Numerical simulation of unsteady turbulent flow in axisymmetric sudden expansions

This paper is concerned with the numerical simulation of unsteady turbulent flows behind sudden expansions without inlet swirl. Time dependent simulat ions have been carried out using the VLES approach with the standard k - ep silon model. The expansion ratio investigated is in the range from 1.96-6.0 . The simulations show that the flows in axisymmetric sudden expansions are inherently unstable when the expansion ratio is above a critical value. Th e processing phenomenon, which features self sustained precession of the gl obal flowfield around the expansion centerline, is predicted successfully u sing CFD, with simulated oscillation frequencies that are in general agreem ent with reported data. For the case of expansion ratios from 3.5-6.0, a co mbination of a precession motion and a flapping motion in a rotating frame of reference is predicted in terms of the jet movment. Largescale structure s are identified in the downstream flowfield. Other important phenomena, su ch as the transition of the oscillation patterns, have also been predicted.