| Summary: | 碩士 === 國立中興大學 === 土木工程學系所 === 98 === ABSTRACT
The installation of a vertical drop pool in an open channel is aimed to dissipate the flow energy, which can diminish the destruction of the down-stream channel. Many previous experimental works have been conducted on the hydraulic properties of the flow over the drop pool. This paper tells that FLOW-3D solver, the numerical model based on coupling the RANS equation and the RNG turbulent transport equation, is presented to analyze the characteristics of the skimming flow over the drop pool. The simulated results presented in the paper are first verified according to previous results of flow visualization. The characteristics of flow fields, including velocity distributions, vorticity distributions, turbulent energy dissipations and pressure distributions etc. are then analyzed. The empirical formula of the energy loss between the approaching flow and the down-stream flow is also presented in this paper.
The present simulations are conducted for a specified supercritical approaching flow on a fixed drop height H but different heights of the end sill h. As a result, the ratios h/H are ranging between 0 and 1. The fixed length of the drop pool is considered in the simulations. The simulated results show that an air pocket exists between the falling jet and pool if the ratios h/H are smaller than 2/14. The skimming flow will be formed in the cases of h/H ≧ 4/14 that the dropping flow slides over the pool and forms a sliding jet. The momentum exchange between the sliding jet and the pool formed the clockwise circulation flow in the pool. The maximum vorticity occurs at the impact zone near the end sill in the case of falling jet, but in the case of skimming flow, the maximum vorticity occurs at the drop corner where sliding jet starts. It is also found that the turbulent energy dissipations take mainly place between the sliding jet and the pool, or that perhaps the turbulent energy dissipations take place at the momentum exchange zone. The results of pressure distributions show that negative pressure occurs in the falling jet which air pocket exists. The pressure on the wall of the end sill is found to be pseudo hydrostatic distribution for larger h/H. As for the energy loss due to the vertical drop, it is found that the head loss decreases as the h/H increases. However, the supercritical flow with high Froude number exists in the down-stream, implying that much attention should be paid to the protection of bed scour.
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