MULLER, UK: The interaction of body- and tail-generated flows controls the performance of undulatory swimmers

Fish reach excellent swimming efficiencies, -manoeuvrability and -speeds by running a transverse wave down their body to propel themselves. Undulatory swimmers can control thrust production by modulating their body wave, in particular the speed with which this wave travels down their body. As the slip (the ratio of forward swimming speed to body wave speed) approaches unity, thrust drops to zero. At a slip above unity, the fish is generating negative thrust. The wake behind the fish should change accordingly from a reverse Karman vortex street at a slip below unity to a Karman vortex street at a slip above unity. Mediofrontal cross-sections through the wake behind swimming mullets Chelon labrosus confirm these qualitative expectations. The generation of the wake is more complex than the predictions, however, as body wake and tail wake interact to form the total wake behind the swimming fish. The body generates a flow field containing weak semicircular flow regions that are ultimately shed as the body wake. This body wake interacts with the wake generated by the oscillating tail. The pattern of interaction between body and tail wake depends on the body wave. Observations over a range of slip values indicate that the fish controls the phase between body flows and tail flows to modify its thrust and its wake.