本研究以實驗方式來探討圓柱表面流場於雷諾數為1.53×105~3.92×105間之流場轉換現象。實驗利用基部壓力隨雷諾數變化，再配合圓柱左右兩側θ=±90˚之擾動壓力訊號來判斷圓柱從預臨界區到單分離泡區至雙分離泡區的分界。研究內容主要利用基部壓力係數及圓柱左右兩側θ=±80˚、±90˚之壓力係數搭配熱線探針(Hot-wire)來觀察流場之瞬時訊號。由於單分離泡會造成尾流偏移，吾人同時在單分離泡出現時，置一熱線探針於尾流區，固定X軸及Z軸位置，移動Y軸於一範圍內量測尾流訊號，並同時也量測預臨界區及雙分離泡區做比較。
另外，同時使用熱絲觀念(Thermal tuft)在圓柱θ=±92˚量測，經由訊號可判定此角度下流場為順流抑或是逆流，進而判定流體是否已在圓柱表面分離，並由即時訊號得到流場當下流況。接著再於單分離泡及雙分離泡出現時，轉動圓柱改變熱絲感測器(MEMS sensors)之角度，期望藉由訊號顯示之順逆流找出流動分離及回覆再接觸現象發生之位置。
至於瞬時頻率之分析，藉由希爾伯特-黃轉換進行數據處理，得到瞬時溢放頻率及能量大小，此方法貢獻了快速傅立葉轉換無法得到之瞬時頻率。

This study aims to investigate the transition phenomenon of flow around a circular cylinder in the critical regime at Reynolds numbers between 1.53×105~3.92×105.The base-pressure coefficients and the root-mean-square of the pressure coefficients on two sides of a cylinder were used to determine the pre-critical regime, the one bubble regime, and the two bubble regime. The main purpose of this study was to analyze the instantaneous signals which were measured by the pressure transducers on the cylinder surface and a hot-wire in the wake region. Because the wake flow was shifted in lateral direction while one bubble developed on the cylinder surface, a hot-wire probe was employed to measure the wake velocity profiles at X/D=2 and Z/D=0. Meanwhile, the wake velocity profiles were obtained with respect to the pre-critical regime and the two bubble regime for comparative study.
Besides, self-made MEMS thermal films sensors located at θ=±92˚ were carried out . A thermal tuft method was used to determine the flow direction at the position measured. Moreover, in order to determine the position of separation point and reattachment point, the cylinder was rotated while the bubble appeared stably.
The instantaneous behavior of vortex shedding was studied by analyzing the measured data with Hilbert-Huang transformation. This method offers instantaneous information that fast Fourier transformation can not provide.

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