Vortex wakes of a apping foil
Teis Schnipper
Fluid•DTU
Department of Physics
Abstract:
Periodic vortex wakes are found many places in nature. Examples include the well known von Kármán vortex wake behind cylindrical structures in a free stream and the thrust-indicating inverted von Kármán wake behind swimming fish. This talk presents an experimental investigation of the vortex wakes behind a simple model fish, i.e. a rigid fin flapping in a flowing soap film. By adjusting frequency and amplitude of the pitching oscillation we map out a botany of impressive vortex wakes, including the von Kármán vortex wake as well as novel wakes with up to sixteen vortices shed each flapping period. Using high-speed video, we identify the vortex origins of the 2P wake (where 2 vortex pairs are shed each period) and show the importance of the timing of vortex shedding. Finally, we propose a simple model for the vorticity generation and discuss it in relation to certain wake transitions in the map.