Thus a gapless by surface blowing very potential. Circulation control is a high-lift device with CC is able to generate the required lift viable active flow control approach that has been considered [19, 20]. The low drag coefficients during climbing, achiev- to meet the NASA Subsonic Fixed Wing project’s Cruise able with these powered high-lift systems, could also allow Efficient Short Take-Off and Landing goals (Jones et al., the use of new low-noise trajectories, which would further [4]). Recent interest in circulation control (CC) technology reduce the noise exposure at ground level. The absence of has been prompted by the National Aeronautic and Space slats might allow laminar flow in cruise flight, thereby re- Administration (NASA) Cruise Efficient Short Take-Off and ducing the drag in this flight segment. Even with taking into Landing (CESTOL) initiative and the Air Force Research account the additional system weight associated with the Laboratory (AFRL) Advanced Joint Air Combat System distribution for a gapless high-lift system, there is a Aerodynamic Surface Circulation Control Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1 3 chance of reducing the total weight of the aircraft and possi- particular geometrical and other physical characteristics bly the cost, hence rendering slats and fowler flaps redun- of application dant. 3. To identify research and development progress in circu- The flow of a fluid over curved surfaces has long been lation control technology in academics and industries studied for a variety of applications [1]. The most prevalent 4. To identify new directions and applications. application of circulation control works by increasing the near surface velocity of the airflow over the Following the work of the author and colleagues (Hamid (LE) and/or trailing edge (TE) of a specially designed air- et al., [25]; Djojodihardjo et al., [26]; Djojodihardjo et al., craft wing using a series of blowing slots that eject high ve- [27]; and Djojodihardjo et al., [28]), the present work re- locity jets of air [21]. These augmented wings normally have views the applications of Coand jet within the framework of a rounded trailing edge, and eject the air tangentially, aerodynamic surface circulation control, and identifies key through these slots inducing the Coand effect [22]. This principles and various techniques that have been applied to phenomenon prolongs boundary layer separation while in- take advantage of Coand-jet for aerodynamic performance creasing circulation around the airfoil and thus increases the enhancement for aircrafts. With due considerations to other lift generated by the wing surface due to the relaxation of the flow control methods, the advances in trapped vortex cavity . Initially, at very low blowing values, the jet technologies are discussed in analyzing the benefits of these entrains the boundary layer to prevent aft , two technologies for various applications. and thus is a very effective form of boundary layer control In this connection, the paper will start with the basic theo- (BLC) [23]. As blowing levels are increased, the jet contin- retical foundation of circulation control which facilitates our ues to wrap around the Coand surface causing a rise in the understanding and interpretation of the principle, CFD simula- local static pressure. tion and experimental work associated with it, and followed This pressure increase, along with viscous shear stress, by research and development work. Selected recent patents are and centrifugal forces, leads to jet separation from the reviewed and a critical assessment concludes the paper. rounded TE resulting in a new stagnation point on the lower It should be noted, however, that another perspective can surface of the airfoil. The direct effect of altering streamlines be considered from the point of view of synthetic jets, which and stagnation points around an airfoil is lift augmentation. has been comprehensively discussed by Glezer [29]. In addi- In comparison, flaps and slats are effective in increasing lift, tion, with the progress of modern airplanes of larger dimen- but accompanied with a penalty of increased drag and added sions and innovative configurations such as Blended-Wing- hardware. In this conjunction, a critical performance measure Body (BWB) airplanes, and modern fighters with higher of circulation control by blowing is the overall aerodynamic maneuverability and agility, the utilization of thick wings performance gain associated with the required added energy. and trapped vortex cell came into view, and is conceived to In this connection, the benefit of the CC wing is that no extra be beneficial. To overcome the drawback of thick airfoils drag is created from the movement of surfaces into the air- due to the high value of their drag coefficient and early flow flow around the wing and the CL is greatly increased. separation phenomenon, even for small incidence angles, the Rogers [24] elaborates the technical challenges in en- use of intense trapped vortex cavity to force the potentially hancing the performance of Subsonic Transport Aircraft, separated flow to remain attached has been given serious which implies the reduction of drag in addition to enhanced considerations, such as in the “Vortex Cell 2050” [30], a lift. Experiments on 2-D Coand-fitted airfoil at Georgia European funded research project launched at the end of Tech are focused on performance, while NASA experimental 2005. The effort is focused on the possibility to control the data are focused on CFD validation. Of particular interest are flow separation using trapped vortex cavities equipped with conditions at the jet exit, which will define the boundary a suction system aimed to stabilize the vortex. conditions for CFD and the boundary layer state. The 2-D Englar Circulation Control (CC) Airfoil has been used for II. BASIC PRINCIPLES AND ANALYSIS OF SUR- benchmarking of Large Eddy Simulation (LES) at NASA FACE BLOWING with the objective to investigate improved turbulence models As a basis for airfoil surface blowing, one may start with that are capable to correctly predict separation and lift, and to trailing edge blowing. A high velocity, tangentially blown air obtain reliable prediction of the separation location on CC jet remains attached to a convex surface due to the balance airfoils. Large Eddy Simulation is utilized to create a data- between centrifugal forces and the sub-ambient pressure in base to guide turbulence model development. This Large the jet sheet. The trailing edge jet can be directed downward Eddy Simulation should be validated against experimental data acquired for the same configuration. Typical problems as a “jet-flap” which has been the subject of a great deal of analysis. For further references, the well-known Kutta- and results are illustrated in Fig. (2). Joukowski theorem is re-derived to apply to these cases. With such background, the objectives of the present pa- Consider the situation of a self-propelled, 2-D wing in a uni- per are four-fold: form, steady flow as depicted in Figure, which has been 1. To elaborate principles and key features of circulation adapted from Keen [31] and Jones et al. [4], ignoring body control technology forces. Using first principles, the force on this body is given by (Karamcheti [32]): 2. To elaborate salient application principles and key fea- = & tures of circulation control technology as appropriate for Fi pndc mjj() u U (1) C 4 Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1 Djojodihardjo and Thangarajah

Fig. (2). Large Eddy Simulation (LES) of the 2D-Englar Circulation Control (CC) Airfoil [24].

Fig. (3). Schematic of a body or airfoil with blown section and control surfaces.

Aerodynamic Surface Circulation Control Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1 5 where p is the pressure, dc is a differential length along a where is now the total modified circulation of the wing- control surface, and mp is the mass flow rate per unit span. surface blowing system, and the second term describes the reaction lift due to the jet. In the above equation, denotes Knowing the value on the integral over the far-field con- the width of the jet. trol surface, the value of the integral over any surface enclos- ing the body will also be known, including one which is right Circulation control wing (CCW) technology is known to on the body surface. The force expression can be written as be beneficial in increasing the bound circulation and hence (Djojodihardjo et al., [28]): the sectional of airfoil. Circulation control is implemented by tangentially blowing a small high-velocity

jet over a highly curved surface, such as a rounded trailing edge. This causes the boundary layer and the jet sheet to re- Fn=Ud () qcmuU & ()  i jj main attached along the curved surface due to the Coand C (2) effect (a balance of the pressure and centrifugal forces) and where q is the difference between the total fluid velocity at a causing the jet to turn without separation. The rear stagnation point and the free stream value, n the normal vector to a sur- point location moves toward the lower airfoil surface, pro- ducing additional increase in circulation around the entire face element dc (two-dimensional equivalence), and  airfoil. The outer irrotational flow is also turned substan- C tially, leading to high value of lift coefficient comparable to encloses the aerodynamic body. It can be shown (Keen, [31]; that achievable from conventional high lift systems. These Djojodihardjo et al. [26]) that the total force on the body can techniques are depicted in Figs. (4 & 5), illustrating trailing then be written as: edge and leading edge blowing, respectively. The ability of circulation control technology to produce large values of lift =  +& FkUUwithout jet ()()uj ymsin kiju j U  is also advantageous for since any in- crease in the magnitude of the lift force (while keeping drag (3) small, and L/D high) will immediately contribute to a corre- Here i and k are the unit vectors in the direction of and sponding increase in induced thrust and torque [34-37]. Har- perpendicular to U, respectively. Lanchester, Kutta, and ris [38] also proved that the 96 degrees arc corner at the trail- Joukowski laid the foundation for a quantitative theory relat- ing edge produces more deflection than corners with the ing the lift to an infinite wing through the integration of the same radius and with a greater arc length at higher jet thrusts velocity field along a streamline. at the same Coand jet momentum.  =V·dL (4) The Coand-effect works best when the slot height is  about 1% to 5% of the curved surface radius and the slot where lift is height is between 1 and 2 per mil of the chord length [20, 39]. Profiles with trailing edge blowing using Coand prin- Lift = L = U · (5)  ciple can generate high lift coefficients. At high velocity, Equation (3) is the modified Kutta-Joukowski theorem tangentially blown air jet remains attached to a convex sur- for an airfoil with additional blowing. The addition of sur- face due to the balance between centrifugal forces and the face blowing at the trailing edge modifies the original (or sub-ambient pressure in the jet sheet. baseline) circulation, and due to the nature of the resulting The driving and critical parameter for the Coand-effect jet, produces a net thrust. To justify the results of Coand-jet is the dimensionless momentum coefficient C of the jet, study and to give a physical explanation of the effect of associated with the second term of equation (3), which can Coand-jet, one may attempt to carry out simple calculation be defined as follows: using similar first principle and Kutta-Joukowski law for potential flow. Using Kutta-Joukowski law and considering VVh()Vm& = Coanda jet Coanda jet Coanda jet  Coanda jet Coanda jet Coand-jet contribution to the lift, Djojodihardjo et al. [26- C 1122 VS VS 28] arrive at the formula: 22 (8) =  + FkUVVhoriginal Coanda jet() Coanda jet Coanda jet It is important to notice that the increase of the lift coeffi- (6) cient is much higher than the dimensionless momentum co- efficient used. The augmentation can be as large as eighty where h is the moment arm of the Coand-jet with Coanda jet times the applied C [39-41]. So the lift gain is due to flow respect to the airfoil aerodynamic center. is now the total separation control and super-circulation and does not arise modified circulation of the wing-surface blowing system, because the momentum of the jet is directed downwards and the second term describes the reaction lift due to the jet. [41]. In the above equation, denotes the width of the jet. A deri- vation carried out by Siestrunck [33] comes to an analogous III. HISTORICAL DEVELOPMENT OF COAND expression: TECHNIQUE In the early 1900s, Coand [42] and Coand and France =  ++ 2 FkUpVtotal Coanda jet Coanda jet i [43] conducted experiments with unusual mounted engine in (7) front of an aircraft, and discovered a strange phenomenon 6 Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1 Djojodihardjo and Thangarajah

Fig. (4). Coand-jet configuration (adaptation from [9] and [37]).

Fig. (5). Dual Radius Circulation Controlled Wing (CCW) Airfoil with LE Blowing (Englar [34-37]).

a b c

Fig. (6). (a) and (b) Custer Channel-Wing Anonymous, aircraft (Liska, [44, 45]; Englar and Campbell, [46]); and (c) Antonov Izdeliye 181, experimental aircraft, at the State Aviation Museum, Zhuliany Airport, Kiev, Ukraine1. that the hot airflow from the engine nozzle was attracted to were much lower than the lower surface, just like an aircraft nearby surfaces (aircraft ). This Coand engine was wing. an experimental plane powered by a ducted fan. For this dis- He construed that the lift is “due to the speed of air, not covery and patent, Coand referred it as “Method and appa- the airspeed” (Liska [44, 45]). Custer’s novel design of the ratus for the deviation of a fluid into another fluid”. This aircraft features a half circular wing aircraft to place the pro- invention was further discussed by the leading aerodynami- peller so that the upper surface of that part of the wing can cist during that time, Theodore von Karman who named it as take advantage of the speed of air from the , as Coand effect. Sometimes in 1920s, another inventor, shown in Fig. (6), exhibiting such principle. Willard Custer, took shelter in a barn during a hurricane, when suddenly, the roof of the barn was blown away (“lifted-off”) and soared through the air. This event led him to realize, that what happened was due to the notion that the 1http://www.geolocation.ws/v/W/File%3AAntonov%20Izdeliye%20181. pressure forces acting on the upper surface of the barn roof jpg/-/en, accessed 25 December 2013. Aerodynamic Surface Circulation Control Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1 7

