本論文以實驗方式探討渦流溢放過程低頻調變現象與三維性之瞬時特性，包括渦流溢放信號之振幅變化（Aw(t)）、渦流溢放頻率變化（fw(t)）、渦流溢放相位變化（q(t)）。實驗雷諾數範圍為1.8×103至2.7×104，以一平板鈍形體為實驗模型，使用煙線視流法觀察渦流溢放之流向與側向結構，並以熱線測速儀量測近尾流區之渦流溢放信號，再以小波分析法求得此信號之瞬時特性。
實驗結果顯示，渦流溢放信號低頻變化為近尾流流場之整體性的變化，Aw(t)與渦流形成長度具有明顯的相關性，而Aw(t)與fw(t)之相關性係數可達-0.7。在流場三維性方面，使用三根熱線同時量得側向間距分別為1倍與2倍特徵長度之信號，可得到相位變化差Dq(t)，Dq(t)之最大值約為35°。Aw(t)與|Dq(t)|表現出明顯的負相關性，其相關性係數約為-0.4。此外Aw(t)之無因次化積分時間尺度（It）約為2，與Dq(t)之It 值1.86相當接近，且此積分時間尺度在實驗條件下顯示與雷諾數無關。
在同一雷諾數條件下，根據渦流形成長度的不同，以兩倍的特徵長度為分界，本論文提出鈍形體渦流溢放可區分為兩種模式，長渦流形成區模式(mode L)與短渦流形成區模式(mode S)。在mode L，fw(t)較小，Aw(t)較大，Dq(t)較小，表現出較不明顯的三維性;在mode S，fw(t)較大，Aw(t)較小，Dq(t)之值大於20°，此外mode S發生的機率小於5%，為鈍形體渦流溢放過程之猝發(burst)特徵。

The characteristic behaviors of low-frequency modulations embedded in the vortex shedding process were studied experimentally with a normal plate situated in a low-speed wind tunnel at Re = 1.8×103 to 2.7×104. Hot-wire signals and smoke-wire visualization images were acquired simultaneously to examine the correlation between low-frequency modulations and the vortex formation length. Wavelet analysis was performed to extract the instantaneous properties from the raw hot-wire signals measured in the region upstream of the normal plate model. Results show that the variations of instantaneous vortex shedding frequency appear to be correlated in a negative manner with the low-frequency modulations, that the cross-correlation coefficient can reach –0.7 in value. This substantiates that the low-frequency modulations observed are linked with the vortex shedding process.
Further experiments using three hot-wires situated at different spanwise locations at the edge of separated shear layer were performed to investigate the three-dimensionality of the vortex shedding. Meanwhile, the three-dimensionality was evident by the streamwise vortices and the spanwise incoherence in the separated shear layer as visualized using the smoke-wire technique. It is noted that the phase difference of vortex shedding between the two signals with spanwise separation of two characteristic length may reach as high as 35°, at which the amplitude of vortex shedding sensed by either of the hot-wires appeared to be minimal. More specifically, the correlation coefficient of the spanwise phase differences of vortex shedding and the amplitude of vortex shedding reduced from the hot-wire signals measured amounts to –0.4, signifying that the linkage between the low-frequency modulations and three-dimensionality is noticeable. As a result, it is suggested that the low-frequency unsteadiness and the three-dimensionality of vortex shedding are can be described into two modes; namely, the long formation region mode, called mode L, and the short formation region mode, called mode S. In mode S, the instantaneous vortex shedding frequency appears to be higher, the instantaneous vortex shedding amplitude detected at a point outside the separated shear layer is weaker, and the three-dimensionality appears to be more pronounced. The mode S, corresponding to the events of vortex shedding with the spanwise phase difference larger than 20°, occupies less than 5% of the total time measured. In this study, the mode S is referred as the burst mode in vortex shedding.

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