Numerical study on periodic pressure fluctuations driven by air–water interactions in a dropshaft-tunnel system

Air–water interactions in deep tunnel systems can cause safety risks. However, the multiphase flow dynamics under continuous air entrainment from upstream and release from downstream, which are the typical case in engineering practice, remain insufficiently understood. In this study, a three-dimensional computational fluid dynamics model was developed and validated based on an experimental study to investigate the periodic air–water behaviors in a dropshaft-tunnel system. With the numerical model, the features of air entrainment and release across various flow regimes were quantified, and the evolution of air–water flow corresponding to different stages of pressure fluctuations was clarified, which could hardly be measured from experiments. The simulation results revealed that the periodic pressure fluctuations were driven by the cyclic process of slug flow development in the tunnel, and the amplitude was mainly governed by the amount of water rushing into the air release shaft. Furthermore, the effects of different layout designs on the pressure fluctuations were explored numerically. It was found that increasing the distance between the air entrainment and release points could significantly amplify the pressure amplitude and prolong the fluctuation period. These findings contribute to new knowledge on air–water flow dynamics and design considerations of deep tunnel systems.