Although the design failed to be commercialized due to tures downstream of the vehicle. Circulation control has prevailing circumstances, Custer channel-wing concept been implemented in various applications since its inception. proved that the Coand effect takes place since the flow en- Among these applications that have been studied in the past trained to the top of the wing surface increases the overall are high lift fixed wing aircraft, vertical/short takeoff and circulation as well as the lift forces (Englar [35]; Englar and landing aircraft (V/STOL), and anti-torque systems for ro- Campbell, [46]). As exhibited in Fig. (6c), a new initiative torcraft. West Virginia University (WVU), was among those has taken place in to experiment on the channel that conducted a study on two circulation controlled cam- wing aircraft. In 1952, another attempt to design a Coand bered airfoils, for fixed wing aircraft applications. effect aircraft was proposed by John Carver Meadows Frost, Two models were developed to prove that the addition a British aircraft designer, who was given an initial funding of a high velocity jet of air ejected tangentially around the by and the Canadian government to start a pro- airfoil would produce a greater lifting force on the airfoil [1] ject to design a vertical takeoff landing (VTOL) aircraft, due to the Kutta-Joukowski theorem, as modified in Equa- called “Avrocar”. Frost believed that the Coand effect is tion (4). Their work allowed the creation of a non- able to provide a powerful ground cushion that could have dimensional relationship to be established between ejection both the VTOL and high performance aircraft capabilities. slot height, airfoil chord, and trailing edge radius. This in- After a remarkable breakthrough, in 1954 the United States formation became important in choosing an airfoil profile for Air Force and later the United States Army agreed to con- CC applications, as there was a region where the Coand tinue funding the project (Pugliese and Englar, [47]; Englar operation was most effective, as postulated by Englar and et al. [48]; Englar and Huson [49]; Englar [50]; Englar [51]; illustrated in Fig. (7) [1]. The airfoils which were designed to and Rose and Butler, [52]). However, the project was offi- fall within the shaded region would be able to take more cially stopped in 1961 due to financial problem and other advantage of the benefits of the CC technology than ones technical factors [52]. The noteworthy capabilities of the which fell outside of this region. In 1976, the inaugural CC Coand effect were also implemented on many other air- flight demonstrator was built and flown at WVU. The pio- crafts such as Boeing YC-14 and C-17 Globemaster III, An- neer plane was equipped with a retractable Coand surface tonov An-72 Coaler, McDonnel Douglas YC-15, and the No- on the TE of the aircraft, as shown in Fig. (8) [56]. It was Tail-Rotor (NOTAR) (with a Coand effect tail). found that CC flap deployment increases the wing chord by In addition, the applications were further used in other areas 20% for increased performance with blowing. The flap is such as in marine technology, automotive industry, air condi- articulated with a bell crank but is not rigidly connected to tioning system, medicine (as a ventilator), and meteorology. the piano hinge at the sharp TE. Its sliding connection allows the rounded TE to thermally expand up to  inches when the The Coand effect takes place when there are jet flows hot gas bleed air is routed from the auxiliary turbine situated over a round or curved surface. The jet flow will be en- in the rear seat of the aircraft. The of this aircraft trained to the nearby surface due to the balance of the radial produced lift augmentation increases of the local C value pressure gradient and centrifugal forces. The flow direction L from 2.1 to 5.3 with a C of 0.12. will change without separation, and in case of airfoil, pro- ducing an additional increase in circulation, which then will With regards to the large rounded trailing edge (TE) on substantially turn the outer irrotational flow, giving extra lift the early Circulation Control Wing (CCW) designs, high comparable to that of conventional system. In 1979, the US drag penalty in case the jet is turned off may result. Tong- Navy sponsored a full-scale flight test program on an A- chitpakdee [57] countered that issue by designing the airfoil 6/CCW STOL demonstrator to improve the fixed wing air- TE lower surface as a flat surface, while keeping the upper craft operation on carriers [49]. However, the full application surface highly curved to produce a large jet turning angle, of the CC or Coand configured airfoil design has some dis- leading to high lift. The forward rotation of the lift vector advantage, particularly in the drag penalty. Englar et al. [50] contributes to the generation of power from wind turbines, have carried out some research to reduce the drag associated and any increase in the magnitude of the lift force (while with the CC or Coand configured airfoil using a smaller keeping drag small, and L/D high) will immediately contrib- cylinder TE and hinged flap. The result shows that, the CL ute to a corresponding increase in induced thrust and torque values could increase around 120% higher than a conven- (Tongchitpakdee et al., [58]). tional Fowler flap, or a 140% increase in the usable lift coef- If there is some power consumed in the generation of the ficient at takeoff/approach angles of attack, by only using jet, there should be a net positive increase in power gener- available bleed air from the engines (Englar et al., [48]; ated for this concept to be attractive and potential for in- Englar and Huson, [49]; and Englar, [51]). New approach on creasing wind turbines power generation. Although, a crude circulation control by Zha and Paxton [53] has shown that estimate can readily be made to take into account the power another Coand-jet related configuration, i.e. the co-flow jet consumed in generating the Coand-jet, the present study airfoil configuration, is able to enhance the lift and reduce only addresses the aerodynamic aspects of this problem. the drag not only at certain flight condition but also for the whole flight mission. Coand-jet has also been utilized for Circulation Control Wing (CCW) technology for in- improving the aerodynamic characteristics, performance, creasing the bound circulation and hence the sectional lift economics, handling and safety of heavy vehicles (Englar coefficient of the airfoil has been extensively investigated [50-54]). In their drag reduction study on a motor vehicle both experimentally and numerically over many years. One (car - 2-D study), Geropp and Odenthal [55] show that acti- of the well-known techniques in implementing circulation vation of the Coand effect flow control has a great potential control is by tangentially blowing a small high velocity jet to overcome the separation from the turbulent vortex struc- over a highly curved surface, in particular over a rounded 8 Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1 Djojodihardjo and Thangarajah

Fig. (7). Englar's original hypothesis of the region of most effective Coand operation [1, 11].

ing the upper surface in the vicinity of the TE highly curved, since such geometry will produce a large Coand jet turning angle, hence higher lift. Circulation control technology to produce large values of lift advantageous for wind turbine design while keeping drag small (and L/D high) will imme- diately contribute to a corresponding increase in induced thrust and torque. Numerical simulation carried out by Tongchitpakdee [58] looked into two approaches of intro- ducing Coand-jet, i.e., at the leading edge and at the trailing edge, both at the appropriately chosen locations. A leading edge blowing jet was found to be helpful in increasing power generation at leading edge separation cases, while a TE blowing jet otherwise. The effects of pulsed jets on a naturally separating low-pressure turbine boundary layer were studied Fig. (8). Trailing edge Coand surface extended from WVU flight in 2002 at the Advanced Flight Systems Department of demonstrator [56]. Lockheed Aeronautical Systems Company. These vortex generator jets were pulsed over a wide range of frequencies TE, which will produce early flow separation otherwise. Us- at constant amplitude. The resulting wake loss coefficient ing such technique, the boundary layer and the jet sheet will versus pulsing frequency data documented a minimal de- remain attached along the curved surface due to what is pendency on amplitude. Vortex generator jets were shown to known as the Coand effect thus causing the jet to turn with- be highly effective in controlling the location of laminar out separation. Consequently, the rear stagnation point will boundary layer separation. This behavior suggested that move rearward toward the lower airfoil surface. This flow some economy of jet flow may be possible by optimizing the change will then produce an additional increase in circula- pulse frequency for a particular application. At higher puls- tion around the entire airfoil. Looking into the outer irrota- ing frequencies, when the flow is fully dynamic, the bound- tional flow, through the action of the boundary layer, the ary layer is dominated by periodic shedding and separation outer flow will also turn accordingly, producing higher value bubble migration [4]. This study opened the door to research of lift coefficient comparable to that achieved from conven- into pulsing flow circulation control applications as well as tional high lift systems. Tongchitpakdee et al. [57] and conservation of onboard CC pressurization. Tongchitpakdee [58] observed that early Circulation Con- Pneumatic control and distributed engine technologies trolled Wing designs typically had a large-radius rounded TE were explored by NASA Langley Research Center in 2003 to maximize the lift benefit. However, such favorable effect as a means to maximize performance of a new civil aircraft will be accompanied by higher drag penalty when the concept called the Personal Air Vehicle [4]. A morphing Coand jet is not activated. Tongchitpakdee [58] also was designed capable of enhancing propulsive effi- pointed out that to reduce such penalty one can make the ciency throughout the flight envelope [59, 60]. The applica- lower surface of the airfoil TE a flat surface, while maintain- tion of Circulation Control to the nacelle of a shrouded fan Aerodynamic Surface Circulation Control Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1 9

Fig. (9). Schematic of trailing edge Coand configuration of WVU flight demonstrator [56].

Fig. (10). Notable airfoils used in past research and their comparison to “The Region of Most Effectiveness” (adapted from [1, 38, 40, 71, 73]). has been carried out in a Morphing Nacelle design [59, 60] for fluid flow. Coupled with a computer code to compute to enhance off-design performance of the shrouded fan. and predict circulation control airfoil characteristics, this Typically, a fixed geometry shroud is efficient at a single method sets out to predict the interaction between variables operating condition. Modification of the circulation about the involved with circulation control systems. Using flow visu- fixed geometry is intended to provide a means to virtually alization, quantitative, and qualitative methods, it was possi- morph the shroud without moving surfaces, which could ble to compare the aerodynamic forces achievable by adding enhance off-design-point performance with efficiency. Such circulation control. Although the particular experiment is concept could be an attractive propulsion option for Vertical only useful for selected ranges of variables such as pressure Takeoff and Landing (VTOL) aircraft. Such conceptual Per- and density, the overall modeling can be used to predict the sonal Air Vehicle (PAV) configurations were proposed by effect of adding circulation control to a model operating at NASA. Experimental results showed that off-design static any range of variables, so long as the variables are all called thrust performance of the model was improved when the CC out at the beginning of the simulation. Kweder et al. [1] devices were employed under certain conditions, within cer- stipulate that through the study of the applications of circula- tain trade-off conditions, as elaborated in [61, 62]. tion control and previous experimentation, it was possible to A rapid predictive method was studied for the implemen- add further specific airfoils to Englar’s original three dimen- tation of circulation control techniques at the GTRI [63]. In sional airfoil comparison study in 1971 to see exactly where this study, two-dimensional circulation control performance all the models and applications fall in comparison to the “re- calculations were made using the Navier-Stokes Equations gion of most effectiveness,” as exhibited in Fig. (10). In this 10 Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1 Djojodihardjo and Thangarajah figure, the previous noted work has been added to show how each compares to the most effective circulation control re- gion [62-72]. This figure can be very useful in selecting fu- ture airfoils for circulation control applications, as the most important airfoil dimensions (hj, c, and r) are related to one another. This reduces the need to research an airfoil profile in a wind tunnel environment before selecting the appropri- ate model for the application. How Golden and Marshall more recent design [73] fares in comparison to the “region of most effectiveness” is also exhibited in Fig. (10). A recent focus on revolutionary aerodynamic concepts has highlighted the technology needs of general aviation and personal aircraft [4]. New and stringent restrictions on these Fig. (11). Artist Concept of Personal Air Vehicle [4]. types of aircraft have placed high demands on aerodynamic performance, noise, and environmental issues. Improved high lift performance of these aircraft can lead to slower takeoff and landing speeds that can be related to reduced noise and crash survivability issues. In recent years, there has been an increasing interest in revolutionary concepts applied to general aviation and per- sonal aircraft shown in Fig. (11) [4]. New and stringent re- quirements on these types of aircraft include aerodynamic performance, noise, and environmental issues. The use of Pulsed Pneumatic High Lift Technology has the potential to revolutionize by reducing wing area, reduc- ing part count, lowering weight, and reducing potential run- way takeoff and landing requirements.

Traditionally, Circulation Control Wings (CCW) are restricted to a pneumatic modification of the flow field through a Coand effect. This Coand effect shown in Figs. Fig. (12). Trailing edge example of Coand effect [4]. (12 & 13) can be described by a 2-D wall bounded jet that exits from a slot tangential to a convex curved surface. The wall bounded jet flows along the surface and has the nature of a boundary layer near the wall but becomes that of a free jet at a larger distance from the wall. The degree of jet turn- ing can be related to the slot height, surface radius, jet veloc- ity, and the Coand surface geometry. The jet turning angle can approach and in some cases exceed 180o. The jet will remain attached to the curved surface because of a balance between the sub-ambient pressure in the jet sheet and the centrifugal force around the curvature of the surface.

Although the Coand effect is very effective for bound- ary layer control (BLC), the interest in this technology Fig. (13). Coand influence on streamlines [4]. comes from its ability to further augment the circulation and lift with flow turning and control of leading edge stream- examined two dimensional flows to better understand the lines, and is thus named Circulation Control (CC). Lanches- blowing effect. In this regard, they obtained larger overall ter, Kutta, and Joukowski laid the foundation for a quantita- lift when the blowing coefficient was increased. In next the tive theory relating the lift to an infinite wing through the step, they collected lift and drag data in three dimensional integration of the velocity field along a streamline [4]. flow. Four flap deflections (0°, 30°, 60°, and 90°) and differ- ent blowing coefficients were tested. Their research arrived IV. RECENT DEVELOPMENT at some favorable configurations to meet their objectives. De la Montanya et al. [74] carried out research using the For an angle of flap deflection of 30o and blowing coeffi- Coand effect to shorten Landing and Takeoff of airplanes, cient of 0.35, the shortest takeoff distance of 2400ft was in the framework of NASA Ames Research Center search achieved. The shortest landing distance was 2000 ft at an for Extreme Short Takeoff and Landing (ESTOL) vehicles to angle of flap deflection of 90o and blowing coefficient of reduce the airplanes takeoff and landing length. In this re- 0.34. Also the lift coefficient gained was up to 3.5. De la gards, a jet is tangentially blown from a slot near the airfoil Montanya et al. [74] compared lift coefficient CL versus trailing edge. In the initial stage, De la Montanya et al. [74] Coand -jet momentum coefficient C with earlier Harvell Aerodynamic Surface Circulation Control Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1 11

