LES-based evaluation of the turbulent Schmidt numbers for confined coaxial jets

Abstract

This paper is devoted to the challenges arising in Reynolds-Averaged Navier-Stokes (RANS) modeling of confined coaxial jet mixing. The main drawbacks of the gradient diffusion hypothesis and of the constant turbulent Schmidt number approach for the considered flow type are elucidated. In the first part of the present work, the mean and fluctuating velocity and scalar fields obtained in RANS simulations are directly validated against experimental data and compared with large-eddy simulation (LES) results. Three different turbulence models and three different turbulent Schmidt numbers values are selected for the present RANS computations. The accuracy given by LES is superior in comparison to RANS results especially for scalar mixing. The inability of the chosen RANS approach to represent counter-gradient diffusion in the axial direction characteristic for the considered test case is discussed. In the second part of the present study, the time-averaged mean and fluctuating velocity and scalar fields from LES are used for the evaluation of turbulent viscosities, turbulent scalar diffusivities, and turbulent Schmidt numbers as they appear in the RANS equations. Specific problems of the evaluation of these parameters from LES data are addressed. The importance of accounting for the turbulent scalar fluxes in all coordinate directions for evaluation of turbulent scalar diffusivity is revealed. The values obtained from LES results are compared with those given by RANS modeling. The deviations and their consequences for RANS predictions are discussed.