本研究分別以熱線測速儀(Hot-Wire Anemometry)及粒子影像測速儀(Particle Image Velocimetry) 在高、低雷諾數(〖Re〗_D=3799；9499)下探討二維圓柱體近域尾流區渦流的形成和脫落過程以及小尺度流流場結構。PIV和HWA的取樣頻率均為10 kHz。利用快照特徵正交分解法（Snapshot Proper Orthogonal Decomposition）與PIV數據將紊流結構分成若干頻率子集(subsets)。為了排除背景噪聲且從POD分析的高階模式中選擇正確的尺度紊流結構，需要使用連續小波變換（CWT）分析各階模態對應的時間係數。由於PIV的高取樣頻率（10 kHz），快照POD不僅可以描述尾流渦流脫落過程相關的相干性大尺度結構（第一對低階模式），還可以表徵泰勒微尺度的相干性結構(二、三次諧波高階模態)。本研究利用HWA數據通過自相關函數分析驗證高階模式的尺度是否達到泰勒微尺度，如下式：
├ (d^2 R)/(dτ^2 )┤|_(τ=0)=(-2)/γ^2
式中γ為泰勒微尺度，結果顯示使用上式所得數據卻能佐證流場之泰勒微尺度。在雷諾數分別為3,799和9,499下，本研究比較二維近域尾流大尺度與小尺度的重建結果並且討論之。
關鍵字：粒子影像測速儀、熱線測速儀、小波轉換、正交特徵分解法、相位平均、

The vortex formation and shedding process of air flow in the near wake region of a long circular cylinder, which is in nature a 2-D flow, is studied by means of a high-speed particle image velocimetry (PIV) and an X-type hot wire anemometry (HWA). Two cases with Reynolds numbers of 3,799 and 9,499 which are of turbulent flows but in the subcritical wake regime, are investigated. Both sampling rates of the employed PIV and HWA are 10 kHz. The snapshot proper orthogonal decomposition (POD) is applied with the PIV data to separate the turbulence structure into a number of subsets of frequency. To select correctly small-scale turbulence structures instead of background noise from the high-order modes of POD analysis, the time coefficients to which the high-order modes correspond have to be analysed using continuous wavelet transform (CWT). Due to the high sampling rate (10 kHz) of PIV, the snapshot POD can characterize not only the coherent large-scale structure (the first pair low-order modes), that is associated with the wake vortex shedding process, but also the Taylor microscale, which is of high-order mode in this study. Identification of the high-order mode of Taylor microscale in each case is made with the support through the auto-correlation function R(τ) made with the HWA data by the following formula
├ (d^2 R)/(dτ^2 )┤|_(τ=0)=(-2)/γ^2
where γ is the Taylor microscale. Different reconstruction results of the 2-D near wakes obtained from two Reynolds numbers of 3,799 and 9,499 are compared and discussed.
Key words: PIV; HWA; POD; Harmonic; Taylor microscale

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