Abstract
Air supply to free-surface water flows in tunnels is a key safety issue for high-head dams. The present study analyses the effects of approach water velocity and area residual of tunnel cross-section on air demand, using the volume of fluid (VOF) method in combination with the k–ε turbulence model. For a tunnel area residual less than 0.4, the attenuation of air velocity above the free-surface water flow decreases with an increase in water velocity. For larger area residuals, the air velocity distributions remain unchanged. The maximum ratio of mean air velocity to water velocity is obtained if the relative area residual is equal to 0.4. A relationship for the mean air velocity above the water flow is suggested. Without considering the flow aeration condition, the numerical simulations reproduce the air demand in the tunnel flow normally with an average error of ±25%. These results provide reference for air ventilation design of flood discharge tunnels.
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