CFD modeling of transient flow in pressurized pipes

Abstract

The aim of this article is the analysis of the hydraulic transient flows in pressurized pipes using Computational Fluid Dynamics (CFD). The model uses a three-dimensional efficient mesh with a refined mesh in the viscous sublayer, corresponding to the best compromise between the maximum accuracy and the minimum computational effort. CFD results have been compared with collected data from an experimental pipe-rig and an excellent fitting was observed, providing that a hyperbolic time-domain function be used to describe the valve closure. Calculated velocity profiles have shown two regions with different behaviors: the wall region, dominated by the fluid viscosity, in which flow changes are faster and with sharp gradients; and the pipe core, strongly dependent on the fluid inertial forces, that tends to maintain its initial steady-state shape and to have memory of the past time history of the velocity distribution. Immediately after the valve closure, the flow is cancelled in the valve section, an invert flux is generated and a vortex sheet is formed (i.e., a cylindrical surface composed of vortices in the circumferential direction), propagating to the upstream end. The transient wall shear stress has shown a strong dependence on the time history of the local velocity variation.