Fig. (14). Lift coefficient CL versus Coand–jet momentum coefficient C; comparison between De la Montanya et al.’s and Harvell et al.’s [75] data. et al. [75] CFD studies; the comparison, as exhibited in Fig. by the blown jet momentum. This provides a secondary con- (14), showed excellent agreement, although there was a little trol in the form of jet momentum with which the lift gener- difference in the jet slot place and shape of the airfoils. De la ated can be controlled. Trade-off analysis has been per- Montanya et al. [74] utilized various values of airfoil x/c and formed for baseline Super STOL configurations, and a thick h/c, namely 89.86% and 0.0016 and for GTRI case the corre- supercritical airfoil modified for circulation control has been sponding values were x/c = 88.75% and h/c = 0.0019, re- suggested which performs adequately in extracting the bene- spectively. fits of circulation control during takeoff and landing and at Nishino, Hahn and Shariff [76] and Rumsey and Nishino the same time producing reasonable values of L/D during [77] carried out a numerical study over a nominally two- cruise. dimensional circulation control airfoil using a large eddy Three-dimensional CC blowing simulations have been simulation (LES) code and two Reynolds-averaged Navier- carried out by Liu [78] by investigating two cases. The first Stokes (RANS) codes, comparing both approaches on a cir- is a stream-wise tangential blowing on a wing-flap configu- culation control airfoil. Different Coand jet blowing condi- ration, which demonstrated that a gradually varied CC blow- tions are investigated, as well as the influence of grid den- ing can totally eliminate the flap-edge vortex, thus reducing sity, using incompressible and compressible flow solvers. the flap-edge noise. The second case involves span-wise The incompressible equations are found to yield negligible tangential blowing over a rectangular wing with a rounded differences from the compressible equations up to at least a . Although CC blowing cannot totally cancel or jet exit Mach number of 0.64. The effects of different turbu- eliminate the tip vortex, it can control and modify the loca- lence models are also studied. Models that do not account for tion of the tip vortex, and increase the vertical clearance be- streamline curvature effects tend to produce jet separation tween the wing and the tip vortex, thus reducing the blade from the Coand surface too late. vortex interaction and the resulting noise. Naqvi [59] performed a computational approach using In line with the general concern for the need of further Navier Stokes Equations (Computational ) experimental results to validate CFD simulation in the study DSS2, a two dimensional RANS code developed by Profes- of circulation control, many recent investigations contribute sor Lakshmi Sankar of Georgia Institute of Technology for to such venture, such as those by Burnazzi and Radespiel analyzing circulation control airfoil characteristics associated [79] and Economon and Milholen [80]. Burnazzi and with NASA proposed Super Short Take Off and Landing and Radespiel’s [79] work represents an advanced step in the Extremely Short Takeoff and Landing (ESTOL) Aircraft. multidisciplinary design of an active high-lift system for The prediction capability produced by this research effort commercial aircraft. The airfoil configuration developed can be integrated with the current conceptual aircraft model- within the framework of the Collaborative Research Centre ing and simulation framework. SFB880 is composed of an active Coand flap and a droop- The Circulation Control relies on the tendency of an nose device. The power required to implement circulation emanating wall jet to independently control the circulation control is provided by electrically-driven compact compres- and lift on an airfoil. Since circulation control airfoils utilize sors, positioned along the wing behind the . This a round trailing edge, the rear stagnation point is free to solution could reduce the additional engine power needed for move, and the location of rear stagnation point is controlled the active high-lift system. Air is provided to the compact 12 Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1 Djojodihardjo and Thangarajah

Fig. (15). High-lift configuration developed by the Collaborative Research Center [79].

(a) (b)

Fig. (16). (a) Analyzed high-lift configuration; (b) DLR F15 airfoil with active Coand flap [79]. compressors by means of a suction slot located on the suc- Introducing such new devices in commercial aircraft raises tion side of the airfoil, which represents an opportunity to many technological issues. The Collaborative Research Cen- increase the aerodynamic performance of the airfoil. The ter SFB880 addresses these issues with multidisciplinary present work investigates the aerodynamic sensitivities of research efforts, which allow to simultaneously deal with shape and location of the suction slot, in relation to the high- different aspects of the development of the system and its lift performance of the airfoil and to the total pressure recov- integration into the aircraft. ery achieved at the end of the suction duct. A significant The means adopted to provide compressed air to the benefit is achieved by suction and the presented analysis high-lift device is a set of electric compact compressors inte- yields physical insight into the flow dynamics around the grated into the wing near the flap, as shown in Fig. (15). airfoil. Burnazzi and Radspiel [79] study is conducted as Here aerodynamics plays an important role, as it is responsi- subproject of the Collaborative Research Center SFB880 in ble for the aerodynamic efficiency of the high-lift system, as Braunschweig. The motivation of the Center is rooted in the well as for providing air to the compact compressors by realization that European airports used by commercial airlin- means of a suction slot located on the suction side at a suited ers are currently operating at rather high capacity. There are position on the wing. The present work focuses on the design two ways to cope with future increases in air traffic: a) to aspects of the suction, based on the following design objec- build new major airports and b) to develop technologies for tives to achieve high lift coefficients and high angles of at- new classes of commercial airplanes that can operate from tack at maximum lift and high total pressure recovery at the existing European airports not in use for commercial pur- end of the suction duct. poses because of short runways or their proximity to popu- lated areas. This class of airplane will have to be quiet and In order to take into account both objectives, a method is use very short runways for takeoff and landing, while offer- proposed to compare and evaluate the overall performance of ing high cruise efficiency as well. Special attention is there- the tested geometries. The high-lift configuration analyzed in fore given to high-lift systems, devices that promise signifi- [79] is equipped with Coand trailing edge flap, droopnose cant flight speed reductions at takeoff and landing. Prelimi- leading edge and suction slot, as depicted schematically in nary design of cruise efficient aircraft states that substantial Fig. (16a). The improvement of the lift is obtained by careful reductions of runway length are only possible by increasing adjustments of the design parameters of the trailing edge the maximum lift coefficient by significant factors accompa- device while the leading edge is geometrically fixed. Steady nied by a moderate increase of the installed engine thrust [1]. blowing is carried out to produce suited turbulent wall jets Another problem is that currently used high-lift systems em- that exploit the Coand effect for effective flow turning. The ploy gaps, which are major sources of aerodynamic noise most important design parameters are flap deflection angle, during landing. Both these aspects should be improved by momentum coefficient of blowing and blowing slot height the new technologies that are being developed. In particular, [4]. While flap angle and blowing momentum coefficient the high lift coefficient needed to allow low flight speeds is should increase for increased lift targets, optimal slot heights obtained by active circulation control. This will also lead to a are rather small, with values of around 0.0006 times the air- significant noise reduction if gap-less devices at both the chord length. It was found that the optimum slot height trailing edge and the leading edge of the wing are used. is independent of the flap angle. Figure 16b displays a typi- Aerodynamic Surface Circulation Control Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1 13

(a) (b)

Fig. (17). (a) The effect of the suction at 85% c on the flow over the flap, C = 0.016,  = 10°, left without suction, right with suction; (b) The effect of the suction at 85% c on the flow over the flap at condition (without suction at 17°, left, and with suction at 15.25°, right), C = 0.016 [79]. cal design result, where the transonic airfoil DLR F15 is equipped with an internally set at 65° deflection angle. Burnazzi and Radespiel study deals with the implementa- tion of the suction slot on the previously designed configura- tion. Figure 17 displays the influence of installing Coand - Jet on such airfoil as results of the study. The detailed curva- ture distribution of the Coand surface used as flap knuckle shape was found to be less important. Values of the radius of curvature of around 0.07 times the chord length are a reason- able design choice. Also the flap length suited to achieve high lift gains could be identified. Best lift gain factors were obtained with flap lengths of 0.25-0.30 times the airfoil chord [3]. With these design choices typical lift gains over blowing momentum (lift gain factor) of 80 are obtained at a Fig. (18). Effect of blowing momentum on the angle of maximum lift coefficient around 4 whereas this value is reduced to 55 lift for the DLR F15 airfoil with 65° flap angle, computed for M = 0.15, Re = 1.7 x 106 [79]. at a lift coefficient around 6. In order to control the pressure distribution, the shape of the clean nose was morphed. As one can see in Fig (19), that the camber-line and the thick- ness are increased, resulting in a reduction of the suction peak over the nose. The morphed shape allows to distribute the low pressure area on a wider surface, reducing the mini- mum values. In Fig. (19), one can see a comparison between the clean nose and the droopnose configuration. The Cp dis- tributions refer to stall conditions, and result in the coeffi- cients shown in Table 1. An approach has been proposed to compare and evaluate slot geometries and locations taking into account both the aerodynamic performance of the airfoil and the total pressure recovery inside the duct. Furthermore, wall suction is found to be about twice as effective when applied in presence of a droopnose leading-edge device, Fig. (19). Cp distributions at stall conditions, C = 0.06 [79]. rather than the clean nose configuration.

Circulation control is an active flow control method with Table 1. Effects due to the Droopnose, C = 0.06 [78]. the potential to generate more lift than that obtained from a conventional airfoil or finite wing geometry. In general, circu- lation control consists of an air jet blown tangentially from a thin slot along the trailing edge of an airfoil or wing, which through the Coand effect can be utilized for separation control or super-circulation control. The net results are a change in the circulation strength around the airfoil and, consequently, lift augmentation (Economon and Milholen, [80]). NASA consid- congestion and a need for noise abatement. In this connec- ers circulation control to be a viable flow control method for tion, noting that for planning purposes one could utilize lift augmentation, and subsequently has incorporated it into computational fluid dynamics (CFD) studies, 2004 achieving the NASA Subsonic Fixed Wing program's Cruise NASA/ONR workshop revealed that CFD studies on circula- Efficient Short Take-Off and Landing (CESTOL) goals. The tion control geometries have been widely unreliable. There- CESTOL goals have been prompted by recent airport fore, the NASA Langley Research Center has undertaken a

14 Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1 Djojodihardjo and Thangarajah

(a)

(b) 4 Fig. (20). (a) Effect of NPR (Nozzle Pressure Ratio) on CL (left) and Cd (right). (M = 0.1, Re = 5.74 x 10 ); (b) Cp comparisons for NPR = 1.1 (left) and NPR = 1.4 (right). (M = 0.1, Re = 5.74 x 104) [80]. research program to provide CFD validation data for circula- gram to more complex circulation control geometries and, tion control geometries. These validation data could provide ultimately, full-scale implementation. Lift and drag are of better understanding of the complex flow physics involved to primary importance due to the high-lift goals of CC. Sub- build a database for CFD validation of circulation control stantially large lift values and relatively low drags are re- cases. Through better understanding and validation, CFD quired in order for the technology to become advantageous. would be able to provide reliable predictions and aid in the In the CFD parametric study, Economon and Milholen eventual implementation of full-scale circulation control. [80] investigated the influence of the momentum coefficient, Economon and Milholen [80] performed a parametric or blowing intensity using a baseline grid and the k -  turbu- CFD study using NASA's TetrUSS software in order to cre- lence model. The blowing condition was varied from a Noz- ate a database for 2-D circulation control CFD validation. zle Pressure Ratio (NPR) of 1.05 up to 1.60 while all other Several important parameters were varied and investigated boundary conditions and variables were kept constant. These such as the amount of blowing, or momentum coefficient values correspond to C values between approximately 0.03 (C), the size and shape of the computational grid, and the and 0.30. The lift and drag results are presented in Fig. (20). turbulence models employed in the computations. Their in- Figure 20a shows that CL values obtained from CFD compu- vestigation yielded a set of results offering better physical tations increased to a maximum of approximately 5.7 follow- understanding of the 2-D circulation control phenomenon ing a parabola-like curve. Cd values varied almost linearly including the effects of varying key parameters. Further, the with NPR to a maximum of 1. If the blowing intensity was results will act as part of a database for circulation control increased, both lift and drag rose markedly. Within the in- CFD validation and will aid in progressing the present pro- vestigated range, a 52% increase in NPR resulted in lift and Aerodynamic Surface Circulation Control Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1 15 drag coefficients growing by factors of 4.7 and 9, respec- butions in Fig. (21a), the SST model over-predicts the flow tively. turning angle and results in more suction on the lower, trail- ing edge surface. The k- model behaves similarly, although In general, the computational results followed the ex- to a lesser extent. The SA model most closely resembles the pected trend of increased lift and drag with increased blow- ing. However, comparing the CFD computations to the ex- experimental Cp data in the separation region, and thus is likely to have most accurately located the separation point. perimental data obtained, large discrepancies were observed. All these experimental observations show that the separation Wind tunnel measurements data indicated that the maximum point occurs slightly higher on the trailing edge than even the lift and drag coefficient values for the investigated geometry SA model predicts. were 4.77 and 0.24, respectively. Further, the slopes of in- crease with NPR were lower. These are clearly exhibited in A particular remark is in order regarding turbulence Fig. (20a). modeling utilization. Turbulence Modeling in CFD is very However, the CFD and experimental results for drag do essential in the choice of grid fineness and for obtaining the not agree at any location, the lift data agree at NPR values correct simulation of particular flow field. For this purpose, one has to choose a particular model out of a host of avail- lower than 1.10, or for C values between approximately 0.03 and 0.07. able turbulence models developed to date. One of the models being user friendly is the k- turbulence model that without To gain insight into the disparity in lift, surface Cp distri- disregarding other models may be suitable for the purpose of butions around the airfoil geometry were examined at a low the work carried out by the author in [26-28]. The k- turbu- and high NPR value as shown in Fig. (20b). The peaks seen in lence model was first proposed by Harlow and Nakayama the CFD predictions near the trailing edge of the jet pressure at [81] and further developed by Jones and Launder [82]. Con- the slot exit are method dependent results. For NPR = 1.1, tinuing extensive research has been carried out to understand experimental and the CFD Cp data agree well, including excel- the nature of turbulence, and this theory has been further lent agreement along the lower, trailing edge near the separa- elaborated and many other theories have also been intro- tion point. A minor disagreement occurs along the upper, trail- duced such as elaborated in [83], which seem to be satisfac- ing edge surface where the experimental data exhibit slightly tory for only certain classes of cases. more suction. However, at an NPR value of 1.4, poor agree- ment was observed. The computational results largely over- The turbulent viscosity models based on Reynolds- predict the experimental Cp values along the upper surface, averaged Navier-Stokes (RANS) equations are commonly and very poor agreement can be observed on the lower surface employed in CFD codes due to their relative affordability near the trailing edge. Economon and Milholen [80] con- [84]. However, since the choice of turbulence model and tended that the large over-prediction of Cp values in suction associated physical phenomena addressed are relevant in surface side as well as the large amount of suction near the modeling and computer simulation of the flow situation near trailing edge offered partial explanations for the variance be- the airfoil surface considered here, a closer look at turbu- tween the CFD and experimental lift and drag results, respec- lence models utilized by the CFD code chosen will be made. tively, at higher NPR values. Although the practical implementation of turbulence model, especially, for the near wall treatment, has been somewhat of Of particular note is the discrepancy between the Cp dis- tributions on the lower, trailing edge near the separation a mystery [83], the numerical implementation of turbulence point. Each of the three turbulence models provided a dis- models has a decisive influence on the quality of simulation tinct shape in their pressure distributions, which suggested results. In particular, a positivity-preserving discretization of that they each modeled the separation location differently. the troublesome convective terms is an important prerequi- Figures 21b-21d depict the separation points and streamline site for the robustness of the numerical algorithm [85]. The  turning at the trailing edge of the geometry for each turbu- k- model introduces two additional transport equations and lence model. As expected, each model predicted a unique two dependent variables: the turbulent kinetic energy, k, and  separation location, which certainly aids in explaining the the dissipation rate of turbulence energy, . Turbulent viscos- differing lift and drag values between the models. By com- ity is modeled by using the Komolgorov-Prandtl expression paring the separation points in Figs. (21b-21d) to the distri- for turbulent viscosity

