Effect of cooperative injection and suction jet on power extraction characteristics of a semi-active flapping airfoil

A novel cooperative injection and suction jet was employed in this work to improve the power extraction characteristics of a semi-active flapping airfoil(S-AFA).The combined Taguchi method and numerical simulations was used to optimize the characteristic parameters of the cooperative injection and suction jet that affect the power extraction characteristics of the S-AFA.Under the Reynolds number of 6×10^(4)and the pitching axis location at the center of the ellipse airfoil,the effects of jet position,jet width,jet coefficient as well as phase difference between the cooperative injection and suction jet and the active pitching motion on the power extraction characteristics of the S-AFA are systematically examined.It is concluded that the power extraction characteristics of the S-AFA can be enhanced by employing cooperative injection and suction jet.Compared to the S-AFA without jet,the maximum efficiency is increased 11.06%for the S-AFA with optimized cooperative injection and suction jet.The flow structure behavior analysis shows that the vortex strength,the low pressure range in the suction surface of the airfoil as well as the pressure value in the pressure surface of the airfoil are increased due to the cooperative injection and suction jet,and it is why the power extraction characteristics of S-AFA can be improved.

AERODYNAMIC FORCE AND FLOW STRUCTURES OF TWO AIRFOILS IN FLAPPING MOTIONS

Aerodynamic force and flow structures of two airfoils in a tandem configuration in flapping motions axe studied, by solving the Navier-Stokes equations in moving overset grids. Three typical phase differences between the fore- and aft-airfoil flapping cycles are considered. It is shown that: (1) in the case of no interaction (single airfoil), the time average of the vertical force coefficient over the downstroke is 2.74, which is about 3 times as large as the maximum steady-state lift coefficient of a dragonfly wing; the time average of the horizontal force coefficient is 1.97, which is also large. The reasons for the large force coefficients are the acceleration at the beginning of a stroke, the delayed stall and the 'pitching-up' motion near the end of the stroke. (2) In the cases of two-airfoils, the time-variations of the force and moment coefficients on each airfoil are broadly similar to that of the single airfoil in that the vertical force is mainly produced in downstroke and the horizontal force in upstroke, but very large differences exist due to the interaction. (3) For in-phase stroking, the major differences caused by the interaction are that the vertical force on FA in downstroke is increased and the horizontal force on FA in upstroke decreased. As a result, the magnitude of the resultant force is almost unchanged but it inclines less forward. (4) For counter stroking, the major differences are that the vertical force on AA in downstroke and the horizontal force on FA in upstroke are decreased. As a result, the magnitude of the resultant force is decreased by about 20 percent but its direction is almost unchanged. (5) For 90 degrees -phase-difference stroking, the major differences axe that the vertical force on AA in downstroke and the horizontal force on FA in upstroke axe decreased greatly and the horizontal force on AA in upstroke increased. As a result, the magnitude of the resultant force is decreased by about 28% and it inclines more forward. (6) Among the three cases of phase angles, inphase flapping produces the largest vertical force (also the largest resultant force); the 90 degrees -phase-difference flapping results in the largest horizontal force, but the smallest resultant force.

Numerical analysis of the s1020 airfoils in tandem under different flapping configurations

The objective of this project is to improve the performance of the efficiency, thrust and lift of flapping wings in tandem arrangement. This research investigates the effect of the arrangement of the airfoils in tandem on the performance of the airfoils by varying the phase difference and distance between the airfoils. Three flapping configurations from an earlier phase of a research which gives high efficiency, thrust and lift are used in the tandem simulation. It is found all the different flapping configurations show improvement in the efficiency, thrust or lift when the distance between the two airfoils and the phase angle between the heaving positions of the two airfoils are optimal. The average thrust coefficient of the tandem arrangement managed to attain more than twice that of the single one （4.84 vs. 2.05）. On the other hand, the average lift coefficient of the tandem arrangement also increased to 4.59, as compared to the original single airfoil value of 3.04. All these results obtained will aid in the design of a better ornithopter with tandem wing arrangement.

