流體經過縱列雙圓柱產生週期性擺盪與柱間互制之馮卡門渦列(Von　Kármán　vortex　street)，渦列週期以單點時序列速度做頻譜分析求得時，常無法獲得空間分佈特性，而以質點影像量測法(Particle　Image　Velocimetry ,　PIV)，雖具有不干擾流場及全域同步量測之優點，然會面臨如粒子(particle)分布不均造成的流場偏差、週期的獲得方式、平均流場的時間選取…等困難。本文即針對質點影像量測法分析週期性流場上會遇到的問題，提出解決之方法與建議。
　　經由特性雷諾數2400、圓心距5D(D 為圓柱直徑)之縱列雙圓柱之影像分析，發現若以空間取樣範圍G×B內，各子區域X軸之速度u做頻譜分析，將各子區域所得週期取整體平均(ensemble　average)作為代表週期時，以代表週期為基準發展的週期平均法，可降低瞬時流場粒子分布不均的影響；且前柱升力係數(Lift coefficient,　Cl)之變化週期與洪(2005)以三維紊流計算所得之相當，後者週期稍小5.39%。
　　本文所建立之週期性質點影像法所求得之週期，不管與前人研究或數值模式比較都相當，可供往後研究者參考。在分析流場特性發現，提升流場準確度之關鍵在於互
相關函數訊雜比之設定，建議當瞬時流場空洞數目大於某一臨界值則視為錯誤流場，將此流場淘汰不用，再以篩選過之瞬時流場進行週期平均將可大幅提升平均流場的正確性。

　Periodic flow such as the Von Kármán vortex street can form and interact within influence region of two aligned cylinders. Though particle image velocimetry(PIV) can obtain the spatial information that any one-point measuring method can not provide,some problems of using PIV for periodic flow remain to be solved. The study concerns about how to find the time interval for velocity averaging of the periodic flow and how to reduce the erroneous flow filed caused by non-uniform seeding.Main parameters of this study are the longitudinal velocity u, the diameter of the cylinder,D,and the image area formed by the gap of the cylinders and the channel width, G×B.
　It is found that the representative period for the tests with Re of 2400 is closely related to the frequency of locking for the aligned cylinders.This representative period is the average of the frequencies obtained from the spectral analysis to u velocities of all interrogation areas.The period is comparable with the value computed from 3-D model,and for the cylinder distance of 5D case, the difference is 5.39% less.The present PIV method is shown to be capable of obtaining both of the velocity distributions of phase and ensemble values for the Re of 2400 cases.
　However,the analyzing method can not give good results for other cases of different Reynolds numbers.Insufficient image resolution may be the causation of this deficiency. Further studies to resolve this problem are still required.

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