(a) (b) (c) (d) Fig. (21). Separation locations for the k -  (left), SA (center), and SST (right) turbulence models. (M = 0.1, Re = 5.74 x104, NPR = 1.4) [80]. 16 Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1 Djojodihardjo and Thangarajah

2 2  =  k  = k  C or  C (9)   where C is a model constant. In expression (9),   is based on the eddy viscosity assumption introduced for convenience of further analysis by Boussinesq [86] to draw analogy be- tween the momentum transfer caused by turbulent eddies and the viscosity in laminar flow. The dimensionless distance in the boundary layer, sub-layer scaled, representing the vis- cous sublayer length scale, plays significant role in capturing relevant physical turbulence phenomena near the airfoil sur- face commensurate with the grids utilized in the numerical computation. In this regard, the flow field in the vicinity of the airfoil surface is usually characterized by the law of the wall, which attempts to identify intricate relationships be- tween various turbulence model scaling in various sub- layers.

For example, the wall functions approach (wall functions were applied at the first node from the wall) utilized by k- Fig. (22). Effect of turbulence models on the upper surface, mid- chord boundary layer. Typical y + values in the turbulent boundary turbulence model uses empirical laws to model the near-wall region (Kuzmin et al., [84]) to circumvent the inability of the layer and velocity profiles in wall units for subsonic flow over flat k- model to predict a logarithmic velocity profile near a plate, M = 0.2, Re = 1  106, medium grid; Nozzle Pressure Ratio = wall. The law of the wall is characterized by a dimensionless 1.4. [80, 87, 88]. distance from the wall defined as: u y Golden and Marshall [73] recently analyzed and de- y+ =  (10) signed multiple circulation control (CC) dual radius flap con-  figurations with varying specific flap parameters. Considera- Subject to local Reynolds number considerations, the tions are given to supercritical airfoil most commonly used for CC purposes, with large leading edge radius, in anticipa- wall y + is often used in CFD to choose the mesh fineness requirements in the numerical computation of a particular tion of NASA N+2 Cruise Efficient Short Take-Off and flow. The CFD code utilized using Land (CESTOL) for 100 passenger . It is desired that y + value of 11 and as suggested in [83] was found to yield satisfactory results as CCW configuration will prevent stall during super circula- indicated there. Figure 22 below shows the influence of tion, and there should be sufficient space for leading edge y + in representing the boundary layer near the wall [80, 87, 88]. plenum as well as trailing edge plenum and slot, and it ac- commodates a radial surface favorable cruise performance A study has been performed by Hall et al. [89], to inves- by delaying shock-induced drag. To this end, the design fea- tigate the effect of Coand-jet blowing on the upper or lower tures dual-radius CCW flap configuration for lower cruise duct surface of a natural blockage thrust reverser. configuration pressure drag, increased jet thrust recovery, The Coand-jet blowing on the lower duct surface (blocker) smaller radius turns flow and larger radius keeps jet flow increases axial reverse thrust force and prevents separation. attached. Figure 23 depicts the Dual-Radius Geometry con- While, Coand-jet blowing on the inlet ramp has a detrimen- ceived. tal effect on the reverse thrust force generated. Sellars et al. In the Dual-Radius Geometry Design [73], the flap was [90], successfully carried out a joint experimental and a CFD designed to r1, because it is the first radius the flow encoun- investigation of circulation control, on a sharp leading edge ters. The following configuration was adopted for setting the delta with two leading edge sweeps using CCW dual-radius parameters: Coand blows over a semi-circular TE and showed that Coand blowing circulation control on a delta wing can pro-  Trailing Edge duce a significant lift increment. Later, Frith and Wood [91] hh r performed an experimental investigation on a full span delta slot,TE ===0.00238; slot,TE 0.04857;1 0.04900 wing test configuration in a closed-loop wind tunnel at crc  1 25m/s to investigate the concept of applying Circulation Control (CC) as a means of trimming an aircraft. It was  Leading Edge found that only low values of momentum jet coefficient, C hh up to 0.002, would be required to trim the aircraft at the slot,LE=  slot,TE 0.60= 0.001428 range of investigated. The variation of pitch- cr1 ing moment about the quarter-chord indicates that circulation control could be used to trim an aircraft, as well as providing Parametric studies carried out by varying the dual radius high lift. Moreover, the differential blowing could provide ratio and the chord length with flap, among others, lead to roll control, as the roll increment seemed to vary reasonably some agreeable configurations, whose details are elaborated in linearly with momentum jet. [73]. Aerodynamic Surface Circulation Control Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1 17

Fig. (23). Sketch of CCW Dual-Radius Geometry (adapted from Golden and Marshall [73]); Leading edge slot is intended to prevent upper surface separation during circulation control.

(a) (b) (c) Fig. (24). Velocity fields of S809 airfoil (a) With and (b) Without Coand-jet [Djojodihardjo et al. [27, 28]] (c) Observation of Nishino et al. [76].

Numerical simulation carried out by Tongchitpakdee [58] corners with the same radius and a greater arc length at looked into two approaches of introducing Coand jet, i.e., at higher jet thrusts with the same Coand jet momentum. the appropriately chosen point in the vicinity of the trailing The case studies performed here use normalized dimen- as well as leading edge. A leading edge blowing jet was sion: airfoil chord length of 1m and free-stream velocity of found to be helpful in increasing power generation at high 10m/sec. These correspond to Reynolds number in the order wind speeds. The progress of high speed computers exempli- of 106, which is considered to be a typical situation. A simple fied by the availability of new generations of notebooks has two-dimensional CFD Modeling using k- turbulence model made possible the use of first-principles-based computational is utilized to reveal the key elements that could exhibit the approaches for the aerodynamic modeling of wind turbine desired performance criteria for a comprehensive series of blades, to name an example. These approaches are compre- configurations. Parametric study performed indicates that hensively based on the fundamental laws of conservation of Coand configured airfoil can only be effective in certain mass, momentum, and energy, and hence they should be able range of trailing edge radius, Coand-jet thickness and mo- to capture much of the physics in great detail. Such ap- mentum jet size; the location of the Coand-jet was found to proaches should also be particularly helpful at high wind be effective when placed close to the trailing edge. The re- speeds, where appreciable regions of separation are present sults are compared with existing experimental data for and the flow is unsteady. With such background Djojodi- benchmarking. Three dimensional configurations are synthe- hardjo and Hamid work (Hamid [25], Djojodihardjo et al., sized using certain acceptable assumptions. [26, 28]), searches for favorable Coand -jet lift enhanced A trade-off study on the S809 Coand configured airfoil configuration for wind turbine designs. For this purpose, is needed to judge the optimum configuration of Coand-jet after a rigorous review of Coand-jet circulation control air- fitted Wind-Turbine design (Djojodihardjo et al. [28]). CFD foil, a generic proof of concept approach in two-dimensional numerical computations for the flow-field around two- subsonic flow is performed. Numerical simulation using dimensional airfoil S809 have been carried out with the ob- commercially available Navier-Stokes CFD method is car- jective to study the extent to which the introduction of ried out and a critical scrutiny of the computational proce- Coand-jet enhances the aerodynamic performance of the dure and grid generation is performed. Tongchitpakdee [58] airfoil, here represented by the L/D value, and lead to the found that best lift enhancing effect could be facilitated by following observations. The flow field in the vicinity of the designing the lower part of the trailing edge surface to be flat TE for both configurations is shown in Figs. (24a & 24b). and this finding is also incorporated in the present numerical investigation. Harris [34] also proved that the 96 degrees arc Figure 24c reproduced from Nishino et al. [76] is also corner at the trailing edge produces more deflection than shown for verification purposes. Careful inspection of these 18 Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1 Djojodihardjo and Thangarajah

Fig. (25). (a) The effect of jet thickness on the L/D and lift augmentation (Re = 1 x 106, RTE = 30 mm); (b) Flow separation with t = 0.5 mm; (c) Flow separation with t = 3.0 mm (Djojodihardjo et al., [28]).

6 Fig. (26). The effect of Coand-jet location on the L/D and lift augmentation (Re = 1  10 , RTE = 10mm, tjet = 1mm); (a) C = 0.005; (b) C = 0.010; (reproduced from Djojodihardjo et al. [28]). figures may lead to the identification of the geometry of the sent study indicates that the optimum jet thickness is com- flow that could contribute to increased lift, in similar fashion mensurate with the airfoil dimension. Care should be exer- as that contributed by flap, jet flap, or . Figure cised to avoid flow separation at larger Coand-jet thickness. 24b typifies the flow field around Coand configured S809 With specific design specifications related to the Coand-jet airfoil without Coand-jet (only with its back-step configura- thickness and TE rounding-off size, the Coand-jet momen- tion), which is used here to get insight to the action of the tum needed to improve the performance (lift augmentation Coand-jet. due to jet) should not be excessive but sufficient to delay separation until the tip of the TE (where the upper surface To a certain extent, smaller TE radius produced better meets the lower one). L/D than larger ones. It is also noted that after certain value, further increase in TE radius does not give significant lift In addition, the Coand-jet should be placed sufficiently augmentation, as indicated by Fig. (25). Their results also close to the TE to avoid premature separation. With due con- exhibit the effect of Coand- jet location on the L/D and lift siderations of prevailing three-dimensional effects, the two- augmentation for various values of Momentum coefficient dimensional numerical study can be used to direct further C., as exhibited in Fig. (26). There is a range of effective utilization of the CFD computational procedure for Wind- Coand-jet size designs, depending on their thickness. Turbine blade studies and their design optimization. Numeri- Within the limits of local boundary layer thickness, there cal results presented have been confined to zero angle-of- attack case, which has been considered to be very strategic in is a certain range of effective Coand-jet thickness. The pre- exhibiting the merit of Coand-jet as lift enhancer. Aerodynamic Surface Circulation Control Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1 19

