Investigation of Non-Swirling and Swirling Turbulent Jet Flows using Unified LES-RANS Models

Abstract

Large-eddy simulation (LES) has proven to be an accurate and computationally feasible approach for swirling turbulent jet flow simulations. However, the inflow conditions of swirling turbulent jet flows are often determined by a nozzle flow, which has a significant influence on the jet flow development. LES simulations of the nozzle flow is often infeasible: the simulation of near-wall fluid motions requires computational costs that are comparable with those of direct numerical simulation. Hence, nozzle flow simulations have to be performed using Reynolds-averaged Navier-Stokes (RANS) methods. The numerical predictions of RANS models are unreliable when no experimental or benchmark data is available for comparison. Hence, there is a need for the development of models which can simulate both the nozzle flow and the jet flow region accurately. This study investigates the combined numerical simulation of a nozzle flow and swirling turbulent jet flow by a unified turbulence model. The unified turbulence model combines RANS methods in the near-wall region with LES methods away from the wall. The numerical simulations performed using the unified model have a much lesser computational cost than the combined nozzle and jet flow simulations using LES. The accuracy of the numerical predictions are validated against experimental data for non-swirling and swirling turbulent jet flows. The application of the validated model to studies of the mechanism of swirl effects shows the following. Swirl breaks apart the typical ring structures of non-swirling turbulent jets into two modes: a helical mode and streamwise braid structures. The interaction of these two modes generates unorganized turbulence that enhances the turbulent mixing.