无人机机翼减阻技术研究

American Institute of Aeronautics and Astronautics1Drag Reduction of Light UA V Wing with Deflectable Surfacein Low Reynolds Number FlowsMasoud Darbandi * and Ali Nazari †Sharif University of Technology, Tehran, P.O. Box 11365-8639, Iran Gerry E. Schneider ‡University of Waterloo, Waterloo, Ontario, N2L 3G1, CanadaThe most effective approach to drag reduction is to concentrate on the components that make up the largest percentage of the overall drag. Small improvements on large quantities can become in fact remarkable aerodynamic improvements. Our experience shows that the use of light material in constructing human-powered airplanes and unmanned-air-vehicles UAVs has a few side effects on the aerodynamic characteristics of their wings. One important side effect is the unwanted deflection on wing shell. It is because of high flexibility and low solidity of the light material, which covers the wing skeleton. The created curvature has direct impact on the separation phenomenon occurred over the wing in low Reynolds number flows. In this work, we numerically simulate the flow over a UAV wing with and without considering the generated deflection on its shell. It is shown that the curvature on the wing surface between two supporting airfoil frames causes total drag coefficient reduction. Indeed, this drag reduction is automatically achieved without benefiting from additional drag-reduction devices and/or drag-reduction considerations. The current investigation has been conducted on a UAV wing with fxmp-160 airfoil section. This airfoil normally provides high lift coefficient in low Reynolds flows because of having suitable camber. The drag of a wing with this airfoil section can be reduced by the proper usage of low weight material as its wing shell providing that the wing shell deflects between its supporting frames during stretching the shell in manufacturing stage.Nomenclatureα = angles of attack C d =total drag coefficientC dp =profile drag C ds =skin friction drag C l = two-dimensional lift coefficientC Lthree-dimensional lift coefficient L/D =lift-drag ratio Re = Reynolds numberI. IntroductionRAG reduction is one of the major objectives to the air vehicle designers and manufacturers 1. The study of airvehicles at their cruise shows that there are two main sources of drag force including lift-induced and skin-friction drags. It is reported that these two sources of drag are approximately one-third and one-half of the total drag, respectively, in civil transport aircraft. Reneaux 2 emphasizes that hybrid laminar flow technology and innovates wing tip devices offer the greatest potential for drag reduction. With respect to lift-induced drag, the classical way to reduce drag has been to increase the wing aspect ratio, which is automatically provided in UAV wings. However, for the wings with low aspect-ratio, it is suggested to use various winglet devices such as wing tip sails, wing grid, *Associate Professor, Department of Aerospace Engineering. †Graduate Student, Department of Aerospace Engineering. ‡Professor and Chair, Department of Mechanical Engineering, AIAA Fellow.D3rd AIAA Flow Control Conference5-8 June 2006, San Francisco, CaliforniaAIAA 2006-3680blended winglet, spiroid, and so on2,3. Unfortunately, such unconventional devices cause a complex geometry, which is difficult to be optimized suitably4.In low speed vehicles, laminar flow can be maintained by shaping the airfoil; however, the application of suction in the region of the leading edge has to be applied for high speed vehicle to control the pressure gradient there5. As an alternative strategy, Saric, et al.6 show that artificial roughness can be applied to the leading edge of the wing as a transition control mechanism. Another idea to improve the drag reduction is to use adhesive film coating on wing. Preliminary experiments in the low turbulence wind tunnel at the Langley Research Center has shown that smoothing a wing surface area by applying an adhesive film coating could reduce its drag by as much as 12%, which translated to a 23% reduction in the overall drag of an airplane7. Another mechanism to reduce the