Numerical simulation of propulsion mechanism of small aquatic creatures by ciliated swimming legs (Influence of clearance between cilia of single leg on propulsion force)

Abstract

The goal of this study is elucidation of propulsion mechanism of small creatures by ciliated swimming legs. For this purpose, a ciliated pereiopod of an opossum shrimp was introduced and it was modeled with a straight cylindrical stem with 10 much narrower cylindrical poles perpendicularly projecting on the stem. Flow drag acting on the pereiopod in a steady flow, which corresponds to the thrust force by paddling of the pereiopod, was computed with a variety of clearance between each adjacent pair of cilia d and the flow velocity U. It was found that the drag increased with the increase in d to be saturated with a certain value in the range of d ≥ 0.04 mm where influence of boundary layers of adjacent cilia is negligible, and the drag agreed well with that estimated from drag of a two-dimensional cylinder. This result suggests that the opossum shrimp is propelled with the drag-based swimming and the ciliated pereiopods supply greater thrust force than tabular pereiopods with the same projection area. It was also found that the drag decreased in the range of d > 0.07 mm in which some cilia come into the boundary layer of body of the shrimp. It was optically observed that distance between adjacent cilia is around 0.07 mm on the stem and cilia near the root of the pereiopod are shorter than those near the tip. The present results suggest that the ciliated pereiopod is well designed to efficiently generate the thrust force within a limited length and mass by preventing influence of the boundary layers.

Keywords

• opossum shrimp
• ciliated pereiopod
• swimming
• propulsion mechanism
• numerical simulation