Flow Structure and Unsteady Loading over a Pitching and Perching Low-Aspect-Ratio Wing

Abstract

This study addresses the flow structure and unsteady loading arising over a low-aspect-ratio rectangular wing subjected to high-amplitude pitching and perching maneuvers under low-Reynolds-number conditions of interest in small unmanned aerial vehicle operation and gust interactions. Simulations are performed employing an extensively-validated high-fidelity computational approach capable of accurately capturing the complex unsteady transitional flows. Canonical pitching motions for a non-translating wing in the presence of a constant freestream are first investigated in order to elucidate the effects of pitch rate and Reynolds number on the flow structure and aerodynamic loading. The wing is pitched about its quarter-chord axis to a maximum incidence of 45◦ with nominal non-dimensional angular rates 0.05 ≤ Ω ≤ 0.2. The Reynolds number based on the wing chord varied from 10 to 4 × 10. For the highest pitch rate, good agreement between the computed three-dimensional flow structure and recent experimental measurements is demonstrated. The 3D dynamic stall process is characterized by the formation of an initial spanwise-oriented leading-edge vortex which evolves into an arch-type structure with legs anchored on the wing surface. This dominant flow element of low-aspect-ratio wings exhibits a long residence time and establishes a well-defined low-pressure region and swirling pattern on the wing surface. Increasing pitch rate promotes an angular delay in the flow evolution and corresponding aerodynamic loads. Even for the lowest pitch rate, a significant increase in maximum lift is achieved relative to the static situation. The low-aspect-ratio of the wing also promotes further delay of dynamic stall and a much smoother behavior of the aerodynamic coefficients when compared to a pitching 2D wing section. The formation of the arch vortex and swirling pattern are found to be qualitatively similar over the range of Reynolds number studied. At fixed pitch rate, higher Reynolds number promotes a more compact arch vortex, a stronger suction on the wing and further stall delay. Canonical perching maneuvers involving simultaneous rotation and deceleration to rest are considered at two values of pitch rate. The reduction in translating velocity halts the development of the leading-edge vortex system which eventually moves upstream in front of the wing. Attempts to correlate the perching loads with the constant-freestream pitching cases indicate that agreement can only be achieved for the initial portion of the maneuver. Discrepancies arise at high angles of attack which depend on the value of pitch rate itself.