Please use this identifier to cite or link to this item: http://hdl.handle.net/11455/2587
標題: 以小波方法分析並列雙圓柱尾流場鎖定特性
Wavelet Analysis on Locked-on Characteristics of Wake behind Two Side-by-Side Circular Cylinders
作者: 陳信安
Chen, Hsin-An
關鍵字: locked-on;鎖定;side-by-side cylinder;wavelet transformation;並列圓柱;小波分析
出版社: 機械工程學系
摘要: 
本文係以實驗與小波方法研究具有不同直徑的並列雙圓柱下游尾流被鎖定之特性,藉以了解外加擾動頻率對於並列雙圓柱下游尾流結構被鎖定的影響。本文的實驗是在低速循環迴路式的水槽中進行,以螢光染液釋放法紀錄流場可視化,以瞭解並列雙圓柱近域尾流定性演化的情形,並佐以雷射都卜勒測速儀系統對尾流場進行整個截面之定量流場量測。
主要內容為:固定切線速度擾動振幅,變化外加擾動振幅以及外加擾動頻率並利用小波分析,探討具有不同直徑(d/D=1/2)的並列雙圓柱下游尾流被鎖定的特性。研究結果顯示:(1)當具有大直徑之圓柱受到外加擾動影響時,並列雙圓柱下游的寬、窄域尾流被鎖定的區域均往單一圓柱特徵頻率的方向偏移;且在適當的擾動頻率時,雙圓柱均被鎖定在不同頻率段,形成雙鎖定的現象;(2)當小圓柱受到外加擾動時,寬域尾流的鎖定範圍以不受擾動的寬域尾流特徵頻率(fw*=fw/fos= 0.9)為中心向兩側擴張,而窄域尾流則是在無因次擾動頻率為fe*=0.9~1.2與fe*= 2.2~3.2間發生被鎖定現象,其中頻段fe*=0.9~1.2中則是寬域和窄域尾流同時發生被鎖定現象;而在頻段fe*=2.2~3.2之鎖定則僅有窄域尾流被鎖定。(3)藉由小波分析,間隙流中之特徵頻率因外加擾動的影響,從原與窄域尾流相同頻率轉為與外加擾動相同頻率。由此可知:外加擾動直接影響被擾動的尾流結構,也確實是經由間隙流的媒介而間接影響到另一個尾流結構。

The experimental investigation is to study, based on wavelet analysis, the locked-on characteristics of wakes behind two side-by-side circular cylinders with various excitation frequencies in the low-speed water channel. Quantitative velocity measurements and qualitative dye flow visualization were performed by way of the LDA system and the laser sheet technique, respectively.
In this study, two side-by-side circular cylinders has different diameters (d/D=1/2). The net gap ratio between two cylinders is 3D/4, and the Reynolds number based on the large cylinder diameter is 1160. One of the cylinders is perturbed in front of an oscillatory rotation about it's center with a constant amplitude of tangential velocity.
It is found that: (1) When the large cylinder is perturbed at various excitation frequencies, the locked-on band of the narrow and the wide wakes behind each cylinder tend to shift towards the natural shedding frequency of the individual cylinder. When the excited at proper frequency, both the wide and the narrow wakes behind two circular cylinders are locked-on simultaneously but at the distinct frequency. This is called double lock-on phenomena. (2) When the small cylinder is perturbed at various excitation frequencies, the locked-on bandwidth of the wide wakes is located with fe*=0.7~0.9. The locked-on bandwidth of the narrow wakes is located at fe*=2.2~3.2. And the double locked-on of both wakes happens to be within fe*=0.9~1.2. (3) By wavelet transformation and analysis, it is shown that the characteristics frequency of the gap flow is dominated by the frequency of narrow wake when unperturbed. However, the responding frequency in gap flow is dominated by the characteristics frequency of the wide wake when the wide wake is excited near it's nature shedding frequency. This reveals the direct evidences of flow coupling between the wide and narrow wakes.
URI: http://hdl.handle.net/11455/2587
Appears in Collections:機械工程學系所

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