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Experimental Investigation of Flow Separation Control over Airfoil by Upper Surface Flap with a Gap

Experimental Investigation of Flow Separation Control over Airfoil by Upper Surface Flap with a Gap This paper focused on the effects of the flap with a tiny gap on the flow separation over the NPU-WA-180 airfoil. The effects of the geometric parameters of the flaps, such as the flap gap height, angle, and position, were investigated. The study showed that the flap can significantly improve the stall features of airfoil in a limited phase of angle of attack (AoA), and increase lift and reduce drag at a high AoA. It can increase the lift coefficient and drag coefficient in the case of high AoAs, and the angle range of the lift augmentation and drag reduction can reach more than 9°. Furthermore, an excessively large gap is not conducive to the improvement of the airfoil stall performance. The flap angle plays a key role in the airfoil stall characteristic. As the flap angle decreases, the angle range of improving airfoil stall characteristics becomes larger, the pitching moment increment becomes smaller. However, the maximum lift increment and the effect of the drag reduction will decrease. And the effects of the position of the flap on the airfoil performance were also studied. Considering the maximum lift coefficient and drag coefficients in large AoA, the Type1 installed at the 0.7c position has the best effect; from the perspective of delayed stall, the Type1 installed at the 0.6c position has the best delay effect. These results can provide the data and theoretical support for the flap application in engineering. Keywords Flap · Flow control · Airfoil · Stall characteristic · Wind tunnel experiment Abbreviations 1 Introduction C Chord Biomimetic engineering has stimulated a new area of flow Cl Lift coefficient control by combining two different concepts of biology Cd Drag coefficient and fluid mechanics. Choi et al. [1] discussed some recent Cm Pitching moment coefficient biomimetic flow control results for aerodynamic perfor- Cn Normal force coefficient mance enhancement and skin-friction reduction. Chen et al. α Angle of attack [2] proposed a novel biomimetic drag reduction surface, and α Zero-lift angle of attack conducted water tunnel experiments to clarify its efficiency Re Reynolds number and mechanism combining computational fluid dynamics k Slope of the normal force line (CFD) analysis. The researchers [3, 4] conducted a wing f Position of the separation point reconstruction with reference to free flying birds, and inves- θ Flap angle tigated the aerodynamics performance of the wing/airfoil by numerical simulation. In flight of birds, the feathers on the upper surface of the wings stand up, while birds land (Fig. 1). Meyer et al. [5] and Schatz et al. [6] conducted detailed experimental and numer- ical investigations on the mechanism of feather movement and its aerodynamic characteristics. The results showed that B Lishu Hao the flap prevented the development of the reversed flow from haolishu@nwpu.edu.cn the trailing edge to the suction peak position, bringing benefit School of Aeronautics, Northwestern Polytechnical of the flow separation delay. University, Xi’an 710072, People’s Republic of China 123 860 International Journal of Aeronautical and Space Sciences (2022) 23:859–869 Fig. 2 Model and flap: (a) Three-segment airfoil model; (b)Flap mounted on the model Fig. 1 Bird’s wing posture when landing Therefore, the effects of flap with gap on the aerodynamic performance of the airfoil were experimentally studied. The influences of key geometry parameters (such as gap height, There is a body of research focused on the effect of angle, and position) on the lift, drag, and pitching moment the upper surface flap on flow separation. Johnston et al. coefficients were explored in detail. This research attempts [7, 8] designed a lift-enhancing effector device, which was to explore the flow mechanism of the flap with gap, and at arranged on the upper surface of the airfoil at different angles. the same time provides theoretical and data basis for the flap The results showed that the device could increase the lift and with gap in aeronautical engineering application. stall angle of attack (AoA). Kernstine et al. [9] explored the influence of parameters of passive flap on the flow separation, including its size, chord placement, configuration, material, etc. Bramesfeld et al. [10] designed a high-lift device, and 2 Equipment and Procedure investigated the influence of different flap positions and mul- tiple flaps on the pressure distribution of the airfoil. Schlüter The experiments were performed in the NF-3 low-speed wind [11, 12] placed a passive flap on the different airfoils. The tunnel, at Northwestern Polytechnical University, China. The experiments showed that the flap could improve the stall turbulence intensity of the wind tunnel was 0.045%, and the characteristics. Traub et al. [13] studied the effect of several test section size was 1.6 m × 3.0 m. spoiler geometric parameters by water tunnel experiments. In this work, the study was inspired by the wing shapes The experiments showed that the device could improve the of birds during takeoff and landing, and hoped to study the stall characteristics. Wang [14] designed a self-activated flap effects of the trailing-edge flap with gap on aerodynamic to place the flap to the wing of the aircraft. The experiments forces at large AoAs. When birds take off and land, the flow showed that the flap could improve the stall characteristics. separation will raise the feathers on the trailing edge of the Montefort et al. [15] described a thin-wing vibration control wings, and the raised feathers inhibit the development of the method using an array of small flexible flaps attached near the separation flow and play a certain role in flow control. In this leading edge on the suction surface to manipulate the leading condition, bird wings present typical trailing-edge separa- edge vortex structures. Brücker et al. [16] carried out research tion characteristics. Therefore, we selected the trailing-edge on the effects of self-adaptive hairy flaps on the stall delay separation airfoil for this research. The NPU-WA-180 airfoil of an airfoil in its ramp-up motion. Liu et al. [17] designed a used in this experiment is a typical trailing-edge separated device to extend the trailing edge of NACA0012 airfoil. The airfoil. The span length of the airfoil was b  1.6 m and the experiments showed that the lift coefficient was significantly chord length was c  0.6 m. The experimental wind speed increased. Zhuang et al. [18] used morphed trailing-edge flap was 27.4 m/s, and the corresponding experimental Reynolds (MTEF) to control the aerodynamic performance of large number was Re  1.1 × 10 . During the experiments, if the wind turbine blades and found that MTEF could significantly AoA is greater than 8°, the AoA interval is one-degree; oth- change the pressure distribution and air flow. erwise, a two-degree interval is used. Considering the effect The above research showed that flap could be used as a of the flow in the corner area of the wind tunnel sidewall viable method of lift enhancement and stall characteristics on the two-dimensional flow of the model when the flow is improvement. Nevertheless, it can still be considered as a separated, the experimental model was designed as three seg- relatively novel method, and therefore, it is worthy of further mentations in this article, as shown in Fig. 2a. The middle study. Particularly, the flap with gap has not been studied in section of the model used for force measurement in the bal- detail. ance was made of metal, while the two sides of the model 123 International Journal of Aeronautical and Space Sciences (2022) 23:859–869 861 were made of wood. Meanwhile, taking the specific