TYTELL, E.D.*; LAUDER, G.V.; Harvard University: Kinematics and wake structure of steadily swimming eels (Anguilla rostrata)

Swimming kinematics and wake flow fields of steadily swimming eels (Anguilla rostrata) were characterized using high-speed video and high-resolution digital particle image velocimetry (DPIV) to examine three previous wake flow models: a global circulation model based on computational results, an unlinked vortex ring model observed for small eels, and a linked vortex ring model commonly seen in carangiform swimmers. Approximately 20cm long eels were filmed at 250Hz swimming steadily at a range of speeds on the bottom of a flow tank. The observed wakes contained strong laterally-directed flows, alternating in direction, separated by elongated vortex cores. They do not seem to contain the velocity deficit behind the tail or the global circulation predicted by previous computational models. Instead, the wake seems to be intermediate between the unlinked and linked vortex wake models, because sequential vortex rings in the wake are partially, but not completely, separated. We examine the relationship between the swimming kinematics and wake morphology and detail the production and evolution of the wake. Because the momentum of the wake is almost entirely directed laterally, we analyze it quantitatively to help explain how the swimming fish derives forward momentum from the laterally-directed jets in the wake. Finally, we compare anguilliform swimming kinematics and wake structure to previous studies of carangiform swimming to help describe the functional differences between the two swimming modes.