請用此 Handle URI 來引用此文件: http://hdl.handle.net/11455/1491
標題: 縱列雙圓柱流場之軸向效應實驗研究
Experimental Investigation of Spanwise Effects for Flow Around Two Circular Cylinders in a Tandem Arrangement
作者: 李思賢
關鍵字: Two Circular Cylinders
雙圓柱
Spanwise
Hilbert transform
軸向效應
出版社: 機械工程學系
摘要: 本論文研究為風洞實驗與分析,主要探討雙圓柱模型軸向流場之相關性,以及相位差之變化。實驗主要是量測軸向尾流渦漩剝離頻率、軸向表面壓力、雙穩態現象,並利用煙線可視化技巧,來觀察尾流場的結構。 實驗在開放式風洞中進行,測試模型為直徑45mm之圓柱,主要雷諾數為4500。首先針對靜態縱列雙圓柱研究,結果發現,流場相關性與柱體無因次間距(Z/D)有密切的關係。中心點與軸向量測點相位差的變化,隨著相關係數的下降,而相位差的震盪振幅隨著加大。在臨界間距(L/D = 3.88 - 4.22)時,有兩種流場交替發生,此時流場的特徵為存在兩種穩定狀態的狀況,稱為雙穩態流場。而雙穩態流場的交替並無週期性。本研究利用相位差量測,嘗試找出雙穩態流場變化與軸向流場的相關性,並作進一步的探討。 動態的探討上與靜態量測相同外,藉由改變上游柱體之震盪頻率與震盪振幅,探討不同震盪模式下流場發生鎖定現象與軸向流場之相關性,結果發現當流場鎖定時,軸向流場的相關性亦隨著提昇,而相位差的變化振幅亦隨著下降。實驗中並利用相位差的比較,來研究動態系統中之差異,探討雙穩態流場之特性。
The spanwise effects for flow around two identical circular cylinders in a tandem arrangement have been studied experimentally. The experiments were performed in an open-type wind tunnel for Reynolds number of 4500. In addition to the case of two stationary cylinders, the oscillating case involves a rotationally oscillating upstream cylinder. The spanwise vortex shedding frequency, and spanwise surface pressures are described. This study employs the cross-correlation as well as Hilbert transformation to examine the spanwise effects caused by the oscillation of upstream cylinder. At the critical spacing (L/D = 3.88 - 4.22) dual vortex shedding frequencies have been observed for the stationary case well as for the oscillating case. The occurrence of dual shedding frequencies indicates switching of the flow patterns between two stable modes and is referred to as the bistable flow. In the stationary case, as the spanwise cross-correlation coefficient decreases it is also found that difference measured using Hilbert transformation increases. In the oscillating case, research is emphasized on the phase difference when bistable flow occurs. This is attained by rotationally oscillating the upstream cylinder while holding the downstream cylinder stationary. The bistable flow can also occur when the vortex shedding is locked-on to the oscillating upstream cylinder. The lock-on of vortex shedding to the oscillating upstream cylinder leads an increase in the spanwise cross-correlation and a decrease in the phase difference.
URI: http://hdl.handle.net/11455/1491
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