![]() Yanase and Saarenrinne (2015) observed that the velocity distribution within the boundary layers of rainbow trout swimming at 1.0 L s −1 oscillated above and below the classical logarithmic law of the wall for a turbulent boundary layer with body motion ( von Kármán, 1930). Thus, PIV offers significant advantages for the direct determination of the surface-normal gradient of longitudinal velocity in a highly heterogeneous flow field. PIV is an optical method of flow visualization that is based on the statistical correlation of small interrogation areas with high particle density. This was the first time that the boundary layers of a swimming fish were measured using the particle image velocimetry (PIV) technique. Yanase and Saarenrinne (2015) have successfully measured the boundary layers of Oncorhynchus mykiss, a subcarangiform swimmer, swimming at a low swimming speed of 1.0 L s −1 in a controlled experimental flow channel. Kunze and Brücker (2011) demonstrated that the tangential flow-velocity ( u) distribution within the boundary layer over the surface of a fish-like moving plate with increasing amplitude downstream oscillated between a laminar and a turbulent flow profile throughout the cycle of undulatory motion at that c/ U ratio. ![]() At the c/ U ratio of 1.2, the net power input that is required to counteract incoming flow was found to be at its minimum. (2003) confirmed these results numerically for turbulent flow over the surface of a smooth wavy wall that was undergoing transverse motion in the form of a stream-wise travelling wave with constant amplitude. BOUNDARY LAYER FREEThey demonstrated that the boundary layer over the surface of a motor-driven undulatory plate laminarized at the wave-crest when the ratio of the velocity ( c) of the wave travelling downstream to the free stream velocity ( U) was 1.2. Therefore, there is still a lack of quantitative evidence to support the boundary layer laminarization over the undulatory fish surface.Īn experiment using a mechanical model that emulated the undulatory motion of an aquatic animal had been performed previously by Taneda and Tomonari (1974). In this regard, however, the surface motion phase-related characteristics of the unsteady boundary layer are not fully documented in Anderson et al. The experiments revealed the oscillation between the laminar and turbulent boundary layers, which was in agreement with such known boundary layer models as the Blasius or Falkner−Skan equation for the laminar boundary layer, and the law of the wall for the turbulent boundary layer, respectively. (2001) investigated the boundary layer over the body surfaces of two marine species, the scup ( Stenotomus chrysops a carangiform swimmer) and the smooth dogfish ( Mustelus canis an anguilliform swimmer). Experiments conducted on swimming fish by Anderson et al. (2001) and the authors' previous measurements ( Yanase and Saarenrinne, 2015). Despite the critical roles of the boundary layer in swimming hydrodynamics and lateral line flow sensing in fishes, the boundary layer structure of a swimming fish has rarely been studied experimentally. Furthermore, the boundary layer over the surface of a fish's body plays a major role in determining the signals detected by a lateral line mechanoreceptor (reviewed by McHenry et al., 2008). Therefore, the boundary layer is an important component of the hydrodynamics of fish swimming. ![]() BOUNDARY LAYER SKINSkin frictional drag, which is the dominant factor in the total drag of a swimming fish, is created in the boundary layer due to the viscosity of the fluid ( Webb, 1975). This spatial gradient in flow is known as the boundary layer. ![]() The viscosity of water causes the flow close to the surface of any biotic or abiotic object to move more slowly ( Schlichting, 1979). These findings suggest an energy-efficient swimming strategy for rainbow trout in a turbulent environment. This was consistent with the condition of the boundary layer laminarization that had been confirmed earlier using a mechanical model. The ratio of the body-wave velocity to the swimming speed was in the order of 1.2. Flow separation was postponed until vortex shedding occurred over the posterior surface ( l x=163☒2 mm, N=3). 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. The tangential flow velocity distributions in the pectoral and pelvic surface regions (arc length from the rostrum, l x=71☘ mm, N=3, and l x=110☑3 mm, N=4, respectively) were approximated by a laminar boundary layer model, the Falkner−Skan equation. The boundary layers of rainbow trout, Oncorhynchus mykiss, swimming at 1.6☐.09 L s −1 ( N=6) in an experimental flow channel (Reynolds number, Re=4×10 5) with medium turbulence (5.6% intensity) were examined using the particle image velocimetry technique. ![]()
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