Please use this identifier to cite or link to this item: http://hdl.handle.net/11455/2865
標題: 撓性尾鰭下游流場結構與推力之數值分析
Simulations on Flow Characteristics and Thrust of Flexible Oscillating Fin
作者: 李國瑋
Li, Kuo-Wei
關鍵字: 可撓性魚尾鰭;flexible fin;硬式魚尾鰭;推進效率;逆卡門渦列;控制體積;動態網格;自定義函數(UDF);rigid fin;propulsive efficiency;reverse Karman vortex;control volume analysis;User Defined Function (UDF)
出版社: 機械工程學系所
引用: 1.J.M.Miao,M.H.Ho.(2006).Effect of flexure on aerodynamic propulsive efficiency of flapping flexible airfoil.Journal of Fluids and Structures,Vol.22,pp.401-419. 2.M.Sfakiotakis,D.M.Lane,J.B.C.Davis.(1999).Review of fish swimming modes for aquatic locomotion.IEEE Journal of Oceanic Engineering,Vol.24(2),pp.237-252 3.Tuyen Quang Le,Jin Hwan. (2010).Effect of Chord Flexure on aerodynamic Performance of a Flapping Wing.Journal of bionic Engineering,Vol.7,pp.87-94. 4.W.B.Tay,K.B.Lim.(2010).Numerical analysis of active chordwise flexibility on the performance of non-symmetrical flapping airfoils.Journal of Fluids and Structures,Vol.26,pp.74-91. 5.M.A.Ashraf,J.Young,J.C.S.Lai.(2011).Reynolds number,thickness and camber effects on flapping airfoil propulsion. Journal of Fluids and Structures,Vol.27,pp.145-160. 6.Oleksandr Barannyk,Bradley J.Buckham,Peter Oshkai.(2012).On performance of an oscillating plate underwater propulsion system with variable chordwise flexibility at different depths of submergence.Journal of Fluids and Structures,Vol.28,pp.152-166. 7.S.Heathcote,I.Gursul.(2007).Jet switching phenomenon for a periodically plunging airfoil.Physics of Fluid, Vol.19,027104-1~027104-12. 8.S.Heathcote,I.Gursul.(2008).Effect of spanwise flexibility on flapping wing propulsion.Journal of Fluids and Structures,Vol.24,pp.183-199. 9.K.D.von Ellenrieder,S.Pothos.(2008).PIV measurements of the asymmetric wake of a two dimensional heaving hydrofoil.Experiments in Fluid,Vol.44,No.5,pp.733-745. 10.謝偉麟.(2010).撓性尾鰭下游之流場結構與推力之實驗探討.國立中興大學機械工程學系碩士論文.
摘要: 
本文以控制體積的觀點利用流體力學(CFD)分析軟體探討具有不同撓性之尾鰭以不同擺動頻率游動時,對應之流場結構特性以及推進效率。本文利用數值模擬輸出所需的各項數據計算作用於控制體積入、出口壓力差、經由控制表面的動量流出率以及在控制體積內的動量變化率。入流速度為5 cm/s(即Re=1000)、擺動角度為±10°和±20°,尾鰭擺動頻率介於0.3至1.5Hz時,分析各項受力以及對應之流場結構特性,探討可撓性與非可撓性尾鰭推進效率。
研究結果顯示:在魚尾鰭擺動一週期內,不論撓性尾鰭或是非撓性尾鰭,控制體積動量變化率的總合力幾乎為零,因此在週期內作用於控制體積上受力主要來源有兩大項,亦即控制體積入、出口壓力差和控制表面的動量流出率等。當魚尾擺動產生阻力時,控制體積入、出口壓力差所產生之阻力占總合力之比例較進出控制表面的動量淨流出率高出甚多,因此在低史卓荷數時,流場對魚尾鰭的阻力主要源自控制體積入、出口壓力差。當史卓荷數大於0.3時,不同撓性魚尾鰭都產生正向推力,此時控制表面的動量淨流出率均為正值,且和控制體積入、出口壓力差所產生的推力約占總合力各50%。當達到最高推進效率時,控制表面的動量淨流出之貢獻甚至超出控制體積入、出口壓力差所產生之推力。當史卓荷數持續提高時,動量淨流出率的貢獻則略低於入、出口壓力差。
在低擺動角度時,非可撓性魚尾鰭(Fin A)推進效率比可撓性魚尾鰭(Fin B)好;反之,在高擺動角度且史卓荷數小於0.6時,可撓性魚尾鰭推進效率比非可撓性魚尾鰭佳,而在史卓荷數大於0.6則是非可撓性魚尾鰭較佳。對於更可撓性魚尾鰭(Fin C)而言,因擺動頻率的增高,造成魚尾鰭尾端振幅減少,導致對流體之推力減少,推進表現不如其他的撓性魚尾鰭。

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.
URI: http://hdl.handle.net/11455/2865
其他識別: U0005-2708201217400500
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