condi- tions of the experiment into account, that were, the limits of the size and range of the balance, the span length of the mid- dle force measurement section was designed to 400 mm, and the span lengths of the other two sections were all 599 mm with 1 mm gap with the middle section. In the experimental model designed in this work, the mid- dle metal section is used to measure aerodynamic force, which is divided into upper and lower halves. First, install the lower half, then install the balance, and finally cover the upper half to realize the effect of burying the balance in the Fig. 3 Diagram of the flap (Type1) metal section. The two sides are wooden models, and its pro- file shape is consistent with the metal section, which is used to ensure the two-dimensional flow through the metal section. In addition, there is a gap of less than 0.5 mm between the Table 2 Geometric information of the six types of flaps metal section and the wood section model. Through a spe- Configuration θ Gap location Gap height cial structure, it can not only ensure that the flow field is not damaged, but also ensure that the balance can only measure Type0 14° 70%c 0.0 mm the aerodynamic force felt by the metal section. There was a Type1 14° 70%c 0.5 mm steel frame inside the model to realize the connection of the Type2 17° 70%c 0.5 mm three-segment model, that was, the model was a whole and Type3 7° 70%c 0.5 mm could achieve the same AoA change. Fix the force measur- Type4 14° 70%c 1.0 mm ing balance on the middle section of the steel frame, and then Type5 14° 70%c 1.5 mm fix the metal part model on the balance, so that the balance can only be used to measure the force of the metal model in the middle. The measurement range and calibration results of the balance are shown in Table 1. was arranged on the suction surface (or upper surface) of the In this experiment, the 16-bit A/D acquisition card of NI airfoil through its fixed surface. Company was used to collect the voltage signal output of On the above basis, the included angles between the wind- the balance, the sampling rate was set to 1000 Hz, and the ward and fixed surfaces are changed to 17° and 7°, forming sampling time was 5 s. two flaps. Similarly, the gap heights between the windward Six types of bionic flap were designed, including three and fixed surface are changed to 1.0 mm and 1.5 mm, form- gap lengths and three flap angles. The spanwise lengths of the ing two flaps. The geometric information of the six types of fixed and windward surface are 200 mm, the chord lengths are bionic flaps is shown in Table 2. 120 mm and 180 mm, respectively, as shown in Fig. 3.The thicknesses of the windward surface and the fixed surface are both 1.28 mm. In the wind tunnel experiments, eight small flaps were arranged at the same chord length to ensure 3 Results and Discussion that there are small pieces on the middle metal part and the wooden parts on both sides. The influence of parameters including the gap height, angle, In this article, the plane where the flap faced the incoming and position of bionic flaps on the static aerodynamic char- flow was defined as the windward surface, and the plane fixed acteristics of airfoil was discussed in this section. The clean to the model was defined as the fixed surface. The small flap airfoil without bionic flap was denoted as "Baseline". Table 1 Measurement range and Item Y Mz X Z calibration results of the balance Design load (N, N·m) 1000 100 150 500 Calibration load (N, N·m) 1000 72 160 480 Absolute error (N, N·m) 1.0 0.1 0.16 0.48 Accuracy (%) 0.1 0.1 0.1 0.1 Precision (%) 0.02 0.02 0.03 0.03 123 862 International Journal of Aeronautical and Space Sciences (2022) 23:859–869 Fig. 4 Separation point and normal force coefficient of reference airfoil 3.1 Effect of Flap Gap The purpose of this article was to use the flap to improve the aerodynamic characteristics of the stall phase of the airfoil, so it was necessary to obtain the flow separation information of the airfoil. Unfortunately, only the aerodynamic force of the experimental model was collected in this experiment, and the surface pressure information of the model was not obtained. Considering that the trailing-edge separation airfoil was used in this work, the Kirchhoff flow theory [19] was used to ana- lyze trailing-edge separation. A specific case of this theory is to simplify the trailing-edge separation phenomenon into a simple model, as shown in the following equation: 1+ f (α) Cn(α)  k(α − α )( ),(1) where k is the slope of the normal force line, α is the zero-lift AoA, Cn is the normal force coefficient, and f is the position of the separation point. For the reference airfoil (Baseline), the airfoil separation point calculated using Eq. (1)isshown in Fig. 4. The flow separation point position at stall AoA calculated according to Kirchhoff’s theory is x/c≈0.77. To make the flap work near the stall, the flap needs to be installed near x/c  0.77. In the research of this section, the flap position was placed at 0.7c on the upper surface to study the effects of flap gap on the aerodynamic characteristics of the airfoil. The aerodynamic coefficients of the airfoil with different flap gap heights are shown in Fig. 5. The gap heights of the flap are 0 mm, 0.5 mm, 1.0 mm, and 1.5 mm, respectively. Fig. 5 Aerodynamic characteristics of airfoil with different flap gap As can be seen from the figure, the flaps with three different heights: (a) Cl; (b) drag polar curve; (c)Cm gap heights can increase the pitching moment, and improve airfoil stall characteristics. The maximum lift coefficients of the four gap heights of 0 mm, 0.5 mm, 1.0 mm, and 1.5 mm are increased by 6.1%, 6.0%, 6.0%, and 3.6%, respectively. 123 International Journal of Aeronautical and Space Sciences (2022) 23:859–869 863 Fig. 6 Effects of gap height on Cd at large AoA phase It can be seen that the lift enhancement abilities of the con- figurations with gap of 0 mm, 0.5 mm, and 1.0 mm are the same. The drag reduction effect in large AoA phase is calculated using Eq. (2) in this work, which can be seen in Fig. 6 Cd − Cd Baseline δCd  × 100%. (2) Cd Baseline The circle marked in the figure is the stall AoA for each configuration. In the large AoA range, ideal drag reduction effects can all be obtained using the four flap configurations in this article. In the range of α < 16.5°, the configuration with a gap of 0.5 mm (Type1) has the best drag reduction effect, which can reduce up to 14.53%. While when α > 16.5°, the configuration with a gap of 1.0 mm (Type 4) shows a better drag reduction effect. In the range of α ≥ 16.0°, the configuration with a gap of 0 mm (Type0) has the worst drag reduction effect, due to that the gap at large AoA can leak a part of the separated flow. In terms of Cl and Cd in the max large AoA range, we consider that the configuration with a gap of 0.5 mm (Type1) is the best. Therefore, the flap gap is set to 0.5 mm in Sects. 