Please use this identifier to cite or link to this item: http://hdl.handle.net/11455/1479
標題: 具不同直徑與淨間距之並列雙圓柱尾流鎖定特性之研究
Lock on Characteristics of Wake behind Two Side by Side Circular Cylinders with Different Diameters and Various Gaps
作者: 周士欽
CHIN, CHOU SHIH
關鍵字: lock-on
鎖定
double lock-on
anti-phase
in-phase
side-by-side cylinder
雙鎖定
異相位
同相位
並列圓柱
出版社: 機械工程學系
摘要: 摘 要 本文係以實驗方法研究具有不同直徑、不同淨間距的並列雙圓柱下游尾流被鎖定之特性,藉以瞭解外加擾動的頻率與振幅的變化,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,並佐以雷射都普勒測速儀系統進行定量流場量測。 本研究的主要內容分為:(1)固定切線速度擾動振幅,變化淨間距以及外加擾動頻率,探討具有相同直徑(D1/D2=1)的並列雙圓柱下游尾流被鎖定的特性。(2)當淨間距G/D2=1時,探討切線速度擾動振幅與頻率變化時,對具有相同直徑(D1/D2=1)的並列雙圓柱下游尾流被鎖定的特性。(3)固定切線速度擾動振幅時,探討淨間距以及外加擾動頻率變化,對具有不同直徑比(D1/D2=1/2)的並列雙圓柱下游尾流被鎖定的特性。 當具有相同直徑之並列雙圓柱下游的尾流受到外加擾動影響時,其結果顯示:(1)隨著兩圓柱的淨間距增大時,並列雙圓柱下游的寬、窄域尾流被鎖定區域的頻寬皆有縮小的現象,同時兩者的鎖定頻段均往單一圓柱特徵頻率(fe*=1)的方向偏移,最後並匯合在一起;(2)當外加擾動頻率接近單一圓柱特徵頻率時,並列雙圓柱下游的寬、窄域尾流結構具有雙鎖定的特性。此時,並列雙圓柱下游的渦流結構具有同相位或異相位交互脫離的現象。(3)當淨間距為G/D2=1時,並列雙圓柱下游寬、窄域尾流被鎖定的範圍,隨著外加擾動振幅持續增大,而有向兩側擴張的趨勢,同時兩個鎖定區域均往單一圓柱之特徵頻率方向匯合。在寬、窄域尾流同時被鎖範圍的重疊區域內,並列雙圓柱下游的尾流具有雙鎖定的現象。(4)當外加擾動頻率接近未擾動前並列雙圓柱下游之寬、窄域尾流特徵頻率時,具有不同直徑比的並列雙圓柱下游的寬、窄域尾流通常會被鎖定。
Abstract The present investigation is to study the lock-on characteristics of wakes behind two side-by-side circular cylinders with different diameters and various gaps. Quantitative velocity measurements and qualitative dye flow visualization were performed by way of the LDA system and the laser sheet technique, respectively. Three aspects are investigated in this study: (1) the lock-on characteristics of the wakes behind two side-by-side circular cylinders as functions of excitation frequency and the net gap ratios when excitation amplitude is held constant. (2) the lock-on characteristics as functions of excitation frequencies and amplitudes while the net gap ratio is kept constant at G/D2=1. (3) the lock-on characteristics of two side-by-side circular cylinders with the different diameter(D1/D2=1/2), as functions of excitation frequencies and the net gap ratios. It is found that (1) when the net gap ratio increases, the lock-on band of the narrow and wide wakes behind each cylinder shrink in bandwidth and tend to shift towards the characteristic frequency of a single circular cylinder(fe*=1). (2) When the excitation frequency approaches fe*=1, both the wakes behind two circular cylinders are locked at the excitation frequency. This is called double lock-on phenomena. When double lock-on characteristic occurs, the flow patterns exhibit either anti-phase or in-phase shedding that appears at random time interval. (3) when the net gap ratio is kept at G/D2=1, the lock-on bandwidth of both narrow and wide wakes become widened with increasing excitation amplitudes. Within the overlapped region of the each lock-on bandwidth, double characteristics of of both narrow and wide wakes occur. (4) When the diameter ratio of two side-by-side circular cylinders becomes unequal (D1/D2=1/2), the lock-on characteristics of either the narrow or the wide wake occurs while the excitation frequency approaches the characteristics frequency of either wake (narrow or wide) behind two side-by-side cylinders.
URI: http://hdl.handle.net/11455/1479
Appears in Collections:機械工程學系所

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