Flapping foil in a hydrodynamic tunnel: asymmetric vortex shedding

The flapping foil is a simple model that contains key dynamical elements, such as the creation and organization of vorticity, that are crucial in every system involving flapping wings or fins as a means of producing propulsive forces. The flow structures in the wake can be visualized as we show here by injecting dye filaments upstream of the foil. Two cases are shown at Reynolds number Re=1035 and Strouhal number St=fA/U=0.4 (f and A being, respectively, the frequency and amplitude of the flapping motion and U the upstream velocity in the hydrodynamic tunnel). First a rigid foil, where the vortex shedding in the wake is asymmetric, and afterwards a flexible foil made of a silicone elastomer (poly-vinyl siloxane), which is shown to suppress the symmetry breaking of the vortex street. For details on the physics behind these observations see: C. Marais; B Thiria; Wesfreid, J. E. & R. Godoy-Diana. Stabilizing effect of flexibility in the wake of a flapping foil. Journal of Fluid Mechanics, 710 : 659-669 (2012). http://dx.doi.org/10.1017/jfm.2012.390 https://blog.espci.fr/ramiro/ https://www.pmmh.espci.fr/Biomimetics-and-Fluid-Structure-Interaction