The numerical studies could be extended to higher an- tional blowing where a jet is injected tangentially all around gle-of-attack to obtain more comprehensive information, for the wing perimeter. The wake structure of the CC wing is which the choice of turbulence model will be more crucial. compared to that of a wing without CC but at the same lift The study also shows that the maximum total energy output coefficient. For the CC wing, the stream-wise roll up of the of Coand configured airfoil may exceed that predicted by trailing vortex wake is delayed relative to that of a wing with- Betz limit [Djojodihardjo et al., [26-28]]. With all the results out CC. Turbulence Modeling was found to be a critical factor obtained thus far, it is felt that the present work is by no in circulation control simulations. The wake structure (velocity means exhaustive. Other issues may still be explored, such and vorticity) of the CC wing is then compared to that of the as how could the ambient air energy input that can be drawn same wing at equal lift coefficient but without CC. The lift for by the Coand-jet configured Wind-turbine either from the the case without CC is generated by placing the wing at the nacelle or elsewhere be utilized to energize the Coand-jet, proper angle of attack. Using commercial software FLU- and for that matter, to lower the cut-in speed of Horizontal ENT6.3TM, the aerodynamic phenomena and performance Axis Wind Turbine (HAWT) or the starting speed of Verti- characteristics associated with the disc were determined by cal Axis Wind Turbine (VAWT). exercising care on the turbulence model, the computational mesh commensurate with the regions of high momentum ex- The Coand propulsion concept is intended to fill the change between wall jet and free shear layers. efficiency gap between the rotary wing and fixed wing micro aerial vehicles (MAV), studied by Schroijen and van Tooren [92]. It should have the capability to hover efficiently as well V. BRIEF REVIEW OF TRAPPED VORTEX CAVITY as cruise efficiently. This puts the concept in direct competi- TECHNIQUE tion with the flapping wing MAVs, however the Coand Trapping vortices is a technology for vortex shedding propulsion concept could benefit from the reduced number prevention and drag reduction in flows past bluff bodies, as of moving parts. The preliminary wind tunnel test and Euler well as maintenance of lift for flows past streamlined bodies computation showed a difference in the pressure coefficient at high angles of attack. The European Vortex Cell 2050 of a factor of 2, which is probably caused by viscous effects, program, for example, has the objective of delivering a new like boundary layer formation and separation (Schroijen and technological platform by combining two cutting-edge tech- van Tooren, [92]). Schroijen and van Tooren design was nologies, the trapped-vortex cell (TVC) and the active flow based upon an annular wing wrapped around a centrifugal control for the prevention of vortex shedding and reduction flow generator, potentially creating a vehicle with no exter- of drag, and is envisioned for applications to the next- nal moving parts, reduced vehicle aerodynamic losses com- generation thick-wing aircraft (Vortex Cell 2050, [30]). pared to previous V/STOL technologies and substantially eliminating induced drag. It appears to offer greater potential The interest of such new flow control and aerodynamic at a micro-aerial vehicle scale with regard to fundamental surface performance enhancement is illustrated by a series of “lift to weight ratio” performance parameter and shows that continuing and recent research efforts, such as the pioneering such a wing works best with a thick airfoil section. They also work of Rossiter [76] and Gharib and Roshko [77], to men- experienced efficiency losses on their design, mainly occur- tion only a few examples. Large vortices created in high- ring from annular flow expansion and the problem on choos- speed flows past bluff bodies are shed downstream, and pro- ing the best blower slot heights. The modified approach was duce an unsteady wake. These vortices and associated un- further explored and proved viable with the use of pure up- steady wake give rise to an increase in drag and unsteady per surface blowing with Coand effect. Saeed and Gratton loads on the body. Figure 27, which is reproduced from De [93] have conducted a research program in exploring a new Gregorio and Fraioli [96], could give some insight into the lift system for Vertical/Short Take-Off and Landing potential of trapped vortex cavity in aerodynamic perform- (V/STOL) aircraft to set a path for further research in the ance enhancement and its application for flow control on a design of future V/STOL aircraft. Using a Coand surface at high thickness airfoil. The influence of the TVC on the the duct trailing edge, in his study on Ducted Fan Aerody- model without mass transfer has been investigated for differ- namics and Modelling, Ohanian [94] demonstrated that by ent Wind-Tunnel speeds (from 15 to 30m/s) and different turning of the stream-tube exiting the duct, a normal force is angles of attack. The flow field and pressure coefficient for created and a decrease in results. At the all these cases indicated that the flow separates from the cav- leading edge, steady and synthetic jet blowing caused sepa- ity tip and does not reattach downstream the cavity. This ration on the duct lip at high angles of attack. This separation effect is more noticeable as the wind speed or the angle of reduced thrust and also decreased the nose-up pitching mo- attack is increased. Therefore, lower velocities (such as ment. 15m/s) have been given more attention. At this point, one could mention new initiatives to utilize Looking from another perspective, synthetic jets can be Coand effect for propulsion, in addition to those carried out introduced by the advection and interactions of trains of dis- in [60, 61], among others. In this conjunction the work of crete vortical structures and have zero net mass flux. A dis- Ragab et al. [95] is of great interest. In their work, a numerical tinctive feature of synthetic jets is that they are formed by study of 3D circulation control (CC) using Coand effect was intermittent suction between successive ejections through an carried out on a low aspect ratio wing of circular planform. orifice in the flow boundary, therefore introducing trains of The Reynolds-Averaged Navier-Stokes (RANS) equations and discrete vortical structures that originate entirely from the a second-order closure model for the Reynolds stresses are fluid of the surrounding flow system, and thus transfer mo- solved using commercial software (FLUENT6.3TM). The flow mentum to the cross flow without net mass injection across field around the wing is analyzed for the case of omnidirec- the flow boundary. 20 Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1 Djojodihardjo and Thangarajah

Fig. (27). Flow field without suction in front the model (left) and with suction (right) (De Gregorio and Fraioli [96]).

Fig. (28). Flow field velocity inside the cavity without suction/blowing mass flow (left) and with (right) (De Gregorio and Fraioli [96]).

Such jets can be produced by actuators integrated in the pressure coefficient considered in the Wind-Tunnel investi- boundary of a cross flow, such as by the motion of a dia- gation by De Gregorio and Fraioli [96], the flow separates phragm that is built into one of the walls of an otherwise from the cavity tip and does not reattach downstream the sealed cavity below the surface. However, when the actua- cavity. This effect is more noticeable as the wind speed and/ tion frequency is sufficiently high to be decoupled from or the angle of attack of the airfoil is increased. Figure 28, global instabilities of the base flow, changes in the aerody- reproduced from De Gregorio and Fraioli [96], shows the namic forces are attained by controlling the generation and prevailing flow field pattern inside the cavity and immedi- regulation of ‘trapped’ vorticity concentrations close to the ately downstream the airfoil surface for a wind speed of surface to modify its aerodynamic shape (Glezer [29]). 15m/s and for six different values of angles of attack  (A,  = 5.66° and F,  = 12.66°). Figure 28 reveals the magnitude A trapped vortex can manifest itself as a steady separa- of the flow stream lines and shows that for the smaller angle tion eddy above an aerofoil at high angle of attack. However, ( = 5.66°) a weak vortical structure inside the cavity results, with the use of an appropriate cavity, a trapped vortex cell with its center located far from the geometrical center of the (TVC) will be produced and a dramatic change in aerody- cavity and moved toward the shear layer region. Further- namic performance will take place. Practical utilization of the trapped-vortex idea poses a challenge, since the trapped more, the vortex is elongated toward the exit of the cavity. The vortex strength is not sufficiently large to force a flow vortex should be almost steady and should remain in the reattachment. As the angle of attack  increases, the center close vicinity of the body. Therefore, the stabilization of a of the vortex moves in the direction of the shear layer region trapped vortex has been considered as a major challenge in and it is drawn outside the cavity, which in turn induces fur- the European Vortex Cell 2050 project [30]. The influence of ther vortex shedding. At the same time, the fluid at the sepa- the TVC on the model without mass transfer has been inves- tigated by many researchers, such as Gregorio and Fraioli rated region downstream from the cavity is drawn inside the cavity. [96] and Donelli et al. [97-99]. For the flow field and the

Aerodynamic Surface Circulation Control Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1 21

Table 2. Summary of Some Coand Effect and Trapped Vortex Cavity Technique Development and Application (Djojodihardjo [102]).

Some Coand Effect Development and Application Example / Remark

1 Invention by Coand Examples / Applications

2 Willard Custer, Englar and Campbell, Channel Wing Aircraft; The lift is “due to the Coand Effect Aircraft speed of air, not the airspeed”

3 Coand effect aircraft Avrocar

4 Enhanced lift by Coand effect Circula- Boeing YC-14 and C-17 Globemaster III, An- tion enhancement tonov An-72 Coaler, McDonnel Douglas YC-15, and the NOTAR helicopter

5 Coand effect CCW STOL Demonstrator Wake Vortex Wingtip-Turbine Powered CC High-Lift System

6 Co-Flow Jet Co-Flow Jet

7 California Polytechnique Group, Mar- Combined Blowing Circulation Control Applica- shall, de la Montanya, Lichtwardt, etc. tion to Extreme short takeoff and landing

8 Automotive Applications [GG] Utilization in Formula-1 cars; Increase of base pressure, Coand effect through exhaust gas for better traction

Trapped Vortex Cell Examples / Applications

1 Pioneers and active contributors, among Without any mass flow suction the TVC did not others: Roshko, Rossiter, Gharib and induce flow reattachment produced by vortex Roshko; TVC2050 Research Group; De shedding Gregorio, Fraioli, Donelli

2 Kasper Wing and the EKIP Aircraft The Kasper Wing (US Patent: No. 3831885) and the EKIP aircraft (US Patent: No. 5417391)

3 European TVC2050 Initiative Prevention of vortex shedding and reduction of drag for next generation thick-wing aircraft

22 Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1 Djojodihardjo and Thangarajah

De Gregorio and Fraioli [96] placed the pressure taps in lay or eliminate flow separation. Common vortex generators the cavity between 54% and 66% of the model chord. The Cp have a height that is similar to the boundary layer thickness values show that the typical pressure exhibits a clear expan- and this produces parasitic drag. At supersonic speeds, shock sion on the upper leading edge region up to a maximum value wave interaction with turbulent boundary layer diminishes until the cavity region. At this point, i.e. in the cavity, the CP aerodynamic performance of the inlets due to shock-induced value forms a plateau, and the flow is separated and unsteady. flow separation, stagnation pressure loss and increase in Further downstream from the cavity, the pressure continues to boundary layer thickness. By bleeding the flow at the shock remain constant; this indicates that the flow is fully separated. impingement, separations can be suppressed and this leads to At this relatively low velocity and low angle of attack of 5.66° an increase in pressure recovery as well as thinning of the and 6.66°, it is possible to characterize the typical pattern of boundary layer. the trapped vortex inside the cavity. Passive control has been However, bleeding the flow involves removing a portion investigated by Donelli et al. [99] to examine the behavior of of the incoming mass flow, requiring larger inlets to com- the TVC without any mass flow suction. pensate for the lost mass flow. This leads to weight and drag The results showed that without any mass flow suction increase. Hence, improved flow control devices that can re- the TVC did not induce flow reattachment due to vortex duce or eliminate bleeding are sought after. shedding. Massive steady suction, however, induces flow An invention by Babinsky et al., US Patent reattachment due to the stabilization of an intense vortex in US20120018021 in 2012 [103] provides a device which gen- the cavity. Donelli et al. [99-101] observed that by increas- erates stream-wise vorticity in a boundary layer. This device ing the mass flow rate, the recovery-pressure increases and is capable of ensuring delayed flow separation and makes when a mass flow suction rate of 25.8m3/h is applied, the way for an airfoil to operate at higher angles of attack. A flow attaches up to a 95% of the chord. The vortex is stead- split-ramp vortex generator (see Fig. (29)) is comprised of ily located in the center of the cavity and the resulting flow is two ramp elements with front and back ends, and ramp sur- fully attached. From their research, De Gregorio and Fraioli, faces extending between both ends. Vertical surfaces extend [96] concluded that passive TVC flow regulation is not able between the front and the back ends adjacent to the ramp to control the flow separation. The vortex will not be re- surfaces. A flow channel is located between both ramps. The stricted in the cavity and in addition there occurs vortex back ends of the ramps have a height greater than that of the shedding which decreases the aerodynamic performance of front ends while the width of the front ends is greater than the airfoil without TVC. However, active TVC flow control that of the back ends. is able to govern the flow separation; full reattachment has been achieved for limited values of the blowing coefficient. Vorticity is created by the flow spill over the peak edges of the ramp pair, which are at an angle to the free stream The above brief review serves to illustrate the major flow. This split-ramp vortex generator channels the flow challenge in developing trapped vortex cavity by a combina- between the ramp elements to the centre of the split-ramp tion of trapped-vortex technology with active flow control. vortex generator, improving boundary layer characteristics Associated with such concern, the European Vortex Cell downstream. This reduction of flow separation enables the 2050 [30] project has formulated specific major objectives, split-ramp vortex generator to reduce drag, improving the among others, to develop a software tool for designing a aerodynamic capability of external surfaces. thick airfoil with a trapped vortex assuming that the flow is stable, apart from small-scale turbulence, and to develop a Parallel to the aforementioned invention, Corke et al., methodology and software tools for designing a system of 2011 (US Patent US20110120980) [104] have incorporated stabilization of such a flow. Some of the development and stream-wise vortex generators (SVGs) in their design. applications of Coand-jet as elaborated in the paper are Stream-wise vortex generators (SVGs) are capable of gener- summarized in Table 2, as an extension of that presented by ating longitudinal vortices that are effective in maintaining Djojodihardjo [102]. attached flow over a surface. However, passive devices like these are always deployed, regardless of need and they lower VI. PATENTS ON CIRCULATION ENHANCEMENT the efficiency of vehicles by adding drag to them. This in- TECHNOLOGY vention consists of a number of plasma stream-wise vortex generators (PSVGs) (1102) (see Fig. (30a)), which comprise Ongoing research in the field of circulation control has two electrodes sandwiched between dielectric layers. The led to the introduction of new concepts and technologies that longitudinal axis of the first electrodes is placed in the mean improve aircraft efficiency by means of lift enhancement, flow direction. This produces body forces in the cross-flow drag reduction and delaying flow separation. For the purpose direction. As the PSVGs are located on the surface where of this review, the present technologies on circulation control flow passes, the cross-flow oriented body forces cause the are divided into three main groups, namely, passive flow downstream flow to roll up, transforming into either a single controls which are typically design and surface blowing ori- co-rotating or pairs of counter-rotating streamwise-oriented ented, active flow control which generally incorporates flow vortices (see Fig. (30b)). actuators or jets and flow enhancement control related to . The vortices while being similar to those produced by passive SVGs are manipulable as the PSVGs can be operated 7.1. Passive Flow Controls Vortex generators (VGs) are used to generate more circu- 2Numbers in bold characters here refer to similar number lation of the airflow in the boundary layer as a means to de- appearing in the original patents quoted. Aerodynamic Surface Circulation Control Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1 23

Fig. (29). Plan view of the split-ramp vortex generators having parallel and non-parallel centrelines [103].

