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Experiments of flow field and scour around the bending vegetation
|關鍵字:||倒伏植株;Bending Vegetation;紊流動能;沖淤型態;ADV;TKE;Scour;ADV||出版社:||水土保持學系所||引用:||Ackerman, J. D. and Okubo, A. (1993). “Reduced mixing in a marine macrophyte canopy,” Functional. Ecology, 7, 305-309. Angelina, A. J. and James, C. S. (2003). “Experimental study of bed load transport through emergent vegetation,” Journal of Hydraulic Engineering, 129, 6, 474-478. Brown, G. and Roshko, A. (1974). “On density effects and large structure in turbulent mixing layers,” Journal of Fluid Mechanics, 64, 775–816. Chen Su-Chin, Hsun-Chuan Chan, Yu-Hsiu Li, (2012), “Observations on flow and local scour around submerged flexible vegetation”, Advances in Water Resources, 43, 28-37 Elizabeth M. Follett and Heidi M. Nepf, (2012), “Sediment patterns near a model patch of reedy emergent vegetation”, Geomorphology, 179, 141-151 Ghisalberti, M. and Nepf, H. M. (2002). “Mixing layers and coherent structures in vegetated aquatic flows,” Journal of Geophysical Research, 107, NO. C2, 10.1029/2001JC000871. Green, J. C. (2004). “Modeling flow resistance in vegetated streams: review and development of new theory,” Hydrological Processes, 19, 6, 1245-1259. Ikeda, S. and Kanazawa, M. (1996). “Three-dimensional organized vortices above ﬂexible water plants,” Journal of Hydraulic Engineering, 122, 634-640. Kouwen N. and Unny, T. E. (1973). “Flexible roughness in open channels,” Journal of the Hydraulics Division, 99, 5, 713-728. Nepf, H. M. (2012). “Flow and transport in regions with aquatic vegetation,” Annual. Review of Fluid Mechanics, 44, 123-42. Nezu, I. and Onitsuka, K. (2001). “Turbulent structures in partly vegetated open-channel flows with LDA and PIV measurements,” Journal of Hydraulic Research, 39, 6, 629-641. Nezu, I., Sanjou, M., 2008. “Turbulence structure and coherent motion in vegetated canopy open-channel flows”. J. Hydro-env. Res. IAHR, 2, 62–90. Raudkivi A. J. and Ettema R., 1983, “Clear-water scour at cylindrical piers,” Journal of Hydraulic Engineering., ASCE, 109(3): 338-349. Raupach, M., Finnigan, J. and Brunet, Y. (1996). “Coherent eddies and turbulence in vegetation canopies: the mixing-layer analogy,” Bound.-Layer Meteorology, 78, 351-382. Rose, C. W., Yu, B., Hogarth, W. L., Okom, A. E. A. and Ghadiri, H. (2003). “Sediment deposition from flow at low gradients into a buffer strip - a critical test of re-entrainment theory,” Journal of Hydrology, 280, 33-51. Sand-Jensen, K. (2003). “Drag and reconfiguration of freshwater macrophytes,” Freshwater Biology, 48, 271-283. Winant, C. and Browand, F. (1974). “Vortex pairing: the mechanism of turbulent mixing-layer growth at moderate Reynolds number,” Journal of Fluid Mechanics, 63, 237–255. Zong Lijun and Heidi Nepf (2012), “Vortex development behind a finite porous obstruction in a channel”, Journal of Fluid Mechanics, 691, 368-391.||摘要:||
本研究在探討受水流衝擊後彎曲倒伏於底床之植株，對於底床之沖淤以及周圍流場之影響。實驗模型前端採用剛性之塑膠彎管模擬植株遭水流沖擊後根莖部份彎曲處，後段則使用柔性塑膠投影片模擬植株葉片，試驗模型總共分成葉片15公分、20公分與25公分三組，水流條件以底床粒徑之啟動流速分為高於以及低於啟動流速兩部分，在量測倒伏植株周圍流場狀況時採用的流速為低於啟動流速，而進行植株周圍泥砂運移觀測時採用的是高於啟動流速之條件。試驗過程中利用ADV(Acoustic Doppler Velocimeter)流速儀對於模型周圍進行流場量測，並使用雷射測距儀量測底床之沖淤型態，探討不同葉片長度下模型周圍流場變化以及沖淤情形。
This study experimentally investigated the effects of bending submerged vegetation on the characteristics of flow and configuration of deposition. The bending submerged flexible vegetation, which was made by plastic pipe and the P.P.C. film, and the length of the films are 15cm, 20cm and 25cm. Experimental flow velocity, sediment initial velocity (Vc), performed for two types velocities, higher than Vc and lower than Vc. Vertical distributions of time-averaged velocity and turbulent intensity at various streamwise distances were measured with an acoustic Doppler velocimeter (ADV). And use the laser distance meter to measure the bedform.
According to the experimental results, the longest blade will impact the flow field around the vegetation greater. The swing blade may disturb velocity between the X, Y and Z direction. There is a property of jet flow motions after the flow leaving the vegetation because the kinetic energy exchange. Swing motion and turbulence intensity increase with the blade length. The impact range also increase with the blade length, but the range will not be over the model high 1.5cm. When the flow entered the model, the turbulent flow began to develop toward the right side of the model, and began vibrating as it ran through it. It was similar to the Karman vortex, occurring after the flow runs through the obstacle and emerges behind it. The area with a low flow velocity around the blades could effectively trap the sediment that was carried away because of a local scour at the root section of the model.
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