CFD Investigations of Transient Cavitation Flows in Pipeline Based on Weakly-Compressible Model

In hydraulic systems, transient flow often occurs and may results in cavitation in pipelines. In this paper, the Computational Fluid Dynamics (CFD) method based on the Fluent software was used to investigate the cavitation flow in pipeline; the density-pressure model was incorporated into the continuity equation by using further development of UDF (user defined function), which reflects the variable wave speed of the transient cavitation flow, and the related algorithms were established based on weakly compressible fluid Reynolds Average Navier-Stokes (RANS) techniques. Firstly, the numerical simulations of the transient non-cavitation and cavitation flows caused by the fast closing valve in the reservoir-pipe-valve system were carried out by using the grid slip technique. The simulation results can enrich the flow field information such as velocity, pressure and vapor volume fraction. Through the evolution process of the pressure field, the propagation characteristics of pressure waves can be analyzed qualitatively and quantitatively. Through the evolution process of the velocity field, it can be seen that the velocity distribution in the wall area changes rapidly and has a high gradient, which mainly depends on the viscosity. However, the change of the velocity distribution in the core region is related to the velocity distribution of the history of the past time, which mainly depends on the diffusion. The formation, development and collapse of the cavity can be successfully captured, and it can be clearly and visually observed that the uneven distribution of vapor cavity in the direction of pipe length and pipe diameter, and the vapor cavity move slowly along the top of the pipe wall. Rarefaction wave's propagation into pressure decreasing region and pressure increasing region can lead to different results of cavitation flow. The accuracy and reliability of the weakly compressible fluid RANS method were verified by comparing the calculated results with the experimental data.