Turbulence characteristics of flows passing through tetrahedron frames in a smooth open-channel
光纖雷射杜普勒流速儀 ； 明渠流 ； 透水框架 ； 總剪應力 ； 阻力係數 ； fiber-optic laser Doppler velocimeter ( FLDV ) ； open channel ； tetrahedron frame ； total stress ； drag coefficient
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台灣地勢陡峭，颱洪時期水流湍急，經常造成河床沖刷劇烈與水工結構物破壞。近年來四面體框架於水利工程設計之應用頗多，其特點為減速促淤，在拋投區可達到減少沖刷之效果。惟目前文獻中，關於四面體框架如何消能，及其水力特性與減沖促淤機制則有待更深入之研究。 本研究首先利用二維光纖雷射杜普勒流速儀 ( 2D-FLDV )，於光滑明渠流中量測水流通過單一四面體框架附近流場。除利用量測資料探討基本物理機制外，亦針對浸沒流場進行模式率定與驗證。量測資料顯示，四面體具有遲滯流速、擾動水流增加耗能與減小底床處總剪應力之功效。經修正Carollo等人 (2002) 明渠流通過水生植物之流速分佈理論，吾人可快速求出鄰近透水四面體框架下游處之流速剖面，並獲得合理模擬成果。此外，本研究配合 ANSYS CFX 11套裝軟體，可以LES模式初步模擬出單顆四面體附近之流場特性。 其次，量測多排數四面體群，除基本物理機制外，亦探討排數效應對流場之影響。實驗結果顯示，緩坡與陡坡條件下物理機制差異頗大，緩坡條件下僅需較少排數其減阻效應即可達極限值，而陡坡條件下排數效應影響較為顯著。此外，根據浸沒流場下之實驗量測資料，本研究提出減速率與阻力係數之相關迴歸式，藉此吾人可快速推估水流通過四面體群框架後之相關物理特性參數。
Due to the steep topography, rivers in Taiwan usually carry rapid flows during typhoon seasons, which frequently cause severe riverbed deformations and hydraulic structure failures. Recently the use of tetrahedron frames in the hydraulic engineering design has an increasing trend. The main characteristics of these tetrahedron frames include generation of turbulence, retardation of the flows and inducing sediment deposition near the structures. However, basic research related to the investigation of the physical mechanism of the related phenomena is still limited. First, a two-dimensional fiber-optic laser Doppler velocimeter ( 2D-FLDV ) was adopted to measure the flow field near a tetrahedron frame in a smooth boundary open channel. The experimental data were used not only to clarify the physical mechanism of the flow retardation caused by the tetrahedron frame, but also to calibrate the simple velocity-profile model under the submerged flow conditions. The experimental results revealed that the tetrahedron frame can reduce the mean longitudinal flow velocity, boundary shear stress, and increase the turbulence intensities. With proper modifications of Carollo et al.’s (2002) theory for channel flows passing through vegetation, the velocity profiles immediately downstream of the tetrahedron frame can be quickly predicted with reasonable accuracy. Moreover, the flow field near a single tetrahedron can be approximately predicted by the LES model in ANSYS CFX 11 software package. As to the tetrahedron frame group tests, the experimental data were used to clarify the physical mechanism and the effect of row number on the flow. The experimental results indicated that only few rows of tetrahedron frames are needed for the deceleration rate to reach a limit under a mild slope condition. Conversely, the effect of row number is sensitive for a steep slope condition. Furthermore, the multiple regressions of deceleration rate and drag coefficient have been performed using the measured data. The developed empirical equations can be applied to predict the physical parameters for the flows passing through the tetrahedron frames.
工程學 > 土木與建築工程