applied sciences

Article Experimental Study on Aerodynamic Characteristics of a Gurney Flap on a Wind Turbine Airfoil under High Turbulent Flow Condition

Junwei Yang 1,2,3 , Hua Yang 1,2,*, Weijun Zhu 1,2 , Nailu Li 1,2 and Yiping Yuan 4 1 College of Electrical, Energy and Power Engineering, Yangzhou University, Yangzhou 225127, China; [email protected] (J.Y.); [email protected] (W.Z.); [email protected] (N.L.) 2 New Energy Research Center, Yangzhou University, Yangzhou 225009, China 3 College of Hydraulic Science and Engineering, Yangzhou University, Yangzhou 225009, China 4 Jiangsu Key Laboratory of Hi-Tech Research for Wind Turbine Design, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China; [email protected] * Correspondence: [email protected]; Tel.: +86-138-1583-8009



Received: 20 September 2020; Accepted: 14 October 2020; Published: 16 October 2020


Abstract: The objective of the current work is to experimentally investigate the effect of turbulent flow on an airfoil with a Gurney flap. The wind tunnel experiments were performed for the DTU-LN221 airfoil under different turbulence level (T.I. of 0.2%, 10.5% and 19.0%) and various flap configurations. The height of the Gurney flaps varies from 1% to 2% of the chord length; the thickness of the Gurney flaps varies from 0.25% to 0.75% of the chord length. The Gurney flap was vertical fixed on the pressure side of the airfoil at nearly 100% measured from the leading edge. By replacing the turbulence grille in the wind tunnel, measured data indicated a stall delay phenomenon while increasing the inflow turbulence level. By further changing the height and the thickness of the Gurney flap, it was found that the height of the Gurney flap is a very important parameter whereas the thickness parameter has little influence. Besides, velocity in the near wake zone was measured by hot-wire anemometry, showing the mechanisms of lift enhancement. The results demonstrate that under low turbulent inflow condition, the maximum lift coefficient of the airfoil with flaps increased by 8.47% to 13.50% (i.e., thickness of 0.75%), and the Gurney flap became less effective after stall angle. The Gurney flap with different heights increased the lift-to-drag ratio from 2.74% to 14.35% under 10.5% of turbulence intensity (i.e., thickness of 0.75%). However, under much a larger turbulence environment (19.0%), the benefit to the aerodynamic performance was negligible.

Keywords: wind tunnel experiment; wind turbine airfoil; turbulence; Gurney flap; aerodynamic characteristics

1. Introduction Wind power generation technology has been maturely developed in the past decades. Airfoil is a basic element of a wind turbine blade, and its aerodynamic characteristics have a major influence on the wind energy conversion efficiency. Among the conventional rotor aerodynamic design strategy, the blade add-ons were of particular interest to further improve wind energy efficiency. Therefore, mounting flap to the airfoil trailing edge was one of the most feasible methods to improve the aerodynamic performance of wind turbines. In addition to power production, such a technique can also effectively reduce the aerodynamic loads both of wind turbine blades and tower. If a sophisticated controller was implemented, the flap can further reduce turbulence-induced fatigue loads, so that longer lifetime was guaranteed. After the pioneering work of Liebeck [1], a large number of studies have been conducted to explain the phenomena induced by the presence of this device. More recently,

Appl. Sci. 2020, 10, 7258; doi:10.3390/app10207258 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 7258 2 of 21 research objects were most focused on airfoil attached various shapes of flaps, such as Gurney flaps [2,3], triangular flaps [4], separate trailing edge flaps [5,6] and deformable trailing edge flaps [7,8]. For experiment tests, Zhang et al. [9] and Amini et al. [10] studied the aerodynamic effect of a Gurney flap based on the airfoil through wind tunnel experiments. T Lee et al. [11] carried out a wind tunnel test of the lift force and pitching moment coefficients of both trailing edge flaps and Gurney flaps with different shape parameters. Based on a 5 MW reference wind turbine, Chen et al. [12] designed and optimized a trailing edge flaps such that the blade mass can be further reduced but still maintain the desired power performance. Medina et al. [13] explored the flow mechanisms of a flap at a high deflection angle. When the flap works in a separated flow region, he provided some ideas for realizing instantaneous action or alleviating extra aerodynamic loads on wind turbines. Elsayed et al. [14] studied the flap tip vortexes and characterized the flow structures behind a flap in a low-speed wind tunnel by using particle image velocimetry. Little et al. [15] designed a trailing edge flap by using a single medium plasma driver which resulted in a higher lift force. Bergami et al. [16] designed an active controller of a trailing edge flap on a 5 MW reference wind turbine and proved that the flap could effectively control aerodynamic loads. Edward et al. [17] also conducted experiments on a flaps noise drop. According to the wind tunnel tests based on a full-size rotor, Straub et al. [18] found that flaps could also be used to control noise and vibration such that the noise generated by blade vortex interaction could be reduced by 6 dB. For numerical simulations and theoretical analyses, Traub et al. [19] fitted a semi-empirical equation by summarizing the performance of a large number of flaps. Lario et al. [20] numerically solved the unsteady flow field of Gurney flaps at high Reynolds numbers through the discontinuous Galerkin method. By analyzing dynamic characteristics of an airfoil with Gurney flaps through the numerical simulation, Li et al. [21] found that flaps were capable to reduce unsteady aerodynamic loads of wind turbines. Zhu et al. [22] numerically simulated the airfoil with trailing edge flaps using the immersed boundary method and found that flaps could be combined with the paddle movement to adjust the aerodynamic loads of a wind turbine airfoil. Ng et al. [23] investigated the trailing edge flap together with an aeroelastic analysis. It was noted that the trailing edge flap could be a smart device to control aeroelastic deformation. All the above researches on the flaps were built on the uniform inflow of low turbulence intensity. In the past, there were few studies carried out by flow over flaps under high turbulence intensity [24,25], however, such a flow condition often occurs on wind turbines operating in a wind farm. Considering wind turbines operate in a turbulence environment, the conclusions obtained from the previous studies might not be accurate. Therefore, to simulate wind turbines under turbulence environment, active and passive wind tunnel turbulence generation methods can be used. The active technology includes a vibrating grille and multi-fan wind tunnel; the maximum turbulence intensity can reach more than 20% [26]. The passive control structure was relatively simple, which can be divided into grille, wedge, and rough square types among the passive turbulence generators, it was more convenient to construct grilles, which have gain very popular use. In this experiment, specific turbulence levels were passively controlled by a grille with proper grid size. The investigations presented in this paper were focused on the coupled effects of Gurney flap and turbulence inflow. The Gurney flaps were experimentally investigated under various turbulence intensities and flap configurations. The desired turbulent flow passes the airfoil was achieved by changing the grille size as well as the distance between the grille and the airfoil model. The hot-wire anemometer was used to record the wind speed and turbulence intensity in a flow cross-section. The quantitative information obtained during the experiment includes: (1) turbulent field descriptions, (2) airfoil pressure coefficients, (3) lift-to-drag coefficients, (4) wake measurements. On that basis, the aerodynamic performance of Gurney flaps with different heights and thicknesses was researched. The values of lift and pitching-moment coefficients were obtained through the integration of surface pressures. The wake rake array was also used to determine the values of drag coefficient. Furthermore, the flow fields near the trailing edge of airfoil were tested which could further Appl. Sci. 2020, 10, 7258 3 of 21 verify the reason for lift improvement. Finally, concluding remarks accompany the discussion of the experimental investigations.

2. Experimental Setup The experiments were carried out in a wind tunnel located at Yangzhou University. It was a close-loop type wind tunnel which contains two experimental sections. The measurements were conducted in the smaller section with the cross-section parameters of 3 m 1.5 m, and the length × is 3 m. The operational wind speed range was 0~50 m/s and the calibrated maximum turbulence intensity in the free stream was 0.2%. The DTU-LN221 airfoil model [27], as shown in Figure1, was adopted which has a chord length of 0.6 m and a span length of 1.5 m. The airfoil model was vertically placed in the test section. The bottom part of the airfoil section was connected by the rotating shaft, which was fixed on a rotational plate, as demonstrated in Figure2. Therefore, the angle of attack can be remotely controlled via a shaft connection with a motor below the wind tunnel. The airfoil model was made of aluminum alloy where small taps were drilled on the surface of the middle section, with the location sketched in Figure1. The arc length between the taps was approximately spaced every 2.5% of chord length starting from the leading edge to 90% of the trailing edge. The hollow plastic hoses were connected on the reverse side of the airfoil surface which has an outer diameter of 1.2 mm. Thus, a total of 77 pressure taps were arranged for the pressure measurement on the surface of the airfoil. The pressures on the airfoil surface were sensed by the electronic pressure acquisition system of PSI (Pressure Systems Inc., Hampton, VA, USA). The pressure system used in the experiment has a sampling frequency of 333.3 Hz, a measuring range of 2.5 kPa and a measuring accuracy of ± 0.05%. Figure1 also shows the chord-wise position, so the lift and pitching-moment of the test airfoil ± were achieved by integrating the pressure and the position of the taps on the surface. The lift coefficient and pitching-moment coefficients were given by

C = Cn cos α Ct sin α l − R1  (1) Cm = Cpl Cpu (0.25 xˆ)dxˆ 0 − − where R1  Cn = C Cpu dxˆ pl − 0 (2) yˆuR max  Ct = C C dyˆ p,be − p,a f yˆl max where Cl and Cm are the lift coefficient and pitching-moment coefficient, respectively; αrepresents the angle of attack; xˆ is the relative chord length; yˆ is the thickness value relative to the chord length; Cpu and Cpl are the pressure coefficient on the suction and pressure sides of the airfoil, respectively; Cn and Ct are the normal force coefficient and tangential force coefficient, respectively. Cp,be and Cp,af are the pressure coefficient before and after the maximum thickness of airfoil; yˆu max and yˆl max represent the maximum thickness values of the suction and pressure sides relative to chord length. Appl. Sci. 2020, 10, 7258 4 of 21 Appl.Appl. Sci. Sci. 2020 2020, ,10 10, ,7258 7258 44 of of 21 21

600600mmmm

150150mmmm PPressureressure taps taps lift lift, ,pitching pitching- - normalnormal force force ∫ ∫integralintegral momentmoment obtained obtained liftlift

DragDrag ( pressure(pressure drag drag) )

c

c / Pitching / Pitching wakewake rake rake array array momentmoment tangentialtangential force force

Thickness y Thickness Chord line Drag obtained Thickness y Thickness Rotating center Chord line Drag obtained Rotating center (pressure drag + AngleAngle of of (pressure drag + attackattack frictionalfrictional drag drag) ) 00 00.1.1 00.2.2 00.3.3 00.4.4 00.5.5 00.6.6 00.7.7 00.8.8 00.9.9 11.0.0 ChordChord-wise-wise position position x x/c/c InflowInflow ConnectConnect with with servoservo motor motor throughthrough a ashaft shaft

FigureFigure 1. 1. Schematic Schematic diagram diagram of of measuring measuring position position. position. .

GGrillerilless

AirfoilAirfoil section section Wake rake array AirfoilAirfoil section section 420420mmmm Wake rake array GurneyGurney flap flap

incomingincoming flow flow 15001500mmmm

WWakeake rake rake array array pressurepressure taps taps 16001600mmmm PitotPitot tube tube rotaryrotary plate plate 30003000mmmm

FigureFigure 2. 2. Experimental Experimental test test devicedevice. device. .

