Transient pressures of multiple entrapped air pockets in a tunnel with double-side inflows: a numerical investigation

ABSTRACT Deep drainage tunnel systems have been adopted as effective flood control infrastructure for conveying and storing excess rainwater. However, air in tunnels is easily entrapped during rapid filling under double-sided inflow conditions, causing unpredictable pressure oscillations. This study aims to quantify the transient pressures of multiple entrapped air pockets, and a 3-D CFD model with two shafts and one tunnel was first constructed and validated. Subsequently, numerical simulations are performed for both uniform and non-uniform air volume distributions in tunnels with variations of air pockets numbers and positions. The results showed that multiple air pockets undergo varying degrees of deformation, splitting, and displacement during the water filling process. The pressure oscillations are influenced by the total air volume, spatial arrangement, numbers, and sizes of air pockets, and generally exhibit irregular patterns. The maximum pressure exceeds that of a single pocket with the same total air volume in some cases; but in most conditions, the relative extreme pressure ranges between 0.85 and 1.00. The peak pressure of uniformly distributed air pockets is not necessarily higher than that of non-uniformly distributed ones, and the air pockets in odd-numbered cases of this study often generate higher peak pressures.