Comparison of 2-D and 1-D Simulations of Water-Hammer Pressure Pulse Damping Using Steady and Unsteady Friction Terms

The condition known as water-hammer problem is in fact, a transient condition which may occur as a result of the worst-case loadings such as pump failures, valve closure in a pipe line system. In such cases, a positive pressure wave travels upstream and a negative wave downstream from the point of interruption. The pressure in the water hammer can vary in such a way that in some cases, it may increase and cause destruction to the hydraulic systems. The experimental evidence of dependence of friction on frequency in oscillating flows induced researchers to propose 1-D models in which this effect is taken into account explicitly. In fact, the velocity profiles in unsteady flows show greater gradients, and thus greater shear stresses, than the corresponding values in steady flows. 1D- model approaches give rise to an under- estimation of the friction forces.In this work, a 2-D model for unsteady transient flow in a pipeline with a reservoir-pipe-valve system was simulated for laminar and turbulent flows. In turbulent flow, the Prandtl mixing length was used for turbulence modeling. Results of this simulation were compared with the results obtained from the 1-D models with steady and unsteady (Brunone model) friction terms. This simulation shows that the unsteady flow phenomena can be more accurately modeled by means of 2-D models, in which the velocity profiles in the cross section, is taken into account. However, the 2D model is more time consuming than 1D model. Through comparisons with the experimental data, results show that applying 2D model can improve the magnitude of the predicted duration of the pressure pulse.