3.2 and 3.3. To better analyze the effects of small flaps on the aero- dynamic characteristics of the airfoil, the difference of aerodynamic coefficient curves is plotted here for further analysis and discussion. The differences of the aerodynamic coefficient curves of the airfoil with different flap gap heights are shown in Fig. 7. The change of lift coefficient can be divided into three regions: reduction, transition, and augmentation, and the cor- Fig. 7 Differences of aerodynamic coefficients of the airfoil with dif- responding cut-off line angles are 8° and 13°, respectively. ferent flap gap heights: (a) Cl; (b)Cd; (c)Cm The cut-off line is the critical point at which a sudden change of the curve trend occurs in the increase or decrease area. Before α  8°, the air flow through the airfoil surface is attached, and then, when it encounters a small flap, there 123 864 International Journal of Aeronautical and Space Sciences (2022) 23:859–869 will be a separation vortex under the flap, resulting in a drop in Cl. The amount of drop in Cl decreases slowly as the AoA increases. When the AoA is between 8° and 13°, the amount of drop in the Cl decreases rapidly as the AoA increases, and then, the Cl continuously increases. This is because there is a formed vortex between the upper surface of the flap and the airfoil, which is conducive to inhibit the further development of the separation vortex over the airfoil [20]. After α  13°, due to the existence of the flap, two smaller separation vor- texes are formed on the upper and lower surfaces of the flap, which plays a good role in weakening the development of stall flow on the airfoil, and the Cl is significantly improved. In the reduction and transition regions, the gap height has no significant effect on Cl. The change of drag coefficient can be divided into two regions: reduction and augmentation, and the corresponding cut-off line angle is 11°. The amount of increase in Cd grad- ually decreases with the increase of the AoA, and drops to zero at 8°. It reaches the first extremal value at 11°, and then, it starts to oscillate. This is because 11° is an inflection point of the drag coefficient curve, as shown in Fig. 7b, where sig- nificant flow separation occurs on the airfoil surface. In the phase of large AoAs, the gap height has a certain effect on Cd. The change of Cm can also be divided into two regions: the linear reduction and fluctuation reduction regions, and the corresponding cut-off line angle is 9°. This is because a vortex is formed between the flap upper surface and airfoil at 9°, which causes the pitching moment coefficient to fluctuate. For the flap with gap, the gap heights of 0.5 mm has the smallest decrease in lift at a small AoA, and the maximum increase in lift and the largest decrease in drag at a high AoA, which is also the best one. The higher the flap gap height is, and the weaker the ability to improve the airfoil stall characteristics becomes. Within the current limited range of gap height variation, the influence of gap height variation on airfoil aerodynamic characteristics is not evident. 3.2 Effect of Flap Angle The variations of the aerodynamic coefficients of the airfoil with different flap angles are shown in Fig. 8. Here, the flaps with the same gap height (0.5 mm) were located at 70%c on the suction surface of the airfoil trailing edge according to Kirchhoff’s flow theory in Sect. 3.1, and the angles of the flap are 7°, 14°, and 17°, respectively. As can be seen, the flaps of three different angles can increase the pitching moment, and improve airfoil stall characteristics. However, the price Fig. 8 Aerodynamic characteristics of airfoil with different flap angles: of both the reduction in lift, and the increase in drag and (a) Cl; (b) drag polar curve; (c)Cm pithing moment at a small AoA is necessary. The maximum lift coefficients of bionic flap with θ  7.0°, θ  14°, and θ 1 2 3 17° are increased by 4.2%, 6.0%, and 5.9%, respectively, 123 International Journal of Aeronautical and Space Sciences (2022) 23:859–869 865 and the drag coefficients are decreased by 3.6%, 11.6%, and 9.4%, respectively. The differences of the aerodynamic coefficients of the air- foil with different flap angles are shown in Fig. 9. As can be seen from the figure, the change of lift coefficient can be divided into three regions: reduction, transition, and augmen- tation, and the corresponding cut-off line angles are 8.0° and 13.0°, respectively. In the reduction region, the flap angle is smaller, and the amount of drop in Cl is smaller; in the tran- sition region, the flap angle is smaller, and the initial AoA of improving lift is smaller; in the augmentation region, the flap angle is smaller, the ability to increase lift becomes weaker. The effective AoA range for improving the lift characteris- tics can reach more than 8°. The flap angle has a significant effect on Cl. The change of drag coefficient can be divided into two regions: reduction and augmentation, and the corresponding cut-off line angle about is 11°. The flap angle is smaller, the angle at which the amount of increase in the drag coefficient drops to zero is smaller, but the angle at which the change of the drag coefficient reaches the first extreme value is still 11°, and then, the drag coefficient begins to oscillate. The effective AoA range for reducing the drag can reach more than 11°. The flap angle has a significant effect on Cd. The change of the pitching moment coefficient can also be divided into two regions: the linear reduction and fluctuation reduction regions, and the corresponding cut-off line angle is 9°. The flap angle is smaller, and the amount of increase in the pitching moment coefficient is smaller. The flap angle has a significant effect on the pitching moment coefficient. The flap angle is smaller, the initial AoA to improve airfoil stall characteristics is smaller, and the initial AoA to reduce the drag is also smaller. At the same time, the angle of the flap is smaller, the range of improving airfoil stall characteristics is larger, and the pitching moment increment is smaller. How- ever, the maximum lift coefficient increment will decrease, and the effect of the drag reduction will also decrease. For the improvement of airfoil stall characteristics, the flap angle is a key factor. 3.3 Effect of Flap Position It can be seen from Sect. 3.2 that the best control effect can be achieved when the angle of flap is 14°. Therefore, a flap with an angle of 14° was used to study the effects of the flap installation position on the aerodynamic characteristics of the airfoil in this section. As in Sect. 3.2, the gap of flap is 0.5 mm. Fig. 9 Differences of aerodynamic coefficients of the airfoil with dif- The flap positions are 60%c, 70%c, and 80%c on the suction ferent flap angles: (a)Cl; (b)Cd; (c)Cm surface of the airfoil, called P1, P2, and P3, respectively. The variations of the aerodynamic coefficients of the airfoil with different flap positions are shown in Fig. 10. As can be seen, the flaps of three different positions can increase Cm, reduce Cd, and increase Cl at a large AoA. 123 866 International Journal of Aeronautical and Space Sciences (2022) 23:859–869 The differences of the aerodynamic coefficients of the air- foil with different flap positions are shown in Fig. 11. As can be seen from the figure, the change of lift coefficient can be divided into three regions: reduction, transition, and augmen- tation, and the corresponding cut-off line angles are 8° and 13°, respectively. In the augmentation region, the flap is too close to the trailing edge, and the effect in the lift increase is significantly weaker, which is not conducive to improve the airfoil stall characteristics. The change of drag coefficient can be divided into two regions: reduction and augmenta- tion. The flap is closer to the trailing edge, and the range of AoA to decrease the drag is larger. The change of the pitch- ing moment coefficient can also be divided into two regions: the linear reduction and fluctuation reduction regions, and the corresponding cut-off line angle is not 9°. The P2 is the best. This is because the P3 is too close to the trailing edge, which has limited inhibitory effect on the development of vortex separation, and P3 can only increase the lift within the finite AoA. The P1 is too close to the max- imum thick of airfoil, which is a blocking effect for airflow over the airfoil. The initial AoA to improve airfoil stall char- acteristics and initial AoA to reduce the drag are largest. For the improvement of airfoil stall characteristics, the flap posi- tion is a key factor. 