drag of fluid is to oscillate the solid wall in the appropriate directions. Jung, et al.8 demonstrated that the skin friction drag of a turbulent channel flow can be reduced by a factor of 40% if one of the walls oscillates in the spanwise direction. Baron and Quadrio9 showed that there would be a 10% net energy saving if the wall was suitably oscillated. Choi, et al.10 and Choi and Clayton11 reached to a similar conclusion with less and more quantifications by investigating on a flat plate. Beside the classical approaches, an emerging technology to control the separation over wing is MEMS, which will effectively contribute to enhance the drag reduction. This technology helps to control the flow through an active manipulation of the coherent structures developed in near-wall region of the boundary layer12.The experience has shown that one primitive idea to reduce the drag of wings is to use flexible wings. The idea has taken into investigation since many decades ago13. Indeed, a flexible aerodynamic surface can be regarded as a possible mean of delaying transition and reducing skin-friction drag. Gyorgyfalvy13 investigated the issue and indicated that the theoretically possible drag reduction is most significant within the Reynolds number ranges from 3 to 50x106 . Hence potential field of flexible wing application would be in sail planes, helicopters, small and medium subsonic airplanes, hydrofoils, torpedoes, speed boats, and miniature submarines. Indeed, the idea of using flexible wings returns to the flow behavior behind the airfoil section utilized in low Reynolds number flow applications. The aerodynamic characteristics of airfoil in low Reynolds number regimes, i.e., Re=5x104 to 1x106, are of importance in a variety of sailplanes and high altitude unmanned aerial vehicles14-15. Tatineni and Zhong14 performed two-dimensional computation over the APEX airfoil and showed that the flow is unsteady with periodic vortex shedding. The vortex shedding is caused by the instability of the separated flow. Redioniottis, et al.16 employed the idea of utilizing an active skin to reduce turbulence drag. The active skin is deformed to take the shape of a traveling wave profile through active material actuation. They indicated that for typical Reynolds numbers encountered during flight condition in UAV’s, the optimum parameters of a surface wave would be a wave length of around 25mm, an amplitude of around 30 µm, and a frequency in the range of a few hundred Hertz. Sinha17 develops a flexible composite surface which is affixed to surfaces of aircraft wings in order to reduce the profile drag. A drag reduction around 18%-37% is reported in flight tests on light aircraft and sailplanes.The wing flexibility issue has been also targeted in wings with low thickness. Shyy, et al.18 treated low Reynolds number environment, i.e., Reynolds as low as 7.5x104 around an elastic, massless membrane as a portion of the upper airfoil surface. They showed that more favorable lift-to-drag characteristics would be obtained when the Reynolds number decreased. Additionally, they deduced that a flexible profile yields better overall performance than a similar rigid profile in an oscillatory free stream. The idea of using curvature of the wing section in improving its aerodynamics characteristics has been similarly touched in the other fields of aerodynamics. For example, Ashil19 utilizes the bump concept and locally (and slightly) modifies the airfoil surface in the shock region. This way, a normal standing shock is weakened into a lambda shock configuration and its strength is reduced by the presence of compression waves.Contrary to the above traditional and innovates drag reduction mechanisms and technologies, we are faced with unwanted deflections on UAV wings20. In reality, the need for designing the light wings for special applications such as UAVs promotes the designer to use light material such as thin plastics to construct its surface shell. The use of such light shells causes unwanted deflections on wing surface. The deflection may be caused by either stretching the skin surface or loading the aerodynamics forces. For