Evolution of wake structure with aspect ratio behind a thin pitching panel

The evolution of wake structures behind thin pitching panels at Reynolds number 1000 is studied numerically for a wide range of aspect ratios (0.54 <= AR <= 16). At the maximum pitching angle (theta (max)) of 5(degrees) and 15(degrees), simulations are performed for pitching frequency St = 0.5,1. The higher pitching angle and frequency promote thrust generation for all the AR. Although the propulsive power grows with AR in the thrust cases, the propulsion efficiency remains insensitive to the AR. Higher AR imparts stabilizing effect on the flow in the drag regime, though it triggers instabilities with asymmetric vortex pairing in the thrust cases. Wake topology for the drag cases reveals the emergence of horseshoe vortices and bridgelets-type vortex structures within the range of AR studied. The wake consists of entangled vortices undergoing subsequent reconnections in the thrust cases. A smaller theta(max) and St induces organized 3D structures for AR <= 4, although the end instabilities diffuse for increasing AR, resulting in a 2D wake signature at larger AR. The spanwise circulation data show the dominant influence of primary instability in rendering a high horizontal velocity-compressed separated region in the drag regime.