Boundary layer control by a fish: Unsteady laminar boundary layers of rainbow trout swimming in turbulent flows.
The boundary layers of rainbow trout, Oncorhynchus mykiss [0.231±0.016 m total body length (L) (mean±s.d.); N=6], swimming at 1.6±0.09 L s-1(N=6) in an experimental flow channel (Reynolds number, Re=4×105) with medium turbulence (5.6% intensity) were examined using the particle image velocimetry technique. The tangential flow velocity distributions in the pectoral and pelvic surface regions (arc length from the rostrum, lx=71±8 mm, N=3, and lx=110±13 mm, N=4, respectively) were approximated by a laminar boundary layer model, the Falkner-Skan equation. The flow regime over the pectoral and pelvic surfaces was regarded as a laminar flow, which could create less skin-friction drag than would be the case with turbulent flow. Flow separation was postponed until vortex shedding occurred over the posterior surface (lx=163±22 mm, N=3). The ratio of the body-wave velocity to the swimming speed was in the order of 1.2. This was consistent with the condition of the boundary layer laminarization that had been confirmed earlier using a mechanical model. These findings suggest an energy-efficient swimming strategy for rainbow trout in a turbulent environment.
Department of Mechanical Engineering and Industrial Systems, Tampere University of Technology Korkeakoulunkatu 6, Tampere FI-33101, Finland kazutaka.yanase@gmail.com.
Kazutaka
http://orcid.org/0000-0003-4701-7320
K
Yanase
Department of Mechanical Engineering and Industrial Systems, Tampere University of Technology Korkeakoulunkatu 6, Tampere FI-33101, Finland.
Pentti
P
Saarenrinne
Drag reduction; Oncorhynchus mykiss; Particle image velocimetry; Swimming performance