本研究應用MEMS熱膜感測器探討鈍型體於流場中，渦流溢放過程中造成的非定常三維現象。實驗中以MEMS熱膜感測器陣列貼附於圓柱鈍型體、圓柱鈍型體附加延長平板、梯形鈍型體以及梯形鈍型體附加延長平板模型表面於次臨界雷諾數下使用多組感測器陣列進行同步量測。實驗為達到非侵入式近壁面量測，微製程技術被應用於設計及製造可撓式熱膜感測器陣列。在訊號處理方面，本研究利用小波轉換法分析MEMS熱膜感測器以及熱線測速儀(Hot-wire)於鈍型體流場量測到之渦流溢放訊號。發現小波分析法內含濾波功能，能夠有效濾除訊號雜訊，適用於鈍型體流場瞬時溢放頻率及其相位之探討。此外，爲分析渦流溢放過程中，翼展間溢放相位延遲所造成之側向波數，二維傅立葉轉換(2D-FFT)亦被利用分析MEMS熱膜感測器陣列之訊號。進而探討非定常三維流動分離下，圓柱鈍型體以及梯形鈍型體翼展間側向波數變化。實驗數據經由小波分析法以及二維傅立葉轉換，鈍型體流場時間變化下之瞬時特性；其中包含瞬時渦流溢放頻率和相位、翼展間側向波數、渦流位錯事件等將被量化描述。
由實驗訊號推判，鈍型體流場週期性渦流溢放訊號存在著非定常之低頻擾動。經統計結果發現，圓柱體瞬時渦流溢放頻率與振幅低頻調變呈現負相關特性，負相關係數分布於-0.2至-0.4之間。其中，實驗亦發現，負相關係數不確定度受渦流位錯事件所影響。此外，在梯形鈍型體附加延長平板模形實驗發現，延長平板可以有效抑制非定常三維特性，避免尾流渦流位錯事件之發生。因此，其瞬時渦流溢放頻率與振幅低頻調變負相關係數可增至-0.7。然而，若是於圓柱體後緣增加延長平板卻發現三維非定常特性不僅不能被抑制，反而更為嚴重。上述現象原因歸咎於梯形鈍型體分離點固定發生在平板前銳角處。而圓柱鈍型體分離點遊走於圓柱表面分離區處，屬於非固定模式。因此在圓柱流體中，增加延長平板反而增加流場之複雜性。
上述論點由量化數據描述，鈍型體流場之三維非定常低頻擾動現象亦可被估算。實驗發現，圓柱體流場在吾人實驗條件下，渦流位錯事件發生之機率約為10%。而於梯形鈍型體流場中機率為3%，梯形鈍型體附加延長平板則不會有渦流位錯事件之發生。實驗統計結果以及陣列式訊號觀察再驗證幾何外型差異對於非定常流場特性之影響。

This study is aimed to investigate the unsteady, three-dimensional characteristics of vortex shedding from a bluff body cylinder. The Reynolds numbers studies fall in the sub-critical regime. Flexible-skin MEMS thermal film sensors were fabricated to investigate the aerodynamic characteristics around the bluff cylinder. The bluff bodies employed for experiment include a circular cylinder with and without a splitter plate, a trapezoidal cylinder and a T-shaped cylinder. In order to examine the characteristics of flow quantitatively, the techniques of Wavelet transformation and 2D Fourier transformation were employed to reduce the instantaneous vortex shedding frequency and phase, the characteristic spanwise wave numbers of the instantaneous vortex shedding frequency, and the events of vortex dislocation.
The results for the circular cylinder show that the variations of instantaneous vortex shedding appear to be correlated in a negative manner with the low-frequency modulations that the correlation coefficients fall largely in a range between -0.4 and -0.2. It is further noted that the variation of negative correlation was affected by the occurrences of the events of vortex dislocation. Furthermore, it was found in the experiment of T-shaped cylinder that the instantaneous vortex shedding frequency and its low-frequency modulus was highly correlated. Moreover, it was found the addition of a splitter plate behind the trapezoidal cylinder could successful suppress the wake flow modulations. In this case, one realizes that the unsteady modulations of wake flow behave two-dimensional like. On the other hand, for circular cylinder flows, the addition of a splitter plate did not help to suppress the three-dimensionality of vortex shedding. This is attributed to the fact that the flow separation point on a circular cylinder is not fixed. Thus, the addition of the splitter plate further complicates the development of flow separation and the process of vortex shedding.
The data obtained from four bluff cylinder cases were compared. The discrepancies between the results obtained indicate that for the case of the circular cylinder there was about 10% of time that vortex dislocations developed along the span of the cylinder, whereas it was merely 3% in the case of a trapezoidal cylinder, and no events of vortex dislocation are seen in the case of the T-shaped cylinder.

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