Analysis of non-symmetrical flapping airfoils

Simulations have been done to assess the lift, thrust and propulsive efficiency of different types of non-symmetrical airfoils under different flapping configurations. The variables involved are reduced frequency, Strouhal number, pitch amplitude and phase angle. In order to analyze the variables more efficiently, the design of experiments using the response surface methodology is applied. Results show that both the variables and shape of the airfoil have a profound effect on the lift, thrust, and efficiency. By using non- symmetrical airfoils, average lift coefficient as high as 2.23 can be obtained. The average thrust coefficient and efficiency also reach high values of 2.53 and 0.61, respectively. The lift production is highly dependent on the airfoil＇s shape while thrust production is influenced more heavily by the variables. Efficiency falls somewhere in between. Two-factor interac- tions are found to exist among the variables. This shows that it is not sufficient to analyze each variable individually. Vorticity diagrams are analyzed to explain the results obtained. Overall, the S1020 airfoil is able to provide relatively good efficiency and at the same time generate high thrust and lift force. These results aid in the design of a better ornithopter＇s wing.

Influence of Flap Parameters on the Aerodynamic Performance of a Wind-Turbine Airfoil

A numerical method has been used to analyze the flow field related to a NACA 0015 airfoil with and without a flap and assess the influence of the flap height and angle on the surface pressure coefficient,lift coefficient,and drag coefficient.The numerical results demonstrate that the flap can effectively improve the lift coefficient of the airfoil;however,at small attack angles,its influence is significantly reduced.When the angle of attack exceeds the critical stall angle and the flap height is 1.5%of the chord length,the influence of the flap becomes very evident.As the flap height increases,the starting point of the separation vortex gradually moves forward and generates a larger wake vortex.Optimal aerodynamic characteristics are obtained for 1.5%(of the chord length)flap height and a 45°flap angle;in this case,the separation vortex is effectively reduced.

Bi-objective path optimization of flapping airfoils based on a surrogate model

A bi-objective optimization problem for flapping airfoils is solved to maximize the time-averaged thrust coefficient and the propulsive efficiency. Design variables include the plunging amplitude, the pitching amplitude and the phase shift angle. A well defined Kriging model is used to substitute the time-consuming high fidelity model, and a multi-objective genetic algorithm is employed as the search algorithm. The optimization results show that the propulsive efficiency can be improved by reducing the plunging amplitude and the phase shift angle in a proper way. The results of global sensitivity analysis using the Sobol’s method show that both of the time-averaged thrust coefficient and the propulsive efficiency are most sensitive to the plunging amplitude, and second most sensitive to the pitching amplitude. It is also observed that the phase shift angle has an un-negligible influence on the propulsive efficiency, and has little effect on the time-averaged thrust coefficient.

Thrust Optimization of Flapping Wing via Gradient Descent Technologies

The current work aims at employing a gradient descent algorithm for optimizing the thrust of a flapping wing. An in-house solver has been employed, along with mesh movement methodologies to capture the dynamics of flow around the airfoil. An efficient framework for implementing the coupled solver and optimization in a multicore environment has been implemented for the generation of optimized solutionsmaximizing thrust performance & computational speed.

Uncertainty quantification for a flapping airfoil with a stochastic velocity deviation

An outdoor flapping wing micro air vehicle （FWMAV） should be able to withstand unpredictable perturbations in the flight condition. The responses of the time-averaged thrust coefficient and the propulsive efficiency with respect to a stochastic flight velocity deviation were numerically investigated. The deviation was assumed to obey the Gauss distribution with a mean value of zero and a specified standard deviation. The probability distributions of the flapping performances were quantified by the non-intrusive polynomial chaos method. It was observed that both of the time-averaged thrust coefficient and the propulsive efficiency obeyed Gauss-like but not the exact Gauss distribution. The velocity deviation had a large effect on the time-averaged thrust coefficient and a small effect on the propulsive efficiency.

Numerical simulation of Gurney flap on SFYT15thick airfoil

A two-dimensional steady Reynolds-averaged Navier-Stokes （RANS） equation was solved to investigate the effects of a Gurney flap on SFYT15 thick airfoil aerodynamic performance. This airfoil was designed for flight vehicle operating at 20 km altitude with freestream velocity of 25 m/s, The chord length （C） is 5 m and the Reynolds number based on chord length is Re = 7.76 × 10^5. Gurney flaps with the heights ranging from 0.25%C to 3%C were investigated. The shear stress transport （SST） k-ω turbulence model was used to simulate the flow structure around the airfoil. It is showed that Gurney flap can enhance not only the prestall lift but also lift-to-drag ratio in a certain range of angles of attack. Specially, at cruise angle of attack （ω = 3°）, Gurney flap with 0.5%C height can increase lift-to-drag ratio by 2.7%, and lift coefficient by 12.9%, respectively. Furthermore, the surface pressure distribution, streamlines and trailing-edge flow structure around the airfoil are illustrated, which are helpful to understand the mechanisms of Gurney flap on airfoil aerodynamic performance. Moreover, it is found that the increase of airfoil drag with Gurney flap can be attributed to the increase of pressure drag between the windward and the leeward sides of Gurney flat itself.

