Air–Water Flows in a Deep Tunnel Downstream of a Dropshaft

Dropshafts of deep tunnel systems can entrain a large amount of air, and the resulting air–water interactions cause potential safety issues. In this study, the air–water flow characteristics along a tunnel under the effect of air entrainment of an upstream dropshaft and air release from a downstream airshaft were investigated experimentally with a large-sized physical model. Various regimes of air–water flow along the tunnel under different conditions of water inflow rates and downstream water levels were observed. The pressure fluctuations along the tunnel exhibited periodic features in amplitude, which were analyzed with variational mode decomposition and the Hilbert transform. The cause of pressure fluctuations in the tunnel was then explored. The results showed two major peaks of the pressure data corresponding to the frequencies of 0.2 and 1.8 Hz. The pressure component of 0.2 Hz was caused by the slug frequency of the air–water flow, and the other was related to the periodic air release from downstream in the form of air pockets. The findings of this study can contribute to a better understanding of the air–water flow features in deep tunnel systems.