For design and management of large scale tunnels, it is important to model the transitions between pressurized and free-surface flows inside them. The pressurized and free-surface flow regimes are separated by a discontinuity with different wave speeds on two sides, which makes efficient and accurate simulations still challenging. A robust and conservative space-time adaptive mesh refinement (AMR) strategy is proposed to simulate such transient mixed flow. In the present AMR, the mesh resolution is refined adaptively near the interface between two flow regimes, and the flow states at child meshes are determined by a conservative approach which can avoid numerical oscillations. The numerical results from the AMR strategy are validated with analytical and experimental results, which indicates its good ability to capture the interface between two flow regimes and prevent numerical oscillations. The numerical test in a field scale tunnel shows that AMR can speed up simulations of transient mixed flow by 60 times.

Transition between free-surface and pressurized flows is a crucial phenomenon in many hydraulic systems. During simulation of such phenomenon, severe numerical oscillations may appear behind filling-bores, causing unphysical pressure variations and computation failure. This paper reviews existing oscillation-suppressing methods, while only one of them can obtain a stable result under a realistic acoustic wave speed. We derive a new oscillation-suppressing method with first-order accuracy. This simple method contains two parameters, P_a and P_b, and their values can be determined easily. It can sufficiently suppress numerical oscillations under an acoustic wave speed of 1000 ms⁻¹. Good agreement is found between simulation results and analytical results or experimental data. This paper can help readers to choose an appropriate oscillation-suppressing method for numerical simulations of flow regime transition under a realistic acoustic wave speed.

The emergency control of Menglou~Qifang inverted siphon, which is about 72 km long, is the key to the safety of the Northern Hubei Water Transfer Project. Given the complicated layout of this project, traditional emergency control method has been challenged with the fast hydraulic transient characteristics of pressurized flow. This paper describes the application of model predictive control (MPC), a popular automatic control algorithm advanced in explicitly accounting for various constraints and optimizing control operation, in emergency condition. For the fast prediction to the pipe-canal combination system, a linear model for large-scale inverted siphon proposed by the latest research and the integrator-delay (ID) model for open canals are used. Simulation results show that the proposed MPC algorithm has promising performance on guaranteeing the safety of the project when there are sudden flow obstruction incidents of varying degrees downstream. Compared with control groups, the peak pressure can be reduced by 17.2 m by MPC under the most critical scenario, albeit with more complicated gates operations and more water release (up to 9.75 × 10⁴ m³). Based on the linear model for long inverted siphon, this work highlights the applicability of MPC in the emergency control of large-scale pipe-canal combination system.

Numerical simulation of free-surface and pressurized mixed flow in urban sewer networks is important for the protection of urban areas. The numerical model needs to accurately represent the flow regime transitions and maintain horizontal still water conditions when the sewer network is closed (i.e., the well-balanced property). This paper presents a well-balanced finite volume model for sewer networks based on the HLLS (Harten–Lax–van Leer + Source term) solver. A novel boundary solver is proposed, which directly incorporates the source term to maintain stationary flow conditions and efficiently manages transitions between various flow regimes at interfaces between pipes and junctions. In addition, the update procedure for pipes and junctions to guarantee mass conservation and solution stability is illustrated. The numerical model is tested in various scenarios to validate its accuracy and stability. The results demonstrate that the model can accurately predict the sewer network filling and emptying while maintaining the well-balanced property.