Vortex wakes of a flapping foil in a flowing soap film

We present an experimental study of an oscillating, symmetric foil in a vertically flowing soap film. By varying frequency and amplitude of the oscillation we explore and visualize a variety of wake structures, including von K´arm´an wake, reverse von K´arm´an wake, 2P wake, and 2P+2S wake. We characterize the transition from the von K´arm´an wake (drag) to the reverse von K´arm´an wake (thrust) and discuss the results in relation to fish swimming. We visualize the time evolution of the vortex shedding in detail, identify the origins of the vortices comprising the wake, and propose a simple model to account for the transition from von K´arm´an like wakes to more exotic wake structures.