| Summary: | 碩士 === 國立中興大學 === 機械工程學系所 === 100 === This study, based on the control volume analysis, employs the simulation software (Fluent/Ansys) to analyze the flow structures and the corresponding efficiency in relation to the Strouhal number while the fin with different flexibility oscillates at different frequencies. This paper analyzes the relative proportion of the rate change of the momentum within the control volume, the pressure force acting on the surface and the net momentum flux through the control volume. The inflow velocity is 5 cm / s (Re = 1000), the angular displacement are ±10° and ±20°, respectively; and the oscillation frequency ranges from 0.3Hz to 1.5Hz.
It is found that within an oscillating cycle, the change of the momentum within control volume is zero; thus, the streamwise force mainly results from the pressure force on and the net momentum flux across the control volume. While the fin oscillates at low Strouhal number, the net momentum flux across the control volume is always negative, but the pressure force varies periodically with negative cyclic mean value, leading to a drag force on the fin. In such cases, the cyclic mean pressure forces outweigh the net momentum flux across the control volume. As the Strouhal number is greater than 0.3, the net momentum flux across the control volume gradually becomes positive. In such cases, the pressure force also varies periodically with a small negative cyclic mean value. Sum of these two forces results in a thrust force on the fin. At this range of Strouhal number, the net momentum flux across the control volume becomes comparable with and surpasses the cyclic mean pressure forces. At even higher Strouhal number, the net momentum flux across the control volume reduces slightly and becomes smaller than the cyclic mean pressure force.
At small oscillation amplitude, the propulsion efficiency of rigid fin (Fin A) is better than the flexible fins (Fin B and C). However, while oscillating at large amplitude and a Strouhal number 0.6, the propulsion efficiency of flexible fin (Fin B) is better than the rigid one. While the Strouhal number exceeds 0.6, the propulsion efficiency of rigid fin (A) is better than the flexible fins (B and C). For the most flexible fin C, the thrust efficiency at high oscillation frequency becomes poor because the fin is too flexible. For such a fin, the oscillating amplitude reduces at high oscillating frequency; thus the thrust efficiency of the most flexible fin C is the worst among the fins studied.
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