AtAt aa smallsmall angleangle ofof attack, attack, thethe aerodynamic aerodynamic dragdrag largelylargely consistsconsists ofof frictionalfrictional drag,drag, whilewhile thethe datadata measuredmeasured by by surface surface pressure pressure tapstapstaps cannot cannot accurately accurately represent represent the the drag drag force, force, so so thethe dragdrag cancan bebe measuredmeasured byby thethe momentummomentum method method more more precisely precisely.precisely. .As As shown shown in in F FigureFigureigure 22, ,2, aa a wakewake wake rakerake rake arrayarray array waswas placed placed at at 0.7 0.7 chord chord lengthlen lengthgth behind behind the the trailing trailing edge edge of of the the airfoil airfoil and and at at the the same same vertical vertical level level as as thethe pressure pressure taps.tap taps.s. The The measurement measurement range range ofof the the wake wake probes probes was was 80.8 80.8 cm, cm, 102 102 total total pressure pressure pipes pipes (with(with anan an outerouter outer diameterdiameter diameter of of of 1.2 1.2 1.2 mm) mm) mm) and and and 4 static4 4 static static pressure pressure pressure pipes pipes pipes (with (with (with an outeran an outer outer diameter diameter diameter of 2 mm)of of 2 2 weremm)mm) wereaveragelywere averagely averagely arranged, arranged, arranged, with an with with 20 cm an an apart20 20 cm cm for apart apart the staticfor for the the pressure static static pressure pressure pipes. To pipes. pipes. prevent T Too airprevent prevent leakage, air air allleakage, leakage, pressure all all pressuretubespressure were tubes tubes connected were were connected connected by plastic by hosesby plastic plastic to the hose hose pressuress to to the the measurement pressure pressure measurement measurement device. Besides, device device two. .Besides, Besides, Pitot tubes two two werePitotPitot installedtubestubes were were at 1.6installedinstalled m measured at at 1.6 1.6 m m from measured measured the downstream from from the the downstream downstream of grilles where of of grilles grilles the free where where stream the the velocity fr freeee stream stream was velocityrecorded.velocity was was The recorded. recorded. drag coe ffiThe Thecients drag drag were coefficients coefficients given as were were follows given given as as follows follows Z r r  2 22 PPPPP01PPPP01P  P 0101 P  C =CC01− 111 01 − ∞ ds dsds (3) d dd    (3) c ccw P P0 P00P P P− P PP0 P PP w w −00∞− ∞ where Cd dis the drag coefficient; c is the chord length, w is the range of integration; s is the wherewhereC dCis is the the drag drag coe ffi coefficient;cient; c is thec is chord the chord length, length,w is the w range is the of integration;range of integration;s is the coordinate s is the coordinate along the thickness direction of the wake rake array; P∞ and P0 are the static alongcoordinate the thickness along direction the thickness of the wake direction rake array; of theP and wakeP0 rakeare the array; static pressureP∞ and andP0 are total the pressure static ∞ 01 measuredpressurepressure and byand the total total Pitot pressure pressure tubes; P measured andmeasuredP01 are by theby the the static Pitot Pitot pressure tubes; tubes; and P P and totaland P P pressure01 are are the the measured static static pressure pressure by the wake and and raketotaltotal array.pressure pressure It should measured measured be noted by by the thatthe wake wake there rake rake is a totalarray. array. pressure It It should should loss be be along noted noted the that that Pitot there there tubes is is toa a total thetotal wake pressure pressure rake array,lossloss along along therefore the the thePitot Pitot total tubes tubes pressure to to the the loss wake wake should rake rake be array, array,added thereforetherefore to each total the the pressure total total pressure pressure measuring loss loss point should should of the be be wakeaddedadded rake to to each array.each total total pressure pressure measuring measuring point point of of the the wake wake rake rake array. array. FigureFigure3 3 presents3 presentspresents the thethe schematic schematic schematic diagram diagram diagram of the of of grillethe the grille grille geometry. g geometry.eometry. The grilles The The grillesgrilles assembled assembled assembled with many with with squaredmanymany squared squared alloys alloys withalloys geometry with with geometry geometry specified speci speci byfiedfied four by by parameters four four parameters parameters a, b, c a, anda, b, b, c d.c and and These d. d. These smallThese small squaressmall squares squares were werewere bolted bolted together together andand onon top top of of the the grille grille the the rubber rubber pads pads were were attached. attached. During During the the measurements,measurements, two two types types of of grilles grilles werewere implemented. implemented. The The dimensions dimensions of of the the two two grilles grilles were were Appl. Sci. 2020, 10, 7258 5 of 21

Appl.provided Sci. 2020 in, 10Table, 7258 1. The width of the longitudinal grille and the transverse grille were denoted5 by of 21 a and c, respectively; the width and the height of the spacing were denoted by b and d, respectively, the thickness along the flow direction was e. bolted together and on top of the grille the rubber pads were attached. During the measurements, two types of grilles were implemented.Table 1. Dimensions The dimensions of the experiment of the two grilles grilles. were provided in Table1. The width of the longitudinal grille and the transverse grille were denoted by a and c, respectively; a (cm) b (cm) c (cm) d (cm)e (cm) the width and the height of the spacing were denoted by b and d, respectively, the thickness along the Scheme 1 3 34 3 32 3 flow direction was e. Scheme 2 6 31.4 6 29.8 3 a Rubber pads

b d c

Anchor bolts (a)

Section8 Section1

Grille

200 200 100 100 100 100 200 mm mm mm mm mm mm mm Anchor bolts

1000mm 1000mm (b)

Figure 3. Schematic diagram of experimental grille. ( a) Front view; (b) side view.

Behind the grille, there Tablewere 1.eightDimensions measurement of the experiment sections designed grilles. for velocity recording. At each of the section, 15 measurementa (cm) points b were (cm) uniformly c (cm) d sp (cm)aced. e The (cm) center of the topmost measurement point corresponds to the geometrical center of the wind tunnel section. The horizontal Scheme 1 3 34 3 32 3 and vertical spacing of measuring points were 10 cm, as displayed by the black spots in Figure 3a. In Scheme 2 6 31.4 6 29.8 3 addition, we set eight measurement sections one to two meters behind the grille, as shown in Figure 3b. The wind speed was measured through a Dantec hot-wire probe (type 55P61). The sampling frequencyBehind was the 5 grille, kHz, thereand the were test eight wind measurement speeds were sections consistent designed with forthat velocity in the airfoil recording. experiment. At each ofFigure the section, 4 displays 15 measurement the device of points turbulence were uniformlymeasurement. spaced. Besides The, center wake ofmeasurements the topmostmeasurement in this paper pointwere carried corresponds out on to an the electronically geometrical centercontrolled of the traverser wind tunnel consisting section. of a The two horizontal-dimensional and probe, vertical as spacingdisplayed of in measuring Figure 5. points were 10 cm, as displayed by the black spots in Figure3a. In addition, we set eight measurement sections one to two meters behind the grille, as shown in Figure3b. The wind speed was measured through a Dantec hot-wire probe (type 55P61). The sampling frequency was 5 kHz, and the test wind speeds were consistent with that in the airfoil experiment. Figure4 displays the device of turbulence measurement. Besides, wake measurements in this paper were carried out on an electronically controlled traverser consisting of a two-dimensional probe, as displayed in Figure5. Appl.Appl. Sci. Sci.2020 2020, ,10 10,, 7258 7258 66 of of 21 21 Appl. Sci. 2020,, 10,, 72587258 6 of 21

Figure 4. Grille turbulence generated device. Figure 4. GrilleGrille turbulence turbulence generated device.

Figure 5. Wake measurements. Figure 5. WakeWake measurements measurements... The influence of the flap height and thickness were two key parameters which influence the overallThe aerodynamicinfluence influence of the performance flap flap height of andthe airfoil. thickness For were this reason,two key a parameterstotal of nine which types influence influenceof flaps werethe overalldesigned aerodynamic in the experiments, performance namely, of of the the the airfoil. flap withFor For this thisheights reason, reason, h of a a 1%,total total 1.5% of of nine nine and types 2% chord of flaps flaps length were (6 designedmm, 9 mm inin theandthe experiments, experiments,12 mm, respectively) namely, namely, theand the flap withflap with relativewith heights heights thicknesses h ofh 1%,of 1%, 1.5%d of 1.5% 0.25%, and and 2% 0.5% chord2% chordand length 0.75% length (6 chord mm, (6 mm,9length mm 9 andmm (1.5 12 and mm, mm, 12 3 respectively)mm,mm andrespectively) 4.5 mm, and withrespective and relativewith ly).relative thicknesses The diagramthicknesses dof of0.25%, the d of flap 0.25%, 0.5% used and 0.5% in 0.75%the and experiment chord0.75% lengthchord was length(1.5given mm, (1.5in 3F iguremm, mm and3 6. mm As 4.5 shown,and mm, 4.5 respectively). themm, flap respective has an The Lly). shape; diagram The diagramthe of bottom the flapof thepart used flap has in used a the width in experiment the of experiment7 mm waswhich given was was giveninattached Figure in F6 igure. to As the 6. shown, pressureAs shown, the side flap the offlaphas the anhas airfoil L an shape; L shape; trailing the the bottom edge. bottom part During part has has the a widtha experiments,width of of 7 7 mm mm the which which flap was was was attachedvertically to to attachedthe the pressure pressure near side theside of position the of airfoilthe airfoil of trailing 100% trailing edge.of the edge.During chord During the length, experiments, the see experiments, Figure the 2 flap with wasthe the flapvertically specific was verattachedexpeticallyrimental near attached the setup. position near The the coordinate of position 100% of ofsystem the 100% chord behind of length, the thechord see trailing Figure length, edge2 with see wasF theigure defined specific 2 with for experimental the subsequent specific expesetup.wakerimental The measurements, coordinate setup. Thesystem also coordinate the behind x and the system y trailing axis behind were edge setwas the in definedtrailing the vertical for edge subsequent was and defined the wake forward for measurements, subsequent directions, wakealsorespectively. the measurements,x and y The axis black were also spots set the in thein x and the vertical F yigure axis and 6were the represented forward set in the directions, the vertical hot-wire respectively. and measuring the forward The positions black directions, spots set in at respectively.thedownstream Figure6 represented Theof the black trailing thespots hot-wireedge. in the measuringFigure 6 represented positions set the at hot downstream-wire measuring of the trailing positions edge. set at downstream of the trailing edge. x x y d y d h h 2mm 2mm 7mm 7mm Figure7mm 6. Flap geometry. Figure 6. Flap geometry. Figure 6. Flap geometry. Appl. Sci. 2020, 10, 7258 7 of 21

