水流流經植被區域之阻力研究

Abstract

水流流經植被渠道時，植被阻力分成浸沒式和非浸沒式二種。本研究著重在植被為浸沒式之情況，採用之植被模型為圓柱狀，改變植被橫向及縱向間距進行實驗，並探討不同流量對於內部流場所產被之阻力、流速變化、阻力係數與植被糙度變化之影響。 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，對於植被渠道流場之流速變化缺乏整體、有效的了解。因此本研究利用非侵入式彩色質點影像測速法(color particle image velocimetry, CPIV)，進行植被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，並利用流經植被前後之流速及水位變化，計算植被之阻力；除CPIV，並使用陳式自製移動平台(陳宥達,2008)，此平台連接至拉力計，當水流流經此平台，受到平台上植被影響而拖曳，此時拉壓力計讀得數值即為植被阻力。 經實驗結果，利用CPIV法得到流經植被區域前後之流速與水位，並計算得到植被阻力；並與陳氏自製阻力量測技術(陳宥達,2008)測得之植被阻力相比較進行驗證，可明顯得知植被阻力受到水深和流速影響，而阻力係數與相對植被密度比有上升之趨勢；達西威斯巴哈摩擦因子與相對植被密度比有大幅度上升之趨勢；曼寧值亦是如此。在植被排列方面，除了植被橫向間距對於達西威斯巴哈摩擦因子與曼寧值有很大之影響，植被縱向間距亦能影響之，縱向間距越小，植被之糙度越大。 關鍵詞：植被、彩色影像質點測速法、植被阻力

There are two types of the resistances of the vegetation: submerged and unsumberged. This study focuses on the submerged type to investigate the influence of different vegetated density on the vegetation resistance, the variation of velocity, and the variation of drag coefficient, Darcy-Weisbach factor, and Manning’s n. The cylinder is used as the model plant. This study changes the effects of lateral spacing and longitudinal spacing, discussing the different of the flow rate. 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 Except the CPIV, the study makes a moving platform (Yo-Ta Chen, 2008) that is connected force gauge, act as water and flow through this platform, vegetation it influences to be dilatory, force gauge read value (N) at the platform. The result is exported by the computer, examine the resistance of the vegetation directly. After measuring the vegetated-channel flow field with the CPIV method, the vegetation resistance can be calculated and be compared with the value of the measured (Yo-Ta Chen, 2008). The results demonstrate that flow resistance is greatly influenced by both depth and velocity but that the effects are opposite in sign. The drag coefficient increases in only a slightly nonlinear fashion with the relative density ratio (total vegetation stomatal area/bed cross sectional area), but Darcy-Weisbach factor increases in a highly nonlinear manner with increasing the relative density ratio, so does Manning,s n. Expect there are effects of the lateral spacing of the vegetation, the longitudinal spacing of the vegetation can also influence it. The less longitudinal spacing is, the bigger roughness of the vegetation is. Keywords: vegetation, CPIV, vegetation resistance