Unsteady flow structure and loading of a pitching low-aspect-ratio wing

This study addresses the flow structure and unsteady loading arising over a pitching low-aspect-ratio rectangular wing under low-Reynolds-number conditions of interest in small unmanned aerial vehicle operation and gust interactions. Simulations are performed employing a high-fidelity computational approach capable of accurately capturing the complex unsteady transitional flows. The wing is pitched about its quarter-chord axis to a maximum incidence of 45 degrees over time intervals ranging from four to 16 convective time scales. The Reynolds number based on the wing chord varied from 10(3) to 4 x 10(4). For the highest pitch rate, good agreement between the computed three-dimensional (3D) flow structure and recent experimental measurements is demonstrated. The 3D dynamic stall process is characterized by the formation of an initially spanwise-oriented leading-edge vortex which evolves into an arch-type structure with legs anchored to the wing surface. The normal vorticity in the arch vortex legs establishes a low-pressure region and swirling pattern on the wing surface. A distinct characteristic of the arch vortex is its upstream propagation and persistence over the wing, postulated to be the result of the self-induced velocity of the vortex and its image underneath the plate. Increasing either pitch rate or Reynolds number promotes a more coherent arch vortex and circulation pattern, and delays the onset of stall to a higher angle of attack. Even for the lowest pitch rate considered, a significant increase in maximum lift is achieved relative to the static situation.