The wake structure and thrust performance of a rigid low-aspect-ratio pitching panel

Abstract

Thrust performance and wake structure were investigated for a rigid rectangular panel pitching about its leading edge in a free stream. ForRe_C=O(10⁴), thrust coefficient was found to depend primarily on Strouhal numberStand the aspect ratio of the panelAR. Propulsive efficiency was sensitive to aspect ratio only forARless than 0.83; however, the magnitude of the peak efficiency of a given panel with variation in Strouhal number varied inversely with the amplitude to span ratioA/S, while the Strouhal number of optimum efficiency increased with increasingA/S. Peak efficiencies between 9% and 21% were measured. Wake structures corresponding to a subset of the thrust measurements were investigated using dye visualization and digital particle image velocimetry. In general, the wakes divided into two oblique jets; however, when operating at or near peak efficiency, the near wake in many cases represented a Kármán vortex street with the signs of the vortices reversed. The three-dimensional structure of the wakes was investigated in detail forAR= 0.54,A/S= 0.31 andRe_C= 640. Three distinct wake structures were observed with variation in Strouhal number. For approximately 0.20 <St< 0.25, the main constituent of the wake was a horseshoe vortex shed by the tips and trailing edge of the panel. Streamwise variation in the circulation of the streamwise horseshoe legs was consistent with a spanwise shear layer bridging them. ForSt> 0.25, a reorganization of some of the spanwise vorticity yielded a bifurcating wake formed by trains of vortex rings connected to the tips of the horseshoes. ForSt> 0.5, an additional structure formed from a perturbation of the streamwise leg which caused a spanwise expansion. The wake model paradigm established here is robust with variation in Reynolds number and is consistent with structures observed for a wide variety of unsteady flows. Movies are available with the online version of the paper.