Appl. Sci. 2020, 10, 7258 7 of 21 3. Experimental Results and Analysis 3. Experimental Results and Analysis In the experiment, the free stream flow velocity was 20 m/s, and the corresponding Reynolds numberIn basedthe experiment, on the airfoil the free chord stream length flow was velocity 0.8 was106, 20 although m/s, and wind the corresponding turbines often Reynolds operate at ×6 windnumber speed based below on the 20 airfoil m/s, but chord the length magnitude was 0.8 of × 10 the, although Reynolds wind number turbines was often up to operate an order at wind of 10 6. 6 Therefore,speed below the experimental20 m/s, but the valuemagnitude of the of the Reynolds Reynolds number number was was chosenup to an to order approach of 10 . Therefore an order, of the experimental value of the Reynolds number was chosen to approach an order of magnitude magnitude corresponding to those obtained from full-scale wind turbines. Two types of grilles generate corresponding to those obtained from full-scale wind turbines. Two types of grilles generate different turbulence levels in the wind tunnel, such that the experiments were mainly separated into different turbulence levels in the wind tunnel, such that the experiments were mainly separated into low and high turbulence cases. To be specific, a grille was placed at the upstream of the airfoil test low and high turbulence cases. To be specific, a grille was placed at the upstream of the airfoil test section which results in different aerodynamic characteristics of flow over the airfoil with and without section which results in different aerodynamic characteristics of flow over the airfoil with and a Gurneywithout flap.a Gurney flap. 3.1. Experimental Results of Grille Turbulence Generator 3.1. Experimental Results of Grille Turbulence Generator TheThe measurements measurements werewere firstfirst conducted for for the the two two grilles. grilles. Figure Figure 7 illustrates7 illustrates the time the time series series of ofthe the instantaneous instantaneous velocity velocity at atthe the central central measuring measuring point point of the of 6th the section. 6th section. As shown As shown in Figure in Figure7, the 7, theincoming incoming wind wind became became highly highly disturbed, disturbed, for this for reason, this reason, a statistical a statistical analysis analysis was done was for done the flow for the flowfield. field. The decay The decay of turbulence of turbulence intensity intensity from the from grilles the towards grilles the towards airfoil thetest airfoilsection testwas sectionreported was reportedin Figure in 8a Figure. Within8a. Withinthe scope the of scope the test of, thethe test,averaged the averaged turbulence turbulence intensity intensitywas recorded was along recorded 8 alongdownstream 8 downstream sections sections (each measured (each measured with 15 with points). 15 points). Besides, Besides, the margin the marginof error ofin errorFigure in 8a Figure was 8a wasexpressed expressed as as the the standard standard deviation deviation of of each each section section data. data. According According to F toigure Figure 8a8, a, the the average average turbulenceturbulence intensity intensity of of downstream downstream in schemein scheme one one dropped dropped from from 13.5% 13.5% at the at position the position of 1.0 of m 1.0 behind m thebehind grille tothe 9.4% grille at to the 9.4% position at the ofposition 2.0 m; of the 2.0 average m; the turbulenceaverage turbulence intensity intensity of the downstream of the downstream in scheme twoin was scheme higher two than was scheme higher 1, than which scheme decreased 1, which from decreased 27.7% at the from position 27.7% of at 1.0 the m position behind theof 1.0 grille m to 15.9%behind at thethe positiongrille to 15.9% of 2.0 at m. the The position standard of 2.0 deviations m. The standard of turbulence deviation intensitys of turbulence of the intensity two schemes of dropsthe two quickly schemes towards drops the quickly test section. towards The the test change section. in turbulence The change level in turbulence indicates level that thereindicates were that very highthere turbulence were very fluctuations high turbulen justce fluctuation behind thes grillejust behind which the yielded grille which very unstable yielded very flow unstable field. The flow flow becamefield. Thegradually flow became stabilized gradually owing to stabilized the mutual owing dissipation to the mutual over a propagation dissipation over distance. a propagation To ensure a homogeneousdistance. To andensure isotropic a homogeneous turbulent and field, isotropic so the distance turbulent 1.6 field, m (from so the the distance leading 1.6 edge m of(from the airfoilthe leading edge of the airfoil to the grille) was selected for placing the leading edge of the airfoil. The to the grille) was selected for placing the leading edge of the airfoil. The turbulence intensity measured turbulence intensity measured at the 6th section of the two schemes was 10.5% and 19.0%, at the 6th section of the two schemes was 10.5% and 19.0%, respectively. Figure8b illustrates the power respectively. Figure 8b illustrates the power spectral density (PSD) of the wind speed at the two spectral density (PSD) of the wind speed at the two grille schemes measured at the central measuring grille schemes measured at the central measuring point of the 6th section (1.6 m measured from point of the 6th section (1.6 m measured from grille to the hot-wire probes). Obviously, the larger grille to the hot-wire probes). Obviously, the larger the turbulence intensity was generated, the thelarger turbulence the amplitud intensitye of was the generated, power spectral the larger density the was amplitude observed. of theMoreover, power spectralthe power density spectra was observed.amplitude Moreover, in the downstream the power direction spectra amplitudewas consistent in the with downstream the corresponding direction Karman was consistent spectra, and with theit correspondingdropped down rapidly Karman after spectra, the frequency and it dropped over 30 Hz, down which rapidly suggests after that the the frequency grille distance over was 30 Hz, whichmore suggests proportional that theto the grille turbulent distance kinetic was energy. more proportional to the turbulent kinetic energy.

24 Scheme 1 22 Scheme 2 20 18 16 14 12

wind velocity (m/s) 10 8 0 2 4 6 8 10 Time (s) FigureFigure 7. 7. TheThe time time series series of the of instantaneous the instantaneous streamwise streamwise velocity (the velocity 6th section, (the 6thcentral section, measuringcentral measuringpoint). point). Appl. Sci. 2020, 10, 7258 8 of 21

Appl. Sci. Sci. 20202020,, 1010,, 7258 7258 88 of 21 21 T.I. of scheme 1 0.4 T.I. of scheme 2 20

u 0.1 Average velocity of scheme 1 S T.I. of scheme 1 Average velocity of scheme 2 18 0.4 T.I. of scheme 2 20 0.01

0.3 u 0.1 Average velocity of scheme 1 S 16 Average velocity of scheme 2 18 0.001 0.3 0.01 0.2 14 16 Experiment (Scheme 1, 1.6m) 0.0011E-4 Karman(Scheme 1, 1.6m)

0.2 12 Power spectral density 14 Experiment(Scheme 2, 1.6m)

0.1 velocityaverage wind (m/s)

Downwind turbulence intensity ExperimentKarman (Scheme (Scheme 2, 1.6m) 1, 1.6m) 1E-5 1E-4 Karman(Scheme 1, 1.6m) 10 1 10 100 1000 1.0 1.2 1.4 1.6 1.8 2.0 12 Power spectral density Experiment(Scheme 2, 1.6m) 0.1 velocityaverage wind (m/s) Frequency（Hz） Downwind turbulence intensity Karman (Scheme 2, 1.6m) Distance (m) 1E-5 10 1 10 100 1000 1.0 1.2 1.4 1.6 1.8 2.0 (a) (bFrequency) （Hz） Distance (m) Figure 8. Description of the flow field behind the grilles. (a) Turbulence intensity and average (a) (b) velocity (Re: 0.8 × 106); (b) Comparison of the PSD (1.6 m behind the grille). Figure 8. 8. DescriptionDescription of of the the flow flow field field behind behind the grilles. the grilles. (a) Turbulence (a) Turbulence intensity intensity and average and average velocity 3.2. Experimentalvelocity(Re: 0.8 (Re:10 60.8Results); ( b× )10 Comparison6 );of ( bthe) Comparison Baseline of the Airfoil PSD of the (1.6 PSD m behind (1.6 m thebehind grille). the grille). × To verify the accuracy of the experiment, the experimental data from the wind tunnel of LM 3.2. Experimental Experimental Results Results of of the the Baseline Baseline Airfoil Airfoil Wind Power were selected as a reference [28]. The aerodynamic data of the DTU-LN221 airfoil To verify the accuracy of the experiment, the experimental data from the wind tunnel of LM Wind underT o uniform verify the inflow accuracy were of measured the experiment, in the the LM experimental wind tunnel data experiment from the atwind the tunnel approximate of LM Power were selected as a reference [28]. The aerodynamic data of the DTU-LN221 airfoil under uniform ReynoldsWind Power number were of selected 1.5 × 10 as6. Thea reference lift and [2 drag8]. The data aerodynamic were selected data as a of cross the - DTUvalidation-LN221 case. airfoil In inflow were measured in the LM wind tunnel experiment at the approximate Reynolds number of considerationunder uniform of inflowthe interference were measured effect of in wind the LM tunnel wind wall, tunnel the maximum experiment blockage at the approximate ratio in this 1.5 106. The lift and drag data were selected as a cross-validation case. In consideration of the experimentReynolds× number was about of 1.5 8.4%, × 10 so6. the The experiment lift and drag measured data were data selected of the airfoil as a cross were-validation corrected by case. after In interference effect of wind tunnel wall, the maximum blockage ratio in this experiment was about theconsideration reference [29] of ,the and interference the experimental effect data of wind analyzed tunnel below wall, were the all maximum corrected. blockage In order ratio to perform in this 8.4%, so the experiment measured data of the airfoil were corrected by after the reference [29], and the aexperiment comparison, was Figure about 9 8.4%,shows so the the comparison experiment of measured experimental data dataof the of airfoilthe DTU were-LN221 corrected airfoil by under after experimental data analyzed below were all corrected. In order to perform a comparison, Figure9 uniformthe reference inflow [29] conditi, and theon .experimental To minimize data the analyzed uncertainty, below measurements were all corrected. were In performed order to perform several shows the comparison of experimental data of the DTU-LN221 airfoil under uniform inflow condition. timesa comparison, for comparison Figure 9 test shows at the the approximatecomparison ofReynolds experimental number data of of 1.5 the × DTU106, YZU1,-LN221 YZU2,airfoil YZU3under To minimize the uncertainty, measurements were performed several times for comparison test at the representsuniform inflow the results conditi ofon . theTo repeated minimize experiments. the uncertainty, As measurementsshown in Figure were 9, performed the lift and several drag approximate Reynolds number of 1.5 106, YZU1, YZU2, YZU3 represents the results of the repeated coetimesfficients for comparison were very similar test at thebefore approximate the× stall angle, Reynolds while numberthere were of some 1.5 × 10discrepancies6, YZU1, YZU2, in the YZU3 stall experiments. As shown in Figure9, the lift and drag coe fficients were very similar before the stall state.represents Consider the the results cause of of theflow repeated separation, experiments. the deviation As of shownresults of in drag Figure is quite 9, the considerable, lift and drag but angle, while there were some discrepancies in the stall state. Consider the cause of flow separation, thecoe fficientsstall angles were tested very in similar the two before wind the tunnels stall angle, were nearlywhile therethe same. were The some reason discrepancies for this may in the be thatstall the deviation of results of drag is quite considerable, but the stall angles tested in the two wind tunnels althoughstate. Consider the airfoils the cause were of of flow the separation,same type, thethey deviation were individually of results of manufactured, drag is quite considerable, for example thebut were nearly the same. The reason for this may be that although the airfoils were of the same type, roughnessthe stall angles of the tested airfoil in the surface two wind may tunnels affect thewere experimental nearly the same. results. The Besides,reason for there this may were be some that they were individually manufactured, for example the roughness of the airfoil surface may affect the differencealthough thes in airfoilsdrag measurements, were of the same LM type,adopted they the were model individually surface pressure manufactured, distribution for example in a certain the experimental results. Besides, there were some differences in drag measurements, LM adopted the rangeroughness of the ofangle the of airfoil attack, surface while YZU may used affect the the wake experimental probes at all results. angles. Besides, Similar phenomena there were somewere model surface pressure distribution in a certain range of the angle of attack, while YZU used the wake seendifference in differents in drag wind measurements, tunnel laboratories LM adopted [30–3 2the]. In model the following, surface pressure for reason distribution of comparisons, in a certain the probes at all angles. Similar phenomena were seen in different wind tunnel laboratories [30–32]. In the measuredrange of the data angle are ofaveraged. attack, while YZU used the wake probes at all angles. Similar phenomena were seenfollowing, in different for reason wind of tunnel comparisons, laboratories the measured [30–32]. In data the are following, averaged. for reason of comparisons, the

measured data are averaged. 0.16 1.5 0.14 LM YZU 0.12 1.0 0.16 YZU-2 1.5 0.10 YZU-3 0.14 LM 0.5 0.08 YZU

Cl

Cd 0.12 1.0 YZU-2 0.06 0.0 LM 0.10 YZU-3 0.5 0.04 YZU 0.08

Cl -0.5 YZU-2 Cd 0.02 0.0 YZU-3 0.06 LM 0.00 -1.0 YZU 0.04 -15 -10 -5 0 5 10 15 20 -10 -5 0 5 10 15 20 -0.5 YZU-2 0.02 α/ (°) YZU-3 α/ (°) -1.0 0.00 -15 -10 -5 0 5 10 15 20 -10 -5 0 5 10 15 20 (a)α/ (°) (b) α/ (°) FigureFigure 9. 9. ComparisonComparison of of experimental experimental data data of of the the DTU DTU-LN221-LN221 airfoil airfoil (uniform (uniform inflow, inflow, Re: Re: 1.5 1.5 × 101066).). (a) (b) × ((aa)) Lift Lift coefficient coefficient comparison; comparison; ( b)) drag coefficient coefficient comparison comparison.. Figure 9. Comparison of experimental data of the DTU-LN221 airfoil (uniform inflow, Re: 1.5 × 106). (a) Lift coefficient comparison; (b) drag coefficient comparison. Appl. Sci. 2020, 10, 7258 9 of 21 Appl. Sci. 2020, 10, 7258 9 of 21 3.3. Experimental Results of the Gurney Flap under Uniform Inflow

