Meeting Abstract
Larval fish swimming differs from adult swimming in its kinematics, morphology and fluid mechanics. Compared with adults, larvae swim at lower Reynolds numbers and rely on drag-based mechanisms to generate most of their propulsive force. They use body undulations with a larger amplitude and frequency. They have a finfold rather than a pronounced tail fin. In this study, to develop a mechanical explanation that relates larval swimming morphology and kinematics to larval flow regime, we simulated forward cyclic swimming using a three-dimensional computational fluid dynamics approach that simulates free swimming by coupling hydrodynamics and body dynamics. Our three-dimensional, high-resolution map of the thrust and drag distribution on the body shows that thrust is generated not just at the tail tip, but along the entire body posterior to the centre of mass. By examining both horizontal and vertical cross-sections of the flow, we found that vertical flow elements contribute considerably to thrust production at Reynolds numbers in the 100 order of magnitude: the flow separates at the finfold and forms strong and complex vortex structures at the upper and lower edges of the finfold. Thrust generation is concentrated at these edges, indicating a causal relationship between edge vortex and thrust generation. Aside from finding that the finfold has a significant propulsive function in larval fish, this study shows show that larval fish generate a substantial proportion of their propulsive forces by flows that manifest in vertical rather than horizontal cross-sections. Hence, thrust estimates based solely on horizontal cross sections will underestimate total thrust production.
