| Summary: | 博士 === 國立清華大學 === 動力機械工程學系 === 100 === In this thesis, the hydrodynamics of a carangiform fish swimming with the pectoral fins abducted and adducted were investigated with three- and two- dimensional simulations, respectively. For Strouhal numbers in a range 0.1–0.8, the numerical results reveal a pair of pectoral-fin vortices is formed behind the abducted pectoral fins of a swimming fish. There exist hydrodynamic interactions between the pectoral-fin vortices and the undulating fish body. For Strouhal numbers in a range 0.2–0.8, the undulating fish body produces a locally high pressure in the region downstream of the pectoral-fin vortices. This downstream high-pressure region adversely suppresses the detachment of pectoral-fin vortices, resulting in vortices closely attached behind the abducted pectoral fins. In contrast, for Strouhal number = 0.1, the pectoral-fin vortices are shed from the pectoral fins and drift downstream. The low-pressure suction force arising from the shed pectoral-fin vortices facilitate lateral movements of the fish body, decreasing the power consumption. We regard this mechanism as significant to harvest energy from the shed pectoral-fin vortices.
We also propose a biohydrodynamic analogy between a fish swimming with the pectoral fins abducted and a fish swimming behind an upstream D-shaped obstacle. Through examination of the energy-saving mechanism pertaining to pectoral-fin vortices and that pertaining to a Kármán vortex street shed by an upstream D-shaped obstacle, we found that, as long as there exist environmental vortices, a fish can readily initiate energy-saving actions. Although the manners of operation of these energy-saving actions vary, the exploitation of environmental vortices is common in fish.
For slip numbers in a range 0.409–0.732, the proto vortex caused by the undulating fish body produces a low-pressure kernel attached to the tail. The low-pressure kernel is beneficial for the decreased energy expenditure because of facilitating the lateral movement of a fish body but still unfavorable in terms of the forward movement of a fish due to an increased form drag. For slip number = 0.409, the enlargement of the energy expenditure to maintain the quasi-steady swimming of a fish is greater than the decrease of the energy expenditure provided by low-pressure kernel, leading to a net increased energy expenditure of a fish. Although a decreased slip number results in an increased energy expenditure of a swimming fish, the maneuvering capability to execute longitudinal and lateral maneuvers is enhanced. This condition indicates that there exists a hydrodynamic compromise between the energy expenditure and maneuvering capability of a swimming fish.
A timing of a fish swimming with the pectoral fins adducted to execute maneuvers from a straight-line swimming state is investigated numerically. In one undulation cycle of the fish, the numerical results reveal there are four time instants preferable for a fish to execute the longitudinal maneuvers–two for the linear acceleration and two for the linear deceleration, whereas only two time instants for the sideway maneuvers–one for turning right and one for turning left.
The energy-saving mechanism and maneuvering capability revealed in this thesis provide a useful biomechanical foundation for the design of future biomimetic vehicles with a view to diminish power consumption and execute maneuver.
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