3.3. ExperimentalThis subsection Results starts of to the investigate Gurney Flap the under aerodynamic Uniform Inflow characteristics of the effect of Gurney flap in low turbulent inflow condition, the Reynolds number was decreased to 0.8 × 106. Seen from the This subsection starts to investigate the aerodynamic characteristics of the effect of Gurney flap Figure 10, three lift and drag curves were compared with the original at the angle of attack ranged in low turbulent inflow condition, the Reynolds number was decreased to 0.8 106. Seen from the from −9.6° to 14.4°. As shown, the lift coefficients increased with the increase ×of the height of the Figure 10, three lift and drag curves were compared with the original at the angle of attack ranged from Gurney flap before the stall angle, while it was also realized that the effect of the Gurney flap was 9.6 to 14.4 . As shown, the lift coefficients increased with the increase of the height of the Gurney limited− ◦ at a high◦ angle of attack. The lift coefficient slopes and the stall angles of the flapped airfoil flap before the stall angle, while it was also realized that the effect of the Gurney flap was limited remain essentially unchanged compared to the baseline airfoil, but the flapped airfoil caused a at a high angle of attack. The lift coefficient slopes and the stall angles of the flapped airfoil remain leftward shift compared to the baseline airfoil. In order to show the changes of aerodynamic essentially unchanged compared to the baseline airfoil, but the flapped airfoil caused a leftward shift characteristics clearly, the following lift and drag differences were defined: compared to the baseline airfoil. In order to show the changes of aerodynamic characteristics clearly,

the following lift and drag differences were defined:clbs- cl gf cl= 100% (4)

clbs clbs clgf ∆cl = − 100% (4) cdclbs- cd gf × cd =bs 100% (5) cdbs cd cd bs − gf ∆cd = cldbs- cld gf 100% (5) cld=cdbs × 100% cld (6) bs cldbs cldgf ∆cld = − 100% (6) where clbs and cdbs are the lift and dragcld coefficientsbs × obtained from the baseline airfoil, respectively. and are the lift and drag coefficients obtained from the flapped airfoil. where clbs andclgf cdbs arecd thegf lift and drag coefficients obtained from the baseline airfoil, respectively. cl and cd are the lift and drag coefficients obtained from the flapped airfoil. cld and cld are the cldgfbs and gfcldbs are the lift-to-drag ratio obtained from the baseline airfoil and flappedbs airfoil.bs lift-to-drag ratio obtained from the baseline airfoil and flapped airfoil.

2.0 0.16 0.14 1.5 0.12 Flap 12mm*4.5mm Flap 9mm*4.5mm 0.10 1.0 Flap 6mm*4.5mm 0.08 Baseline airfoil

Cl 0.5 Cd 0.06 Flap 12mm*4.5mm 0.04 0.0 Flap 9mm*4.5mm Flap 6mm*4.5mm 0.02 -0.5 Baseline airfoil 0.00 -10 -5 0 5 10 15 -10 -5 0 5 10 15 α/ (°) α/ (°) (a) (b)

-0.06 120 Flap 12mm*4.5mm 90 -0.08 Flap 9mm*4.5mm Flap 6mm*4.5mm 60 Baseline airfoil -0.10 30

Cm -0.12 Cl/Cd 0 Flap 12mm*4.5mm -30 Flap 9mm*4.5mm -0.14 Flap 6mm*4.5mm -60 Baseline airfoil -0.16 -90 -10 -5 0 5 10 15 -10 -5 0 5 10 15 α/ (°) α/ (°) (c) (d)

FigureFigure 10. 10. VariationVariation of of the the aerodynamic aerodynamic characteristics characteristics with with different different heights heights of of Gurney Gurney flaps flaps (uniform(uniform flow flow,, Re: Re: 0.8 0.8 × 10106). 6 ().a) (Lifta) Lift coefficient coefficient comparison; comparison; (b) drag (b) drag coefficient coefficient comparison; comparison; (c) × pitching(c) pitching-moment-moment coefficient coefficient comparison comparison;; (d) (liftd)- lift-to-dragto-drag ratio ratio comparison comparison.. Appl. Sci. 2020, 10, 7258 10 of 21

When the flap height changes to 6 mm, 9 mm and 12 mm, the maximum lift coefficients were increased by 8.47%, 9.56% and 13.50% at 9.4 degrees, respectively. Looking into the drag coefficients shownAppl. in Sci.Figure2020, 1010b,, 7258 within the range of −9.6~10.4°, the drag coefficients show regular change10 of where 21 the frictional drag dominates as expected. The drag coefficients rise rapidly in the stall state, at this time the changeWhen the in flapdrag height coefficient changes was to 6 dominated mm, 9 mm andby pressure 12 mm, the drag. maximum For example, lift coeffi cientsat an wereangle of attackincreased of 12.4°, by with 8.47%, the 9.56% flap heights and 13.50% of 6 at mm, 9.4 degrees, 9 mm, 12 respectively. mm the drag Looking coefficients into the dragwere coe increasedfficients by 39.67%,shown 59.92% in Figure and 10 113.43%,b, within respthe rangeectively. of 9.6~10.4 According◦, the to drag Figure coeffi cients 10c, the show Gurney regular flap change generated where a − prominentthe frictional increase drag in dominates the pitching as expected.-moment The coefficient drag coe fficomparedcients rise to rapidly the baseline in the stall airfoil, state, atthe this value increasedtime thewith change the heights in drag of coe thefficient Gurney was flaps dominated and reached by pressure negative drag. peak For example, values at at approximatel an angle of y 4.4°. Basedattack ofon 12.4 Figure◦, with 10d, the the flap lift heights-to-drag of 6ratios mm, were 9 mm, compared. 12 mm the When drag coe theffi cientsangle wereof attack increased was less by 39.67%, 59.92% and 113.43%, respectively. According to Figure 10c, the Gurney flap generated than 8.4°, the lift-to-drag ratios were all larger than the baseline airfoil. However, the maximum a prominent increase in the pitching-moment coefficient compared to the baseline airfoil, the value lift-toincreased-drag ratios with of the the heights Gurney of flap the Gurneyat three flaps heights and were reached smaller negative than peak the values baseline at approximatelyairfoil. When the flap heights were 6 mm, 9 mm and 12 mm, the corresponding maximum lift-to-drag ratios were 4.4◦. Based on Figure 10d, the lift-to-drag ratios were compared. When the angle of attack was less decreasedthan 8.4by◦ ,17.16%, the lift-to-drag 22.79% ratiosand 24.47%, were all respectively. larger than theIn the baseline range airfoil. after the However, stall angle, the maximum the presence of thelift-to-drag Gurney flap ratios reduces of the Gurney the lift flap-to- atdrag three ratios heights and were the smaller higher than the the flap baseline height, airfoil. the more When the lift efficiencyflap heightsdecreases. were When 6 mm, the 9 mm angle and of 12 attack mm, thewas corresponding 12.4°, for example, maximum the lift-to-drag lift-to-drag ratios ratios were of the flap withdecreased heights by 17.16%, of 6 mm, 22.79% 9 and mm 24.47%, and 12 respectively. mm were In decreased the range after by 26.55 the stall%, angle, 38.11% the and presence 52.20%, respectively.of the Gurney Meanwhile, flap reduces we observed the lift-to-drag the same ratios trend and from the higherthe other the thicknesses flap height, of the the more Gurney the lift flaps (i.e., e offfi ciency 1.5 mm), decreases. WhenWhen the flap the angle height of attack changes was to 12.4 6◦ , mm, for example, 9 mm the and lift-to-drag 12 mm, ratios the maximum of the flap lift coefficientswith heights were ofincreased 6 mm, 9 mmby 9.27%, and 12 9.78% mm were and decreased 14.08% at by 9.4 26.55%, degrees. 38.11% and 52.20%, respectively. Meanwhile, we observed the same trend from the other thicknesses of the Gurney flaps (i.e., of 1.5 mm), The following study in this section will be focused on the effect of the thickness of the Gurney When the flap height changes to 6 mm, 9 mm and 12 mm, the maximum lift coefficients were increased flap. Figure 11 illustrates the changes in the aerodynamic characteristics of the baseline airfoil and by 9.27%, 9.78% and 14.08% at 9.4 degrees. airfoil withThe a following height of study 9 mm in this of section Gurney will flaps be focused under on uniform the effect flow. of the thicknessAs seen of from the Gurney the figure, flap. by changingFigure the11 illustrates thickness the of changes the Gurney in the aerodynamic flap, the lift characteristics-to-drag curves of the of baseline the flapped airfoil and airfoils airfoil with differentwith thicknesses a height of 9 almost mm of Gurneyoverlaps, flaps it seems under uniformthat the flow.flap thickness As seen from does the not figure, have by obvious changing effect the in the aerodynamicthickness of the performance. Gurney flap, The the lift-to-drag results imply curves that of thethe flappedsmall increase airfoils with of the diff erentflap thickness thicknesses does not changealmost overlaps,the degree it seemsof downward that the flap turning thickness of the does mean not haveflow obviousand recirculation effect in the flow aerodynamic around the trailingperformance. edge of the The airfoil. results imply that the small increase of the flap thickness does not change the degree of downward turning of the mean flow and recirculation flow around the trailing edge of the airfoil.

2.0 0.16 0.14 1.5 Flap 9mm*4.5mm 0.12 Flap 9mm*3mm Flap 9mm*1.5mm 1.0 0.10 Baseline airfoil 0.08

Cl 0.5 Cd 0.06 Flap 9mm*4.5mm 0.0 Flap 9mm*3mm 0.04 Flap 9mm*1.5mm 0.02 Baseline airfoil -0.5 0.00 -10 -5 0 5 10 15 -10 -5 0 5 10 15 α/ (°) α/ (°) (a) (b)

Figure 11. Cont. Appl. Sci.Appl. 2020 Sci., 202010, 7258, 10, 7258 11 of11 21 of 21

-0.06 120 Flap 9mm*4.5mm Flap 9mm*3mm 90 -0.08 Flap 9mm*1.5mm Baseline airfoil 60 -0.10 30

Cm -0.12 Cl/Cd 0 Flap 9mm*4.5mm -30 Flap 9mm*3mm -0.14 -60 Flap 9mm*1.5mm Baseline airfoil -0.16 -90 -10 -5 0 5 10 15 -10 -5 0 5 10 15 α/ (°) α/ (°) (c) (d)

