ANDERSON, E.J.*; JIANG, H.; GROSENBAUGH, M.A.: Jet flow in swimming squid
Jet flow from live swimming squid (L. pealei) and boundary layer flow in the vicinity of the jet orifice were visualized using PIV and PTV. The squid were swum in a flume at 0.25 – 2 body lengths per second (BL/s) (i.e. 9 – 62 cm/s; BL = 30 – 38 cm). From these data, the fluid structure of the jet, vorticity, jet velocity and the contribution of the jet to the dynamic balance in squid locomotion were determined. Peak jet velocities in swimming squid were 30 – 120 cm/s. The ratio of jet velocity to swimming speed, uj/U, varied inversely with swimming speed, U, with the suggestion of a local minimum of close to 1 at about 0.7 BL/s. The inverse relationship reveals that hydrodynamic efficiency during jetting, in steady swimming, increases with swimming speed, as in propeller driven vehicles. Interestingly, 0.7 BL/s is the approximate cruising speed of the similarly sized squid T. pacificus during impressive migrations. The structure of the jet in steadily swimming L. pealei most resembled a weakly stable momentum jet. That is, a long central core of fast moving fluid surrounded by relatively undisturbed flow. The jet eventually went unstable leaving behind a long pocket of turbulent flow rather than rolling up into a coherent vortex ring. This finding is in agreement with jet volume output data taken by the first author, who demonstrated that L. pealei consistently eject a long, narrow plug of fluid–as much as 20 – 40 times as long as in diameter. Jet flows from pipes were also visualized using PIV in an attempt to replicate the flow structure observed in live squid. The ratio of jet velocity to the flume flow parallel to the pipe had a significant impact on the ultimate structure of the jet. The observed effects can be linked to the differences in vorticity in the boundary layers of the inner and outer walls of the pipe.
