本研究中主要利用粒子影像測速儀(Particle Image Velocimetry)於低、中、高雷諾數(ReD = 3840、9440、12320)之條件下探討二維圓柱近域尾流區之渦旋的產生與脫離過程以及流場中的組織性結構，並使用擁有高時間解析及足夠達到統計穩定的樣本數之熱線測速儀(Hot-Wire Anemometry)加以佐證其速度量測之精確性。圓柱尾流存在具有組織性的大尺度相干性結構，本實驗使用正交特徵分解(Proper Orthogonal Decomposition, POD)進行降維分析，分析尾流渦旋脫離過程，並使用頻譜分析辨認出大尺度相干性結構。 在低、中、高速實驗中，由流場的上游(0.5 - 5.5 d)、中游(5.5 - 10.5 d)、下游(10.5 - 15.5 d)三區段相干性結構能量貢獻來觀察其相干性結構衰退的狀況，發現其能量隨著與圓柱的距離增加而衰變，衰變速率亦與雷諾數呈正相關性關係。
本研究針對正交特徵分解時的三項參數進行分析。第一項為拍攝視場大小之影響，拍攝視場大小的選擇主要為完整拍攝週期性尾流並分析其能量；根據斯特勞斯哈爾數(Strouhal number)可求得，於本研究約需使用5 d (d :圓柱直徑)來進行分析，研究發現使用3 d、7 d分析結果與5 d有著不小的差異；3 d大小不足以擷取最大週期之大尺度運動；7 d大小造成分析結果解析度變差，。第二項為使用樣本(sample)的數量之影響，透過遞增樣本數並與總樣本數計算誤差找出描述上游、中游、下游的足夠樣張數。第三項為流場重建時所使用模態的數量，透過本實驗誤差所得之能量計算標準藉以精確決定所需之模態後重建紊態流場，另外，組織性結構在下游多分解(break down)為小尺度紊流，對於小尺度紊流要越後面的模態才能表示，越後面的模態相對波數會越大，結果符合物理意義。

The generation and dissipation processes of a coherent structure in the near-wake region at three Reynolds numbers (3840, 9440, and 12,320) are extracted using particle image velocimetry (PIV) along with the proper orthogonal decomposition (POD) analysis method. The information of each instantaneous flow field was projected to different modes. A large-scale Karman vortex street dominates the flow field in the lower-order mode. As the mode becames higher, its energy contribution accounts for the lower ratio and small-scale vortex decomposed by the Karman vortex street, which verifies whether the position gradually moves from the upstream close to the cylindrical position to the downstream, where the flow field energy gradually attenuates. In our experiment, the coherent structure energy contributions of the three regions, which are the upstream (0.5-5.5d), midstream (5.5-10.5d), and downstream (10.5-15.5d) regions of the flow field, are used to identify the degradation of the coherent structure. It was observed that the energy decayed as the distance from the cylinder increased, and the decay rate also had a positive correlation with the Reynolds number.
Three POD parameters are analyzed in the study. The field of view (FOV) was taken as the first parameter, where field of view was defined as a complete shot of a coherent structure, after which its energy was analyzed,which is obtained according to the Strouhal number, which was approximated in this research, where need to use 5d (d: cylinder diameter) for analysis. The results of using 3d and 7d analysis are quite different from 5d; 3d size is not enough to capture period of large-scale motion; 7d size reduces analysis resolution. The second parameter was the number of samples, by increasing the number of samples, find out enough samples to describe upstream, midstream, and downstream regions of the flow field. The third parameter is the number of modes used in the reconstruction of the flow field, the energy standard obtained through the equipment error determines the required modes to reconstruct turbulent flow field.

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