FigureFigure 11. Variation 11. Variation of the of theaerodynamic aerodynamic characteristics characteristics with didifferentfferent thicknesses thicknesses of Gurneyof Gurney flaps flaps (uniform flow, Re: 0.8 6 106). (a) Lift coefficient comparison; (b) drag coefficient comparison; (uniform flow, Re: 0.8 × 10×). (a) Lift coefficient comparison; (b) drag coefficient comparison; (c) pitching(c) pitching-moment-moment coefficient coeffi comparisoncient comparison;; (d) lift (d)- lift-to-dragto-drag ratio ratio comparison comparison.. 3.4. Experimental Results of Gurney Flap under 10.5% Turbulence Intensity 3.4. Experimental Results of Gurney Flap under 10.5% Turbulence Intensity This subsection starts to investigate the aerodynamic characteristics of the Gurney flap under Thisa turbulent subsection inflow starts with to an investigate intensity of the 10.5%. aerodynamic Figure 12 characteristicsshows the changes of the in Gurney the aerodynamic flap under a turbulentcharacteristics inflow with of the an baseline intensity airfoil of and 10.5%. the Gurney Figure flaps12 shows with a the thickness changes of 4.5 in mmthe aerodynamicwhen the characteristicsturbulence of intensity the baseline of the incoming airfoil and flow the was Gurney 10.5%. flaps The measured with a thickness angle of attack of 4.5 ranged mm when from the 9.6 to 20.4 . As shown, under the present turbulence intensity of 10.5%, all cases induced a stall turbulence− ◦ intensity◦ of the incoming flow was 10.5%. The measured angle of attack ranged from −9.6° delay.to 20.4°. The As stall shown, angle was under nearly the delayed present from turbulence the original intensity 9.4◦ to aboutof 10.5%, 16.4◦ allunder cases turbulent induced flow. a stall delay.In The Figure stall 12 anglea, with was the increasenearly delayed of turbulence from intensity,the original the maximum9.4° to about lift coe16.4°fficient unde ofr theturbulent baseline flow. airfoil reaches 1.611 and increases by 17.59% as compared with uniform inflow condition. An interesting In Figure 12a, with the increase of turbulence intensity, the maximum lift coefficient of the baseline observation was that in such a turbulence environment, the lift coefficients were not significantly airfoil reaches 1.611 and increases by 17.59% as compared with uniform inflow condition. An different from each other under the current three flapped configurations. For the flap heights of 6 mm, interesting9 mm and observation 12 mm, the was maximum that in lift such coe affi cients turbulence were increased environment, by 15.21%, the lift 17.19% coefficients and 17.26% were at not significantly18.4 degrees, different respectively. from each The increaseother under rate was the found current larger three than flapped that under configurations. the uniform turbulence For the flap heightscondition. of 6 mm, The 9 marginmm and of 12 error mm, in Figurethe maximum 12a was expressed lift coefficients as the variancewere increased of the time-averaged by 15.21%, 17.19% lift and 17.26%coefficient. at As18.4 can degrees be seen, from respectively. the Figure The12a, the increase pressure rate uncertainty was found increased larger with than the that increase under of the uniformthe angleturbulence of attack, condition. and the liftThe coe marginfficient of fluctuations error in Figure caused 12a by was the pressureexpressed fluctuations as the variance were the of the time-mostaveraged obvious lift pronouncedcoefficient. As when can the be attackseen from angle the reaches Figure the 12 stalla, the angle. pressure According uncertainty to Figure increased 12b, with thethe increase increase in of the the lift ofangle the flappedof attack, airfoil and was the at lift a cost coefficient of increasing fluctuations drag. The dragcaused coe ffibycient the ofpressure the fluctuationsflapped airfoils were the was most also non-linearly obvious pronounced raised in the stall when state. the For atta example,ck angle when reaches the angle the of stal attackl angle. was 20.4 , the drag coefficients were increased by 12.50%, 37.50% and 44.53%, respectively. As can be According to◦ Figure 12b, the increase in the lift of the flapped airfoil was at a cost of increasing drag. seen from Figure 12c, compared with the baseline airfoil, the Gurney flap also generated a prominent The drag coefficient of the flapped airfoils was also non-linearly raised in the stall state. For example, increase in the pitching-moment coefficient, the negative peak values delay to about 6.4◦compared whenwith thethe angle uniform of attack inflow was condition. 20.4°, Asthe demonstrated drag coefficients in Figure were 12d, increased the peak value by 12.50%, of the lift-to-drag 37.50% and 44.53%,ratio respectively becomes less. As as compared can be seen with from the uniform Figure flow12c, conditioncompared in with all cases. the Unlikebaseline the airfoil low turbulence, the Gurney flap condition, also generated the maximum a prominent lift-to-drag increase of Gurney in the flap pitchingwith different-moment heights coef slightlyficient increased, the negative relative to peak valuesthe delay baseline to about airfoil. 6 The.4°compared maximum lift-to-dragwith the uniform raised 14.35%, inflow 9.15% condition and 2.74%. As respectivelydemonstrated when in theFigure 12d, flap the heights peak value were 6 ofmm, the 9 mm, lift- andto-drag 12 mm, ratio respectively, becomes with less the as maximum compared lift-to-drag with the ratio uniform occurred flow conditionat the in smallest all cases. flap Unlike height. the Besides, low turbulence in contrast tocondition, the situations the maximum of 1.5 mm thicknesslift-to-drag of of the Gurney Gurney flap with flaps,different the Gurney heights flaps slightly also achieved increased a lift relative increase to under the baseline 10.5% turbulence airfoil. The intensity. maximum The maximum lift-to-drag raisedlift 14.35%, coefficient 9.15% was and increased 2.74% by respectively 14.77%, 14.83%, when and the 16.44% flap heights for height were equals 6 mm, to 6 9 mm, mm, 9 and mm and12 mm, 12 mm, respectively. The two thickness configurations both had maximum lift-to-drag efficiency at respectively, with the maximum lift-to-drag ratio occurred at the smallest flap height. Besides, in 6 mm height, for the three heights, respectively. contrast to the situations of 1.5 mm thickness of the Gurney flaps, the Gurney flaps also achieved a lift increase under 10.5% turbulence intensity. The maximum lift coefficient was increased by 14.77%, 14.83%, and 16.44% for height equals to 6 mm, 9 mm and 12 mm, respectively. The two thickness configurations both had maximum lift-to-drag efficiency at 6 mm height, for the three heights, respectively. Appl. Sci. Sci. 20202020,, 1010,, 7258 7258 12 of 21

2.5 0.20 0.18 2.0 0.16 1.5 0.14 Flap 12mm*4.5mm 0.12 1.0 Flap 9mm*4.5mm

Cl Flap 6mm*4.5mm Cd 0.10 Baseline airfoil 0.5 0.08 Flap 12mm*4.5mm 0.0 Flap 9mm*4.5mm 0.06 Flap 6mm*4.5mm 0.04 Baseline airfoil -0.5 0.02 -10 -5 0 5 10 15 20 25 -10 -5 0 5 10 15 20 25 α/ (°) α/ (°) (a) (b)

-0.06 50 Flap 12mm*4.5mm Flap 9mm*4.5mm 40 -0.08 Flap 6mm*4.5mm Baseline airfoil 30 -0.10 20

Cm 10 -0.12 Cl/Cd 0 Flap 12mm*4.5mm Flap 9mm*4.5mm -0.14 -10 Flap 6mm*4.5mm -20 Baseline airfoil -0.16 -10 -5 0 5 10 15 20 25 -10 -5 0 5 10 15 20 25 α/ (°) α/ (°) (c) (d)

Figure 12. 12. VariationVariation of ofthe theaerodynamic aerodynamic characteristics characteristics with withdifferent diff erentheights heights of Gurney of Gurney flaps (T.I. flaps of 10.5%(T.I. of, Re:10.5%, 0.8 Re: × 10 0.86). (a10) 6Lift). (a coefficient) Lift coe ffi comparison;cient comparison; (b) drag (b) dragcoefficient coeffi cient comparison; comparison; (c) × pitching(c) pitching-moment-moment coefficient coefficient comparison comparison;; (d) (liftd)- lift-to-dragto-drag ratio ratio comparison comparison..

In addition, the effectseffects of variousvarious flap flap thicknesses thicknesses werewere investigated.investigated. Figure 13 indicates the change of the aerodynamicaerodynamic characteristicscharacteristics ofof the the baseline baseline airfoil airfoil and and airfoil airfoil with with a heighta height of of 4.5 4.5 mm mm of ofthe the Gurney Gurney flaps flaps when when the turbulence the turbulence intensity intensity was 10.5%. was As 10.5%. Figure As 13 F shows,igure 13 like shows, low turbulence like low turbulenceconditions, conditions, after changing after thechanging thickness the thickness of the Gurney of the flapGurney under flap turbulence under turbul conditionence condition of 10.5%, of 10.5%,the aerodynamic the aerodynamic characteristics characteristics of flapped of airfoil flapped almost airfoil show almost no change. show no The change. main di ff Theerence main in differencethe Figure in13 thed was Figure concentrated 13d was concentrated in the large lift-to-drag in the large ratio lift-to zone,-drag obviously ratio zone, because obviously the pressurebecause thefluctuations pressure caused fluctuations by the caused turbulence by the and turbulence the flow and separation the flow at separation the airfoil at surface the airfoil result surface in the resultdeviation in the of deviation the measurement. of the measurement.

2.5 0.20 0.18 2.0 0.16 Flap 9*4.5mm 1.5 0.14 Flap 9*3mm 0.12 Flap 9*1.5mm 1.0 Baseline airfoil

Cl Cd 0.10 0.5 Flap 9mm*4.5mm 0.08 0.0 Flap 9mm*3mm 0.06 Flap 9mm*1.5mm 0.04 Baseline airfoil -0.5 0.02 -10 -5 0 5 10 15 20 25 -10 -5 0 5 10 15 20 25 α/ (°) α/ (°) (a) (b) Appl. Sci. 2020, 10, 7258 12 of 21

2.5 0.20 0.18 2.0 0.16 1.5 0.14 Flap 12mm*4.5mm 0.12 1.0 Flap 9mm*4.5mm

Cl Flap 6mm*4.5mm Cd 0.10 Baseline airfoil 0.5 0.08 Flap 12mm*4.5mm 0.0 Flap 9mm*4.5mm 0.06 Flap 6mm*4.5mm 0.04 Baseline airfoil -0.5 0.02 -10 -5 0 5 10 15 20 25 -10 -5 0 5 10 15 20 25 α/ (°) α/ (°) (a) (b)

-0.06 50 Flap 12mm*4.5mm Flap 9mm*4.5mm 40 -0.08 Flap 6mm*4.5mm Baseline airfoil 30 -0.10 20

Cm 10 -0.12 Cl/Cd 0 Flap 12mm*4.5mm Flap 9mm*4.5mm -0.14 -10 Flap 6mm*4.5mm -20 Baseline airfoil -0.16 -10 -5 0 5 10 15 20 25 -10 -5 0 5 10 15 20 25 α/ (°) α/ (°) (c) (d)

Figure 12. Variation of the aerodynamic characteristics with different heights of Gurney flaps (T.I. of 10.5%, Re: 0.8 × 106). (a) Lift coefficient comparison; (b) drag coefficient comparison; (c) pitching-moment coefficient comparison; (d) lift-to-drag ratio comparison.

In addition, the effects of various flap thicknesses were investigated. Figure 13 indicates the change of the aerodynamic characteristics of the baseline airfoil and airfoil with a height of 4.5 mm of the Gurney flaps when the turbulence intensity was 10.5%. As Figure 13 shows, like low turbulence conditions, after changing the thickness of the Gurney flap under turbulence condition of 10.5%, the aerodynamic characteristics of flapped airfoil almost show no change. The main difference in the Figure 13d was concentrated in the large lift-to-drag ratio zone, obviously because the pressure fluctuations caused by the turbulence and the flow separation at the airfoil surface resultAppl. Sci. in 2020the ,deviation10, 7258 of the measurement. 13 of 21

2.5 0.20 0.18 2.0 0.16 Flap 9*4.5mm 1.5 0.14 Flap 9*3mm 0.12 Flap 9*1.5mm 1.0 Baseline airfoil

Cl Cd 0.10 0.5 Flap 9mm*4.5mm 0.08 0.0 Flap 9mm*3mm 0.06 Flap 9mm*1.5mm 0.04 Baseline airfoil -0.5 0.02 -10 -5 0 5 10 15 20 25 -10 -5 0 5 10 15 20 25 α/ (°) Appl. Sci. 2020, 10, 7258 α/ (°) 13 of 21 (a) (b) -0.06 50 Flap 9mm*4.5mm Flap 9mm*3mm 40 -0.08 Flap 9mm*1.5mm Baseline airfoil 30 -0.10 20

Cm 10 -0.12 Cl/Cd 0 Flap 9mm*4.5mm Flap 9mm*3mm -0.14 -10 Flap 9mm*1.5mm -20 Baseline airfoil -0.16 -10 -5 0 5 10 15 20 25 -10 -5 0 5 10 15 20 25 α/ (°) α/ (°) (c) (d)

Figure 13.13. Variation of the aerodynamic characteristics with different different thicknesses of Gurney flapsflaps (T.I.(T.I. of of 10.5%, 10.5%, Re: 0.8 0.8 × 10106).6 ). (a) (a Lift) Lift coefficient coefficient comparison; comparison; (b) (dragb) drag coefficient coefficient comparison; comparison; (c) × (pitchingc) pitching-moment-moment coefficient coefficient comparison comparison;; (d) ( dlift) lift-to-drag-to-drag ratio ratio comparison comparison..