4 Further Analysis The effects of the gap, angle, and installation position of the flap on the overall aerodynamic force of the airfoil were analyzed in detail earlier in the article. It is found that the flap can significantly improve the aerodynamic force during the stall and post stall stages. An in-depth analysis of its impact details and reasons were conducted in this section. It can be seen from Fig. 8 that the flap angle has almost no effect on the slope of the lift line of the linear section, while it moves the linear section of the lift line downward as a whole. The lift coefficient Cl at 0° was used to quantitatively evaluate this downward shift. Additionally, the effect of flap angle on the maximum lift coefficient has been discussed in Sect. 3.2. In this section, the maximum lift coefficient is also redrawn to express this effect more clearly, as shown in Fig. 12. The reason for the effect on the overall lift coefficient of the linear segment is that the flap angle changes the camber of the airfoil. In this experiment, flaps are installed on the Fig. 10 Aerodynamic characteristics of airfoil with different flap posi- upper surface of the airfoil, which makes the airfoil bend tions: (a) Cl; (b) drag polar curve; (c)Cm upward. Therefore, the linear segments of the lift line of all configurations with flaps are moved downward as a whole in this experiment. It can be seen from Sect. 3.2 that the main reason that the flap angle has an effect on the maximum lift 123 International Journal of Aeronautical and Space Sciences (2022) 23:859–869 867 Fig. 12 Effects of flap angle coefficient is to change the flow pattern of the reference airfoil in the separation flow stage. Unlike the effect of flap angle, the installation position of the flap has an effect on the slope of the lift line and the maximum lift coefficient, as shown in Fig. 13. The reason for the effect on the slope of the lift line is that the installation position of the flap changes the thickness distribution of the airfoil, and the thickness of the airfoil will affect the slope of the lift line in turn. It can be seen from Sect. 3.3 that the main reason that the flap installation position affects the maximum lift coefficient is to change the flow pattern of the reference airfoil in the separation flow stage. The flow separation points in different configurations were calculated using Eq. (1), as shown in Fig. 14. It can be seen that after using the flap, the separation point change curve with the AoA moves significantly to the right, indicating that the flap installed on the suction surface of the airfoil used in this article can significantly delay separation. From the perspective of delayed separation, the Type1 installed at the 0.6c position has the best effect (the separation point curve is at the far right). The stall AoA and the AoA corresponding the maxi- mum lift coefficient in different configurations are shown in Table 3. The stall AoA of the Type1 configuration installed at the 0.6c position is 18°, which is greater than the stall AoA of other configurations, and the delay effect is the best, which is consistent with the conclusion of Fig. 13. We conducted a more in-depth study on the influence of flap on typical aerodynamic characteristics (lift line slope, maximum lift coefficient, stall AoA, etc.) in this section. The flap installed on the suction surface changes the camber of the airfoil, resulting that the linear section moves downward Fig. 11 Differences of aerodynamic coefficients of the airfoil with dif- as a whole, and also changes the thickness distribution of the ferent flap positions: (a)Cl; (b)Cd; (c)Cm airfoil, which changes the slope of the lift line. In terms of 123 868 International Journal of Aeronautical and Space Sciences (2022) 23:859–869 Fig. 13 Effects of flap installation position Fig. 14 Separate points of different configurations the maximum lift coefficient and drag coefficients in large AoA, the Type1 installed at the 0.7c position has the best (3) The bionic flap position has a significant effect on the effect (Sects. 3.1, 3.2, and 3.3); while from the perspective airfoil stall characteristics. The flap needs to be installed of delayed stall, the Type1 installed at the 0.6c position has near the separation point. In terms of the maximum lift the best delay effect. coefficient and drag coefficients in large AoA, the Type1 installed at the 0.7c position has the best effect; from the perspective of delayed stall, the Type1 installed at the 5 Conclusions 0.6c position has the best delay effect. (4) The gap, angle, and installation position of the flap have The effects of flap with a tiny gap on airfoil aerodynamic a significant effect on the airfoil aerodynamic charac- performance were studied by low speed wind tunnel experi- teristics. The mechanism is that the flap has the effect of ments. The conclusions were as follows: changing the camber and thickness distributions of the basic airfoil, which can affect the maximum lift coeffi- (1) The flaps can improve the maximum lift coefficient of cient, stall AoA, and the slope of the lift line, etc. airfoil up to more than 5%, and the effective range of AoA to improve lift characteristics up to more than 9°. The next research plan will be carried out from the fol- The configuration with a gap of 0.5 mm (Type1) is the lowing two aspects: on the one hand, for the research on best. Excessive gap height of flaps is not conducive to bionic airfoils, the flaps will undergo corresponding deforma- the improvement of airfoil stall characteristics. tion or displacement changes with the change of the surface (2) The flap angle is smaller, the initial AoA to improve aerodynamic force; on the other hand, we will develop a airfoil stall characteristics is smaller, and the range of three-dimensional bionic airfoil wing research to further improving airfoil stall characteristics is larger. However, study the landing configuration of bird wings. the maximum lift coefficient increment will decrease, and the effect of the drag reduction will also decrease. Table 3 Typical AoA of different Configuration Baseline Type3 Type2 Type1(0.6c) Type1(0.7c) Type1(0.8c) configurations Stall AoA/° 10 15 17 18 16 15 AoA corresponding 15 15 17 18 16 15 Clmax /° 123 International Journal of Aeronautical and Space Sciences (2022) 23:859–869 869 Acknowledgements The present work is supported by National Nat- 7. Johnston J, Gopalarathnam A (2012) Investigation of a bio-inspired ural Science Foundation of China (11502214) and the Fundamental lift-enhancing effector on a 2d airfoil. Bioinspir Biomim 7:036003 Research Funds for the Central Universities (D5000200685). The 8. Johnston J, Gopalarathnam A, Edward J Experimental investiga- authors would like to express their gratitude to Professor Yuqin Jiao tion of bio-inspired high lift effectors on a 2-d airfoil. AIAA Paper, in Northwestern Polytechnical University for his valuable guidance. 2011–3791, 2011. 9. 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Experimental Investigation of Flow Separation Control over Airfoil by Upper Surface Flap with a Gap

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Springer Journals
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Copyright © The Author(s) 2022
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2093-274X
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2093-2480
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10.1007/s42405-022-00488-x
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Abstract

