以肥皂膜觀察單一振動圓柱之尾流研究

Abstract

肥皂膜水洞自從Couder(1984)提出運用後，因其具有架設簡單、彩色、且最接近二維流場的特性，故深受流力研究者喜愛，也有利於流體力學基本知識的推廣。本研究中，吾人建立了一台雷諾數可達300以上之水平連續流動式肥皂膜水洞，也再次證實流場中的三維不穩定現象(Mode A、Mode B)皆不存在，趨勢與Barkely and Henderson(1996)純二維流場模擬吻合。吾人在研究中並發現：在相同高雷諾數下之靜止圓柱尾流，在水平連續流動式肥皂膜水洞所觀察到的，與垂直流動式肥皂膜水洞中所觀測到的略有差異，此可能與在垂直肥皂膜水洞之流速遠較於水平者高有關。 此外，本研究亦在水平肥皂膜流場中，利用其顯影的便利性去觀察單一振動圓柱尾流隨著圓柱振動頻率與振動振幅改變時的變化。吾人發現在低雷諾數(Re=68)及大振幅下可發現文獻所認為不存在的雙偶渦漩(2P)之尾流模態；而此現象在振幅變小時會轉變為單-偶渦漩對(P+S)之尾流模態，此實驗結果與文獻、模擬比較大致吻合；另外，在觀察2S模態轉變為P+S模態的分界，也有發現其隨著雷諾數降低而有向上偏移的趨勢。實驗中也發現在相同雷諾數、振動頻率與振幅的條件下，不一樣之圓柱振動波型，會出現不一樣之尾流型態。

Since the soap film tunnel was applied by Couder in 1984, the researchers like to utilize the soap film tunnel in the study of fluid mechanics for the sake of its simple structure, colorful flow visualization and near to two-dimensional flow field characteristic In this research, the horizontal soap film tunnel which can reach the Re up to 300 was constructed, and the three-dimensional unsteady phenomena (Mode A, Mode B) do not exist in soap film tunnel. The results match well with the 2D simulation by Barkely and Henderson(1996).This research has also shown that the wake of a static cylinder in the horizontal soap film differs from in the vertical one because of the difference in velocity which may be the one reason. Furthermore, the convenience of soap film visualization was applied to observe the effectiveness of the wake by vibrating a single cylinder with various frequency and amplitude. In low Re(=68) and large amplitude, the results show that there has2P mode wake structure which has never been found in the literature, but the 2P mode changes into P+S mode wake structure while in small amplitude. These results almost match with papers and numerical simulations and the boundary between 2S mode and P+S mode shifts upward while the Reynolds number decreases was also found. Besides, the study also shows that there are different wake structures while vibrates the cylinder in different way with the same Re, vibrating frequency and amplitude.