example, Deluca, et al.21 investigated a man-portable Micro-Air-Vehicle and showed that the wings would deform considerably under aerodynamics loading. In this work, we focus on the deflection of the wing surface due to surface stretch. The deflection is observed between the two neighboring airfoil sections, located on the wing’s frame, which supports the wing shell. The effect of this deflection is then studied in reducing the wing drag coefficients.2American Institute of Aeronautics and AstronauticsII.Mathematical DescriptionWe choose three-dimensional Navier-Stokes equations to solve the flow field around our UAV wing. The governing equations are discretized using finite-volume method. We choose SIMPLE algorithm to solve the discretized equations. We also use two-equation standard κ-ε turbulence model for turbulence modeling purposes. The near-wall wall-functions are used to model the kinetic energy of turbulence fluctuations κ and its dissipation rate ε near the wall22. The turbulence source terms are approximated at cell faces using power-law scheme. The pressure and diffusion terms are approximated at cell faces using second-order central scheme. However, the convection fluxes at the cell faces are approximated using a second-order upwind scheme23.Since the boundary layer on the airfoil surface and the wake region behind the airfoil play critical roles in the aerodynamic characteristics of the wing, it is necessary to refine mesh in vicinity of wing surface suitably. Additionally, the past investigations have shown that the use of quadrilateral grids in vicinity of the solid walls can tremendously improve the accuracy of the solution. It is because the flow in boundary layer beside the solid wall can be consistently discretized in the flow direction without generating excessive false diffusion in coarse grid employments24. On the other hand, the use of unstructured triangular grid can remarkably reduce the number of cells far form the boundary layer25. To benefit both of these advantages, we have created a hybrid grid topology consisting of a structured quadrilateral grid beside the solid boundary and an unstructured triangular grid for the rest of our computational domain. We have used a multiblocked strategy to distribute these two type of grid topology26,27.Figure 1 partially illustrates the grid distribution around the airfoil section utilized in our UAV wing. Since we intend to verify the accuracy of our numerical solutions against the experimental data, we model the airfoil section in a wind tunnel section. As it is observed, the computational domain is divided by three zones. One structured zone near the wing surface and two unstructured zones in mid and outer regions to the airfoil. Of course, this figure only shows two-dimensional perspective of our three-dimensional grid distribution. Figure 2 shows the deflection occurred on the surface of the wing. Indeed, the deflection on the wing has been exaggerated in order to3American Institute of Aeronautics and Astronauticsprovide a better illustration of what it really happens to its shell. The figure shows the grid between its two consequent airfoils sections (or frames) in the wing. In order to model the curvature, occurred on the wing shell between two supporting frames accurately, we used digitizers. After careful measurements of the deflection in our UAV wing, the deflected surfaces were simulated using the AutoCAD software. In the mesh generation stage, the three-dimensional grids were suitably distributed around the wing considering the created curvature on the wing shell. The grid lines on the wing shell are partially observed in Fig. 2. As it is seen in this close view, the grid on the wing surface is structured and it is clustered wherever it is desired.Fig. 2: A deflection occurred over the wing shell.Since the flow field is solved in an unsteady manner, we consider zero magnitude velocity field and atmospheric pressure in the solution domain as the initial conditions. To be consistent with the conditions implemented in wind tunnel, we numerically modeled the two-dimensional test cases in a channel. We specified the