Fig. (30). (a) Shows the front view of a vortex generator system. (b) Is an enlarged view of Figure (a) when the vortex generator system is activated [104]. only when needed. Selective activation of AC voltage to perturb the trailing vortices. The perturbation has to be in both sets of electrodes triggers the PSVGs to generate the real-time at suitable frequency where the total lift experi- vortices. Parasitic drag could be alleviated as the PSVGs can enced by the wing will not be drastically changed. The in- be made flush with the surface. Wake vortices are created by vention incorporates a translating flap integrated to an aero- aircraft flying in low speeds (takeoff and landing) and this dynamic surface. An example is shown in Fig. (30), where a creates potential hazards for the subsequent flights in an air- blunt Gurney flap (200) vertical actuator is fixed at the trail- port. Vortices which are too strong could cause physical ing edge of an aerodynamic surface. Gurney flaps are able to damage and loss of control to following aircraft. To alleviate enlarge the section lift of an airfoil without suffering a large this predicament, large longitudinal spacing between air- drag increase as they effectively change the chamber of the crafts and large spacing between parallel runways are airfoil and this increases circulation around it. The small size needed. This is to eliminate any possibility of a wake vortex of Gurney flaps enables them to achieve this while remain- to interact with a trailing aircraft. However, this measure ing inside the airfoil boundary layer. The active translation greatly reduces an airport’s capacity. of the flap along a slot in the aerodynamic surface to induce early destruction of the wake vortex is created. An actuator is A Karman vortex occurs when a flow passes a bluff used to position the flap in air to enable positioning, translat- body at high Reynolds number. Alternating sign vortices will be shed by bluff bodies and the shedding of these vortices ing and stowing the flap. The flap-aerodynamic surface pair could be configured to one of several pairs that are suited for creates an unsymmetrical flow pattern around the wing, thus specific applications, i.e., having additional lifting surfaces, changing the pressure distribution. This generates a periodic better yaw, pitch and roll control and to extend flutter force which causes vibration along the wing. Several ways to boundary. cause perturbations to the tip vortex in order to excite natural destruction for early destruction were studied. They include Supersonic aircrafts typically employ delta wings as the mounting fixed-position passive vortex shedding perturba- wings experience low drag at supersonic lifts. However, tors in the form of transverse jet or a bluff body near the these wings lead to a sudden formation of leading edge vor- wing trailing edges. However having these perturbators in- tices at off-design conditions due to the separation of bound- creases the wing drag and induces unwanted resonance of the ary layer as it travels to the leading edge from the lower sur- wing due to vibration. Therefore, a need to produce rapid face of the wing, forming a shear layer which curves and variations in the position of vortex arises. These rapid varia- rolls into a region of high vorticity. Additionally, a spanwise tions cause vortex generated from each wing to oscillate. outflow is generated on the upper surface, causing another Two vortices would interact and their destruction would oc- flow separation as it approaches leading edge, thus forming cur much earlier relative to natural circumstances. secondary vortex. The proposed invention by Eaton et al., 2010 (US Patent Vortex lift control is related to the stabilization of the US7740206) [105] provides a wake vortex alleviator on vortex shed from the leading edge of the wing, essentially aerodynamic surfaces which perturbs the wake vortex in locking the leading edge vorticity along the spanwise direc- real-time. The challenge in achieving an active wake allevia- tion of the wing. An effective increase of the wing camber tion strategy is finding the appropriate and practical way to and lift results from the reattachment of streamline airflow 24 Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1 Djojodihardjo and Thangarajah passing over the upper surface of the wing and over the As for Short Take-Off and Landing (STOL) aircrafts, the locked vortex. Passive vortex lift control is a means to con- need for short and narrow austere landing fields requires trol unstable vortex upon reaching critical limits, also known glide paths which are accurate and proper directional control. as vortex burst, where wing lift is drastically reduced. Thus, During a STOL landing, the controllability of a STOL air- it is crucial to delay vortex burst and increase wing lift by craft diminishes drastically. Under certain conditions, the controlling vortex stability. airflow across the upper surface of the wing can suffer from partial or substantial separation from the upper surface, lead- Dixon, 1987 (US Patent US4705240) [106] proposed an ing to a loss of control. This stall condition generally occurs invention that has an apparatus to establish vortex lift along at low flying speeds and when the wing is at a relatively high the span of the planform of a high-lift airfoil. This can be angle of attack and maximum lift is substantial. This condi- employed as a wing, side force generator or control surface. tion is also common in other aerodynamic members such as ailerons, and elevators. Large control surfaces are needed for STOL aircrafts due to the large pitching, yawing and rolling moments associated with powered lift arrangements. Thus, a high drag situation is encountered at cruise. For greater lateral response for a given control input at STOL operation, side force controls are to be enhanced by including thrust deflection vanes in the and also having large control surfaces, aforemen- tioned. However, movable vane configurations give rise to large thrust losses and they need complex actuators which are difficult to maintain, leading to reduced reliability of

performance. As a solution, the invention by Thomas, 1987 Fig. (31). Attachment of a Gurney flap at the trailing edge of an (US Patent US4682746) [107] provides a control force gen- aerodynamic surface [105]. erator which can eliminate the common yaw and roll that are normally associated with using conventional control sur- The separated flow is combined into a leading edge vor- faces. The control force generator reduces drag and alleviates tex adjacent to the leading edge. This organized vortex ex- thrust losses by maximizing its aspect ratio while providing tends along the span of the upper surface and remains lateral force which is independent of other control forces. locked. This locked vortex rotates and leads to a main chord Besides, as previously mentioned, the need for large control wise air flow over the airfoil, above the locked vortex and surfaces is removed. A feature of this invention is the use of then deflects to be reattached to the surface aft of the vortex. circulation control in a single Coand surface, with double The airfoil is provided with a leading edge extension or slot to selectively generate lift on otherwise non-lifting sur- (25) (see Fig. (32b)) to aid in organizing the separate face. flow into the vortex. From Fig. (33), a fluid jet is discharged from a control The strake forms a stable vortex at a given angle of at- port (43B) and this jet, upon flowing in a body of relatively tack and then organizes the vortices shed from the leading stagnant fluid, entrains some of the surrounding fluid and edge into the vortex stabilized by the spanwise flow which brings it in motion. As the surrounding fluid is brought along results from the strake vortex. This conversion of the sepa- the sides of the jet, fluid which has been replenished con- rated flow to an organized vortex increases lift, according to tinuously moves into this region. Along the open air, the the increase in angle of attack until vortex burst is achieved average pressure equals to ambient pressure and the average thus reducing lift. Several other possible configurations of pressure on the Coand surface (45) is below ambient as the this invention include appropriately placed panels together replenishing fluid has to flow down an opening between the with small leading edge to delay separation as much as pos- jet boundary and the Coand surface. The pressure gradient sible. The design of this invention thus provides passive vor- forces the jet to move closer and attach to the Coand sur- tex lift control, delays vortex bursting and improves lift to face. This attachment causes the jet to be deflected tangen- drag ratios. These are achieved by having a favorable pres- tially to the surface and results in an entrainment of the fluid sure distribution which enhances aerodynamic performance, flow stream next to the control port. This, in turn leads to the stability characteristics and structural design loads. imparting of flow velocity to stream in the direction of the

Fig. (32). (a) A conventional swept aft airfoil at a positive angle of attack, illustrating the formation of a leading edge vortex. (b) A conven- tional swept aft airfoil having a strake for organizing and stabilizing leading edge flow separation [106]. Aerodynamic Surface Circulation Control Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1 25

The solution is a device which has easy attachment or detachment to the vehicles (see Fig. (34)). The invention is comprised of two portions: an aerodynamic part and a fas- tening part. The aerodynamic portion deflects incident air. The second portion has two sides, a curved first side and a flat second side. The second part secures both portions. A gripping pad with an adhesive side is attached to the flat side. The surface flow enhancement devices can be placed on the vehicle surfaces to divert the fluid flow around the surface of the device. These increase the circulation of the Fig. (33). A representation of the operation of a control force gen- erator [107]. fluid around the vehicle and hinder friction resistance which increases drag.

An invention by Pitt, 2012 (US Patent US8251317) [109] focuses on the benefits of varying the porosity of an aerodynamic surface. Incorporation of variable porosity sys- tem could lead to enhancement of performance and envi- ronmental impact of an aircraft. This design incorporates two layers of pores and an actuator mechanism whereby the lay- ers are slidable relative to one another. The actuator is de- vised from a shape memory material which changes its length according to ambient temperature. The variable porosity system can be applied to any lifting Fig. (34). A close-up of the flow enhancement devices (100) on a surfaces, control surfaces and propulsion system of an air- vehicle surface (510) [108]. craft. Having the variable porosity system in wings alleviates transonic shock and energizes certain regions of the bound- ary layer, especially at points where separation of flow is most likely to occur. This will delay stall at increasing angles of attack. The variable porosity system also eases the airflow from high pressure region to low pressure region, and pro- motes airflow into the engine inlet. An added benefit to this is that the proper configuration of the variable porosity sys- tem could alleviate noise from aircraft, particularly due to the interaction of vortices with the trailing edge flap during landing.

Fig. (35). Cross sectional view of an aerodynamic surface having a Figure 35 indicates that a sliding mechanism is vital for variable porosity system with pores of both layers in alignment with the system. The system utilizes materials of low sliding coef- each other [109]. ficient and a reliable actuator mechanism, made from shape memory alloy which is able to contract upon application of Coand surface. An apparent increase in angle of attack of heat and thus moving the second layer in relation to the first. the fluid flow over the leading edge can be observed due to The shape memory wires could also be trained via appropri- the downstream deflection of the fluid flow stream. This ate heat treatments to stretch when the aircraft is on the generates a lift perpendicular and away from the surface. The ground at relatively high ambient temperature and contract force generator can also be mounted to the bottom surface of as the external temperature decreases, as the aircraft gains the wing, to produce side force on the aircraft. Placing the altitude. An antagonistic configuration of the shape memory generator on the wing results in deflection of propeller or jet wire pair is preferred as they provide a platform for a mini- thrust and the versatility of the generator placement is a sig- mum-power active control arrangement for this system. In- nificant feature of this patent. clusion of a plenum in the system enables it to act as a recir- culation chamber which re-energizes the flow in low pres- A simple mechanism was developed by Evans, 2013 (US sure region, thus reducing boundary layer thickness and Patent US20130228236) [108] as a provision to control fluid weakening shock waves, both of which lead to enhanced flow in order to minimize drag. As vehicles move through aerodynamic performance. fluid, a layer of turbulent fluid flow is created which circulates along the vehicle, which is slow moving relative to the fluid 7.2. Active Flow Control flow that is not in contact with the surface area of the vehicle. The fluid layers of different speeds meet at the back of the Active flow control systems used in various situations as vehicle and form a low pressure region or a vacuum. The vor- a means to modify shear layers, control flow separation by tices and turbulent layer create a drag force which impairs the energizing the boundary layer as well as produce thrust vec- speed and the efficiency of the moving vehicle. Evans came toring by generating jet deflections. A crucial part of active up with a solution to control fluid flow, alleviating the creation flow control technology is the usage of flow actuators. These of vortices which in turn minimizes the drag. actuators produce a steady, oscillating or pulsating acoustic, 26 Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1 Djojodihardjo and Thangarajah

Fig. (36). An active flow control system integrated in an aerodynamic structure [110].

