For clarification of an ecological system or application to ship propulsion mechanisms, studies of fish swimming have been conducted. However, many parts remain unexplained due to complexity and diversity of the swimming mechanism. In this study, we focused on the shape change of the fish larvae’s fin fold and investigated effects of aspect ratio of fish larvae’s fin fold on its propulsion performance by numerical simulations. Flow around the fish model was simulated by the regularized lattice Boltzmann method. To describe curved boundary on a Cartesian grid, the virtual flux method was applied. 4-tired multi block method was employed, and calculation domain was divided into 4 blocks with different grid resolutions. In this simulation, aspect ratio was defined by the ratio of fin fold length to fin fold width at the tail end, and fish models with different aspect ratio of fin fold were applied under constant surface area condition. As a result, vortices were generated along the fish body, and edge vortices pairs rotating in the opposite direction were observed at the fin fold. The edge vortices contributed to generate thrust force by increasing the front-back pressure difference at the fin fold. Compared to lower aspect ratio models, higher aspect ratio models generated more edge vortices pairs during a cycle. Accordingly, at lower aspect ratio, thrust force became larger by the wider fin fold, but amplitude of swimming speed changes in a cycle also became larger. In contrast, higher aspect ratio models swam efficiently by stabilizing its swimming speed.
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