CFD simulation of precession in sudden pipe expansion flows with low inlet swirl

Abstract

In this paper, three-dimensional, time-dependent calculations are carried out using the finite volume CFD code CFX4 and the VLES approach with standard k– ε model to simulate the turbulent swirl flow in an axisymmetric sudden expansion with an expansion ratio of 5.0 for a Reynolds number of 10 5. This flow is unstable over the entire swirl number range considered between 0 and 0.48, and a large-scale coherent structure is found to precess about the centerline. Compared with the unswirled case, inclusion of a slight inlet swirl (swirl number below 0.23) can reduce the precession speed, cause the precession to be against the mean swirl and suppress the flapping motion. Several modes of precession are predicted as the swirl intensity increases, in which the precession, as well as the spiral structure, reverses direction. Accompanying the transition between different modes, abrupt changes in precession frequency are also experienced. Grid sensitivity and comparison with smaller expansion ratio data are also discussed.