Fig. (37). Placement of flow actuators on an aerodynamic structure [111]. momentum or mass flow field over a range of frequencies, and acts as a flow blockage when the momentum is suffi- affecting the flow that is being controlled. ciently high, leading to lift destruction. Boespflug et al., 2013 (US Patent US20130284273) Despite having great flow control in laboratory experi- [110] presented a method to manipulate a primary fluid flow ments, the integration of such flow actuators into hardware over a surface by means of an active flow control system. of aerospace vehicles remains a challenge. These devices are The system includes an active fluid flow device which is to withstand harsh environments and shock loads during capable of providing lift enhancement and lift destruction. their operation. Thus, they have to be robust, reliable and The active fluid flow device can also be triggered to produce have a long life cycle. a secondary fluid flow which can be steady blowing, pulsed An invention by Raghu, 2013 (US Patent US8382043) or oscillating. While a primary flow field is created over the [111] incorporates an aerodynamic flow control device surface, a secondary fluid flow is injected in upstream direc- which has a compact array of a multiple discrete fluidic ac- tion where it opposes the inflow of the primary fluid. This tuator oscillators in a planar or linear fashion (refer to Fig. influences the primary flow as momentum of the secondary (37)) or a circular or non-planar structure. The actuators are fluid flow interacts with it. Lift destruction occurs when a large component of momentum of the secondary fluid flow capable of producing high frequency oscillating or sweeping disrupts the incoming primary fluid flow. Lift enhancement jets which interact with the incoming flow and control flow and stall extension are achieved when the small component separation by energizing the boundary layer with appropriate of momentum of the secondary fluid flow mixes substan- mixing. Integration of the present invention into the wing tially with the incoming primary fluid flow. section of an airfoil at various locations along the chord or the flap of a wing leads to an increase in lift force, decrease The active flow control device could be a synthetic jet in drag and a reduction of downwash during vertical takeoff flow control device such as a dual bimorph synthetic jet and landings. Having the flow control devices in the (DBSJ) actuator and an oscillatory jet. The device may in- or tail wing would result in an increase of control forces re- clude other pressurized fluid sources such as a blowing quired to steer the plane. This leads to a reduction of the tail source which is centralized and conducts the fluid through wing size, thus saving weight and fuel. Application of pre- ducts in the blade. sent invention in wind turbine blades generates increased Figure 36b shows an enlarged view of active flow con- power for a given size of a turbine blade and can operate at a trol section of Fig. (36a). Having injected at least one of a lower wind speed. The compact fluidic actuators also help in steady, pulsed or oscillatory air flow of a chosen strength in noise reduction in jet and rocket engines. This is done by the upstream direction toward the primary incoming air flow, injection of small amounts of momentum in the edge of the an enhancement or destruction of lift could be obtained, de- jet from random oscillating jets, which creates axisymmetric pending on the operation settings. The momentum addition vertical structures which are able to weaken the source of together with vorticity production provides an instantaneous aerodynamic noise. Currently, the arrays are made of both lift when combined with the injected secondary fluid flow’s metals such as aluminum and stainless steel and various acceleration (similar to the Coand effect with a steady jet) types of engineering plastics depending on the application. Aerodynamic Surface Circulation Control Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1 27

The development in materials technology in creating new the contour of tail boom and induces wake at the main rotor. lightweight and high strength materials would benefit the This attachment produces lift on the tail boom which coun- fabrication of these arrays in terms of manufacturing cost teracts the torque of the main rotor. Addition of vortex gen- and also increased strength of the arrays. erators (20) gives rise to production of a pair of trailing vor- tex filaments. These interact with the boundary layer to in- 7.3. Helicopters duce free stream, formed by the main rotor. The interaction delays the boundary layer separation from the tail boom’s Circulation enhancement technology has given a fair surface, thus creating lift as well as enhancing circulation share to the improvement of helicopters. The patents dis- control about the tail boom. cussed will include redesigning the tail boom to be of an airfoil shape and also having circulation control in rotors and yaw control. The torque from the main rotor of conventional helicopters during operation causes the fuselage to rotate in a direction opposite to that of the main rotor. At low forward speeds, the downwash force is at a maximum, requiring greater torque control to oppose the fuselage torque. A means of controlling the fuselage torque during takeoff, landing and low forward speed flight is the utilization of tail Fig. (39). (a) Shows a helicopter in a NOTAR system. (b) Shows rotors. A tail rotor for a conventional helicopter is attached the longitudinal nozzles with integrated vortex generators [113]. to the aft section of the fuselage through a tail boom.

Brand et al., 2013 (US Patent US20130087653) [112] Schmidt, 1978 (US Patent US4131390) [114] invented a have designed a tail boom with the shape of an airfoil (see circulation controlled rotor blade, which eliminates flutter Fig. (38)). The tail boom is manufactured as a single airfoil and provides lift by means of Coand effect. The system shaped member where the side surface (309) acts as the pres- works by introducing a laminar flow over the trailing edge of sure surface of an airfoil and the side surface (311) acts as the blade, which has a deformable tube to control a spanwise the suction surface. Both surfaces are tapered towards each slot. This slot is created by the combination of an upper trail- other to form the leading and trailing edges respectively. A ing edge lip with the deformable tube. The laminar flow is a high pressure region is created near the side surface and a result of compressed fluid directed through the rotor blade low pressure region is generated near the suction surface of and through the slot. Having an adjustable slot provides a the airfoil. This pressure difference results in the movement uniform slot width without interfering with the laminar flow of the tail boom towards the low pressure region and this over the Coand edge or without generating turbulence. motion rotates the tail boom in a lateral direction, opposing From Fig. (39a), a helicopter rotor blade is shown having the fuselage torque. An effective and lightweight solution is comprised of a skin, a thin sandwich of epoxy / graphite core therefore presented for the purpose of an anti-torque device between metal, forming the leading edge at the fold. Span- for a conventional helicopter. ners run along the longitudinal length of the blade forming ducts in the blade’s interior, to facilitate flow of air from rotor hub to the position where the blade is attached. These ducts present a uniform flow of the compressed fluid which is directed to the trailing edge assembly along the blade span. Figure 40 shows the components of the trailing edge assembly - a lip, a fluted core or vanes and a Coand edge (34). A detailed cross-sectional view is exhibited in Fig. (41). Fig. (38). Top view of a tail boom detached from the aft section of Combination of the lip and the Coand edge gives rise to the fuselage [112]. a slot which is responsible for introducing fluid flow over the trailing edge and entraining the fluid medium in which the blade is operating to follow the blade surface. This creates Van Horn, 1990 (US Patent US4948068) [113] incorpo- additional lift, due to Coand effect. Having a suitable valv- rated a solution for NOTAR (No-Tail Rotor) system where ing at the hub modulates the compressed fluid flow, thus the tail rotor for a conventional helicopter is replaced with controlling the magnitude of the lift. This patent proposes a vortex generators in the longitudinal slots or nozzles which critical slot width of approximately 0.002 times the chord of generates the circulation control of the system and combines the blade - currently unachievable due to manufacturing con- with a jet thrusters and fluid resource. The thruster directs air strains. Thus, some means for adjusting the slot width with- to either side of the tail boom in various amounts, providing out having any mechanical means at the slot are crucial since variable side force on the boom. Figure 39 shows a pictorial otherwise turbulence will be created. representation of the NOTAR system. A variable-pitch, axial flow fan is taken in air and feeds the longitudinal nozzles The invention presents a circulation controlled rotor (11). blade with an adjustable trailing edge spanwise slot. Non- turbulent compressed fluid is channeled through the slot and Low pressure air is ejected from two thin horizontal noz- the slot gap is controlled by the aforementioned deforming zles which then generates a thin stream of air which follows tube of the trailing edge assembly. 28 Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1 Djojodihardjo and Thangarajah

Fig. (40). (a) Shows the top plan view of a helicopter rotor blade. (b) Is a perspective view of a section of a rotor blade without trailing edge assembly [114].

mands on aerodynamic performance, noise, and environ- mental issues. Improved high lift performance of these air- craft can lead to slower takeoff and landing speeds that can be related to reduced noise and crash survivability issues. Circulation Control technologies have been around for 65 years, yet have been avoided due to trade-offs of mass flow, pitching moment, perceived noise etc. [4]. NASA [115] has a set of “Green Aviation” research goals which are related to mitigating environmental impacts of aviation. NASA’s goals represent the world’s serious effort to achieve environmen- tally friendly aviation and aircrafts technology, such as to reduce aircraft fuel consumption, emissions and noise simul- taneously, which is a much more difficult challenge than working to reduce them individually. Such effort has been Fig. (41). Cross-sectional view of the blade in Fig. (40a), taken envisioned in Europe as Joint Technology “Clean Sky Initia- along line 4-4 [114]. tive” [116]. As pointed out by Rogers [24], the trend and vision of future air-transportation system will address the A fluted core aids in the reduction of turbulence in the following issues: compressed fluid stream and also supports the upper trailing a). Discover, explore, and develop technology concepts to edge lip. This invention claims to have provided a mecha- improve aerodynamic efficiency for overall system-level nism for the adjustment of helicopter rotor blade slot without benefit to energy efficiency and environmental compati- invoking turbulence or disturbing the lift of the blade. bility. All these patents are tabulated in Table 3, which high- b). Meet energy efficiency challenge by reducing drag. lights major features of the selected patents. Such requirements could be met by the following tech- VII. FURTHER DIRECTIONS, APPLICATIONS AND nology approaches [24]: TECHNICAL CHALLENGES FOR AERODYNAMIC (i) Novel configurations enabling laminar flow, reduced PERFORMANCE ENHANCEMENT wetted areas, higher aspect ratio wings, and synergistic Circulation control is a viable active flow control ap- propulsion/airframe integration proach that can be used to meet the NASA Subsonic Fixed (ii) Revolutionary enabling technologies including boundary Wing project’s Cruise Efficient Short Take-Off and Landing layer ingestion, active flow control, and concept- goals. Currently, circulation control systems are primarily enabling flight control strategies designed using empirical methods. However, large uncer- tainty in our ability to predict circulation control perform- (iii) Improved physical understanding and physics-based ance has led to the development of advanced CFD methods. high-fidelity computational design tools with broad ap- This paper provides an overview of a systematic approach to plicability developing CFD tools for basic and advanced circulation In particular, associated with the aerodynamic develop- control applications. This four-step approach includes ment of fuselage and wings, Tailored Fuselage Initiative is “Unit”, “Benchmark”, “Subsystem”, and “Complete System” focused on exploring and developing technologies enabling experiments. The paper emphasizes on the current and direct skin friction reduction, with a goal of reducing the planned 2-D and 3-D physics orientated experiments with fuselage turbulent boundary layer drag by 10%. Associated corresponding CFD efforts. Sample data are used to high- with N+3 aircraft design, the ultimate goal is to develop light the challenges involved in conducting circulation con- wing shaping control technology, which may incorporate trol computations and experiments [117]. The development Active Flow Control for mechanically simple high-lift sys- outlined in previous sections led to an impression that the tems and improved understanding of flow control physics, recent focus on revolutionary aerodynamic concepts has which is focused on exploring and developing technologies highlighted the technology needs of green transport aircraft, enabling highly coupled, synergistic aero-propulsive-control general aviation and personal aircraft. New and stringent integration. Novel aerodynamic configurations and installa- restrictions on these types of aircraft have placed high de- tion approaches may incorporate boundary layer ingestion Aerodynamic Surface Circulation Control Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1 29

Table 3. An Overview of Selected Recent Patents on Circulation Control.

No Patent Inventor Ref Key Features Type

1 US20120018021: Vortex generators to control boundary layer Babinsky, H., Loth, E., [103] Split-Ramp Vortex Passive interactions (2012) Lee, S. Generator

2 US20110120980: System and method for aerodynamic flow control Corke, T., Thomas, F., [104] Streamwise Vortex Passive (2011) Shatzman, D., Wood, T. Generators

3 US7740206: Translating active gurney flap to alleviate wake vortex Eaton, J., Matalanis, C. [105] Flap Actuators Passive (2010) Wake Vortex Allevia- tors

4 US4705240: Passive vortex lift control (1987) Dixon, C. [106] Delay vortex burst with Passive leading edge extension / strake

5 US4682746: Control force generator (1987) Thomas, A. [107] Single Lifting Coand Passive Surface

6 US20130228236: Surface flow enhancement device having a grip- Evans, R. [108] Increased Circulation Passive ping pad (2013) Easy attachment and detachment of device

7 US8251317: System and method for varying the porosity of an Pitt, D. [109] Variable Porosity Sys- Passive aerodynamic surface (2012) tem Shape Memory Wire Actuator

8 US20130284273: Method of using an active flow control system Boespflug, M., Sad- [110] Active Flow Control Active for lift enhancement or destruction (2013) doughi, S., Bennet, System J.R.G., Opaits, D. DBSJ

9 US8382043: Method and apparatus for aerodynamic flow control Raghu, S. [111] Fluidic actuators Active using compact high-frequency fluidic actuator arrays (2013)

10 US20130087653: Airfoil shaped tail boom (2013) Brand, A., Narramore, [112] Airfoil shaped tail Helicopter J., Harse, J., Lanigan, B. boom Anti-torque Device

11 US4948068: Circulation control slots in helicopter Yaw Control VanHorn, J. [113] No Tail Rotor Helicopter system (1990) (NOTAR)

12 US4131390: Circulation controlled rotor blade (1978) Schmidt, J. [114] Coand Effect Helicopter and distributed propulsion. Associated with these technology pendent blowing systems allow a multi-function system that approaches, Circulation Control technology has been consid- can be used for high lift systems and flight control systems ered to be promising. Despite the present state of the art, such as ailerons and air brakes. The transition from a high various research and development efforts still need to be lift to a cruise configuration depends on the upper and lower carried out. With the progress and availability of various blowing ratios and the free stream velocity. Further research CFD computational codes, validation by Circulation Control could be addressed to alleviate Coand effects in modifying Airfoil Experiments still needs to be continued, among oth- the flow at the trailing edge and the leading edge stagnation ers to establish database for Turbulence Modeling assess- simultaneously that may produce a large pressure gradient at ment and development. Of particular interest are conditions the leading edge, which can lead to premature boundary at the jet exit (these will define the boundary conditions for layer separation and airfoil stall. To avoid conventional lead- CFD) and the boundary layer state. ing edge slats or other flow control techniques a blunt lead- ing edge is desired. Several ideas raised in early 1970s may have to be revis- ited. The optimization of high lift and cruise performance The two most popular Coand shapes typically used for with one airfoil shape gives rise to the pneumatic flap con- CCW applications are based on circular and elliptic profiles. cept [4, 65, 117]. The concept was based on the ability to For low speed applications such as high lift, the circular pro- switch from a high lift configuration to a cruise configura- file is more effective than the elliptic shape. However, the tion without utilizing any mechanical systems. Two inde- elliptic shape is more effective at high-speed cruise condi- 30 Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1 Djojodihardjo and Thangarajah

