Comparison of Subgrid Scale Models for LES of a Variable Density Jet

Abstract

LES models for subgrid scale transport of momentum and mixture fraction are assessed using DNS and LES data of a turbulent round variable density jet. The closure models evaluated are the dynamic variable density extensions of the mixed nonlinear or Clark model and an eddy diffusivity-type model for both momentum and mixture fraction. The work is focused on understanding the coupling between momentum and scalar models. The ability to reproduce means and Reynolds stresses is used to determine model performance. To aid in understanding the coupling between models, SGS dissipation is computed, and it is demonstrated that the choice of SGS scalar model alters the SGS dissipation properties of the momentum closure. Results show that the predictive ability of the simulation depends on the model chosen, with the mixed model in general outperforming the eddy diffusivity type models. The combination of a dynamic mixed model for momentum and a fixed eddy diffusivity for scalar is a cost effective approach for variable density LES problems. I. Introduction. The low-Mach number equations provide a computationally attractive set of conservation equations for implementation in direct numerical simulation (DNS) and large-eddy simulation (LES) of low speed, complex flows with large density and temperature gradients, including for reacting flow applications. LES of reacting flow requires numerous modelling decisions including how to model the unclosed momentum, scalar, and combustion terms. Accurately predicting the scalar concentration fields is essential for the development of precise combustion simulations. Assessment of the subgrid scale (SGS) models for the incompressible momentum equation 1–3 as well as combustion models 4, 5 have seen considerable effort. Previous investigations of SGS models in compressible flows 6–8 focused primarily on higher Mach number regimes and SGS models for the energy equation. There has been some analysis 9–11 of the models used to close the scalar equations. These studies demonstrate the superiority of mixed model closures for momentum in free shear flows, and that for small density gradients the choice of scalar closure has less influence. In this work, we seek to elucidate what effects different subgrid scale (SGS) stress and scalar flux models have on both the mixing properties and turbulent statistics of the flow. Additionally, we aim to understand how the coupling between the SGS stress and scalar flux models alters the predictive abilities of LES. To isolate these effects, we restrict the study to an isothermal axi-symmetric turbulent jet governed by the lowMach number equations. This canonical flow was chosen for both its abundance of available experimental data and its lack of a wall region that requires additional treatment in LES. We study different permutations of two common SGS models. The first is the ubiquitous dynamic Smagorinsky model and its scalar analog, and the other is a dynamic mixed model combining the nonlinear or Clark model with an eddy diffusivity term, for a total of five LES cases. A methane/air jet is selected for the investigation. In addition to its practical importance, this flow has ∂