Please use this identifier to cite or link to this item: http://hdl.handle.net/11455/1589
標題: 空穴剪力流鎖定現象的實驗研究
Experimental Study on the Lock-on Phenomena of Cavity Shear Layer
作者: 楊大葳
Wei, Yang Ta
關鍵字: cavity shear layer;空穴剪力層;externally forcing frequency;lock-on;外加擾動;鎖定現象
出版社: 機械工程學系
摘要: 
本研究是以實驗方法研究空穴結構中分離剪力層不穩定現象之鎖定特性,期能瞭解外加擾動頻率與振幅對於空穴中流場不穩定振動頻率的鎖定現象,達成控制空穴剪力層不穩定波動之目的。本研究之實驗是在低速循環迴路式水槽中進行,流場可視化是以螢光染液釋放法進行,以瞭解空穴結構的定性演化情形,並以雷射都普勒測速儀系統進行定量的流場量測。本實驗模型參數變化如下:空穴長深比L / D = 3.0,振動薄板長度為d =2.5cm,厚度為0.05cm,流體之自由流速 ,其對應的雷諾數為 =2500,振動薄板尾端置於H / D = 2.4 ,Xt / L =0.0處,外加擾動頻率則在fe = 0.32Hz ~ 1.68Hz範圍中變化,另外,外加擾動振幅則控制在0.57mm ~ 0.93mm。
由實驗的結果顯示:當空穴剪力層未受到外加擾動時,空穴中不穩定的剪力層已具有一穩定的波動頻率(fo = 1.12Hz)。當薄平板靜止時,空穴上方的平均流場結構與薄剪力層者完全相同,因此,剪力層不穩定特性並未因外加擾動平板的存在而受到改變。且當外加擾動之振幅固定在0.57mm且,擾動頻率由fe / fo = 0變化至fe / fo = 1.50時,空穴剪力層波動具有三個鎖定頻段,分別為:(1) 5 / 9鎖定頻段,(2) 2 / 3鎖定頻段,(3) 1 / 1鎖定頻段。除了以上三個鎖定頻段外,並發現空穴的鎖定現象亦具有鎖定邊界區域。當外加擾動振幅增加到0.78mm時,在fe / fo = 0.5處會出現另一新的鎖定區域。其中,各個鎖定的頻段是以剪力層不穩定波動頻率之次諧和頻率為中心向兩側做適度的伸展。另外,隨著外加振幅的增加此頻寬亦將逐漸變寬,各鎖定頻段的寬度均會向高、低頻兩側擴張,但其擴張量則比振動圓柱者為小。

The lock-on phenomena of cavity shear layer was studied by the flow visualization and the velocity measurements. Quantitative velocity measurement and quantitative flow visualization were performed by ways of LDA system and the laser sheet technique. The dimension of the cavity model is as follows: The length / depth ratio of the cavity is L / D = 3.0; the length of the vibrating-thin-plate d =2.5cm; the thickness is 0.05cm. The uniform inflow , and the Reynolds number Re is =2500. The external perturbation is generated by the thin plate located above the cavity. The trailing edge of the thin vibrating-plate is located at H / D = 2.4 and Xt / L =0.0. The objectives in this study are to investigate the influence on the cavity shear layer by periodic external forcing. The perturbation frequency varies from fe / fo = 0.32 to fe / fo = 1.50 and the perturbation amplitude is controlled within 0.57mm ~ 0.93mm.
In the present study, three lock-on frequency bands are found to center at fe / fo = 5 / 9, 2 / 3,1 / 1, respectively, while the perturbation amplitude is controlled at 0.57mm. They are defined as 5 / 9 lock-on band, 2 / 3 lock-on band, 1 / 1 lock-on band. However, a new lock-on band centered at fe / fo = 1 / 2 is found while the perturbation amplitude is greater than the threshold amplitude ( 0.78mm ). Similar to that of the perturbed cylinder, the lock-on frequency band is extended on both sides from the individual center frequency. However, the amount of the frequency extension within the cavity is smaller than that of the perturbed cylinder.
URI: http://hdl.handle.net/11455/1589
Appears in Collections:機械工程學系所

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