This paper focused on the effects of the flap with a tiny gap on the flow separation over the NPU-WA-180 airfoil. The effects of the geometric parameters of the flaps, such as the flap gap height, angle, and position, were investigated. The study showed that the flap can significantly improve the stall features of airfoil in a limited phase of angle of attack (AoA), and increase lift and reduce drag at a high AoA. It can increase the lift coefficient and drag coefficient in the case of high AoAs, and the angle range of the lift augmentation and drag reduction can reach more than 9°. Furthermore, an excessively large gap is not conducive to the improvement of the airfoil stall performance. The flap angle plays a key role in the airfoil stall characteristic. As the flap angle decreases, the angle range of improving airfoil stall characteristics becomes larger, the pitching moment increment becomes smaller. However, the maximum lift increment and the effect of the drag reduction will decrease. And the effects of the position of the flap on the airfoil performance were also studied. Considering the maximum lift coefficient and drag coefficients in large AoA, the Type1 installed at the 0.7c position has the best effect; from the perspective of delayed stall, the Type1 installed at the 0.6c position has the best delay effect. These results can provide the data and theoretical support for the flap application in engineering. Keywords Flap · Flow control · Airfoil · Stall characteristic · Wind tunnel experiment Abbreviations 1 Introduction C Chord Biomimetic engineering has stimulated a new area of flow Cl Lift coefficient control by combining two different concepts of biology Cd Drag coefficient and fluid mechanics. Choi et al. [1] discussed some recent Cm Pitching moment coefficient biomimetic flow control results for aerodynamic perfor- Cn Normal force coefficient mance enhancement and skin-friction reduction. Chen et al. α Angle of attack [2] proposed a novel biomimetic drag reduction surface, and α Zero-lift angle of attack conducted water tunnel experiments to clarify its efficiency Re Reynolds number and mechanism combining computational fluid dynamics k Slope of the normal force line (CFD) analysis. The researchers [3, 4] conducted a wing f Position of the separation point reconstruction with reference to free flying birds, and inves- θ Flap angle tigated the aerodynamics performance of the wing/airfoil by numerical simulation. In flight of birds, the feathers on the upper surface of the wings stand up, while birds land (Fig. 1). Meyer et al. [5] and Schatz et al. [6] conducted detailed experimental and numer- ical investigations on the mechanism of feather movement and its aerodynamic characteristics. The results showed that B Lishu Hao the flap prevented the development of the reversed flow from haolishu@nwpu.edu.cn the trailing edge to the suction peak position, bringing benefit School of Aeronautics, Northwestern Polytechnical of the flow separation delay. University, Xi’an 710072, People’s Republic of China 123 860 International Journal of Aeronautical and Space Sciences (2022) 23:859–869 Fig. 2 Model and flap: (a) Three-segment airfoil model; (b)Flap mounted on the model Fig. 1 Bird’s wing posture when landing Therefore, the effects of flap with gap on the aerodynamic performance of the airfoil were experimentally studied. The influences of key geometry parameters (such as gap height, There is a body of research focused on the effect of angle, and position) on the lift, drag, and pitching moment the upper surface flap on flow separation. Johnston et al. coefficients were explored in detail. This research attempts [7, 8] designed a lift-enhancing effector device, which was to explore the flow mechanism of the flap with gap, and at arranged on the upper surface of the airfoil at different angles. the same time provides theoretical and data basis for the flap The results showed that the device could increase the lift and with gap in aeronautical engineering application. stall angle of attack (AoA). Kernstine et al. [9] explored the influence of parameters of passive flap on the flow separation, including its size, chord placement, configuration, material, etc. Bramesfeld et al. [10] designed a high-lift device, and 2 Equipment and Procedure investigated the influence of different flap positions and mul- tiple flaps on the pressure distribution of the airfoil. Schlüter The experiments were performed in the NF-3 low-speed wind [11, 12] placed a passive flap on the different airfoils. The tunnel, at Northwestern Polytechnical University, China. The experiments showed that the flap could improve the stall turbulence intensity of the wind tunnel was 0.045%, and the characteristics. Traub et al. [13] studied the effect of several test section size was 1.6 m × 3.0 m. spoiler geometric parameters by water tunnel experiments. In this work, the study was inspired by the wing shapes The experiments showed that the device could improve the of birds during takeoff and landing, and hoped to study the stall characteristics. Wang [14] designed a self-activated flap effects of the trailing-edge flap with gap on aerodynamic to place the flap to the wing of the aircraft. The experiments forces at large AoAs. When birds take off and land, the flow showed that the flap could improve the stall characteristics. separation will raise the feathers on the trailing edge of the Montefort et al. [15] described a thin-wing vibration control wings, and the raised feathers inhibit the development of the method using an array of small flexible flaps attached near the separation flow and play a certain role in flow control. In this leading edge on the suction surface to manipulate the leading condition, bird wings present typical trailing-edge separa- edge vortex structures. Brücker et al. [16] carried out research tion characteristics. Therefore, we selected the trailing-edge on the effects of self-adaptive hairy flaps on the stall delay separation airfoil for this research. The NPU-WA-180 airfoil of an airfoil in its ramp-up motion. Liu et al. [17] designed a used in this experiment is a typical trailing-edge separated device to extend the trailing edge of NACA0012 airfoil. The airfoil. The span length of the airfoil was b  1.6 m and the experiments showed that the lift coefficient was significantly chord length was c  0.6 m. The experimental wind speed increased. Zhuang et al. [18] used morphed trailing-edge flap was 27.4 m/s, and the corresponding experimental Reynolds (MTEF) to control the aerodynamic performance of large number was Re  1.1 × 10 . During the experiments, if the wind turbine blades and found that MTEF could significantly AoA is greater than 8°, the AoA interval is one-degree; oth- change the pressure distribution and air flow. erwise, a two-degree interval is used. Considering the effect The above research showed that flap could be used as a of the flow in the corner area of the wind tunnel sidewall viable method of lift enhancement and stall characteristics on the two-dimensional flow of the model when the flow is improvement. Nevertheless, it can still be considered as a separated, the experimental model was designed as three seg- relatively novel method, and therefore, it is worthy of further mentations in this article, as shown in Fig. 2a. The middle study. Particularly, the flap with gap has not been studied in section of the model used for force measurement in the bal- detail. ance was made of metal, while the two sides of the model 123 International Journal of Aeronautical and Space Sciences (2022) 23:859–869 861 were made of wood. Meanwhile, taking the specific condi- tions of the experiment into account, that were, the limits of the size and range of the balance, the span length of the mid- dle force measurement section was designed to 400 mm, and the span lengths of the other two sections were all 599 mm with 1 mm gap with the middle section. In the experimental model designed in this work, the mid- dle metal section is used to measure aerodynamic force, which is divided into upper and lower halves. First, install the lower half, then install the balance, and finally cover the upper half to realize the effect of burying the balance in the Fig. 3 Diagram of the flap (Type1) metal section. The two sides are wooden models, and its pro- file shape is consistent with the metal section, which is used to ensure the two-dimensional flow through the metal section. In addition, there is a gap of less than 0.5 mm between the Table 2 Geometric information of the six types of flaps