The effect of phase angle and wing spacing on tandem flapping wings

In a tandem wing configuration, the hindwing of- ten operates in the wake of the forewing and, hence, its per- formance is affected by the vortices shed by the forewing. Changes in the phase angle between the flapping motions of the fore and the hind wings, as well as the spacing between them, can affect the resulting vortex/wing and vortex/vortex interactions. This study uses 2D numerical simulations to in- vestigate how these changes affect the leading dege vortexes （LEV） generated by the hindwing and the resulting effect on the lift and thrust coefficients as well as the efficiencies. The tandem wing configuration was simulated using an incom- pressible Navier-Stokes solver at a chord-based Reynolds number of 5 000. A harmonic single frequency sinusoidal oscillation consisting of a combined pitch and plunge motion was used for the flapping wing kinematics at a Strouhal num- ber of 0.3. Four different spacings ranging from 0.1 chords to 1 chord were tested at three different phase angles, 0°, 90° and 180°. It was found that changes in the spacing and phase angle affected the timing of the interaction between the vor- tex shed from the forewing and the hindwing. Such an inter- action affects the LEV formation on the hindwing and results in changes in aerodynamic force production and efficiencies of the hindwing. It is also observed that changing the phase angle has a similar effect as changing the spacing. The re- suits further show that at different spacings the peak force generation occurs at different phase angles, as do the peak efficiencies.

Study of Eppler 423 Airfoil with Gurney Flap and Vortex Generators

In the past extensive research has been carried out, to study the effect of Gurney flap (GF) on symmetric and cambered airfoil for its usage in low Reynolds number regime. Use of GF at the trailing edge of the airfoil enhances the lift due to increase in the effective camber of the airfoil, which in turn improves the aerodynamic efficiency i.e. Cl/Cd. In the present study, Eppler 423 airfoil is used to first understand the aerodynamics of such a highly cambered airfoil and later GF of various sizes were added on it to understand the change in flow dynamics achieved by adding the GF and their impact on aerodynamic parameters such as Cl, Cd and Cl/Cd. Eppler 423 being a highly cambered airfoil produces high lift coefficient and smoother stall and by adding the GF of various sizes the performance of Eppler 423 improves tremendously and reason for this enhanced performance and effect of size of GF are presented in this paper. Vortex Generators (VG) generate counter rotating vortices that allow the flow to remain attached even at high angles of attack. Also, effect of adding VG at the leading edge of Eppler 423 aerofoil is presented in this paper. At last, results obtained from combination of VG at leading edge and GF at trailing edge on Eppler 423 aerofoil are discussed at length.

Study the Influence of a Gap between the Wing and Slotted Flap over the Aerodynamic Characteristics of Ultra-Light Aircraft Wing Airfoil

The purpose of the study is to assess what the influence of the distance of the gap is between the wing and slotted flap on the aerodynamic characteristics of ultra-light aircraft wing when the flap is retracted. It has been elected numerical approach to the study and it is been realized through applied numerical model of the wing airfoil NACA 2412 for three different lengths of slotted gap size, whose length is expressed as percentages of the airfoil chord. The code ANSYS FLUENT has been applied, as it has been determined RANS （Reynolds-averaged Navier-Stokes） equations and DES （detached-eddy simulation） turbulent model has been used.

Gurney Flap绕流气动特性研究

采用C-H型多块结构网格和SST k-ω湍流模型求解二维定常雷诺平均NS方程,对NACA4412单段翼型后缘附近和NACA63(2)-215B多段翼型襟翼后缘附近的Gurney Flap绕流进行了数值模拟。讨论了局部网格对计算结果的影响,重点研究了Gurney Flap位置和高度对气动特性的影响。计算结果表明:对于NACA4412单段翼型,Gurney Flap位置对气动性能的影响较小,但随着高度的增加,零升攻角和失速攻角减小,最大升力系数增大,同时阻力系数和力矩系数增加;对于NACA63(2)-215B多段翼型,Gurney Flap对气动性能的影响类似于NACA4412单段翼型情形。