3.5. Experimental ResultsResults of Gurney Flap underunder the 19.0% Turbulence IntensityIntensity ToTo studystudy thethe aerodynamicaerodynamic characteristicscharacteristics of the Gurney flapflap under high turbulence conditions, FigureFigure 1414 showsshows the changes of aerodynamic characteristics under the turbulence condition condition of of 19.0%. 19.0%. The measured angle of attack ranged from 9.6 to 24.4 . As seen in Figure 14, the presence of The measured angle of attack ranged from −−9.6°◦ to 24.4°.◦ As seen in Figure 14, the presence of turbulenceturbulence enhanced the lift lift coe coefficientfficient and and drag drag coe coefficifficientent for for all all cases. cases. In FigureFigure 14 14aa,, with the the increaseincrease of of turbulence turbulence intensity, intensity, the maximum the maximum lift coe liftfficient coefficient continually continually increased, increased, and the maximum and the liftmaximum coefficient lift ofcoefficient the baseline of t airfoilhe baseline reaches airfoil 1.781 reaches at 18.5 degree,1.781 at with 18.5 a degree 30.00%, with increase a 30.00% relative increase to that underrelative low to turbulencethat under low condition. turbulence The maximumcondition. liftThe coe maximumfficients increasedlift coefficients by 2.13%, increased 3.37%, by 5.39% 2.13%, at 18.53.37%, degrees, 5.39% at respectively 18.5 degrees relative, respectively to the baselinerelative to airfoil, the baseline when theairfoil, heights when of the the heights flaps were of the 6 flap mm,s 9were mm, 6 andmm, 12 9 mm.mm, Furthermore,and 12 mm. Furthermore, since the pressure since the fluctuations pressure werefluctuations determined were by determined the fluctuation’s by the energyfluctuation of the’s energy turbulent of the flow, turbulent the fluctuations flow, the of fluctuations lift coefficient of lift were coefficient increased were relative increased to that relative under theto that turbulence under the condition turbulence of 10.5%. condition According of 10.5%. to Figure According 14b, theto F dragigure coe 14bffi, cientthe drag of the coefficient baseline airfoilof the changesbaseline more airfoil obvious changes than more those obvious under thanthe low those turbulence under the condition. low turbulence The drag condition. coefficients The were drag no longercoefficients gentle were within no longer the range gentle before within stall, the and range it also before rises stall, sharply and it in also the stallrises state.sharpl Suchy in the a trend stall wasstate. more Such noticeable a trend was after more adding noticeable the Gurney after adding flap. For the example,Gurney flap. at the For angle example, of attack at the was angle 22.4 of◦, theattack drag was coe 22.4°,fficients the increaseddrag coefficients by 32.11%, increased 37.22% by and 32.11%, 37.96%, 37.22% when and the flap37.96%, heights when were the 6 flap mm, heights 9 mm, andwere 12 6 mmmm, respectively. 9 mm, and 12 As mm can berespectively seen from. FigureAs can 14 bec, seen the Gurney from Figure flap also 14c produced, the Gurney a significant flap also increaseproduced in a the significant pitching-moment increase in coe thefficient pitching and- suchmoment situation coefficient still existed and such in turbulent situation flowstill condition.existed in Basedturbulent on Figureflow condition. 14d, unlike Based that underon Figure the condition14d, unlike of that 10.5%, under the the lift gaincondition obtained of 10.5%, with Gurneythe lift g flapain is,obtained unfortunately, with Gurney coupled flap to is, a largerunfortunately, increase incoupled drag. Addingto a larger Gurney increase flap in with drag. diff Addingerent heights Gurney at mostflap with of the different angles reduces heights the at lift-to-drag most of the relative angles to reduces the baseline the lift airfoil,-to-drag and therelative Gurney to the flap baseline doesn’t airfoil, and the Gurney flap doesn’t show obvious effect under the turbulence condition of 19.0%. At the same time the same situation occurred at other thicknesses of the Gurney flaps. Appl. Sci. 2020, 10, 7258 14 of 21

Appl. Sci. 2020, 10, 7258 14 of 21 showAppl. obviousSci. 2020, 10 e, ff7258ect under the turbulence condition of 19.0%. At the same time the same situation14 of 21 occurred at other thicknesses of the Gurney flaps. 2.5 0.28 2.5 0.28 2.0 0.24 2.0 0.24 1.5 0.20 Flap 12mm*4.5mm 1.5 0.20 FlapFlap 12mm*4.5mm9mm*4.5mm Flap 9mm*4.5mm 1.0 0.16 Flap 6mm*4.5mm 0.16 Flap 6mm*4.5mm Cl Baseline airfoil 1.0 Cd

Cl 0.12 Baseline airfoil 0.5 Cd 0.12 0.5 Flap 12mm*4.5mm Flap 12mm*4.5mm 0.08 0.0 Flap 9mm*4.5mm 0.08 Flap 9mm*4.5mm 0.0 Flap 6mm*4.5mm 0.04 -0.5 Baseline Flap 6mm*4.5mm airfoil 0.04 -0.5 Baseline airfoil 0.00 -10 -5 0 5 10 15 20 25 0.00 -10 -5 0 5 10 15 20 25 -10 -5 0 5 10 15 20 25 -10 -5 0 5 10 15 20 25 α/ (°) α/ (°) α/ (°) α/ (°) (a) (b) (a) (b)

-0.08 50 -0.08 50 -0.10 40 40 -0.10 30 -0.12 30 -0.12 20 20

Cm

-0.14 Cl/Cd 10

Cm

-0.14 Cl/Cd 10 0 Flap 12mm*4.5mm -0.16 Flap 12mm*4.5mm 0 FlapFlap 12mm*4.5mm9mm*4.5mm -0.16 Flap Flap 9mm*4.5mm12mm*4.5mm -10 FlapFlap 6mm*4.5mm9mm*4.5mm Flap 9mm*4.5mm Flap 6mm*4.5mm -10 BaselineFlap 6mm*4.5mm airfoil -0.18 -20 -0.18 Baseline Flap 6mm*4.5mm airfoil Baseline airfoil Baseline airfoil -20 -10 -5 0 5 10 15 20 25 -10 -5 0 5 10 15 20 25 -10 -5 0 5 10 15 20 25 -10 -5 0 5 10 15 20 25 ° α/ (°) ° α/ (°) (c) (d) (c) (d) Figure 14. Variation of the aerodynamic characteristics with different heights of Gurney flaps (T.I. of FigureFigure 14. 14.Variation Variation of of the the aerodynamicaerodynamic characteristics with with different different heights heights of ofGurney Gurney flaps flaps (T.I. (T.I. of of 19%, Re: 0.8 × 106). (a) Lift coefficient comparison; (b) drag coefficient comparison; (c) 19%,19% Re:, Re: 0.8 0.810 6 ×). ( 10a)6 Lift). (a coe) ffi Liftcient coefficient comparison; comparison; (b) drag coe (bffi) cientdrag comparison; coefficient (c comparison;) pitching-moment (c) pitching-moment× coefficient comparison; (d) lift-to-drag ratio comparison. coepitchingfficient-moment comparison; coefficient (d) lift-to-drag comparison ratio; (d comparison.) lift-to-drag ratio comparison. Figure 15 exhibits the changes in the aerodynamic characteristics of the baseline airfoil and FigureFigure 15 15 exhibits exhibits the the changes changes inin thethe aerodynamic characteristics characteristics of of the the baseline baseline airfoil airfoil and and airfoil with Gurney flaps of height 4.5mm under the turbulence condition of 19.0%. As can be seen airfoilairfoil with with Gurney Gurney flaps flaps of of heightheight 4.5mm4.5mm under the turbulence turbulence condition condition of of 19.0%. 19.0%. As As can can be beseen seen from it, with the increase of turbulence intensity, obviously, there were more pressure fluctuations. fromfrom it, it, with with the the increase increase ofof turbulenceturbulence intensity, intensity, obviously, obviously, there there were were more more pressure pressure fluctuations. fluctuations. The curves of flapped airfoils with different thickness no longer overlap totally, in the majority of The curves of flapped airfoils with different thickness no longer overlap totally, in the majority of Theangles, curves the ofdifference flapped from airfoils 2% withto 5%, di thofferentugh it thickness may sometimes no longer reach overlap 20%. totally, in the majority of angles,angles, the the di differencefference from from 2% 2% to to 5%, 5%, thoughthough it may sometimes reach reach 20%. 20%.

2.5 0.28 2.5 0.28 2.0 0.24 2.0 0.24 Flap 9mm*4.5mm Flap 9mm*4.5mm 1.5 0.20 Flap 9mm*3mm 1.5 0.20 FlapFlap 9mm*1.5mm9mm*3mm Flap 9mm*1.5mm 1.0 0.16 Baseline airfoil 0.16 Baseline airfoil

Cl 1.0 Cd

Cl 0.12 0.5 Flap 9mm*4.5mm Cd 0.12 0.5 Flap Flap 9mm*3mm 9mm*4.5mm 0.08 0.0 Flap Flap 9mm*1.5mm 9mm*3mm 0.08 0.0 Baseline Flap 9mm*1.5mm airfoil 0.04 -0.5 Baseline airfoil 0.04 -0.5 0.00 -15 -10 -5 0 5 10 15 20 25 0.00-15 -10 -5 0 5 10 15 20 25 -15 -10 -5 0 5 10 15 20 25 -15 -10 -5 0 5 10 15 20 25 α/ (°) α/ (°) α/ (°) α/ (°) (a) (b) (a) (b) Figure 15. Cont. Appl.Appl. Sci. Sci.2020 2020, 10, 10, 7258, 7258 15 of15 21 of 21

-0.08 50

-0.10 40 30 -0.12 20

Cm

-0.14 Cl/Cd 10

Flap 9mm*4.5mm 0 Flap 9mm*4.5mm -0.16 Flap 9mm*3mm Flap 9mm*3mm -10 Flap 9mm*1.5mm Flap 9mm*1mm -0.18 Baseline airfoil -20 Baseline airfoil -15 -10 -5 0 5 10 15 20 25 -15 -10 -5 0 5 10 15 20 25 (°) α/ (°) (c) (d)

FigureFigure 15. 15.Variation Variation of of the the aerodynamic aerodynamic characteristics characteristics with with diff differenterent thicknesses thicknesses of Gurney of Gurney flaps flaps (T.I. of 19%,(T.I. Re: of 0.8 19%,10 Re:6). ( 0.8a) Lift × 10 coe6). ffi(acient) Lift comparison; coefficient ( comparison;b)drag coeffi cient (b)drag comparison; coefficient ( c comparison;) pitching-moment (c) × coepitchingfficient-moment comparison; coefficient (d) lift-to-drag comparison ratio; (d comparison.) lift-to-drag ratio comparison.

