Studies On The Hydrodynamic Mechanism Of Fish-like Swimming With Immersed Boundary Methods

Fish swimming in nature has many advantages, which include high efficient propul-sion, high speed swimming, fast maneuvering and low noise. Studies on the mechanism of fish propulsion are always the highlights in fluid mechanics. The flow environments of swimming fishes are complex where many vortices usually exist. Fishes can perceive the fluctuations of the surrounding flow by their own lateral line systems. According to these fluctuations, fishes can adjust their swimming mode to reduce the energy consumption and improve their propulsion efficiency. Based on the general flapping foil (wing) and wavy foil (wing) models, vortex flow fields can be created by placing a D-section cylinder afront of these fish-swimming models. In this dissertation, the corresponding complex flow fields of swimming fish were simulated by the modified immersed-boundary methods. This dis-sertation mainly focused on the effects of the complex environments on the propulsion behavior of swimming fishes, and some hydrodynamic mechanisms were revealed.Focus on the flapping foil with coupled heave and pitch motion, the effects of the pitch amplitude, flapping frequency, the Strouhal number St and the phase difference be-tween heave and pitch were mainly investigated. Results show that, among the parameter range in this dissertation, when the pitch amplitude equal to 30°, the thrust of the flapping foil become maximum. With the increasing of the St number, the thrust increases, and the propulsion efficiency become maximum when the St number ranges in 0.25 and 0.35.Three modes (which are expanding wake, destructive interaction and constructive in-teraction) of the interaction between the oscillation D-section cylinder and flapping foil observed in the experiments were also implemented in this dissertation by numerical sim-ulation. Based on the hydrodynamic analysis, it can be concluded that the flapping foil's thrust depends on both the interaction between the foil leading edge and incoming vortices and the vortex formation in the wake. With different phase angles between two tandem flapping foils, there are three different patterns for the vortex formation:the leading edge induced mode, trailing edge paired mode and vortex diffusion mode. Thrust of the flap-ping foil in downstream is related to the vortex formations. Its thrust reaches maximum in leading edge induced mode, and minimum in vortex diffusion mode.For the D-section cylinder and wavy foil with tandem arrangement, effects of the dis-tance between the D-section cylinder and the wavy foil, the phase angle of the undulating, the phase angle of the heave motion and the Strouhal number on the foil's propulsion per-formance were investigated systematically. According to the flow structures and the hydro-dynamics behaviors of the wavy foil, the wake field of the D-section cylinder can be divided into three domains:suction domain, thrust enhancing domain and weak influence domain. The undulation of the foil can inhibit the roll-up instability of the shear layers and vortex shedding from the cylinder and consequently significantly enlarge the suction domain. The undulation with a small frequency plays a negative role in the foil propulsion when the foil is located near the cylinder largely in the suction domain and a positive role when the foil in the vortex street. Both the thrust and the propulsion efficiency can be increased signif-icantly when the foil contact with the incoming vortices and has a strong interaction with them. However, the foil bypassing the vortices undergoes both minimum thrust and input power.The linear Euler-Bernoulli beam theory was employed for the study on the interac-tions between shed vortices and a flexible plate. The Reynolds number plays a crucial role in the oscillation behavior of the plate. When the Reynolds number is low, the viscous effect dominates the flow motion, and the initial perturbation of the plate is inhibited. When the Reynolds number is larger than a critical number, the plate begins to oscillate. The higher Reynolds number the larger oscillation amplitude. According to different parameter selec-tions, the surrounding flow fields of the flexible plate can be classified into two modes: 'attached vortex mode' and 'Karman vortex street mode'. Compared with the'attached vortex mode', flexible plate with the'Karman vortex street mode'can extract more energy from the surrounding flow.The effects of the aspect ratios of the flapping wing on its propulsion performance were investigated. In the wake of three-dimensional flapping wing, the tip vortices shed by merging with the leading-edge vortices and the trailing-edge vortices. When the aspect ratio is small, the shed vortices appear as loops. When the aspect ratio is larger, the shed vortices are elongated and appear as arcs. Moreover, both the thrust and propulsion efficiency of the wing also increase with the increasing of the aspect ratio.