A STUDY ON THE MECHANISM OF HIGH-LIFT GENERATION BY AN AIRFOIL IN UNSTEADY MOTION AT LOW REYNOLDS NUMBER

The aerodynamic force and flow structure of NACA 0012 airfoil performing an unsteady motion at low Reynolds number (Re = 100) are calculated by solving Navier-Stokes equations. The motion consists of three parts: the first translation, rotation and the second translation in the direction opposite to the first. The rotation and the second translation in this motion are expected to represent the rotation and translation of the wing-section of a hovering insect. The flow structure is used in combination with the theory of vorticity dynamics to explain the generation of unsteady aerodynamic force in the motion. During the rotation, due to the creation of strong vortices in short time, large aerodynamic force is produced and the force is almost normal to the airfoil chord. During the second translation, large lift coefficient can be maintained for certain time period and (C) over bar (L), the lift coefficient averaged over four chord lengths of travel, is larger than 2 (the corresponding steady-state lift coefficient is only 0.9). The large lift coefficient is due to two effects. The first is the delayed shedding of the stall vortex. The second is that the vortices created during the airfoil rotation and in the near wake left by previous translation form a short 'vortex street' in front of the airfoil and the 'vortex street' induces a 'wind'; against this 'wind' the airfoil translates, increasing its relative speed. The above results provide insights to the understanding of the mechanism of high-lift generation by a hovering insect.

Investigation of steady plasma actuation effect on aerodynamic coefficients of oscillating airfoil at low Reynolds number

In this work, numerical study of two dimensional laminar incompressible flow around an oscillating NACA0012 airfoil is proceeded using the open source code Open FOAM. Oscillatory motion types including pitching and flapping are considered. Reynolds number for these motions is assumed to be 12000 and effects of these motions and also different unsteady parameters such as amplitude and reduced frequency on aerodynamic coefficients are studied. For flow control on airfoil, dielectric barrier discharge plasma actuator is used in two different positions on airfoil and its effect is compared for the two types of considered oscillating motions. It is observed that in pitching motion, imposing plasma leads to an improvement in aerodynamic coefficients, but it does not have any positive effect on flapping motion.Also, for the amplitudes and frequencies investigated in this paper, the trailing edge plasma had a more desirable effect than other positions.

Low-Speed Aerodynamic Characteristics of Supercritical Airfoil with Small High-Lift Devices from Flow Pattern Measurements

In this paper, the lift coefficients of SC-0414 airfoil are estimated by applying modified Yamana’s method to the flow visualization results, which are obtained by utilizing the smoke tunnel. The application of the modified Yamana’s method is evaluated with two calculation methods. Additionally, the lift estimation, wake measurements, and numerical simulations are performed to clarify the low-speed aerodynamic characteristics of the SC airfoil with flaps. The angle of attack was varied from −5° to 8°. The flow velocity was 12 m/s and the Reynolds number was 1.6 × 105. As a result, the estimated lift coefficients show a good agreement with the results from reference data and numerical simulations. In clean condition, the lift coefficients calculated from the two methods show quantitative agreement, and no significant difference could be confirmed. However, the slope of the lifts calculated from ys is higher and closer to the reference data than those obtained from sc, where ys denotes the height where the distance from the streamline to the reference line is the largest, and sc denotes the displacement of the center of pressure from the origin of the coordinate, respectively. In the case of flaps, the GFs have an observable effect on the aerodynamic performance of the SC-0414 airfoil. When the height of the flap was increased, the lift and drag coefficients increased. The installation of a GF with a height equal to 1% of the chord length of the airfoil significantly improved the low-speed aerodynamic performance of SC airfoils.

S型翼缝与尾缘襟翼联合控制风力机翼型气动性能的研究

降低叶片气动载荷对于延长风力机叶片寿命极为重要,尾缘襟翼是降低叶片气动载荷的一种有效手段,然而其在大攻角下对翼型的气动特性控制效果减弱。通过在50%叶片弦长处添加S型翼缝,采用数值模拟方法研究S型翼缝与尾缘襟翼联合调节风力机叶片气动特性的作用。利用剪切应力输运(shear stress transport,SST)湍流模型,计算模拟了S809翼型在雷诺数106下,S型翼缝与尾缘襟翼联合控制对翼型气动特性的影响,并分析了S型翼缝与尾缘襟翼的协同控制机理。结果表明,尾缘襟翼的偏转可显著改变翼缝槽内的气体流量,并有效调节叶片中部的压差,这种联合控制策略可在大攻角下大幅改善翼型的失速现象,从而显著增强尾缘襟翼对升力系数的调控能力。

