(Un)synchronized rowing: importance of phase lag in metachronal swimming performance

Metachronal swimming is a common method of drag-based aquatic locomotion in which a series of swimming appendages are stroked in an oscillatory pattern, such that the movement of each appendage is delayed in time relative to the neighboring appendage. It is often used by crustaceans and other ecologically important marine invertebrates. We developed a dynamically scaled robotic model for a comparative study of metachronal swimming under varying inter-appendage phase lag. Appendage motion profiles were obtained from published hovering and fast-forward swimming kinematics of Euphasia superba (Murphy et al., Mar. Biol., 158, 2011), but the phase lag between adjacent appendage pairs was varied. Swimming performance was characterized by the maximum swimming speed of the self-propelling model, as well as the forward force generated by the model when tethered. Results show that phase lags of 15% and 25% of cycle time, close to the phase lags reported for E. superba, result in the best forward swimming performance when compared to phase lags of 0, 35, and 50%. Additionally, time-resolved particle image velocimetry measurements show that interaction between shear layers of adjacent paddling appendages results in the formation of a continuous wake jet directed in the caudoventral direction, with jet angle and velocity characteristics dependent on the phase lag. Using the jet information found using the robotic model, we developed passive wake mimics to direct flow into a krill-like wake and used arrangements of these wake mimics to determine whether there were any hydrodynamic drag benefits associated with the schooling of metachronal swimmers, compared to freely swimming individuals.