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Abstract
Stepped spillways are commonly used under relatively low unit discharge, where cavitation pitting can be avoided by self-aerated flow. However, there are several dams in China with stepped spillways in combination with X-shaped flaring gate piers with unit design discharge considerably larger than specified in the available guidelines. Consequently, air–water two-phase flow on stepped spillway behind X-shaped flaring gate piers under very high unit discharge was investigated using Computational Fluid Dynamics (CFD) simulations. The 3-D Reynolds-averaged Navier–Stokes equations were solved, including sub-grid models for air entrainment, density evaluation, and drift-flux, to capture self-aerated free-surface flow over the spillway. The pressure on the vertical step faces was compared with laboratory data. In addition, the air–water two-phase flow characteristics and prototype step failure of the simulated prototype spillway were analyzed based on the numerical results of velocity, pressure, and air concentration. Moreover, an optimized bottom-aeration was further studied. The results reveal that the involved models can predict the air concentration near the steps. The cavitation index at the stepped surface is below the threshold value, and the air concentration is insufficient under high unit discharges. Moreover, with the proposed optimization of the aerator air entrainment can be improved and thereby cavitation erosion risk can be reduced.
Highlights
Stepped spillways are commonly used hydraulic structures owing to their construction convenience and excellent energy dissipation performance [1] below the critical unit discharge of roughly 25 m2 ·s−1 [2]
Chen [30] successfully simulated the flow over typical stepped spillways using a for practical design and operation, especially in view of large-scale effects between the laboratory k-ε turbulence model and the volume of fluid (VOF) method
FLOW-3D®, the air entrainment is combined with a mixture model for the single-phase fluid, where air is added to the fluid as passive tracer, i.e., without directly affecting the fluid flow but changes the density of the fluid depending on the air concentration
Summary
Stepped spillways are commonly used hydraulic structures owing to their construction convenience and excellent energy dissipation performance [1] below the critical unit discharge of roughly 25 m2 ·s−1 [2]. Simulated skimming flow over a model and bubble transport model, and presented general distributions of the velocity and pressure pooled stepped spillway with four types of pool weirs, but no air entrainment was considered. Li et al [26] simulated skimming flow over a pooled stepped spillway with et al [27] adopted the two-phase consistent particle method (CPM) for the simulation of a large dam four types of pool weirs, but no air entrainment was considered. Chen [30] successfully simulated the flow over typical stepped spillways using a for practical design and operation, especially in view of large-scale effects between the laboratory k-ε turbulence model and the volume of fluid (VOF) method. The step failure at Ludila dam was analyzed in prototype scale based on the numerical results and an optimization of the corresponding bottom aeration was further studied
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