以兩平行雷射切頁掃描求得穩態明渠流實驗的三維速度場

Abstract

本篇論文的目的是發展一個在穩態明渠流實驗中使用一台攝影機來測量三維速度場的新方法。此方法的原理是利用兩對平行雷射沿著渠道進行掃描，紀錄中性浮力的追蹤粒子通過雷射切葉的過程。追蹤粒子是為了監控在兩個雷射面之間每個粒子分別的軌跡。不同的顆粒軌跡均能利用觀察到的位移與經過時間計算出三個速度分量。 我們的方法藉由兩種版本呈現：一種是不移動的攝影機，適合用於較小視野的情況，而另一種是攝影機與兩個雷射切頁一同沿著渠道移動，適合用於有搭載滑車的長渠道上。 上述的解釋是關於實驗的步驟以及用來分析一系列圖片的方法。我們藉由比較不同斷面的體積流量以及對不動的顆粒進行掃描來驗證我們的方法。我們分別在使用了圓柱體及兩平板，固定底床與砂床，小渠道和大渠道中的不同明渠流場中取得結果。 此方法的可行性已經被證實，而特別之處是能成功的在障礙物及分離流區建立速度分佈。結果的品質與燈光的規劃控制、攝影機的率定與移動模擬結果、任何會遮蔽攝影機的障礙物存在與否以及粒子在流場中的分布情形有著強烈相關性。

The purpose of this thesis is to develop a new method to measure three-dimensional velocity fields in steady open-channel flume experiments, using a single camera. The principle of the method is to image the passage of neutrally buoyant tracer particles across a pair of parallel laser planes scanned along the flume. Particle tracking is used to monitor each particle path between its two laser plane intercepts. The three components of velocity can then be estimated by dividing the observed displacements by the elapsed time, different for each particle path. Two versions of the method are presented: one in which the camera is stationary, suitable for small viewing volumes, and one in which the camera is translated along the flume together with the laser light sheets, suitable for long flumes equipped with a carriage. The experimental steps are presented, as well as the imaging algorithms needed to analyze the acquired footage. The method is checked by comparing the volumetric discharge at different cross-sections, and by scanning stationary particles. Results are obtained for different open-channel flows around cylinders and between vanes, over rigid and sand beds, in small and large flumes, respectively. The feasibility of the method is demonstrated. In particular, the method can successfully map velocity deviations by obstacles and wake separation zones. The quality of the results depends strongly on the illumination, camera calibration and motion identification, the presence or absence of occluding obstacles, and the distribution of tracer particles in the flow.