metal section and the wood section model. Through a spe- Configuration θ Gap location Gap height cial structure, it can not only ensure that the flow field is not damaged, but also ensure that the balance can only measure Type0 14° 70%c 0.0 mm the aerodynamic force felt by the metal section. There was a Type1 14° 70%c 0.5 mm steel frame inside the model to realize the connection of the Type2 17° 70%c 0.5 mm three-segment model, that was, the model was a whole and Type3 7° 70%c 0.5 mm could achieve the same AoA change. Fix the force measur- Type4 14° 70%c 1.0 mm ing balance on the middle section of the steel frame, and then Type5 14° 70%c 1.5 mm fix the metal part model on the balance, so that the balance can only be used to measure the force of the metal model in the middle. The measurement range and calibration results of the balance are shown in Table 1. was arranged on the suction surface (or upper surface) of the In this experiment, the 16-bit A/D acquisition card of NI airfoil through its fixed surface. Company was used to collect the voltage signal output of On the above basis, the included angles between the wind- the balance, the sampling rate was set to 1000 Hz, and the ward and fixed surfaces are changed to 17° and 7°, forming sampling time was 5 s. two flaps. Similarly, the gap heights between the windward Six types of bionic flap were designed, including three and fixed surface are changed to 1.0 mm and 1.5 mm, form- gap lengths and three flap angles. The spanwise lengths of the ing two flaps. The geometric information of the six types of fixed and windward surface are 200 mm, the chord lengths are bionic flaps is shown in Table 2. 120 mm and 180 mm, respectively, as shown in Fig. 3.The thicknesses of the windward surface and the fixed surface are both 1.28 mm. In the wind tunnel experiments, eight small flaps were arranged at the same chord length to ensure 3 Results and Discussion that there are small pieces on the middle metal part and the wooden parts on both sides. The influence of parameters including the gap height, angle, In this article, the plane where the flap faced the incoming and position of bionic flaps on the static aerodynamic char- flow was defined as the windward surface, and the plane fixed acteristics of airfoil was discussed in this section. The clean to the model was defined as the fixed surface. The small flap airfoil without bionic flap was denoted as "Baseline". Table 1 Measurement range and Item Y Mz X Z calibration results of the balance Design load (N, N·m) 1000 100 150 500 Calibration load (N, N·m) 1000 72 160 480 Absolute error (N, N·m) 1.0 0.1 0.16 0.48 Accuracy (%) 0.1 0.1 0.1 0.1 Precision (%) 0.02 0.02 0.03 0.03 123 862 International Journal of Aeronautical and Space Sciences (2022) 23:859–869 Fig. 4 Separation point and normal force coefficient of reference airfoil 3.1 Effect of Flap Gap The purpose of this article was to use the flap to improve the aerodynamic characteristics of the stall phase of the airfoil, so it was necessary to obtain the flow separation information of the airfoil. Unfortunately, only the aerodynamic force of the experimental model was collected in this experiment, and the surface pressure information of the model was not obtained. Considering that the trailing-edge separation airfoil was used in this work, the Kirchhoff flow theory [19] was used to ana- lyze trailing-edge separation. A specific case of this theory is to simplify the trailing-edge separation phenomenon into a simple model, as shown in the following equation: 1+ f (α) Cn(α)  k(α − α )( ),(1) where k is the slope of the normal force line, α is the zero-lift AoA, Cn is the normal force coefficient, and f is the position of the separation point. For the reference airfoil (Baseline), the airfoil separation point calculated using Eq. (1)isshown in Fig. 4. The flow separation point position at stall AoA calculated according to Kirchhoff’s theory is x/c≈0.77. To make the flap work near the stall, the flap needs to be installed near x/c  0.77. In the research of this section, the flap position was placed at 0.7c on the upper surface to study the effects of flap gap on the aerodynamic characteristics of the airfoil. The aerodynamic coefficients of the airfoil with different flap gap heights are shown in Fig. 5. The gap heights of the flap are 0 mm, 0.5 mm, 1.0 mm, and 1.5 mm, respectively. Fig. 5 Aerodynamic characteristics of airfoil with different flap gap As can be seen from the figure, the flaps with three different heights: (a) Cl; (b) drag polar curve; (c)Cm gap heights can increase the pitching moment, and improve airfoil stall characteristics. The maximum lift coefficients of the four gap heights of 0 mm, 0.5 mm, 1.0 mm, and 1.5 mm are increased by 6.1%, 6.0%, 6.0%, and 3.6%, respectively. 123 International Journal of Aeronautical and Space Sciences (2022) 23:859–869 863 Fig. 6 Effects of gap height on Cd at large AoA phase It can be seen that the lift enhancement abilities of the con- figurations with gap of 0 mm, 0.5 mm, and 1.0 mm are the same. The drag reduction effect in large AoA phase is calculated using Eq. (2) in this work, which can be seen in Fig. 6 Cd − Cd Baseline δCd  × 100%. (2) Cd Baseline The circle marked in the figure is the stall AoA for each configuration. In the large AoA range, ideal drag reduction effects can all be obtained using the four flap configurations in this article. In the range of α < 16.5°, the configuration with a gap of 0.5 mm (Type1) has the best drag reduction effect, which can reduce up to 14.53%. While when α > 16.5°, the configuration with a gap of 1.0 mm (Type 4) shows a better drag reduction effect. In the range of α ≥ 16.0°, the configuration with a gap of 0 mm (Type0) has the worst drag reduction effect, due to that the gap at large AoA can leak a part of the separated flow. In terms of Cl and Cd in the max large AoA range, we consider that the configuration with a gap of 0.5 mm (Type1) is the best. Therefore, the flap gap is set to 0.5 mm in Sects. 3.2 and 3.3. To better analyze the effects of small flaps on the aero- dynamic characteristics of the airfoil, the difference of aerodynamic coefficient curves is plotted here for further analysis and discussion. The differences of the aerodynamic coefficient curves of the airfoil with different flap gap heights are shown in Fig. 7. The change of lift coefficient can be divided into three regions: reduction, transition, and augmentation, and the cor- Fig. 7 Differences of aerodynamic coefficients of the airfoil with dif- responding cut-off line angles are 8° and 13°, respectively. ferent flap gap heights: (a) Cl; (b)Cd; (c)Cm The cut-off line is the critical point at which a sudden change of the curve trend occurs in the increase or decrease area. Before α  8°, the air flow through the airfoil surface is attached, and then, when it encounters a small flap, there 123 864 International Journal of Aeronautical and Space Sciences (2022) 23:859–869 will be a separation vortex under the flap, resulting in a drop in Cl. The amount of drop in Cl decreases slowly as the AoA increases. When the AoA is between 8° and 13°, the amount of drop in the Cl decreases rapidly as the AoA increases, and then, the Cl continuously increases. This is because there is a formed vortex between the upper surface of the flap and the airfoil, which is conducive to inhibit the further development of the separation vortex over the airfoil [20]. After α  13°, due to the existence of the flap, two smaller separation vor- texes are formed on the upper and lower surfaces of the flap, which plays a good role in weakening the development of stall flow on the airfoil, and the Cl is significantly improved. In the reduction and transition regions, the gap height has no significant effect on Cl. The change of drag coefficient can be divided into two regions: reduction and augmentation, and the corresponding cut-off line angle is 