velocity at the inlet and pressure at the outlet. Additionally, we implemented no-slip boundary conditions at the solid walls. We also used wall-function to implement the required boundary conditions for our turbulence governing equations. The preliminary investigation and analysis showed that the walls were far enough not to cause adverse effect on the computed aerodynamic coefficients of the model.III.ResultsAt the first stage of our investigation, it is necessary to verify the computational procedure and to evaluate the accuracy of our algorithm. In this regard, we numerically solve the chosen airfoil section at Re=3x105. It is because the two-dimensional measurements are available at this Reynolds. Figure 3 compares the current numerical solutions with those of measurement for a wide range of angles of attack α. The test have been done for a few angles of attack including 0, 2, 4, 6, 8, 10, 12, and 14. As it is seen, the two lift coefficient distributions agree well within the chosen range of α except for α>12, which is reasonable. This validation ensures us to proceed the rest of our study confidently. The numerical investigation showed that the distribution given in Fig. 3 would be slightly affected in the other close low Reynolds number flows, whose results are not presented here.4American Institute of Aeronautics and Astronautics5American Institute of Aeronautics and AstronauticsIV.ConclusionsThe skin friction and lift-induced are two major sources of drag in air vehicles. The latter drag has minor impact in UAVs because of their large aspect ratio magnitudes of UAV wings. However, skin friction and profile drag play important roles in aerodynamics characteristics of UAV wings because of their flight at low Reynolds. To investigate the impact of wing surface deflection in the aerodynamic performance of the light wing utilized in our6American Institute of Aeronautics and AstronauticsAmerican Institute of Aeronautics and Astronautics7UAV, we arranged a numerical study. Because of the limit in measurements, the code validation was performed at a Reynolds number different from the working one. The investigation showsundesirable because the lift coefficient flight with angles of attack greater Re =generated curvature had also direct impact on drag behavior of our UAV. The current investigation shows that an unwanted deflection on the wing shell of UAV can increase the skin friction. However, the profile drag behavior of the wing in low Reynolds advantages can be utilized to enhance UAVs without the need for removing the occurred deflection. Additionally, this privilege can drastically affect the design of wing’s skeleton and reduce utilized as UAVs’ wing shell.V.REFERENCES1Robert, J.P., “Drag Reduction : An Industrial Challenge,” Special Course on Skin Friction Drag Reduction , AGARD Report 786, 1992. 2Reneaux, J., “Overview on Drag Reduction Technologies for Civil Transport Aircraft,” European Congress on Computational Methods in Applied Sciences and Engineering , ECCOMAS, P. Neittaanmäki, T. Rossi, S. Korotov, E. Oñate, J. Périaux, and D. Knörzer (eds.) Jyväskylä, Finland, 24—28 July 2004. 3Bourdin, P., “Planform Effects on Lift-Induced Drag,” AIAA Paper 2002-3151, The 20th AIAA Applied Aerodynamics Conference, St. Louis, June 24-27, 2002.3Reneaux, J. and Blanchard, A., “The Design and Testing of an Airfoil with Hybrid Laminar Flow Control,” First European Forum on Laminar Flow Technology, Hamburg, Germany, March, 1992.4Grenon, R., and Bourdin, P., “Numerical Study of Unconventional Wing Tip Devices for Lift- Induced Drag Reduction,” CEAS Aerospace Aerodynamics Research Conference Cambridge, June 2002.5Arnal, D., Juillen, J.C., Reneaux, J., and Gasparian, G., “Effect of Wall Suction on Leading Edge Contamination,” Aerospace Science and Technology , No. 8, 1997, pp.505-517.6Saric, W., Carrillo, R., Reibert, M., “Leading-edge Roughness as a Transition Control Mechanism,” AIAA Paper 1998-0781, 36th Aerospace Sciences Meeting and Exhibit , Reno, Nevada, January 1998.American Institute of Aeronautics and Astronautics87Beasley, W.D., and McGhee, R.G., “An Exploratory Investigation of the Effects