本研究中利用粒子影像測速儀(Particle Image Velocimetry)於高、低雷諾數(ReD = 3900、9500)之條件下探討二維圓柱近域尾流區之渦漩的產生與脫落過程以及流場中的相關性結構。並加以運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)進行流場測量的驗證，使用正交特徵分解(Proper Orthogonal Decomposition, POD)進行降維分析，並分析如尾流渦漩脫落過程。圓柱尾流存在具有類週期的大尺度相干性結構，並且還會隨著與圓柱的距離所衰減，本研究將使用正交特徵分解及頻譜分析辨認出其大尺度相干性結構，以及其諧波存在的情況。在高速及低速兩組實驗中，由流場的上游(0.5-5d)、中游(5-10d)、下游(10-15d)三段相干性結構能量貢獻來觀察其相干性結構衰退的狀況，可以看到其能量隨著與圓柱的距離增加而減少，不論在高速或低速到達下游時相干性結構的能量貢獻以低於5%。並且使用雷諾分解及週期均值化處理時下游的紊流強度及雷諾應力幾乎沒有差異。
本研究將針對三項參數進行分析。第一項為樣張(sample)的數量，使用6000、8000及10,916張進行各模態能量貢獻的比對，在紊態擾動模態較多的案例中，10,916張是不足的，需要增加更多的樣張。第二項為流場重建所使用模態的數量，使用Kaiser及Sirovich標準觀察流場重建的狀況，發現Kaiser標準無法完整的重現流場細部的擾動值，應使用採用較多模態Sirovich標準。第三項為泰勒尺度與諧波倍頻的關係，發現當諧波倍頻進入到慣性次階區後，將無法在頻譜能量圖中辨認出其諧波倍頻的峰值。

In this research, the generation and shedding processes of vortices in the near wake region and the coherent structure in a flow field are studied at two Reynolds numbers of 3900 and 9500 using proper orthogonal decomposition (POD). POD is a methodology used for the purpose of identifying large-scale eddies (such as Karman vortex) in lower-order modes and for recognizing small scale eddies in high-order modes that contribute to turbulence in the entire flow field. The cylindrical wake possesses a large scale coherent structure, which will be attenuated with distance from the cylinder. POD and a spectrum analysis are used to identify the large scale coherent structure and its harmonics frequency. In the high and low Reynold number experiments, the coherent structure energy contributions of the three regions, which are the upstream (0.5-5d), midstream (5-10d), and downstream (10-15d) regions of the flow field, are used to identify the degradation of the coherent structure. Regardless of whether the Reynolds number is high or low, the energy contribution of the coherent structure is less than 5% in the downstream region. Reynolds decomposition and period-time averaging process exhibit almost no differences in terms of downstream turbulence intensity and Reynolds stress.
Three POD parameters are analyzed in the study. The first parameter was the number of samples, where 6000, 8000, and 10,916 samples were used to compare energy contribution of each mode. However, it reveals that it needs more samples to reach a statistically stationary result. The second parameter was the number of modes used in the reconstruction of the flow field. The Sirovich criterion and the Kaiser criterion were respectively used as the energy contribution bases in the flow field reconstruction. The Kaiser criterion can't reveal the characteristics of small fluctuations. The Sirovich criterion which accumulates more modes is a better choice. The third parameter was the relationship between the Taylor microscale and the harmonic frequency. It was found that when the harmonic frequency falls into the inertia subrange, the peak of the harmonic frequency cannot be identified in the spectrum energy diagram.

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