Please use this identifier to cite or link to this item: http://hdl.handle.net/11455/1584
標題: 均勻流場中旋轉圓柱尾流特性之探討
Investigation on the wake characteristics behind a rotating circular cylinder in a uniform stream
作者: 林威宇
Yu, Lin Wei
關鍵字: rotating circular cylinder
旋轉圓柱
vortex suppression
mean drag coefficient
渦旋抑制
平均阻力係數
出版社: 機械工程學系
摘要: 本文主旨在研究均勻入流中旋轉圓柱尾流之變化,並將針對圓柱尾流渦旋結構之形成及抑制效應、速度分布、相關性以及平均阻力係數的變化等主題作探討。實驗於循環式的水槽中進行,先以流場可視化對圓柱尾流隨轉速改變之流場結構的變化作定性的觀察,再使用FLDV與PIV系統在流場中選定適當位置區域中進行速度場之量測與分析。實驗中之自由流速為10cm/s,對應之雷諾數為1500,圓柱模型之展弦比為23,無因次轉速控制在 =0~3的範圍內。 結果顯示:隨著圓柱之無因次轉速增加,尾流渦漩結構的形成長度變短、尾流中心線平均上仰角度變大、剪力層不穩定波動波長變短或消失、週期性變差、圓柱面上加速區的分離點向下游移動,但減速區的分離點則提前發生等現象。平均阻力的分析中得到旋轉效應可使圓柱於流場中所受之阻力下降的結果。在圓柱上、下兩側之垂直截面上,則發現了沿流向之速度分佈呈現了過衝(overshoot)的現象,在最大過衝值之內、外側分別呈現不同的特徵頻率,分別代表的是圓柱旋轉頻率與圓柱尾流渦漩剝離頻率。而當 =1.5時,旋轉圓柱之尾流場則呈現尾流渦漩上下擺盪幅度較大之週期性剝離現象,此時上、下半截面所產生之渦旋結構則相互切斷,繼而完整脫離尾流而向後方逸流。由頻譜分析結果發現此現象是因為圓柱旋轉之頻率恰為圓柱尾流渦旋剝離頻率之兩倍產生之交互作用所致。 此外,由PIV的分析中發現:旋轉圓柱尾流場隨著轉速增加,渦漩結構會減弱與潰散;渦漩結構中心連線逐漸上仰;通過選定區域(X=2~4cm)之環流量隨轉速增加而減弱等現象。
This study investigates the wake characteristics behind the rotating circular cylinder in the uniform flow. The formation and the suppression of the wake behind the rotating circular cylinder, velocity distribution, auto-correlation, and the alteration of the mean drag coefficient will be emphasized. The experiments are performed in a recirculation water channel. Qualitative flow visualization and quantitative velocity measurements by FLDV and PIV systems are employed to study the details of the wake behind the rotating circular cylinder in a uniform inflow. The Reynolds number base on cylinder diameter is about 1500, the aspect ratio of the circular cylinder is 23, and the dimensionless rotational speed ranges from =0~3. It is found that as the rotational speed increases, the formation length of the wake vortex structure becomes shorter, and the average angle of the upward deflected wake centerline becomes larger. Furthermore the wavelength of shear layer instability becomes shorter, the periodicity of the wake structure becomes worse, and the separation point on the circular cylinder surface moves to upstream at the upper side surface and moves to downstream at the lower side surface. We also found the higher the rotating speed, the lower the drag coefficient. At the cross section near the separation location of the cylinder, the velocity distribution presents an “overshoot” within the near surface region. On both sides of the max overshoot location, there appears different characteristic frequencies. Namely, the frequency of the rotating circular cylinder within the near surface region and the shedding frequency in the region away from the cylinder, respectively. When equals 1.5, the wake behind the circular cylinder becomes organized and stronger. With the spectrum analysis, it is found that the rotating frequency of the circular cylinder is just twice as the shedding frequency at this rotational speed. Through the PIV technique, the vortex structure behind the circular cylinder can be studied via the instantaneous vorticity contour map. The vortex structure behind the rotating cylinder shows a destructive structure as the rotating speed increases. The circulation measured behind the cylinder at a local section (X=2~4cm) is found to decrease in the amplitude with the increasing rotational speed. Further, the periodicity of the wake become worse as the rotating speed increases.
URI: http://hdl.handle.net/11455/1584
Appears in Collections:機械工程學系所

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