When humans kick swim through water, vortices form around their legs, generating the force that propels them forward. However, the mechanisms underlying variations in the structure of these vortices with swimming speed remain unclear.

In a new study published in Experiments in Fluids, researchers analyzed swimmer movement using an optical motion capture system and investigated vortex structure changes with varying speeds. They employed particle image velocimetry to visualize water flow dynamics.

Their results revealed that during underwater undulatory swimming, the vortex structure in the down-kick-to-up-kick transition phase changed as swimming speed increased. Specifically, with rising swimming speed, the direction of the jet flow between the two vortices around the foot shifted to a more vertically downward orientation, a shift hypothesized to enhance forward propulsion during up-kicking.