Flow-field analysis of the underwater flutter kick using particle image velocimetry: Comparison with the dolphin kick

The purpose of this study was to elucidate the propulsion mechanism of an underwater flutter kick (UFK) by comparing the flow fields generated during an underwater dolphin kick (UDK) and a UFK. One male competitive swimmer performed approximately 40 trials of each kicking technique in a recirculating water flume under a uniform inflow velocity of 1.0 m/s. The flow field behind the swimmer's feet was measured using particle image velocimetry, and the data were temporally and spatially phase-averaged to construct a quasi-three-dimensional representation that made it possible to visualize three-dimensional vortex structures. The results show that, similar to the UDK, the UFK produced three-dimensional vortices that contributed to forward acceleration during the kick. In addition, although vertical flows generated near the left and right feet tended to offset each other because of leg crossing, a stronger net downward vertical flow was observed in the UFK. Analysis of the frontal-plane flow field further revealed that the UFK generates asymmetric vortex structures that redistribute momentum laterally, thereby producing rolling and yaw moments acting on the body. These findings indicate that the UFK generates a distinct pattern of propulsion and stabilization, in which asymmetric vortex structures, arising from the kicking motion, contribute to both thrust generation and body moment control.