Numerical studies of hydrodynamics of fish locomotion and schooling by a vortex particle method.

摘要

Aquatic animals, particularly fishes, utilize efficient and effective swimming mechanics to gracefully propel themselves through the water with high performance. The bio-inspired mechanics of fish locomotion generally consist of the interaction of flexible structures with the surrounding fluids to generate propulsive forces. Moreover, fishes tend to swim in a group, and such fish schooling is known to provide hydrodynamic benefits to members of the school. These mechanisms have attracted the interest of scientists and engineers in recent years, particularly in the context of the development of energy-efficient autonomous under-water vehicles. Hence, accurate numerical simulation tools are needed to explore such problems in detail. In this dissertation, we apply a viscous vortex particle method (VVPM) to a two-dimensional fish-like profile and lateral schooling with two and three fish-like bodies, undergoing periodically undulatory motions. This VVPM is extended to continuously deforming bodies and validated on a circular cylinder with periodic deformations. By using this method, we explore the hydrodynamics of fish-like locomotion in both tethered and free-swimming form. We also investigate two and three tethered periodically undulating fish-like shapes in the fluid by changing the separation distance and phase difference between these designed 'fishes' (For simplicity, we still call fish). It is shown that, in mirroring symmetry, the fish in the pair augment each other's thrust even at relatively large separations (up to ten body lengths). At small distances, this augmentation is primarily brought about by a peristaltic pumping in the gap between the fish, whereas at larger distances, the thrust is affected by subtle changes in the vortex shedding at the tail due to the interactions with the other fish. The vorticity shedding pattern has been modified due to the presence of the other fish and the interactions between these fishes, which would have an important influence on the force exerted on the tail. In cases without symmetric undulation, one fish always draws more benefit from the interaction than the other. Furthermore, lateral configurations with three fishes are studied with mirroring symmetry between the neighboring fish. Whereas the center fish draws a net thrust benefit, this comes at the expense of a net drag on the outer two fishes. Each adjacent pair in this arrangement is slightly affected by the presence of the third fish. Finally, a free-swimming fish is explored in this dissertation. The vorticity shedding pattern is nearly matched close to the body between the tethered fish and the free-swimming fish, however, notable differences are found in the wake.