Please use this identifier to cite or link to this item: http://hdl.handle.net/11455/2751
標題: 圓柱對空穴中振盪流場影響的研究
Experimental Investigation on Oscillation Characteristics Within Cavity in the presence of a Circular Cylinder
作者: 林耀宗
Lin, Yaw-Tzung
關鍵字: cylinder wake;圓柱尾流;cavity shear layer;feedback;self-sustaining oscillation;空穴剪力層;迴流;自激式振盪
出版社: 機械工程學系
摘要: 
本研究主要是以實驗方法探討圓柱對空穴中分離剪力層不穩定之特性的影響,期能了解並控制空穴中因剪力層不穩定所造成的振盪特性。本研究是在循環式閉迴路水槽中進行,流場可視化是以螢光染液釋放法進行,以定性的方法了解圓柱對空穴中剪力層的影響,並輔以雷射測速儀系統進行定量速度場的量測。本實驗模型參數變化如下:空穴長深比L/D=2.0,圓柱之直徑為d/D=0.33~0.8,分別置於XL.E/L=0~3.0且圓柱底部和空穴間之垂直距離變化範圍為H/D=0.4~4.0。流體之自由流速為U∞=20cm/s;雷諾數為Re=U∞d/ν=4000~1640。
由實驗的結果顯示:當圓柱置於空穴上方適當高度時,圓柱尾流的週期性速度擾動確實會對空穴中剪力層造成不同程度之影響,並強化了初期剪力層不穩定之波動振幅。當圓柱往空穴下游方向移動時,圓柱與空穴間水平平均速度的加速範圍亦往下游方向移動;然而,其速度增量並沒有明顯的改變。當圓柱接近空穴上游角落且與空穴間之垂直距離降低至H/D=1.2~0.8時,圓柱尾流的週期性速度擾動與空穴剪力層不穩定波動產生強烈的交互作用。當圓柱與空穴間流場的加速程度超越某一臨界值時,剪力層不穩定之振盪模式將躍升至另一振盪模式。此時,空穴中之振盪為多模式共存的特性,且各模式間亦產生相互競爭。總之:當圓柱與空穴間的相對位置愈接近空穴上游角落時,將造成圓柱與空穴間水平平均速度之速度增量變大,並且偏離了線性不穩定理論之tanh曲線。當圓柱與空穴間之垂直距離降至H/D=0.4且接近空穴下游角落時,由於圓柱對於空穴上游角落的屏蔽作用,使得兩者間之水平平均速度產生減速現象。在此條件下,空穴中剪力層不穩定波動振幅則漸減,空穴內部僅有迴流結構。

The objective of this study is to investigate and realize the influence of cylinder wake on the cavity shear layer while a single cylinder is placed at different positions above the cavity. All the experiments were performed in a recirculating water channel. Quantitative velocity measurement and qualitative flow visualization were preformed by ways of fiber laser Doppler's anemometry (FLDA) system and the laser sheet technique. The parameters in this experiment are described as follows: the cavity has a length/depth ratio 2 (L/D=2.0), the diameters of the cylinder in terms of d/D ranges within 0.33-0.8. The vertical distance (H/D) between cylinder and cavity ranges from 0.4 to 4.0, and the streamwise locations vary from XL.E/L=0 to XL.E/L=3.0. For all experimental conditions studied herein, the free stream velocity is U∞=20cm/s and the Reynolds number, based on cylinder diameter, is around Red=
U∞d/ν=4000-1640.
It is found that the periodic perturbation due to cylinder wake strongly affects and enhances the shear layer instabilities across the cavity when a single cylinder is situated above the cavity. When the cylinder moves downstream of the cavity but is maintained at the same height, the streamwise extent of flow accelerating region between cylinder and cavity will also move downstream. However, the amount of speed increment does not change apparently. When the cylinder is located near the upstream corner of the cavity and maintained at a height H/D=1.2-0.8 above the cavity, the cylinder wake will significantly affect the unstable cavity shear layer. In these situations, strong interaction between the periodic wake and the cavity shear layer will take place. When the velocity between cylinder and cavity goes beyond a critical value, the unstable oscillation of the cavity shear layer will jump to another oscillation mode. For these conditions, the oscillation within the cavity is multi-mode, and the competition between each mode will take place and transfer energy in between. When the cylinder is positioned more close to the upstream corner of the cavity, the cross-sectional velocity profile across the cavity does not follow the tanh-velocity profile. This implies that prediction of the oscillating mode within the cavity via linear stability theory is no longer valid. When the height between cylinder and cavity continuously reduces down to H/D=0.4, the blockage of the cylinder will reduce the streamwise averaged velocity. Thus, the unstable oscillation of cavity shear layer will diminish, and only a large-scale recirculation structure in the cavity can be observed.
URI: http://hdl.handle.net/11455/2751
Appears in Collections:機械工程學系所

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