彩色質點影像測速法於植生渠道流場之試驗研究

Abstract

水流流經植生渠道時，植株之反應有直立、擺動及倒伏等三種現象，本研究著重在植株直立情況下，探討不同植生密度對流速變化之影響。本研究採用圓柱狀之聚稀系樹脂為植生模型，其直徑為0.65cm高3cm，共設計7組實驗案例，以特徵體積比來代表植生排列密度，其定義為單位水體積中含有植株體積之比值，流量控制在70(l/min)∼90(l/min)，上游水深控制在5cm。 由於水流受到植株擾動，其流場變化瞬息萬變，若使用單點之量測法量測，對於植生渠道流場之流速變化缺乏整體、有效的了解。因此本文利用非侵入式彩色質點影像測速法(color particle image velocimetry, CPIV)，進行植生渠道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，射至旋轉八面鏡後形成光頁，並由彩色攝影機(CCD)擷取植生流場之流場影像，進行質問窗(interrogation window) 設定與影像分析，以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之渦度分佈值。 經由CPIV法對植生渠道流場進行量測後，可發現渦度值較強之區域，集中在水體與氣體交界處、渠道底床處及近植株處。此外，分析在植株頂端以上流場平均及最大流速變化以及最大流速距水面下距離與特徵體積比間均為指數迴歸關係式，且均具高相關係數。

There are three types of the reactions of the vegetation: erect, waving and prone when flow passes vegetated channel. This study focuses on the erect type to investigate the influence of different vegetated density on the variation of velocity. The cylinder of resin is used as the model plant, with diameter of 0.65 cm and height of 3 cm. There are seven sets of vegetated density. The characteristic volume ratio is used to represent the vegetated density, and it is defined as ratio of the plants’ volume to unit fluid volume. The flow discharge in the experiment is controlled in the range of 70(l/min)∼90(l/min) and the flow depth is 5 cm in the upstream boundary. Because the flow is affected by the vegetation, the flow field changes rapidly. If the traditional single-point measurement at a time is adopted in the experiment, the variation of the velocity field in the vegetated channel cannot be obtained completely and effectively. Hence, this study adopts non-intrusive color particle image velocimety (CPIV) to measure the vertical 2-D flow field in the vegetated channel. The CPIV method uses Argun laser as the light source, the PCAOM separates the light into blue and green lights and they form a light sheet after emitting on a rotating eight-side mirror, the CCD takes the instantaneous images of vegetated channel flow field, and then obtains the velocity field through the set-up of interrogation window and image analysis. Finally, the associated vorticity can be calculated by the central difference scheme based on the measured velocity components at each point. After measuring the vegetated channel flow field with the CPIV method, the stronger vorticities occur in the interface between air and fluid, near the bed of the vegetated channel and the vicinity of the model plant. In addition, the relationships among the increasing rates of the mean and maximum velocities and the distance from the water surface of the maximum velocity at the top of the third-row model plant with respect to the characteristic volume ratio are also analyzed. It can be found that the exponential relationships exist with high correlation coefficients.