Numerical simulation of 3D flow field and fluctuating pressure on radial gate under discharge
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Abstract:
Radial gates are important water retaining and discharging structures in hydraulic engineering. Although radial gates are manufactured to have sufficient stiffness for the design water pressure, large vibrations can be produced due to water pressure fluctuation during discharge causing damage to the gate, especially when the dominant frequency of water pressure fluctuation approaches the natural frequency of the gate, which may cause resonance phenomenon. The fluctuating pressure of water flow under local opening on the radial gate panel is a frequent cause of gate vibration. In the past few decades, the hydraulic characteristics such as average pressure distribution, discharge capacity and flow field around the radial gate have been widely studied through numerical simulation, but the fluctuating pressure acted on the panel of the radial gate has not been solved yet. In order to calculate the fluctuating water pressure, a numerical model of 3D turbulent flow field around radial gate was established using two-equation turbulent model and volume of fluid method for free surface. Two consecutive runs of a steady-state run and a time-dependent transient run were carried out in order to determine the flow velocity at the inlet. Two turbulence models (i.e., k-ε turbulence model and k-ω turbulence model) were applied in the current study, and the accuracy of the k-ε and k-ω turbulence models for the simulation of fluctuating pressure was evaluated and discussed. Based on the k-ω turbulence model, the impact of downstream water level changes on the flow field and fluctuating pressure were investigated. The generation of fluctuating pressure showed close relation to the flow in the boundary layer near the radial gate panel. Reasonable selection of turbulence models and models with near-wall modifications is extremely important for the accuracy of calculating fluctuating pressure results. The combination of k-ε turbulence model and wall function was unable to capture the pressure fluctuating behavior on the gate panel, while the k-ω turbulence model combined with integration method can not only model the flow field around the gate, but also accurately calculate the fluctuating pressure, because of its better performance in the case of boundary-layer flows with a strong adverse pressure gradient. At the stable discharge stage, a large vortex was formed in front of the gate, was the main cause of the fluctuation of water pressure on the gate panel. The amplitude of water pressure fluctuations was influenced by the outflow form of the gate hole and the water level difference between upstream and downstream. Under submerged outflow conditions, a larger water level difference resulted in a higher root mean square value of fluctuating pressure. Conversely, under free outflow conditions, a larger water level difference led to a lower root mean square value of fluctuating pressure. The dominant frequency of fluctuating pressure at each point on the panel under the same operating conditions was identical and was mainly dependent on the orifice flow pattern of the sluice, but independent of the water level difference between upstream and downstream. Under submerged outflow conditions, a large counterclockwise vortex was formed behind the gate, but under free outflow conditions, no obvious vortex will form behind the gate. The dominant frequency of pressure fluctuation under free outflow conditions was higher than that under submerged outflow conditions.
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