本研究旨在針對脈動式管流中之初始不穩定現象進行探討，並嘗試加入一人為變因(粗糙元)，改變邊界層厚度以達到流場控制的效果。透過馬達驅動位於下游之流場控制器旋轉，使流場速度隨時間呈脈動式變化，再利用熱線測速儀量測流場速度，並依據流場控制器之光電輸出信號做流速相位平均(phase-average)，比較不同相位的流速分佈曲線，探討在一個脈動週期內之流場變化趨勢，再以總體經驗模態分離法(ensemble empirical mode decomposition) 去除背景雜訊，將擾動分量擷取出來，以希爾伯特-黃轉換(Hilbert-Huang Transform) 及小波轉換(wavelet transform) 來分析初始不穩定擾動之特徵頻率，並根據Rayleigh instability criterion 所述之發生不穩定性之必要條件，觀察速度一次微分曲線圖之反曲點位置的變化及邊界層發展的情形，同時利用混合層流模型(mixing layer model) 和黏性效應(viscous effect) 之觀點予以解釋，並釐清致使初始擾動發生的機制。
在這項研究中，亦透過不同線徑之金屬絲(粗糙元) 來觀察管內之流體流動行為，不同實驗條件下的結果顯示流動的不穩定現象亦是受到非黏性或黏性機制所影響。此外，加裝金屬絲可以有效的改變邊界層之發展狀況，進而影響非定常速度剖面中反曲點形成的位置，藉此觀察不穩定性發展所受到的改變，經由測試結果說明了安裝金屬絲控制流動不穩定性的有效性，最後再利用上述分析方法找出擾動特徵並與前期實驗結果進行詳細的討論。

This study is aimed at investigating the initial instability in pulsating pipe flow with installing a trip wire at the inlet to manipulate the control of flow instability. The pulsation of pipe flow was produced by a flow regulator situated downstream of the pipe flow, which was driven by a servo motor and monitored by an optical sensor. The instantaneous velocity information at a point in the flow field was obtained by a boundary-layer type hot-wire probe, then phase-averaged according to the output signals of the optical sensor mentioned. For data analysis, the method of Ensemble Empirical Mode Decomposition (EEMD) was employed to extract the disturbance component due to flow instability from the phase-averaged signal trace studied. Then, we employed either Hilbert-Huang Transformation or Wavelet Transformation to obtain the characteristic frequency of the initial unstable disturbance. Moreover, a phase-averaged velocity profile was smoothened by a six-order regression curve, subsequently the inflection points were identified. Then, the flow instability observed was examined with the Rayleigh instability criterion based on a mixing-layer model. By this procedure, one was able to identify the cases of which the flow instability characteristics are complied with the mixing-layer model, which is referred to the inviscid mechanism. Otherwise, the cases not agreed with the inviscid model would be considered due to the viscous effect.
In this study, experimental results obtained with different trip wires under a number of pulsating flow conditions confirm that the flow instability phenomenon observed in the developing region could be due to the inviscid or viscous mechanism. Moreover, the installation of a trip wire could effectively alter the development of boundary layer, therefore affected the locations of the inflection points in the unsteady velocity profile.
As a consequence, the development of flow instability was affected pronouncedly, that the flow instability could be initiated at further upstream location due to either of inviscid or viscous mechanism. The present results illustrate the effectiveness of installing a trip wire to control of flow instability, based on which detailed discussion was carried out concerning the situations associated with the inviscid mechanism and those with the viscous mechanism.

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