Vortical dynamics in the wake of water strider locomotion |
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Authors: | Akira Rinoshika |
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Institution: | (1) Department of Mechanical Systems Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa Yamagata, 992-8510, Japan |
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Abstract: | Abstract Previous studies reported that the hydrodynamic propulsion of the water strider also results from transferring momentum to
the underlying fluid through hemispherical dipolar vortices shed by its driving legs. However, there are no accuracy experimental
measurements of these vortical structures to prove the mechanics of vortical propulsion. Here, we reveal the vortical structures
by reporting the simultaneous measurements of the water strider’s motion and the fluid velocity field with the high-speed
PIV, and proposing a new method of calculating the vortex kinetic energy and vortex momentum. We found that the asymmetrical
vortical structure in each dipolar vortex, generated by one driving stroke, propels the water strider forward, and the outer
elliptic vortex is weaker than the inner circular vortex. The movement of the dipolar vortex is divided into two stages: (1)
translating backward and (2) return curving. In this way, the water strider obtains the maximum velocity with minimal consumption
of energy. The fluid vortical momentum, generated by the driving stroke, accounts for about 64–90% of the water strider’s
momentum. |
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Keywords: | |
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