碩士 / 國立中興大學 / 機械工程學系所 / 102 / This study investigates by CFD to realize the key parameters and related mechanisms that affect the propulsive performance while the flapping fin undergoes the combined pitching-heaving motion with a phase angle of 90 degree. The maximum pitching angle (θmax) and heaving amplitude (hmax) are and 0.4c, respectively. By changing the inflow velocity, the flapping frequency and modifying the heaving motion, the flow characteristics, the thrust forces and the propulsive efficiency are evaluated. Further, the control volume analysis evaluates the cyclic variations of the thrust force, the pressure force difference and the net momentum efflux. Some results are summarized as follows.
As the Strouhal number increases or the Reynolds number decreases for both the rigid and flexible fins, the downstream flow structures can be classified into wavy type, Tai-Chi shape type, and reversed K?rm?n vortex street. As the inflow speed decreases, the averaged input power and thrust force for the rigid fin are greater than those of the flexible fin. For the rigid fin, the maximum propulsive efficiency is 51.76% at Re =40000 , and the pressure force difference contributes 55.65% to the total thrust force. For the flexible fin, the wavelength of the vortex patterns is elongated; the vorticity and circulation, and the leading edge vortex are enhanced as the Reynolds number decreases. Moreover, the flexible flapping fin can harvest energy from the surrounding fluid, saving 16.7~27.3% of the total input power. Under this condition, the pressure difference contributes 64.3% to the total thrust force and attains the maximum propulsive efficiency 52.5% at Re =40000 . As the flapping frequency increases, the total thrust and the input power coefficients also increase as a power function of the Strouhal number. The optimum propulsive efficiency is 43.18% at St=0.27, and 53% of the average thrust force is contributed by the pressure force difference. As the effective angle of attack is modified by harmonic functions, the average total thrust force and the propulsive efficiency are enhanced and the Tai-Chi shape vortex can be transformed into the reversed K?rm?n vortex as well. The control volume analysis finds that the cyclic variation of total thrust force synchronizes with that of the pressure force difference. The cyclic variations of net momentum efflux are always positive with a phase shift of π/5 relative to the pressure force difference.
| Identifer | oai:union.ndltd.org:TW/102NCHU5311040 |
| Date | January 2014 |
| Creators | Kai-An Lee, 李鎧安 |
| Contributors | 郭正雄 |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | zh-TW |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 100 |
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