Local oscillation of the boundary layer over the surface of swimming rainbow trout: implications for active drag reduction

The boundary layers of swimming rainbow trout, Oncorhynchus mykiss, were measured by the Particle Image Velocimetry (PIV) technique at a Reynolds number of 4×105. The flow velocity profile over the fish surface in a particular phase of undulatory motion had characteristics that were similar to the empirically-observed limit to drag reduction that was caused by adding high molecular weight polymers to wall-bounded turbulent flows, the so-called
Virk’s asymptote. The rms velocity fluctuation in the stream-wise component (푢rms+ ) in the reduced-drag flow regime increased with increasing drag reduction as consistently observed in the polymer-induced drag reducing flow. The distribution of the Reynolds stress suggested that turbulence makes a major contribution to the skin friction of rainbow trout that were swimming at the observed swimming speed (1 body-length s−1). The distribution of the turbulent kinetic energy (TKE) and the rate of TKE dissipation suggested that the fish surface that is moving in the direction of the free-stream flow can facilitate to retain the dilution rate of the mucus concentration at a certain level within the
boundary layer. This causes more TKE to be gained from large scale eddies than can be dissipated into smaller scales.