本研究旨詳述流場由次臨界雷諾數進入到預臨界區流場的邊界層初始層─紊流轉換特性，特別針對隨著時間有顯著變化的訊號做細部探討，以實驗方法探討圓柱流場於雷諾數介於1.53×105~3.50×105間之特性。實驗首先量測基部壓力隨雷諾數變化的情形，再利用圓柱左右兩側的壓力係數來判斷圓柱表面流場於預臨界區前後情況，並利用可撓式熱膜感測器(MEMS)平貼於圓柱表面以熱絲感測器(Thermal tuft)的工作原理判斷圓柱表面流場之流動分離現象。吾人利用小波轉換分析法獲得瞬時渦流溢放頻率，統計訊號於各個時間點變化之特性，並輔以相關性分析，得以獲知圓柱兩側流場之對稱性質，透過觀測尾流的訊號亦可發現尾流寬度的變化。由於流場進入到臨界區時，渦流溢放頻率挾帶著三維低頻擾動因此吾人利用經驗模態分離法將低頻擾動濾出，得以觀察圓柱兩側三維性現象。
其主要的發現有：在兩次實驗中分別觀測到Re=2.40×105 與Re=2.54×105時，此定流速下存在著兩種不同性質的流場，即本文所指狀態A流場與狀態B流場，其流體行為分別隸屬於預臨界區與次臨界區的流場特徵，隨著雷諾數增加流場從狀態B型態進入到狀態A型態，從具有高週期性的渦流溢放行為轉為較低週期性的渦流溢放行為，且狀態A流場的渦流溢放有著低頻擾動現象，狀態B流場尾流寬度較狀態A寬，從狀態B流場進入到狀態A流場的分離位置有向下游移動的趨勢。

Experiments were carried out on flow over a circular cylinder at Reynolds number 1.53×105~3.50×105. Pressure measurements on the cylinder surface were made to obtain the base pressure coefficient CPb, and the distribution of CP on the cylinder surface in the pre-critical regime. In addition, thermal tuft method was used to sense the flow-direction on the cylinder surface.
By examining the pressure signals measured on the two sides of the cylinder surface, it is found that the raw signals contain two fluctuating components mainly, one is due to the vortex shedding frequency and the other is due to the low frequency modulation. The two components could be separated by the empirical mode decomposition (EMD). Subsequently, the correlation analysis of each component with respect to the pressure signals obtained on the two sides of the cylinder was carried out.
The major finding of this study is that the States A and B can be bi-stably existed at a fixed free stream speed, evidenced by two separate experiments. While the state A bears the characteristics of flow in the pre-critical regime, the state B bears the characteristics of flow in the sub-critical regime. This finding unveils the flow behavior of transition from the sub-critical regimes to pre-critical regimes.

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