Unsteady flow phenomena in human undulatory swimming: a numerical approach.

The undulatory underwater sequence is one of the most important phases in competitive swimming. An understanding of the recurrent vortex dynamics around the human body and their generation could therefore be used to improve swimming techniques. In order to produce a dynamic model, we applied human joint kinematics to three-dimensional (3D) body scans of a female swimmer. The flow around this dynamic model was then calculated using computational fluid dynamics with the aid of moving 3D meshes. Evaluation of the numerical results delivered by the various motion cycles identified characteristic vortex structures for each of the cycles, which exhibited increasing intensity and drag influence. At maximum thrust, drag forces appear to be 12 times higher than those of a passive gliding swimmer. As far as we know, this is the first disclosure of vortex rings merging into vortex tubes in the wake after vortex recapturing. All unsteady structures were visualized using a modified Q-criterion also incorporated into our methods. At the very least, our approach is likely to be suited to further studies examining swimmers engaging in undulatory swimming during training or competition.

Three-dimensional flow patterns in the upper human airways.

Flow dynamics are studied for different ventilation conditions at a three-dimensional model of the human lung airways. The model is based on Horsfield and Weibel data and bifurcates down to the sixth generation. The flow is analyzed numerically and compared to experimental data received from exactly the same model. Numerical and experimental results agree well. Based on this agreement, flow behavior for conventional mechanical ventilation (CMV) as well as for high frequency oscillatory ventilation (HFOV) conditions can be analyzed. Velocity profiles as well as secondary flow structures are investigated during different phases of the unsteady flow. It is shown that the velocity profiles at peak inspiration and expiration are very similar for CMV and HFOV, probably due to too short branch lengths for the development of a frequency-dependent velocity profile. At the flow reversal times, characteristic zones of bidirectional mass flow emerge with increasing amplitude at higher frequencies. Furthermore, secondary flow structures are analyzed. This investigation reveals that the structures only depend on the local curvature and branch orientation, but are not influenced much by the nearby upper or lower branching generations.

Entraining in trout: a behavioural and hydrodynamic analysis.

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.