Fig. (42). Four levels of CFD validation used to study circulation control [117]. tions than the circular shape. Current research has also been Jones et al. [118] described a systematic approach to addressed to the trade-off between the high lift system and develop experimental and computational databases for im- the dual blowing to optimize the cruise drag. proving CC prediction capability. In general, CFD validation is defined by determining how well the CFD model predicts The momentum coefficient C is a critical parameter in the performance and flow physics when used for its intended understanding the efficiencies of blown systems such as the purposes. The level of CFD validation can be defined by the GACC. The non-linearity of the momentum coefficient due complexity of the code and the experiment being used for to the density ratio has also been given considerations. The momentum coefficient C characterizes the physics related to validation, as described in Fig. (43). These levels of valida- tion are being pursued using NASA’s airfoil, semi-span airfoil performance for different slot heights. Nominally wing, and full-span Hybrid Wing Body geometries have smaller slot heights yield a larger return in lift coefficient at been the focus of many current researches. constant C than do larger slot heights. Empirical methods derived from experiments have been traditionally used in the One of the parameters that CC performance is typically design process. However, the complexities of many flow characterized and must be carefully documented is the thrust control techniques in use today, including circulation con- or momentum coefficient, C. The sensitivity of the airfoil trol, far exceed the experimental databases used to develop performance to C is dependent on the jet characteristics and the empirical methods. By coupling physics-based CFD tools the airfoil geometry, particularly on the surface near the jet with validation experiments, parametric studies that charac- exit. There are two physical regimes that define circulation terize the sensitivities of wing geometries and blowing sys- control as a function of blowing. These regimes are com- tems can be developed to advance empirical techniques nec- monly referred to as separation control and super- essary to design a CESTOL type aircraft. However, the no- circulation control and exhibit different global efficiencies tion that CFD simulations were unable to consistently and as determined by the change in unit lift due to the change in accurately predict the performance of a CC airfoil prompted unit blowing [118]. The transition from one regime to an- renewed experimental initiatives to validate CFD simulations other is not always clearly identified and is dependent on the for the advancement of Circulation Control technology. sharpness of the trailing edge. The systematic study of this Aerodynamic Surface Circulation Control Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1 31

Fig. (43). (a) 2-D CFD prediction of the floor and ceiling walls, AR = 3.26, C = 0.115, NPR = 1.2 Cl = 4.87 (no walls, blue), Cl = 4.43 (walls present, red), h/c = 0.0023, wind tunnel height to chord = 4.88, CCE0020EJ airfoil; (b) 3-D CFD prediction of the wall juncture flow, AR = 3.26, C = 0.23, NPR = 1.4 Cl = 5.09, h/c = 0.0023, CC-E0020EJ airfoil [118]. mechanism should include different trailing edge geometries Another novel project is presently in progress at the that include a hinged flapped geometry (i.e. fixed separation German Aerospace Institute DLR. The recently founded Col- located at tip of flap) and a circular geometry (i.e. separation laborative Research Center SFB 880 located in location free to move up to 180o). Braunschweig, Germany, combines the competencies of Technische Universität Baunschweig, Universität Hannover NASA/GTRI 2-D geometry with an elliptic leading edge and the German Aerospace Center, DLR, for fundamental and a large circular trailing edge profile, shown in the lower and applied research in high-lift of future commercial air- part of Fig. (42), was chosen as the benchmark geometry for NASA’s 2-D effort because it is characteristically simple and craft. Based on the hypothesis that active high-lift systems with high levels of aerodynamic efficiency will add signifi- has a large trailing-edge radius for accurate measurements of cant value to future civil transport, they are working on an jet separation [4, 80, 118]. SFB 880 Reference Configuration, as depicted in Fig. (45), The dual blowing (both upper and lower surfaces) capa- which represents the state of the art in CO2 reductions, low bility will be used to manage the drag characteristics, but noise, and STOL for point-to-point air connections within will have a lower priority in the initial validation database Europe [119]. The motivation of the Research Center SFB experiments. While it has been difficult to compute 2-D cir- 880 is that active high-lift systems with high levels of aero- culation control applications reliably, it has also been diffi- dynamic efficiency will improve future civil transport. These cult to generate 2-D experimental data independent of the active systems will provide much higher flexibility in the influences from wind tunnel walls. The two dimensionality generation of high-lift for aircraft families and aircraft up- of the flow and the wall effects are proportional to the grades, they will allow significant reductions of airframe amount of circulation control hence lift generated by the noise emissions, and they will provide a viable path towards wind tunnel model. Short Take-Off and Landing (STOL) capabilities. The latter The over-predictions of two-dimensional CFD simula- route of research aims at a new transport aircraft segment for tions in comparison to the wind tunnel measurements are use on airports with shorter runway length, that are presently generally related to the walls limiting the streamline turning, not considered in airline operations [119]. The new class of as shown in Fig. (43a), and the wall and model juncture for aircraft will be equipped with advanced technologies for 2-D testing shown in Fig. (43b) [118]. drastic airframe and engine noise reduction. It will represent a community friendly aircraft (labeled as “Bürgernahes An example of the Particle Image Velocimetry (PIV) Flugzeug” in German language) designed for operations data compared to 2-D RANS calculations, shown in Fig. much closer to the home of its passengers than possible to- (44), illustrates how CFD can be used to estimate the AOA day. Coand-jet configured high-lift airfoil with morphing effects of the wind tunnel walls. The leading-edge stagnation droop nose at C=0.035 has been considered in their investi- location measured with the PIV data is at x/c value of 4.03%. gations, as exhibited in Fig. (46). The comparable CFD calculation in free air (i.e. no wind tunnel walls) identifies the stagnation to be at an x/c of The above discussions highlight current research efforts 7.75%. An AOA adjustment of -5° for the CFD simulation in various research institutions (NASA, DLR, for example) resulted in a close match to the experimental results for both concerning circulation control that emphasizes CFD valida- the stagnation location and airfoil pressure distribution. The tion and the creation of benchmark experimental databases to AOA adjustment based on classic wall corrections to account advance our knowledge base and prediction capabilities. The for tunnel wall interference is of this magnitude. Further in- systematic CFD validation approach is focused on founda- vestigation of wall interference using CFD tools is required. tional flow physics research for near-term conventional 32 Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1 Djojodihardjo and Thangarajah

o Fig. (44). Example of PIV stagnation streamline data compared to CFD computations, GACC airfoil at GEOMETRIC = 0 [117].

Fig. (45). SFB 880 Reference Configuration [119].

Aerodynamic Surface Circulation Control Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1 33

Fig. (46). Flow around active high-lift airfoil with morphing droop nose at C = 0.035 [119]. aircraft applications and longer-term hybrid wing-body con- critical. Parametric studies can be considered to be an essen- figurations. The measurement and validation challenges for tial tool for analysis and may offer some clues on relevant small scale circulation control experiments are being pursued parameters which may be utilized in a multi-variable optimi- through various examples as elaborated. The circulation con- zation (and to a larger scale, multi-disciplinary optimiza- trol examples shown highlight recent advances in CFD and tion). The introduction of Coand-jet on both airfoils and experimental methods. These examples set the stage for de- aerodynamic surfaces results in enhanced L/D, which de- veloping wind tunnel databases related to supercirculation pends on the jet velocity or momentum coefficient. Round- that will be required for CFD validation. The outcomes of ing-off of the TE along with the introduction of the Coand- these efforts are expected to improve both experimental and jet seems to be effective in increasing L/D in airfoil specifi- CFD capabilities related to powered-lift and circulation con- cally designed for Wind-Turbine, as exemplified by S809. trol. Provision of a unique data set that will enable research- Recent applications of Coand-jet for STOL, ESTOL and/or ers to separate out the effects of the different blowing pa- CESTOL also indicate the need for doubly-curved trailing rameters will be relevant to better understand the complex edge or flaps. The study on Coand-jet application to wind physics of advanced circulation control applications. It could turbines also shows that the maximum total energy output of be noted that further research is needed for the interaction of Coand configured airfoil may exceed that of the predicted Coand-Jet Wake and Trailing Vortices. Application studies Betz limit. of Coand jet for wind-turbine [25-28, 57, 58, 120] and MAV's [92] have been in progress, but efforts still have to be In the combined theoretical and numerical analyses, one devoted for establishing a working prototype in a dedicated is led to come up with logical cause and effect laws as well program like in aircraft applications. as to find ways to carry out optimization schemes for desired design configurations. In summary, future work may look at VIII. CONCLUDING REMARKS FOR FURTHER various key issues such as the influence and the effectiveness DEVELOPMENT of the Coand enhanced lift for a comprehensive series of configurations, for axial thrust (lift), mass flow rate or torque Progress and development of Circulation Control of producing mode, and the corresponding gain or changes in Aerodynamic Surfaces, with particular reference to Coand lift, drag (or L/D ratio), optimum configuration of the Jet and Vortex Cell have been reviewed and assessed, in Coand effect lift enhanced airfoil, the feasibility and practi- view of their features and capabilities addressing require- cability of Coand configured airfoil for wind-turbine appli- ments and trends of modern aircraft. In this conjunction, cations and the unsteadiness effect of Coand-jet applica- related and selected inventions that have been patented are tions that may determine the success of Coand-jet applica- also reviewed and assessed. The main objectives are to gain tions. an in-depth insight on the fundamental principles of various Circulation Control techniques, with particular attention to A brief review of Trapped Vortex Cavity (TVC) re- Coand-jet, its feasibility and practicability and to identify search shows that TVC is a promising technology, and active salient features essential for its optimal utilization. Funda- research is in progress for trapped vortex stabilization that mental analyses and CFD numerical experiments provide constitutes the crux of the technique. Various parameters are insights for understanding the physics of the problems, and relevant for Coand-jet and Trapped Vortex Cavity, among fundamental and applicatory experiments are essential in others the momentum or blowing coefficient introduced to observing the detail of the phenomena and understanding the original flow. In retrospect, Coand enhanced lift en- their subtlety. CFD numerical experiments have also been hancement technique has to a large extent reached a stage for carried out to elaborate and verify the favorable effects of its practicability and application advantages. Coand configured airfoil for enhanced aerodynamic per- formance and to obtain some guidelines for their critical fea- CONFLICT OF INTEREST tures. The choice of turbulence model and other relevant The authors confirm that this article content has no con- parameters commensurate with the grid fineness desired are flict of interest whatsoever. 34 Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1 Djojodihardjo and Thangarajah

ACKNOWLEDGEMENTS TE = Trailing Edge The author would like to thank Universiti Putra Malaysia TVC = Trapped Vortex Cavity (UPM) for granting Research University Grant Scheme VTOL = Vertical Takeoff and Landing (RUGS) No. 9378200; CC-91933, under which the present research has been initiated, and the Ministry of Higher Edu- VAWT = Vertical Axis Wind Turbine cation Exploratory Research Grant Scheme Project Code No. 5527088, under which the present research is carried out. LIST OF ABBREVIATIONS The authors would also like to thank the editorial assistance c = Chord Length of Assoc. Prof. Dr.-Ing. Surjatin Wiriadidjaja, and the for- matting assistance of Messrs. Mohamad Jafari and Riyadh CL = Lift Coefficient Ibraheem, all from Universiti Putra Malaysia. CD = Drag Coefficient ABBREVIATION Cp = Centre-of-Pressure AR = Aspect Ratio C = Momentum / Blowing Coefficient ATC = Air Traffic Control Cm = Moment Coefficient AUV = Autonomous Underwater Vehicle L/D = Lift to Drag Ratio BWB = Blended-Wing-Body R = Radius CC = Circulation Control Re = Reynolds Number Based on the Wing Chord CC = Coand Configured Length CCR = Circulation Control Rotor / Rudder t = Time CCW = Circulation Control Wing t = Airfoil Thickness CESTOL = Cruise Efficient Short Takeoff and Land- V = Free Stream Velocity ing = Lift Augmentation Factor CFD = Computational Fluid Dynamics u, v, w = Axial, Lateral and Vertical Velocities D = Drag u, v, w = Mean Axial, Lateral and Vertical Velocities ESTOL = Extreme Short Takeoff and Landing u’, v’, w’ = Fluctuation Part of u, v and w GACC = General Aviation Circulation Control U = Free Stream Velocity HAWT = Horizontal Axis Wind Turbine V = Tangential Velocity (of a Vortex) L = Lift p = Pressure LE = Leading Edge n = Normal Vector MAV = Micro-Air-Vehicle = Mass Flow of Jet N+2 aircraft j design = Aircraft Design Associated with Two Gen- S = Wing Area erations Beyond the Present Flying Gen- h = Slot Height eration (2020 Time Frame) x = Distance N+3 aircraft design = Aircraft Design Associated with Three M = Mach Number Generations Beyond the Present Flying k = Turbulence Kinetic Energy Generation (2030 Time Frame)  = Turbulent Dissipation NASA = National Aeronautics and Space Admini- + stration y = Wall Parameter NPR = Nozzle Pressure Ratio Vmax = Maximum Tangential Velocity (of a Vortex) NOTAR = No-Tail-Rotor x, y, z = Coordinates in x-, y- and z-Direction NREL = National Renewable Energy Laboratory yC, zC = Coordinates of the Vorticity Centroid in Half a Plane in y- and z-Direction NSWC = Naval Surface Warfare Centre = Angle of Attack NSRDC = Naval Ship Research and Development Centre HTP = Horizontal Tail Plane Setting RANS = Reynolds-averaged Navier-Stokes = Circulation S/VTOL = Short/Vertical Takeoff and Landing 0 = Initial Value of Circulation  = Departure Angle Aerodynamic Surface Circulation Control Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1 35

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