Vortex wakes of a flapping foil in a viscous uniform flow

Abstract

We perform numerical simulations for two types of rigid and flexible thin foils in a viscous uniform flow to explore the effects of the flexibility on wakes. The thin foils are prescribed by the heaving motions and the relevant non-dimensional parameters are the chord length based Strouhal number and heaving amplitude. When the dynamical features of the wakes are varied with respect to the two parameters, it is possible to make a direct comparison between a both foils. We find three transition boundaries in the wake maps of the thin foils: the transition from a von Kármán to a reverse von Kármán wake, transition from reverse von Kármán to asymmetric reverse von Kármán wake, and a further transition from asymmetric reverse von Kármán to complex wake. The wake transition boundaries of the rigid thin foil are predicted by constant amplitude based Strouhal number(StA) lines, consistent with previous studies. However, contrary to the observation from the rigid thin foil, the wake transition boundaries of the flexible thin foil are not predictable by constant StA lines.