Thrust performance of a flexible low-aspect-ratio pitching plate

We numerically investigate the hydrodynamic performance of an elastic plate of aspect ratio 0.54 and mass ratio 0.1 that pitches around its leading edge in a free stream at Reynolds number 640. It is found that for the rigid plate and the flexible plate with the first-mode deformation, the thrust coefficient nearly collapses onto the same curve when plotted against the Strouhal number that is defined using the tail excursion. Exceptions are found when the plate is overly flexible and higher deformation modes take place. On the other hand, the flexible plate has significantly higher power efficiency than the rigid plate at the same Strouhal number. Like the rigid plate, wake transition is observed for the flexible plate as the Strouhal number is varied. Specifically, at low Strouhal numbers (St < 0.28) the wake consists of a chain of interlocked horseshoe-shape vortices; at high Strouhal numbers (St > 0.28), the wake turns into two trains of closed vortex rings with hairpin legs.