彩色質點影像測速法於浸没植生渠道流場之試驗研究

Abstract

水流流經植生所產生之流場變化極為複雜，於天然河川中，由於植生種類更為多樣性，不確定性之物理量更多，植生部分包括根莖直徑、高度與植生間距等因素，因此，本研究將針對水流通過低莖植生表面之水流現象進行研究，並利用非入侵式彩色質點影像測速法(color particle image velocimentry，簡稱CPIV)之量測方法進行通過植生表面之流場觀測研究，了解植生渠道流場之流速變化。並利用攝影機(CCD)擷取流體影像，進而影像分析，以獲得流場之速度向量分佈與相關之物理量分析，從其中得知渠道植生表面對水流之相關影響。 本研究以兩種不同底床模型進行試驗，分別為光滑平板及水草莫絲，後者之植生高度約為1.5cm。入流量分別為60、80、100（L/min），上游水深分別為5及6cm，共12組試驗案例，比較兩不同底床渠道，植生渠道因底床受到植生影響，通水斷面束縮進而影響流速變化與曼寧n值之差異。此外，根據試驗結果可明顯看出植生渠道因受植生阻力影響，而能量損失率變大，尤其當流量增加或者水深降低時，能量損失率也隨之變大，其底床平板渠道之能量損失平均為9.81%，植生渠道之能量損失平均為11.81﹪，因此，在低莖植生表面之研究中，植生表面不僅會造成水流於近植生表面之流場之改變，亦有消能之功效，對於渠道的穩定有相當之幫助。

Flows through vegetations have extremely complicated variation and are affected by the diameter, height, and spacing of the vegetated stems. Hence, this study tries to understand the phenomenon of varied flow velocities passing the submerged vegetations, using the non-intrusive color particle image velocimetry method (CPIV) to measure the vertical 2-D flow field. The CCD takes the instantaneous images of vegetated channel flow field and makes analysis of the images to obtain the distribution of velocity vectors and the related physical properties in the flow field.

The experiment in this study is constructed under two types of bed materials: one is a smooth flat panel and the other is a vegetated one with a height of 1.5 cm. These two types of bed materials are compared with twelve sets of experimental cases. The flow discharges in the experiment are controlled with levels of 60, 80, and 100 (l/min) with flow depths of 5 and 6 cm in the upstream boundary conditions. From the results, the average energy loss is 11.81% in the case of vegetated bed and 9.81% in the bed with smooth flat panel. Energy loss is accelerated with increasing discharge or decreasing depth. The surface of the vegetation could change the flow field and dissipate energy.