SUMMARY
Rheophilic fish commonly experience unsteady flows and hydrodynamic perturbations. Instead of avoiding turbulent zones though, rheophilic fish often seek out these zones for station holding. A behaviour associated with station holding in running water is called entraining. We investigated the entraining behaviour of rainbow trout swimming in the wake of a D-shaped cylinder or sideways of a semi-infinite flat plate displaying a rounded leading edge. Entraining trout moved into specific positions close to and sideways of the submerged objects, where they often maintained their position without corrective body and/or fin motions. To identify the hydrodynamic mechanism of entraining, the flow characteristics around an artificial trout placed at the position preferred by entraining trout were analysed. Numerical simulations of the 3-D unsteady flow field were performed to obtain the unsteady pressure forces. Our results suggest that entraining trout minimise their energy expenditure during station holding by tilting their body into the mean flow direction at an angle, where the resulting lift force and wake suction force cancel out the drag. Small motions of the caudal and/or pectoral fins provide an efficient way to correct the angle, such that an equilibrium is even reached in case of unsteadiness imposed by the wake of an object.
Experiments on separation control using flexible self-adaptive hairy-flaps are presented herein. The wake-flow behind a circular cylinder is investigated without and with flexible hairy-flaps at the aft-part of the cylinder. Flow dynamics and hair motion were measured by particle image velocimetry and image processing in a range of Reynolds number 5000 < Re < 31 000. The experiments and POD analysis show, that the hairy-flaps alter the natural vortex separation cycle in such a way that the vortices do not shed in a zigzag like arrangement as in the classical von Kármán vortex street but in line in a row with the cylinder wake axis. Thus, the wake-deficit is largely reduced. Furthermore, flow fluctuations are considerably reduced about 42% in streamwise and 35% in transversal direction compared to the reference case without hairy-flaps, too. The condition for this mode change is the lock-in of the vortex shedding with a traveling wave running through the flexible hair bundles in transversal direction at the aft-part of the cylinder. As a consequence, the vortex shedding frequency is increased, the length of the separation bubble is decreased and drag force is decreased, too. The lock-in appears as a jump-like change of the shedding frequency and a jump in the Strouhal-Reynolds number diagram. However, when the characteristic length for the normalized frequency is chosen as the length of the separation bubble instead of the cylinder diameter, the Str-Re dependence is regular again. This hints on the relevance of the resonator model as proposed by Sigurdson and Roshko (1988) [16] on vortex shedding mechanism when boundary conditions are changed such as in our case, where the hairy-flap bundle imposes a flexible wall with visco-elastic coupling in transversal direction.
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