應用影像技術分析濕地流場及物質傳輸變化

Abstract

表面流人工濕地之水力效率與其水力表現有關，意即不同配置下的流場型態與物質傳輸行為才是影響水力效率的主要原因。過去研究多探討不同障礙物、植生、形狀、入出流配置等靜態配置對水力效率的影響，並且常用數值模式來模擬人工濕地之流場與物質傳輸過程；本研究提出動態擾動的配置，以此反映生物利用對於人工濕地之水力表現與物質傳輸過程的影響，受限於數值模式無法模擬動態擾動的限制，本研究建立36場水槽試驗，利用「顏色—濃度預測」與「粒子影像分析」兩個影像技術來獲得全域隨時間變化的流場資料與物質傳輸過程，以解決過去水槽試驗難以獲得全域實驗資料的缺點。水槽實驗結果不僅能夠完整呈現污染物進入人工濕地之後的物質傳輸過程，更能依據內部流場之水理特性將流場分為優勢流、環流區與死水區，對應不同水理特性有不同的主要物質傳輸行為，分別為平流效應、渦流擴散與延散、分子擴散；另外，本研究亦說明了死水區中的物質消散時間受到死水區大小、環流邊界之流速梯度、水深與擾動的影響，對於不同配置下的物質累積問題進行評估；值得一提的是，動態擾動對於物質的傳輸過程有著明顯的不同，加入動態擾動能夠改變局部的物質傳輸行為，能夠有效改善物質累積問題。本研究針對不同的流場型態進行物質傳輸過程的探討，並且發現在不形成累積問題下環流有助於提升水力效率，若有累積問題時，亦可透過動態擾動來縮短消散時間、改善局部的物質傳輸行為，對於人工濕地中的污染物累積問題與後續研究提供水理與物質傳輸過程的觀點。

The treatment efficiency of Free-water surface constructed wetlands (FWSs) cannot be determined without understanding the flow dynamics of individual parcels of water through the wetland. Previous studies have indicated that hydraulic efficiency influences treatment performance, and most of them evaluate hydraulic efficiency with numerical models under different arrangement such as obstructions, vegetation, aspect ratio, shape and inlet-outlet configuration. In this study, we propose a novel dynamic disturbance to show how disturbance from creatures affect hydraulic efficiency. Thirty-six flume experiments were conducted with Reynolds similitude. Different water depth and emergent obstructions were placed to create various flow characteristic, including one kind of innovative disturbing arrangement. We applied two kinds of image techniques, namely color-concentration prediction and particle image velocimetry (PIV), to analyze the pulse input tracer experiment. The results show that image techniques are acceptable for providing time-variant and full-field data, which used to be challenging for flume experiments. In the experiment, we found different sizes and strengths of flow circulation appeared in every case. According to the hydrodynamic of the flow, we then classify the flow characteristic as preferential flow, dead zone, and circulation. Afterward, we identify the contaminant transportation effects as advection, molecular diffusion, eddy diffusion, and dispersion correspondingly. The dissipation time of dead zone is related to its size, velocity gradient of circulation, water depth, and disturbing. We conclude that circulation is positive for wastewater treatment under the condition of no accumulation problem (or low dissipation time). If the accumulation problem is severe, we also provide a solution of adding disturbing in order to change the contaminant transportation condition of circulation, to reach a shorter dissipation time.