An algorithm for calculating air demand in gated tunnels using a 3D numerical model

Abstract

Aeration of high-speed flows issued from gates in tunnels is necessary to prevent high negative pressures and their consequences such as cavitation and vibration. In this research work 3D numerical modeling of complicated air–water flow in a gated tunnel was performed employing FLUENT computer code. Simulation of the high-speed free surface flow was conducted using VOF method with Young’s scheme. Owing to high Reynolds numbers associated with the flow through the gates, the standard two equation k– ε turbulence model was employed. The air–water flow was simulated for both circular and rectangular tunnel cross-sections. Using the results of the numerical model, the rate of air discharge through the vent was determined and validated by available experimental data. The comparison of results with experimental data showed very good agreement. From calculation of the air flow along the tunnel at different conditions, an equation was developed for the velocity distribution of the dragged air over the free surface and the air supplied from the tunnel outlet. The amount of dragged air and air supplied from the tunnel outlet was then calculated knowing the velocity distribution and the area of the air flow. Subsequently an algorithm was presented to design the air supply system in gated tunnels both for circular and rectangular cross-sections. The algorithm was then employed to calculate air demand in number of bottom outlets and comparison with experimental data showed good agreement.