Water Hammer Simulation Using Explicit-Implicit Coupling Methods

The method of characteristics (MOC) with the limitation of Courant's stability condition is widely used in simulation of unsteady flow in a pipeline. However, the relatively complex method of implicit (MOI) provides the advantages of unconditional convergence and mutual independence between time and space mesh parameters. This study combines the MOC and MOI to simulate pipeline unsteady flow and hydropower transient processes. The boundary conditions for the coupled method are introduced and validated by simulating the water hammer in uniform and variable area duct, and the water-level fluctuation in a surge tank. Subsequently, the coupled methods are applied to study the transient processes in two hydropower stations: one is to determine the water-level fluctuation in the surge tank and investigate the effect of water inertia in the connecting pipe on the water hammer pressure, and the other is to determine the solution for the water hammer in a variable-area draft in a pump turbine system by comparing the results with actual measurements. The results show that the coupled method is effective in simulating water hammer in pipelines and transient processes of hydropower system. (C) 2014 American Society of Civil Engineers.