Turbulence characteristics of flows passing through tetrahedron frames in a smooth open-channel

• Main Authors: Tien-Feng Chang, 張天峰
• Other Authors: 盧昭堯
• Format: Others
• Language: zh-TW
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/33672951415333089568

博士 === 國立中興大學 === 土木工程學系所 === 100 === 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.