ANDERSON, E.J.*; GROSENBAUGH, M.A.: The boundary layer and body wake of swimming squid

Flow in the boundary layer and wake of swimming squid (L. pealei) was visualized using combined PIV and PTV. The squid swam in a flume at 0.25 – 2 body lengths per second (BL/s) (i.e. 9 – 62 cm/s; body lengths were 30 – 38 cm). Velocity profiles of the boundary layer were used to determine skin friction, time dependent characteristics of the boundary layer and the effect of surface structures such as the mantle refill orifice and the jet. The wake in swimming squid was usually barely perceptible, suggesting a very stable boundary layer and revealing a very low pressure drag. Therefore the majority of the drag on swimming squid is friction drag and forces resulting from refill. The degree of boundary layer suction during refill was examined. This phenomenon, which has been suggested by previous investigators, may thin the boundary layer and prevent separation. It is during mantle refill that the squid is decelerating and therefore most susceptible to boundary layer separation. During jetting, the squid accelerates; acceleration tends to stabilize the boundary layer around a body. This elegant exchange of stabilizing mechanisms may, in addition to the streamlined shape of the squid, account for the remarkably small wake observed.