of a Thin Plastic Film Cover on the Profile Drag of an Aircraft Wing Panel,” NASA TM-74073, October 1977.8Jung, W.J., Mangiavacchi, N., and Akhavan, R., “Suppression of Turbulence in Wall-Bounded Flows by High Frequency Spanwise Oscillations,” Physics of Fluids A , Vol. 4, No. 8, 1992, pp.1605-1607.9Baron, A., and Quadrio, M., “Turbulent Drag Reduction by Spanwise Wall Oscillations,” Applied Scientific Research , Vol. 55, 1996, pp.311-326.10Choi, K.S., DeBisschop, J.R., and Clayton, “Turbulent Boundary Layer Control by Means of Spanwise-Wall Oscillation,” AIAA J., Vol. 36, No.7, 1998, pp.1157-1163.11Choi, K.S., and Clayton, B.R., “ The mechanism of Turbulent Drag Reduction with Wall Oscillation,” International Journal of Heat and Fluid Flow , Vol. 22, 2001, pp.1-9.12Warsop, C., “Current Status and Prospects for Turbulent Flow Control,” AerodynamicDrag Reduction Technologies , Proceedings of the CEAS/DragNet European Drag Reduction Conference, 19-21 June 2000, Springer, Postdam, Germany. 2001.13Gyorgyfalvy, D. “Possibilities of drag reduction in the use of flexible skin,” Journal of Aircraft , Vol.4, No. 3, 1966, pp.186-192.14Tatineni, M., and Zhong, X., “Numerical simulation of unsteady low-Reynolds-number flows over the APEX airfoil,” AIAA paper 1998-0412, 36th Aerospace Science Meeting and Exhibit, Reno, NV, Jan. 12-15, 1998.15Tatineni, M., and Zhong, X., “A numerical study of low-Reynolds-number separation bubbles,” AIAA paper 1999-523, 37th Aerospace Science Meeting and Exhibit, Anaheim, CA, June 11-14, 1999.16Rediniotis, O., Lagouadas, D., Mani, R., Traub, L., and Allen, R. “Computational and experimental studies of active skin for turbulent drag reduction,” AIAA paper 2002-2830, 1st Flow Control Conference, St. Louis, Missouri, 2002.17Sinha, S.K. “Aircraft drag reduction with flexible composite surface boundary layer control, AIAA paper 2004-2121, 2nd AIAA Flow Control Conference, Portland, Oregon, June 28- July 1, 2004.18Shyy, W., Klevebring, F., Nilsson, M., Sloan, J., Carroll, B., and Fuentes, C., “Rigid and flexible low Reynolds number airfoils,” Journal of Aircraft , Vol.36, No.3, 1999, pp.523-529.19Ashil, P.R., Fulker, J.L., and Shires, A., “A Novel Technique for Controlling Shock Strength of Laminar Flow Aerofoil Aections,” First European Forum on Laminar Flow Technology , Hamburg, Germany, March 1992.20Darbandi, M., Nazari, A., and Saeedi, H., “The Aerodynamics of Light Wings with Flexible Surface in Low Reynolds Number Flows,” Proceedings of the Eleventh Asian Congress of Fluid Mechanics May 22-25 2006, Kuala Lumpur, Malaysia. eluca, A.M., Reeder, M.F., Ol, M.V., Freeman, J., Bautista, I., and Simonich, M., “Experimental investigation into the aerodynamic properties of a flexible and rigid wing micro air vehicle,” AIAA paper 2004-2396, 24th AIAA AerodynamicMeasurement Technology and Ground Testing Conference, Portland, Oregon, June 28- July 1, 2004.22Launder, B.E., and Spalding, D.B., “The Numerical Computation of Turbulent Flows,” Computer Methods in Applied Mechanics and Engineering , Vol. 3, 1974, pp.269-289.23Verrsteeg, H.K., and Malalasekera, W. An introduction to Computational Fluid Dynamics; the finite volume method , Addison Wesley Longman Limited, 1995.24Darbandi, M., Schneider, G.E., and Naderi, A.R., “The mesh orientation impact in performance of physical-based upwinding in structured triangular grids,” Proceedings of the 11th Annual Conference of the CFD Society of Canada , CFDSC, Ottawa, ON, Canada, 2003, pp.688-695.25Darbandi, M., Schneider, G.E., and Vakilipour, S., “A Modified Upwind-Biased Strategy to Calculate Flow on Structured-Unstructured Grid Topologies,” AIAA Paper 2004-0435, the 42nd AIAA Aerospace Sciences Meeting and Exhibit, Reno, Nevada, Jan. 5-8, 2004.26Darbandi, M., Schneider, G.E., and Naderi, A.R., “A finite element volume method to simulate flow on mixed element shapes,” AIAA Paper 2003-3638, 36th AIAA Thermophysics Conference, Orlando, FL, June 23-26, 2003.27Darbandi, M., and Naderi, A.R., “Multiblock Hybrid Grid Finite Volume Method to Solve Flow in Irregular Geometries,” Computer Methods in Applied Mechanics and Engineering , in press, 2006.。