彩色質點影像測速法於明渠後陷階梯流場之試驗研究

Abstract

本文旨在利用非侵入式彩色質點影像測速法(color particle image velocimetry, CPIV)，進行明渠後陷階梯流場之垂直二維流場量測。CPIV法係以Argun雷射為光源，利用多彩聲光調變器（PCAOM）將雷射光源區分為藍、綠相間隔光源，再射至旋轉八面鏡形成光頁，並由彩色攝影機(CCD)擷取明渠後陷階梯流場之流體影像，進而影像分析與質問窗（interrogation window）之設定，獲得流場流速向量分佈，並利用中央差分的方式得到流場中的渦度分佈值。 流場經過後陷階梯，因為複合區內速度剖面變化非常急促，所以剖析此類流場若利用傳統方法如皮托管、熱線、熱膜、LDV等來量測將會十分困難，因此本文應用CPIV法來進行流場之研究，進行後陷階梯流場複合點(reattachment point)之實際位置與變動範圍，以及速度向量予以量測。最後將實驗數據及參數加以無因次分析，探討雷諾數及福祿數對複合長度(reattachment length)之影響，發現無因次複合長度(Xr/Hs)隨著雷諾數(Re)及福祿數(Fr)之增加而減短，其回歸公式如下： Xr/Hs=-0.9589Ln(Re)+13.943 ; Xr/Hs=-1.0889Ln(Fr)+5.2275

The purpose of this study is using non-intrusive color particle image velocimetry method (CPIV) to measure the 2-D vertical flow field in open channel flow over backward facing step. The CPIV method uses Argun laser as the light source, the CCD controlled by the PCAOM to take the instantaneous images of backward facing step flow field, and then obtain the velocity field through the set-up of interrogation window and image analysis. And then obtain the vorticity calculated by central difference base on the measured velocity components. The variation of the velocity profile is irregular in the reattachment zone, which occurs when flow passes a backward facing step. Hence, it is difficult to measure the flow characteristics using traditional measurement method, such as the pitot tube, hot film, and LDV. In this study, the CPIV method is adopted to measure the location of the reattachment point in the backward facing step flow. The velocity vectors of the flow field are also calculated. Finally, on the basis of experimental data and dimensional analysis, the effects of Reynold number (Re) and Froude number (Fr) on the reattachment length decreases as Re or Fr increases, and the regression formulas are following: Xr/Hs=-0.9589Ln(Re)+13.943 ; Xr/Hs=-1.0889Ln(Fr)+5.2275