Effect of aspect ratio on the wake transition behind a thin pitching plate

Abstract

We study the influence of Aspect Ratio (AR) on three-dimensional wake transition past a thin pitching plate at Reynolds number of 1000 by performing computations for the range 0.54≤AR≤16 at pitching frequencies St=0.5,1 and maximum pitching angles θmax=5°,15°. For all AR, larger θmax and St promote thrust generation. However, higher AR imparts a stabilizing influence in the wake of the drag regime. For the ranges of AR, the drag-producing wake consisting of horseshoe vortices and bridgelets-type vortex structures, whereas twin-jet type bifurcated wakes with entangled vortices are observed for thrust-generating wakes. At higher AR, the wakes show a two-dimensional signature in the drag regime, whereas a spatial wake transition is observed in the thurst regime. The spanwise wake width shows the effect of wake compression for larger St even at θmax=5°, which is also substantiated by particle tracking showing wake compression for the thrust cases up to AR≤12. The near wake oscillations are prevalent for higher AR, although the core region remains unaffected by the aiding influence of spanwise instability for larger AR. The time average streamwise velocity for both drag and thrust regimes resembles an apparent feature of the reverse von Kármán vortex street.