本實驗為研究背向階梯流場的結構，以熱線測速儀和質點影像測速儀兩種不同的流場量測法取得流場資訊。實驗流場條件含兩種階梯高以及兩種進口速度，且寬高比皆大於10以確保二維流場特性。熱線測速儀之交叉探針（Dantec, 55P61）由於適用流向角度限制，只能量測剪流層以外的區域；裂膜探針（Dantec, 55R55）則可以探索流場的逆向流，所以剪流層以及主迴流區以此探針量測的資訊為主。本文也建立了裂膜探針之校驗法則，改正前人使用此探針的錯誤量測。而質點影像測速儀之量測結果，將用來探討顆粒在剪流層以及再接觸區因顆粒碰撞或堆積，造成速度量測上的誤差，並以熱線測速儀的量測結果做為比較基準。
實驗結果皆以進口條件12 m/s、階梯高15 mm當作主要討論之數據。熱線測速儀設定取樣頻率為10 kHz（55P61）、6 kHz（55R55），取樣時間為5秒。質點影像測速儀設定雙曝光模式每秒拍攝8000張，4000組資料，每組兩張的時間差為40 μ秒，取樣時間為2.5秒。質點影像測速儀的演算法則使用多重網格疊代逼近，網格會以前次的位移量做變形。以舊有標準一次交互相關所得到的結果，在速度梯度大的情況下，交互相關性峰值下降、變寬、分裂成多個峰值，以至定義位移量就會有極大誤差，然而新的演算法能夠解決速度梯度大的問題。又顆粒當做流場裡的追蹤質點，就必須考慮剪應變大之區域顆粒會堆積造成碰撞效應。可以利用概率密度函數證明顆粒來驗證碰撞效應，還可以檢查受侵入式量測所干擾之區域。而在高速流場下，也須要考慮裂膜探針之圓柱熱線產生的渦流溢放效應。

Experimental study of the backward-facing step flow field is conducted by means of two different flow measurements which are Hot Wire Anemometry (HWA) and Particle Image Velocimetry (PIV). The flow conditions including four different inlets Reynolds numbers, Reh, which are altered by using different inlet mean velocities and the step heights. The aspect ratio is larger than 10 to ensure quasi-2D motion of flows. Due to the flow angle limitation, the cross wire probe in conjunction with CTA system measures the free stream flow regions mainly in the flow field of backward-facing step, which the split fiber film is applied to the shear layer and recirculation zone. This study corrects Hsia’s method (2010) and establishes the calibration of split fiber film in the recirculation zone. The PIV results will be used to investigate the effects of collision and accumulation of the seeding tracers, and HWA database will serve as the standard basis for comparison.
The present results and discussion is focused on the case with 12 m/s inlet velocity and 15 mm step height. HWA sampling frequency is 10 kHz in the cross wire probe and 6 kHz in split fiber film probe, respectively. The sampling times of the both instructive probes are 5 seconds. PIV is set with the double exposure mode and the rate of 4000 samples per second, the time interval between two images in every sample is 40 micro seconds, and the sampling time is 2.5 seconds. PIV algorithm is taken with the multigrid interrogation and iteration in replacement to image deformation. In standard cross-correlation interrogation method, the peak broadens and splits into several individual peaks in the larger velocity gradient flow, but new techniques improve these problems greatly. By comparing the probability density functions (PDF) taken by the PIV and HWA, remarkable different shapes of these two PDF’s are observed in the recirculation zone which similar PDF shapes of these two PDF’s are observed in the free stream region. It implies that effects of inter particle collisions become significant in the recirculation zone and lead to the errors in the PIV measurements.In the high speed flow regions, the split fiber film measurements should consider the vortex shedding effects in the data analysis.

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