11°. The amount of increase in Cd grad- ually decreases with the increase of the AoA, and drops to zero at 8°. It reaches the first extremal value at 11°, and then, it starts to oscillate. This is because 11° is an inflection point of the drag coefficient curve, as shown in Fig. 7b, where sig- nificant flow separation occurs on the airfoil surface. In the phase of large AoAs, the gap height has a certain effect on Cd. The change of Cm can also be divided into two regions: the linear reduction and fluctuation reduction regions, and the corresponding cut-off line angle is 9°. This is because a vortex is formed between the flap upper surface and airfoil at 9°, which causes the pitching moment coefficient to fluctuate. For the flap with gap, the gap heights of 0.5 mm has the smallest decrease in lift at a small AoA, and the maximum increase in lift and the largest decrease in drag at a high AoA, which is also the best one. The higher the flap gap height is, and the weaker the ability to improve the airfoil stall characteristics becomes. Within the current limited range of gap height variation, the influence of gap height variation on airfoil aerodynamic characteristics is not evident. 3.2 Effect of Flap Angle The variations of the aerodynamic coefficients of the airfoil with different flap angles are shown in Fig. 8. Here, the flaps with the same gap height (0.5 mm) were located at 70%c on the suction surface of the airfoil trailing edge according to Kirchhoff’s flow theory in Sect. 3.1, and the angles of the flap are 7°, 14°, and 17°, respectively. As can be seen, the flaps of three different angles can increase the pitching moment, and improve airfoil stall characteristics. However, the price Fig. 8 Aerodynamic characteristics of airfoil with different flap angles: of both the reduction in lift, and the increase in drag and (a) Cl; (b) drag polar curve; (c)Cm pithing moment at a small AoA is necessary. The maximum lift coefficients of bionic flap with θ  7.0°, θ  14°, and θ 1 2 3 17° are increased by 4.2%, 6.0%, and 5.9%, respectively, 123 International Journal of Aeronautical and Space Sciences (2022) 23:859–869 865 and the drag coefficients are decreased by 3.6%, 11.6%, and 9.4%, respectively. The differences of the aerodynamic coefficients of the air- foil with different flap angles are shown in Fig. 9. As can be seen from the figure, the change of lift coefficient can be divided into three regions: reduction, transition, and augmen- tation, and the corresponding cut-off line angles are 8.0° and 13.0°, respectively. In the reduction region, the flap angle is smaller, and the amount of drop in Cl is smaller; in the tran- sition region, the flap angle is smaller, and the initial AoA of improving lift is smaller; in the augmentation region, the flap angle is smaller, the ability to increase lift becomes weaker. The effective AoA range for improving the lift characteris- tics can reach more than 8°. The flap angle has a significant effect on Cl. The change of drag coefficient can be divided into two regions: reduction and augmentation, and the corresponding cut-off line angle about is 11°. The flap angle is smaller, the angle at which the amount of increase in the drag coefficient drops to zero is smaller, but the angle at which the change of the drag coefficient reaches the first extreme value is still 11°, and then, the drag coefficient begins to oscillate. The effective AoA range for reducing the drag can reach more than 11°. The flap angle has a significant effect on Cd. The change of the pitching moment coefficient can also be divided into two regions: the linear reduction and fluctuation reduction regions, and the corresponding cut-off line angle is 9°. The flap angle is smaller, and the amount of increase in the pitching moment coefficient is smaller. The flap angle has a significant effect on the pitching moment coefficient. The flap angle is smaller, the initial AoA to improve airfoil stall characteristics is smaller, and the initial AoA to reduce the drag is also smaller. At the same time, the angle of the flap is smaller, the range of improving airfoil stall characteristics is larger, and the pitching moment increment is smaller. How- ever, the maximum lift coefficient increment will decrease, and the effect of the drag reduction will also decrease. For the improvement of airfoil stall characteristics, the flap angle is a key factor. 3.3 Effect of Flap Position It can be seen from Sect. 3.2 that the best control effect can be achieved when the angle of flap is 14°. Therefore, a flap with an angle of 14° was used to study the effects of the flap installation position on the aerodynamic characteristics of the airfoil in this section. As in Sect. 3.2, the gap of flap is 0.5 mm. Fig. 9 Differences of aerodynamic coefficients of the airfoil with dif- The flap positions are 60%c, 70%c, and 80%c on the suction ferent flap angles: (a)Cl; (b)Cd; (c)Cm surface of the airfoil, called P1, P2, and P3, respectively. The variations of the aerodynamic coefficients of the airfoil with different flap positions are shown in Fig. 10. As can be seen, the flaps of three different positions can increase Cm, reduce Cd, and increase Cl at a large AoA. 123 866 International Journal of Aeronautical and Space Sciences (2022) 23:859–869 The differences of the aerodynamic coefficients of the air- foil with different flap positions are shown in Fig. 11. As can be seen from the figure, the change of lift coefficient can be divided into three regions: reduction, transition, and augmen- tation, and the corresponding cut-off line angles are 8° and 13°, respectively. In the augmentation region, the flap is too close to the trailing edge, and the effect in the lift increase is significantly weaker, which is not conducive to improve the airfoil stall characteristics. The change of drag coefficient can be divided into two regions: reduction and augmenta- tion. The flap is closer to the trailing edge, and the range of AoA to decrease the drag is larger. The change of the pitch- ing moment coefficient can also be divided into two regions: the linear reduction and fluctuation reduction regions, and the corresponding cut-off line angle is not 9°. The P2 is the best. This is because the P3 is too close to the trailing edge, which has limited inhibitory effect on the development of vortex separation, and P3 can only increase the lift within the finite AoA. The P1 is too close to the max- imum thick of airfoil, which is a blocking effect for airflow over the airfoil. The initial AoA to improve airfoil stall char- acteristics and initial AoA to reduce the drag are largest. For the improvement of airfoil stall characteristics, the flap posi- tion is a key factor. 4 Further Analysis The effects of the gap, angle, and installation position of the flap on the overall aerodynamic force of the airfoil were analyzed in detail earlier in the article. It is found that the flap can significantly improve the aerodynamic force during the stall and post stall stages. An in-depth analysis of its impact details and reasons were conducted in this section. It can be seen from Fig. 8 that the flap angle has almost no effect on the slope of the lift line of the linear section, while it moves the linear section of the lift line downward as a whole. The lift coefficient Cl at 0° was used to quantitatively evaluate this downward shift. Additionally, the effect of flap angle on the maximum lift coefficient has been discussed in Sect. 3.2. In this section, the maximum lift coefficient is also redrawn to express this effect more clearly, as shown in Fig. 12. The reason for the effect on the overall lift coefficient of the linear segment is that the flap angle changes the camber of the airfoil. In this experiment, flaps are installed on the Fig. 10 Aerodynamic characteristics of airfoil with different flap posi- upper surface of the airfoil, which makes the airfoil bend tions: (a) Cl; (b) drag polar curve; (c)Cm upward. Therefore, the linear segments of the lift line of all configurations with flaps are moved downward as a whole in this experiment. It can be seen from Sect. 3.2 that the main reason that the flap angle has an effect on the maximum lift 123 International Journal of Aeronautical and Space Sciences (2022) 23:859–869 867 Fig. 12 Effects of flap angle coefficient is to change the flow pattern of the reference airfoil in the separation flow stage. Unlike the effect of flap angle, the installation position of the flap has an effect on the slope of the lift line and the maximum lift coefficient, as shown in Fig. 13. The reason for the effect on the slope of the lift line is that the installation position of the flap changes the thickness distribution of the airfoil, and the thickness of the airfoil will affect the slope of the lift line in turn. It can be seen from Sect. 3.3 that the main reason that the flap installation position affects the maximum lift coefficient is to change the flow pattern of the reference airfoil in the separation flow stage. The flow separation points in different configurations were calculated using Eq. (1), as shown in Fig. 14. It can be seen that after using the flap, the separation point change curve with the AoA moves significantly to the right, indicating that the flap installed on the suction surface of the airfoil used in this article can significantly delay separation. From the perspective of delayed separation, the Type1 installed at the 0.6c position has the best effect (the separation point curve is at the far right). The stall AoA and the AoA corresponding the maxi- mum lift coefficient in different configurations are shown in Table 3. The stall AoA of the Type1 configuration installed at the 0.6c position is 18°, which is greater than the stall AoA of other configurations, and the delay effect is the best, which is consistent with the conclusion of Fig. 13. We conducted a more in-depth study on the influence of flap on typical aerodynamic characteristics (lift line slope, maximum lift coefficient, stall AoA, etc.) in this section. The flap installed on the suction surface changes the camber of the airfoil, resulting that the linear section moves downward Fig. 11 Differences of aerodynamic coefficients of the airfoil with dif- as a whole, and also changes the thickness distribution of the ferent flap positions: (a)Cl; (b)Cd; (c)Cm airfoil, which changes the slope of the lift line. In terms of 123 868 International Journal of Aeronautical and Space Sciences (2022) 23:859–869 Fig. 13 Effects of flap installation position Fig. 14 Separate points of different configurations the maximum lift coefficient and drag coefficients in large AoA, the Type1 installed at the 0.7c position has the best (3) The bionic flap position has a significant effect on the effect (Sects. 3.1, 3.2, and 3.3); while from the perspective airfoil stall characteristics. The flap needs to be installed of delayed stall, the Type1 installed at the 0.6c position has near the separation point. In terms of the maximum lift the best delay effect. coefficient and drag coefficients in large AoA, the Type1 installed at the 0.7c position has the best effect; from the perspective of delayed stall, the Type1 installed at the 5 Conclusions 0.6c position has the best delay effect. (4) The gap, angle, and installation position of the flap have The effects of flap with a tiny gap on airfoil aerodynamic a significant effect on the airfoil aerodynamic charac- performance were studied by low speed wind tunnel experi- teristics. The mechanism is that the flap has the effect of ments. The conclusions were as follows: changing the camber and thickness distributions of the basic airfoil, which can affect the maximum lift coeffi- (1) The flaps can improve the maximum lift coefficient of cient, stall AoA, and the slope of the lift line, etc. airfoil up to more than 5%, and the effective range of AoA to improve lift characteristics up to more than 9°. The next research plan will be carried out from the fol- The configuration with a gap of 0.5 mm (Type1) is the lowing two aspects: on the one hand, for the research on best. Excessive gap height of flaps is not conducive to bionic airfoils, the flaps will undergo corresponding deforma- the improvement of airfoil stall characteristics. tion or displacement changes with the change of the surface (2) The flap angle is smaller, the initial AoA to improve aerodynamic force; on the other hand, we will develop a airfoil stall characteristics is smaller, and the range of three-dimensional bionic airfoil wing research to further improving airfoil stall characteristics is larger. However, study the landing configuration of bird wings. the maximum lift coefficient increment will decrease, and the effect of the drag reduction will also decrease. Table 3 Typical AoA of different Configuration Baseline Type3 Type2 Type1(0.6c) Type1(0.7c) Type1(0.8c) configurations Stall AoA/° 10 15 17 18 16 15 AoA corresponding 15 15 17 18 16 15 Clmax /° 123 International Journal of Aeronautical and Space Sciences (2022) 23:859–869 869 Acknowledgements The present work is supported by National Nat- 7. Johnston J, Gopalarathnam A (2012) Investigation of a bio-inspired ural Science Foundation of China (11502214) and the Fundamental lift-enhancing effector on a 2d airfoil. Bioinspir Biomim 7:036003 Research Funds for the Central Universities (D5000200685). The 8. Johnston J, Gopalarathnam A, Edward J Experimental investiga- authors would like to express their gratitude to Professor Yuqin Jiao tion of bio-inspired high lift effectors on a 2-d airfoil. AIAA Paper, in Northwestern Polytechnical University for his valuable guidance. 2011–3791, 2011. 9. Kernstine KH, Moore CJ, Cutler A, Mittal R Initial Characteri- zation of Self- Activated Movable Flaps. AIAA Paper 2008–369, Declarations 10. Bramesfeld G, Maughmer MD (2002) Experimental investigation Conflict of interest The author(s) declare no potential conflicts of inter- of self-actuating, upper- surface, high-lift-enhancing effectors. J est with respect to the research, authorship, and/or publication of this Aircr 39(1):120–124 article. 11. Schlüter JU Lift enhancement at low Reynolds numbers using pop- up feathers. San Antonio, Texas, USA, AIAA Paper, 2009–4195, Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adap- 12. Schlüter JU (2010) Lift enhancement at low Reynolds numbers tation, distribution and reproduction in any medium or format, as using self-activated movable flaps. J Aircr 47(1):348–351 long as you give appropriate credit to the original author(s) and the 13. Traub LW, Jaybush L (2010) Experimental investigation of sepa- source, provide a link to the Creative Commons licence, and indi- ration control using upper-surface spoilers. J Aircr 47(2):714–717 cate if changes were made. The images or other third party material 14. Wang CJ, Schlüter J (2012) Stall control with feathers: self- in this article are included in the article’s Creative Commons licence, activated flaps on finite wings at low reynolds numbers. CR Mec unless indicated otherwise in a credit line to the material. If material 340:57–66 is not included in the article’s Creative Commons licence and your 15. Montefort J, Pohl N, Liu TS, Gregory J, Crafton J (2013) Thin-wing intended use is not permitted by statutory regulation or exceeds the vibration control using flexible fins. AIAA J 51(9):2218–2230 permitted use, you will need to obtain permission directly from the copy- 16. Brücker C, Weidner C (2014) Influence of self-adaptive hairy flaps right holder. 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Journal

International Journal of Aeronautical and Space Sciences – Springer Journals

Published: Nov 1, 2022

Keywords: Flap; Flow control; Airfoil; Stall characteristic; Wind tunnel experiment

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