Numerical Study of Finite Aspect Ratio Perching Wings

The perching maneuver is a complex motion involving fast change of angle of attack and wing deformation. The salient aerodynamic features can be observed in a simple pitch-up motion coupled with a streamwise deceleration. Parametric experimental study has been performed based on a wall-to-wall mounted flat plate to understand the role of pitch rate, streamwise deceleration, and the blockage effect at a Reynolds number of 20,000. In this paper we study the aerodynamics of finite aspect ratio wings at a low Reynolds number of 500. At this Reynolds number viscosity plays a more important role. The objectives of this paper are (1) to extend the perching wing study to a lower Reynolds number region in which small insect size MAVs operate, (2) to understand the impact of aspect ratio on the perching wing aerodynamics, and (3) to quantify the role of perch rate. The flow field is described by solving the incompressible Navier-Stokes equations on overlapping grids. The wing kinematics is represented by a series of transformation matrices. Comparisons are made between finite aspect ratio wings and 2D simulation. Pitch-up cases without streamwise deceleration are studied. Various reduced frequencies are tested. It was found that as the aspect ratio increases, both the lift curve slope and peak force increase, but the stall angle decreases. At the start of the pitch-up, force histories without streamwise deceleration are very similar; however, force histories deviate from each other when past stall angles. The formation of tip vortices was found to delay the leading edge vortex development. Increasing the pitch rate will delay the formation of tip vortices. Nomenclature