ANDERSON, E.J.*; MCGILLIS, W.R.; GROSENBAUGH, M.A.: Boundary Layer Flow in Fish and Squid

A complete hydrodynamic understanding of aquatic locomotion requires knowledge of the thin boundary layer surrounding swimming organisms. In particular, tangential forces acting on the body surface are dependent on the distribution of velocity throughout the boundary layer. Fluid velocities tangent and normal to the bodies of swimming fish and squid were determined throughout the boundary layer (1-10mm thick) by digital particle image velocimetry (DPIV) and digital particle tracking velocimetry (DPTV). Scup (Stenotomus chrysops), smooth dogfish (Mustelus canis), mottled sculpin (Cottus bairdi) and long-finned squid (Loligo pealei) were observed. The friction drag on swimming fish was determined to be greater than the rigid body friction drag. No separation of flow was observed and the shape of boundary layer velocity profiles suggested that the fish were sensing and responding to the surrounding fluid to prevent separation. Both laminar and turbulent flow were observed in the boundary layer under various conditions, and flow sometimes oscillated between the two states with body phase. The behavior of the boundary layers of the carangiform swimming scup and the anguilliform swimming smooth dogfish showed marked differences. Boundary layer flow in mottled sculpin, which spend most of their time stationary on the substrate, was shown to be greatly impacted by the orientation of the pectoral fins. The findings support the existence of different sensory functions in the lateral line system anterior and posterior to the pectoral fins as suggested by structural differences. In L. pealei, the boundary layer yielded estimates of friction drag, and the effect of boundary layer suction at the mantle intake was observed.