Numerical Research On Three-dimensional Unsteady Flow Field Of Micro Air Vehicle And Aerodynamic Optimization

A great deal of interest has emerged since micro air vehicle was put forward in 1990s. Its advantages of small size, invisibility, and portability make micro air vehicle broad prospects in both military and civilian fields. However, micro air vehicle flies at Reynolds number of 10~5, so classical aerodynamic theory that can accurately predict large-scale aircrafts is generally not applicable for it. In this paper, fixed wing of micro air vehicle is numerically studied to investigate the complicated three-dimensional unsteady aerodynamic characteristics and the laminar/turbulent separation phenomena at boundary layer.Flow around micro air vehicle is simulated by numerically solving three-dimensional incompressible Navier-Stokes equations by artificial compressibility method. Baldwin-Barth turbulence model is employed for turbulent flow calculations. Aerodynamic parameters such as aspect ratio, camber, wing planform, and leading-edge shape are selected to investigate their effects on wing's aerodynamic performance. Results show that at small angles of attack between -12°to 12°, excellent parameters are Zimmerman wing planform, 4% camber, and sharp leading edge with 15°'up' angles. These parameters directly affect the development of boundary layer close to the wing surface and determine the flow structure. As a result, separated bubble on the wing surface is weakened and lift-drag ratio is increased from 26% to 68%.Genetic algorithm code is developed with combination of Navier-Stokes equation simulation to obtain reasonable three-dimensional aerodynamic layout of micro air vehicle wing. In order to decrease huge computational cost generated from the repetitious calling of three-dimensional Navier-Stokes code, a real coded genetic algorithm is developed with small population. An optimization of a multimodal function indicates that present genetic algorithm can converge to the global value with high efficiency. Wing optimization results show that: (1) lift-drag ratios at eight design point are all increased by more than 30%. Especially at 2°angles of attack, double lift-drag ratio is obtained. (2) Optimal airfoils have large leading-edge radius and are cusped near the trailing edge, and have aspect ratios 1.2 and Zimmerman wing planforms.Unsteady Navier-Stokes equations code is developed in order to study the three-dimensional unsteady field of micro air vehicle especially close to the wing surface. Numerically, static stall of thin Zimmerman wing is discussed and wing planform is found to be an important parameter to affect the maximal lift coefficient, stall angle of attack, and hysteresis of lift coefficient. Results show that present numerical method can capture wing's static stall and can comparatively accurately estimate lift coefficient near the stall. Inverse Zimmerman wing has higher maximal lift coefficient with no obvious hysteresis and stall delay of 4°angles of attack than elliptical wing, so it is a reasonable wing planform for micro air vehicle wing.