3.6.3.6. Surface Surface Pressure Pressure Characteristics Characteristics ToT analyzeo analyze the the mechanism mechanism of of turbulent turbulent inflow inflow coupled coupled with with Gurney Gurney flap, flap, Figure Figure 16 16 illustrates illustrates the pressurethe pressure coeffi cientcoefficient distribution distribution of the of baseline the baseline airfoil airfoil and airfoiland airfoil attached attached with with the Gurneythe Gurney flap flap under diunderfferent different turbulent turbulent inflow when inflow Reynolds when Reynolds number Renumber= 0.8 Re10 =6 .0.8 The × x-axis106. The shows x-axis the shows chord-wise the × p positionchord-wise x/c and position the y-axis x/c and shows the they-axis pressure shows coe theffi pressurecients Cp coefficients. The pressure C . coeTheffi pressurecients C pcoefficientson the airfoil surfaceCp on the were airfoil given surface were given pi p0 C = pp−i0- (7) pC = 2 p 0.5ρU 2 (7) 00 where pi is the pressure at the pressure tap of i (i = 1:77), p0 is the free stream static pressure at the where pi is the pressure at the pressure tap of i (i = 1:77), p0 is the free stream static pressure at the airfoilAppl. Sci. tested 2020, 10 by, x theFORPitot PEER tube,REVIEWρ is the air density and U0 is the free stream velocity. 16 of 21 airfoil tested by the Pitot tube, ρ is the air density andU0 is the free stream velocity.

Figure 16 illustrates that the pressure coefficients of the baseline airfoil-3.0 under different -3.0 -3.0 -3.0 -2.5 AOA=-9.6° -2.5 AOA=-9.6° -2.5 AOA=-9.6° -2.5 AOA=-9.6° turbulence conditions Baseline, T.I.=0.2% with small angles were similar to each other. However,-2.0 with the increase of -2.0 -2.0 -2.0 Baseline, T.I.=10.5% -1.5 angle-1.5 of attack, the differences-1.5 become more pronounced-1.5 at the leading edge. In contrast to the Baseline, T.I.=19.0% -1.0 -1.0 -1.0 -1.0 Cp Cp Cp Gurney-0.5 flaps, Figure 16a shows Cp -0.5 that at the angle = −9.6°,-0.5 the differences between-0.5 the flapped airfoils were0.0 small. With the increase0.0 of the angle of attack, as0.0 the angle of −3.6°, compared0.0 with the baseline 0.5 0.5 0.5 0.5 Baseline, T.I.=0.2% Baseline, T.I.=10.5% Baseline, T.I.=19.0% 1.0 1.0 1.0 1.0 airfoils, the intersection of the airfoil Flap 9mm*4.5mm,T.I.=0.2% with Gurney flap moves Flap 9mm*4.5mm,T.I.=10.5% to the leading edge under Flap 9mm*4.5mm,T.I.=19.0% different 1.5 1.5 1.5 1.5 conditions.0.00.20.40.60.81.0 Besides, the pressure0.00.20.40.60.81.0 coefficients (with 0.0 the 0.2 angle 0.4 0.6= −3.6° 0.8 1.0 and 0.0 2.4°), 0.2 the 0.4 pressure 0.6 0.8 1.0 x/c x/c x/c x/c distribution indicate a larger the pressure difference in the range of 0.8 < x/c < 0.9. When the angle of attack was 2.4° and 8.4°, both the pressure coefficient(a) absolute values of the baseline airfoil and the Gurney flap tendency to increase with the increase of turbulence intensity, and the flapped airfoil becomes more obvious than the baseline airfoil. At a large angle of attack, due to the condition of stalling (Figure 16e,f), there was significant flow separation both on the suction side of flapped airfoil and baseline airfoil under uniform inflow. The larger the turbulent inflow, the larger the peak value on the leading edge of the suction side, and also the larger pressure coefficient on the pressure side.

(b)

Figure 16. Cont.

(c)

(d)

(e) Appl. Sci. 2020, 10, 7258 16 of 21

-3.0 -3.0 -3.0 -3.0 -2.5 AOA=-9.6° -2.5 AOA=-9.6° -2.5 AOA=-9.6° -2.5 AOA=-9.6° -2.0 Baseline, T.I.=0.2% -2.0 -2.0 -2.0 Baseline, T.I.=10.5% -1.5 -1.5 -1.5 -1.5 Baseline, T.I.=19.0% -1.0 -1.0 -1.0 -1.0 Cp Cp Cp Cp -0.5 -0.5 -0.5 -0.5 0.0 0.0 0.0 0.0 0.5 0.5 0.5 0.5 Baseline, T.I.=0.2% Baseline, T.I.=10.5% Baseline, T.I.=19.0% 1.0 1.0 1.0 1.0 Flap 9mm*4.5mm,T.I.=0.2% Flap 9mm*4.5mm,T.I.=10.5% Flap 9mm*4.5mm,T.I.=19.0% 1.5 1.5 1.5 1.5 0.0 0.2 0.4 0.6 0.8 1.0 0.00.20.40.60.81.0 0.00.20.40.60.81.0 0.0 0.2 0.4 0.6 0.8 1.0 x/c x/c x/c x/c (a)

Appl. Sci. 2020, 10, 7258 16 of 21 (b)

(c)

(d)

(e)

(f)

Figure 16. The surface pressure coefficients under different angle of attack (Re: 0.8 106). (a) α = 9.6 ; × − ◦ (b) α = 3.6 ;(c) α = 2.4 ;(d) α = 8.4 ;(e) α = 12.4 ; (f) α = 18.4 . − ◦ ◦ ◦ ◦ ◦ Figure 16 illustrates that the pressure coefficients of the baseline airfoil under different turbulence conditions with small angles were similar to each other. However, with the increase of angle of attack, the differences become more pronounced at the leading edge. In contrast to the Gurney flaps, Figure 16a shows that at the angle = 9.6 , the differences between the flapped airfoils were small. − ◦ With the increase of the angle of attack, as the angle of 3.6 , compared with the baseline airfoils, − ◦ the intersection of the airfoil with Gurney flap moves to the leading edge under different conditions. Besides, the pressure coefficients (with the angle = 3.6 and 2.4 ), the pressure distribution indicate a − ◦ ◦ larger the pressure difference in the range of 0.8 < x/c < 0.9. When the angle of attack was 2.4◦ and 8.4◦, both the pressure coefficient absolute values of the baseline airfoil and the Gurney flap tendency to increase with the increase of turbulence intensity, and the flapped airfoil becomes more obvious than the baseline airfoil. At a large angle of attack, due to the condition of stalling (Figure 16e,f), there was significant flow separation both on the suction side of flapped airfoil and baseline airfoil under uniform Appl. Sci. 2020, 10, 7258 17 of 21

Appl.Figure Sci. 2020 16., 10 ,The 7258 surface pressure coefficients under different angle of attack (Re: 0.8 × 106). (a) α 17= of 21 −9.6°; (b) α = −3.6°; (c) α = 2.4°; (d) α = 8.4°; (e) α = 12.4°; (f) α = 18.4°. inflow.As The seen larger from the the turbulent above, the inflow, influence the larger of turbulent the peak inflow value coupled on the leading with Gurney edge of flap the suction on the pressureside, and coefficients also the larger of airfoil pressure was coe complicated.fficient on theTurbulent pressure inflow side. changes the flow around the airfoil surfaceAs and seen restrains from the the above, flow separation, the influence after of attaching turbulent the inflow Gurney coupled flap, withthe pressure Gurney distribution flap on the onpressure both coe suctionfficients and of pressureairfoil was sides complicated. changed Turbulent more significantly. inflow changes Especially the flow in around the turbulence the airfoil condition,surface and the restrains trailing the edge flow pressure separation, distributions after attaching were farther the Gurney apart flap, for the pressureflapped airfoil. distribution In these on situations,both suction the and increased pressure peak sides values changed of more pressure significantly. coefficient Especiallys at the leading in the turbulence edge increased condition, the integralthe trailing area edge of the pressure pressure distributions coefficients, were so the farther lift force apart was for therefore the flapped increased. airfoil. In these situations, the increased peak values of pressure coefficients at the leading edge increased the integral area of the 3.7.pressure Wake coeProfilefficients, Characteristics so the lift force was therefore increased.

3.7. WakeFigure Profile 17 presents Characteristics the distribution of wake velocity measured by the wake rake array under uniform inflow. As can be observed, when the angle of attack was 8.4°, which was before the stall Figure 17 presents the distribution of wake velocity measured by the wake rake array under angle of attack, the wake of airfoil with the Gurney flap was inclined to the pressure side of the uniform inflow. As can be observed, when the angle of attack was 8.4 , which was before the stall airfoil. When the angle of attack was 11.4°, which was after the stall angle,◦ the Gurney flap weaken angle of attack, the wake of airfoil with the Gurney flap was inclined to the pressure side of the airfoil. the ability of the wake position deflection. Moreover, the wake velocity deficit was significantly When the angle of attack was 11.4 , which was after the stall angle, the Gurney flap weaken the ability increased, and the Gurney flap beg◦ an to have side effects on the aerodynamic characteristics of the of the wake position deflection. Moreover, the wake velocity deficit was significantly increased, and the airfoil. The higher the Gurney flap is, the larger the wake velocity deficit is, so it indicates a higher Gurney flap began to have side effects on the aerodynamic characteristics of the airfoil. The higher the mean drag. Gurney flap is, the larger the wake velocity deficit is, so it indicates a higher mean drag.

1.0 1.0

0.9 0.9

0.8 0.8

/U AOA=8.4° /U u u AOA=11.4° 0.7 0.7 Baseline airfoil,T.I.=0.2% Baseline airfoil,T.I.=0.2% Flap 6mm*4.5mm,T.I.=0.2% Flap 6mm*4.5mm,T.I.=0.2% 0.6 0.6 Flap 9mm*4.5mm,T.I.=0.2% Flap 9mm*4.5mm,T.I.=0.2% Flap 12mm*4.5mm,T.I.=0.2% Flap 12mm*4.5mm,T.I.=0.2% 0.5 0.5 -0.4 -0.2 0.0 0.2 0.4 -0.4 -0.2 0.0 0.2 0.4 X(m) X(m) (a) (b)

FigureFigure 17. 17. TheThe wake wake velocity velocity measured measured by bythe thewake wake rake rake array array (uniform (uniform inflow, inflow, Re: 0.8 Re: × 10 0.86). (a)10 α 6=). × 8.4°;(a) α (=b)8.4 α ◦=;( 11.4°b) α. = 11.4◦.