仿灰鲭鲨尾鳍襟翼的翼型气动性能数值研究

为进一步提高翼型的气动性能,基于灰鲭鲨尾鳍上尾叉型线结构,提出一种仿生襟翼结构。利用SST k-ω湍流模型模拟仿生翼型的气动性能和内流特征,分析了仿生襟翼相对位置和角度的影响,得到了气动性能最优的仿生襟翼翼型,并将其与格尼襟翼进行对比。结果表明:安装仿生襟翼后,翼型升阻比较原翼型显著提高;当襟翼相对高度不变时,减小襟翼安装角和增大襟翼与尾缘间的距离均导致翼型失速提前;安装角为45°且逆流向安装在尾缘处的仿生襟翼翼型气动性能最优,其升力系数较格尼襟翼翼型在失速前平均提高5.9%;安装仿生襟翼后流场结构更复杂,流场内涡的位置、数量及大小均有所改变。

Aerodynamic performance enhancement forflapping airfoils by co-flow jet

Introducing active flow control into the design of flapping wing is an effective way to enhance its aerodynamic performance.In this paper,a novel active flow control technology called Co-Flow Jet(CFJ)is applied to flapping airfoils.The effect of CFJ on aerodynamic performance of flapping airfoils at low Reynolds number is numerically investigated using Unsteady Reynolds Averaged Navier-Stokes(URANS)simulation with Spalart-Allmaras(SA)turbulence model.Numerical methods are validated by a NACA6415-based CFJ airfoil case and a S809 pitching airfoil case.Then NACA6415 baseline airfoil and NACA6415-based CFJ airfoil with jet-off and jet-on are simulated in flapping motion,with Reynolds number 70,000 and reduced frequency 0.2.As a result,CFJ airfoils with jet-on generally have better lift and thrust characteristics than baseline airfoils and jet-off airfoil when Cμgreater than 0.04,which results from the CFJ effect of reducing flow separation by injecting high-energy fluid into boundary layer.Besides,typical kinematic and geometric parameters,including the reduced frequency and the positions of the suction and injection slot,are systematically studied to figure out their influence on aerodynamic performance of the CFJ airfoil.And a variable Cμjet control strategy is proposed to further improve effective propulsive efficiency.Compared with using constant Cμ,an increase of effective propulsive efficiency by22.6%has been achieved by using prescribed variable CμNACA6415-based CFJ airfoil at frequency 0.2.This study may provide some guidance to performance enhancement for Flapping wing Micro Air Vehicles(FMAV).

Aerodynamic performance of owl-like airfoil undergoing bio-inspired flapping kinematics

Natural flyers have extraordinary flight skills and their prominent aerodynamic performance has attracted a lot of attention.However,the aerodynamic mechanism of birds’flapping wing kinematics still lacks in-depth understanding.In this paper,the aerodynamic performance of owl-like airfoil undergoing bio-inspired flapping kinematics extracted from a free-flying owl wing has been numerically investigated.The overset mesh technique is used to deal with the large range movements of flapping airfoils.The bio-inspired kinematics consist of plunging and pitching movement.A pure sinusoidal motion and a defined motion composed of plunging of sinusoidal motion and pitching of the bio-inspired kinematics are selected for comparison.The other two NACA airfoils are also selected to figure out the advantages of the owl-like airfoil.It is found that the cambered owl-like airfoil can enhance lift during the downstroke.The bio-inspired kinematics have an obvious advantage in lift generation with a presence of higher peak lift and positive lift over a wider proportion of the flapping cycle.Meanwhile,the bio-inspired motion is more economical for a lower power consumption compared with the sinusoidal motion.The sinusoidal flapping motion is better for thrust generation for a higher peak thrust value in both upstroke and downstroke,while the bio-inspired kinematics mainly generate thrust during the downstroke but produce more drag during the upstroke.The defined motion has similar lift performance with the bio-inspired kinematics,while it consumes more energy and generates less thrust.The unsteady flow field around airfoils is also analyzed to explain the corresponding phenomenon.The research in this paper is helpful to understand the flight mechanism of birds and to design a micro air vehicle with higher performance.