MULLER, UK; LIU, H; VAN LEEUWEN, JL; Wageningen University; Chiba University; Wageningen University: Fluid dynamics of cyclic swimming in larval fish

During cyclic swimming, zebrafish larvae reach swimming speeds of up to 50 body lengths per second (0.2 m/s), beating their tail up to 100 times per second. We studied the swimming kinematics and the flow fields of swimming zebrafish larvae (2 to 5 days post-fertilisation) using 2d high-speed flow visualisation. This technique yields plane slices through the larva�s 3d flow field. The flow adjacent to the body shows the following features. The head acts like a source, pushing water away from itself. Along the head and anterior body are areas of elevated vorticity; vorticity is maximal roughly half a body width away from the body, consistent with a thick boundary layer building up around the anterior body. Along the posterior body, vorticity peaks closer to the body, and the flow speeds adjacent to the body match the speed of the body, reaching up to 0.3 m/s. The vorticity peaks are stronger on the concave side of the body wave, and their rotational sense alternates along the body. This chain of elevated vorticity is shed at the tail and forms a wake of laterally moving vortex pairs. We observed no visible backward flow in the wake, suggesting that the larva generates no net thrust. Vertical slices through the flow field revealed that � except for the source effect of the head � flow is slow across the top and bottom of the larva. The observed swimming kinematics served as input to model the flow field of cyclically swimming zebrafish larvae using computational fluid dynamics (CFD). The CFD flow fields resemble the experimental flow fields. From the CFD model, we obtained viscous and pressure force coefficients, which suggest that during cyclic swimming, the viscous drag coefficient oscillates around 0.18 with an amplitude of ca. 0.08; the net thrust coefficient oscillates around zero with an amplitude ca 0.05.