ByBy comparingcomparingthe the flows flows in in the the near near trailing trailing edge edge region region of the airfoilof the withairfoil Gurney with flap.GurneyThe flap. resultant The resultantvelocitydistribution velocity distribution of the baseline of the airfoilbaseline and airfoil the airfoil and the attached airfoil withattached Gurney with flap Gurney (9 mm flap in (9 height mm inand height 4.5 mm and in 4.5 thickness) mm in thickness at the angle) at the of attackangle of 8.4 attack◦ (before 8.4° the (before stall angle)the stall and angle) 11.4 ◦and(after 11.4° the (after stall theangle) stall under angle) uniform under inflowuniform were inflow depicted were indepicted Figure 18 in. F Theigure vertical 18. The direction vertical Y direction denoted theY denoted forward thedistance forward from distance the hot-wire from probe the hot to- thewire trailing probe edge, to the and trailing the horizontal edge, and direction the horizontal X denoted direction the lateral X denoteddistance the from lateral the hot-wiredistance probefrom the to thehot- trailingwire probe edge. to Asthe showntrailing in edge. Figure As 18shown, the blackin Figure solid 18, spots the blackwere the solid actual spots measurement were the actual positions measurement by using the positions electronically by using controlled the electronically traverser. Ascontrolled shown, traverser.the wake flowsAs shown, shift to the the wake pressure flows side shift in to both the the pressure two situations. side in both When the the two angle situations. of attack When was 8.4the◦, anglethe airfoil of attack with was Gurney 8.4°, flap the hadairfoil a larger with Gurney velocity flap deficit had than a larger the baseline velocity airfoil, deficit which than the suggests baseline an airfoil,increased which drag. sugges Whents thean angleincreased of attack drag. was When 11.4 the◦, the angle velocity of attack deficit was of 11.4°, the baseline the velocity airfoil deficit was also of theless baseline than the airfoil Gurney was flap, also but less there than was the no Gurney obvious flap, diff buterence there between was no the obvious offset difference direction caused between by thethe offset flapped direction airfoil and caused the baselineby the flapped airfoil which airfoil were and the being baseline similar airfoil to the which far field were results being measured similar byto thethe far wake field rake results array. measured by the wake rake array. Appl.Appl. Sci. Sci.2020 2020, ,10 10,, 7258 7258 1818 of of 21 21

0.0 0.0 19.50 0.5 0.5 17.38 1.0 1.0 15.25 1.5 1.5 13.13

2.0 2.0 11.00

Y(cm)

Y(cm) 2.5 2.5 8.875

3.0 3.0 6.750

3.5 3.5 4.625

4.0 4.0 2.500 -3 -2 -1 0 1 2 3 -3 -2 -1 0 1 2 3 m/s X(cm) X(cm) (a) (b)

0.0 0.0 19.00 0.5 0.5 17.50 1.0 1.0 16.00 1.5 1.5 14.50 2.0 2.0 13.00

Y(cm) 2.5 Y(cm) 2.5 11.50

3.0 3.0 10.00

3.5 3.5 8.500

4.0 4.0 7.000 -3 -2 -1 0 1 2 3 4 5 6 -3 -2 -1 0 1 2 3 4 5 6 m/s X(cm) X(cm) (c) (d)

FigureFigure 18. 18. VelocityVelocity distributiondistribution of the region behind behind trailing trailing edge edge (uniform (uniform inflow, inflow, Re: Re: 0.8 0.8 × 10610). 6().a) × (aα) =α 8.4°= 8.4, baseline, baseline airfoil; airfoil; (b) (αb )=α 8.4°= 8.4, Gurney, Gurney flap flap9 mm 9 mm× 4.5 mm4.5; mm;(c) α = (c 11.4°) α =, 11.4baseline, baseline airfoil; airfoil; (d) α = ◦ ◦ × ◦ (d11.4°) α =, Gurney11.4 , Gurney flap 9 mm flap × 9 4 mm.5 mm4.5. mm. ◦ × FigureFigure 19 19 presents presents the the power power density density spectrum spectrum measured measured by by the the region region 1 1 cm cm directly directly behind behind the the trailingtrailing edge edge under under uniform uniform inflow. inflow. As As can can be be observed, observed, at at the the angle angle of of attack attack 8.4 8.4°,◦, both both along-wind along-wind andand across-wind across-wind display display an obvious an obvious peak in peak the power in the density power spectrum, density meanwhile spectrum, the meanwhile across-wind the energyacross- increasedwind energy in the increased flapped airfoil.in the flapped It indicates airfoil. there It are indicates counter there rotating are vorticescounter generatedrotating vorti by theces Gurneygenerated flap, by while the Gurney the wake flap, of baseline while the airfoil wake does of notbaseline show airfoil the existence does not of show vortex the shedding existence [33 of]. Gurneyvortex shedding flaps also [33]. have Gurney a wake flaps structure also similarhave a towake the structure Karman vortexsimilar street to the behind Karman the vortex cylinder street at biggerbehind Reynolds the cylinder number at bigger (i.e., of Reynolds 0.8 106 number), the flow (i.e. oscillations, of 0.8 × 10 were6), the caused flow byoscillations the separating were caused of the × shearby the layer separating on the pressureof the shear side layer and suction on the sidepressure of the side trailing and edge.suction The side Karman of the vortextrailing street edge. both The canKarman be found vortex in uniformstreet both flow can and be turbulent found in flow,uniform so the flow increased and turbulent lift produced flow, so by the the increased Gurney flap lift seemsproduced can beby explained the Gurney in turbulentflap seems flow can condition,be explained either. in turbulent Moreover, flow from condition, the lift coe either.fficients Moreover, shown above,from the size lift ofcoefficients structures shown was aff ected above, by the turbulence size of level. structures Besides, was as affected compared by with turbulence the reference level. dataBesides, under as similarcompared condition with the [33 reference], with the data increase under of similar the Reynolds condition number, [33], with from the Re increase= 1 10 of5 theto × 0.8Reynolds106, the number, Strouhal from number Re = decreased1 × 105 to from0.8 × 0.151106, the (Re: Strouhal 1 105 ,number 2% chord decreased length) to from 0.128 0.151 (Re: 2(Re:10 15 ,× × × × 2%105 chord, 2% chord length) length) and to to 0.088 0.128 (Re: (Re: 0.8 2 × 1056,, 2% 1.5% chord chord length) length). and to 0.088 (Re: 0.8 × 106, 1.5% chord × length). Appl.Appl. Sci. Sci. 20202020, ,1010, ,7258 7258 1919 of of 21 21

0.1 0.1

0.01 0.01

0.001 0.001

along-wind along-wind

Power spectral density S(f) density Power spectral 1E-4 across-wind S(f) density Power spectral 1E-4 across-wind

1 10 100 1000 1 10 100 1000 frequency (Hz) frequency (Hz) (a) (b)

FigureFigure 19. 19. ComparisonComparison of of the the power power density density spectrum spectrum ( (uniformuniform inflow, inflow, Re: 0.8 0.8 × 101066).). (a (a) ) αα == 8.4°8.4, , × ◦ baselinebaseline airfoil; airfoil; ( (bb)) αα == 8.4°8.4, ,Gurney Gurney flap flap 9 9mm mm × 4.54.5 mm mm.. ◦ × 4.4. Conclusions Conclusions InIn summary, summary, this this paper paper presented present aed study a ofstudy the aerodynamic of the aerodynamic performance performance of airfoils with of /without airfoils with/withoutthe Gurney flap the underGurney di flapfferent under turbulence different conditions turbulence by conditions means of by wind means tunnel of wind experiments. tunnel experiments.The experimental The experimental observations observations were as follows: were as follows: TheThe Gurney Gurney flap flap deflect deflectss the the wake wake position position from from the the pressure pressure s sideide of of the the airfoil, airfoil, increasing increasing the the verticalvertical distance distance between between the the airfoil airfoil chord chord and and the the middle arc. The The surface surface pressure pressure characteristics revealedrevealed that that the the pressure pressure difference difference of of the the trailing trailing edge edge was was larger larger for for the the flapped flapped airfoil airfoil in in the the turbulenceturbulence condition. condition. Moreover, Moreover, due due to tothe the special special wake wake structure, structure, the theGurney Gurney flap flap also also indicates indicates the vortexthe vortex generated generated at the at trailing the trailing edge, which edge, result whichs resultsin a greater in a lift greater of the lift airfoil. of the The airfoil. lift increment The lift obtainedincrement from obtained Gurney from flap Gurney was influenced flap was by influenced the turbulence by the intensity, turbulence meanwhile intensity, meanwhilea large increase a large in dragincrease was in observed drag was as observed well. Under as well. a Undermedium a medium range of range turbulence of turbulence intensity intensity (i.e., of (i.e., 10.5%), of 10.5%), the the maximum lift-to-drag ratios increased from 14.35% (flap 6 mm 4.5 mm, a = 12.4 ) to 14.47% maximum lift-to-drag ratios increased from 14.35% (flap 6 mm × 4.5 mm,× a = 12.4°) to 14.47%◦ (flap 6 (flap 6 mm 1.5 mm, a = 12.4 ), the 1.0% Gurney flap case has the best performance, that’s probably mm × 1.5 mm,× a = 12.4°), the 1.0%◦ Gurney flap case has the best performance, that’s probably because thebecause decrease the decrease of boundary of boundary layer thickness layer thickness under turbulent under turbulent conditions, conditions, so the so height the height of 1.0% of 1.0% was optimal.was optimal. However,However, under under a much a much larger larger turbulence turbulence inflow inflow (i.e., of (i.e., 19.0%), of 19.0%), all experimental all experimental data have data shownhave shown that the that impact the impact was negligibly was negligibly small. small. The drag The dragincrement increment was wasfound found to surpass to surpass the thebenefit benefit of theof theincrease increase in lift, in lift, resulting resulting in a in worse a worse lift lift-to-drag-to-drag ratio ratio than than the the baseline baseline airfoil. airfoil. ItIt was was of of importance importance to to note note that that there there are are t twowo kinds kinds of of measurement measurement uncertainties uncertainties could could affect affect thethe acquired acquired results results.. One One was was stochastic stochastic uncertainties uncertainties caused caused by the by thediscrete discrete pressure pressure data data which which can becan eliminated be eliminated by multiple by multiple measurements. measurements. The The other other one one was was systematic systematic uncertainties uncertainties caused caused by by measuringmeasuring instruments instruments which which can can be reduced be reduced by calibration by calibration of the instruments of the instruments before the experiment.before the experiment.Based on the observations from the present measurements, it was found that under very high turbulenceBased on level, the e.g., observations with 19% offrom turbulence the present intensity, measurements, the Gurney it flap was has found very that small under influence very on high the turbulenceairfoil aerodynamic level, e.g., performance. with 19% of Theturbulence measurement intensity, results the show Gurney the flap Gurney has flapvery may small be influence used in areas on thewith airfoil slightly aerodynamic lower turbulence. performance. Nevertheless, The measurement wind turbines results in real show life the were Gurney running flap in may the turbulent be used inatmospheric areas with slightly boundary lower layer, turbulence as shown. Nevertheless, above, the eff windects of turbines Gurney in flap real are life highly were running dependent in the on turbulentturbulent inflow, atmospheric it seems boundary that the Gurney layer, as flap shown would above, be installed the effects as a scalable of Gurney form flapand preferred are highly to dependenthave smaller on size, turbulent such that inflow, it would it seems get morethat the potential Gurney benefits flap would at all periodsbe installed of rotation. as a scalable In addition, form andit should preferred be noted to have that smaller the incoming size, such flow that is often it would a combination get more of potential rotational benefits wake from at all the periods previous of rotation.wind turbines. In addition, Research it should on the be lift noted increment that the of incoming Gurney flap flow was is often not isolated, a combination it always of rotational interacted wakewith such from highly the previous unsteady wind aerodynamic turbines. issues.Research Therefore, on the lift to analyze increment more of preciselyGurney flap the e wasffects not of isolated,combining it always such turbulence interacted levels with with such Gurney highly flap,unsteady further aerodynamic experiments issues. should Therefore, be carried to out analyze for the morerotor blades precisely in highly the effects turbulent of flows.combining This wassuch in turbulence progress on levels a wind with turbine Gurney blade flap, composed further of experimentsseveral DTU-LN221 should wing be carried sections. out for the rotor blades in highly turbulent flows. This was in progress on a wind turbine blade composed of several DTU-LN221 wing sections. Appl. Sci. 2020, 10, 7258 20 of 21

Author Contributions: Conceptualization, J.Y.; conducted the data collection, J.Y. and Y.Y.; funding acquisition, H.Y.; supervision, N.L., H.Y. and W.Z.; formal analysis, J.Y., H.Y., W.Z.; writing—original draft preparation, J.Y.; writing—review and editing, H.Y., W.Z.; All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Postgraduate Research & Practice Innovation Program of Jiangsu Province under grant number KYCX19_2104 and Youth Fund Project of Jiangsu Natural Science Foundation under grant number BK20170510. Acknowledgments: The authors wish to express acknowledgement to the Postgraduate Research &Practice Innovation Program of Jiangsu Province under grant number KYCX19_2104 and Youth Fund Project of Jiangsu Natural Science Foundation under grant number BK20170510. Conflicts of Interest: The authors declare no conflict of interest.

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