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 aircraft aerodynamics, 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 takeoff, 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 airfoil surface blowing of Circulation Enhancement and Control of aerodynamic surfaces. The crux of the TVC active research is their stabilization, while Coand enhanced lift 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, drag 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 wing 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 flap 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 boundary layer 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 wings 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 chord. 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, ailerons, 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 trailing edge 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 boundary layer control 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 airfoils, wings and bodies. Since the wing areas are highlight the necessity for enhanced high-lift systems to sized by the takeoff and landing 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 wing loading 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 airframe 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 bleed air 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