Challenges in modeling scalars in turbulence and LES: Anisotropy, dynamic models, and scale separation

This paper discusses three interrelated aspects of modeling scalar transport, mixing and combustion, in the context of large-eddy simulations (LES). In terms of passive scalar modeling, we find from heated wake measurements that whereas the kinetic energy dissipation tensor tends towards isotropy at small scales, in the presence of a mean scalar gradient the SGS scalar variance dissipation remains anisotropic independent of filter scale. The eddy-diffusion model predicts isotropic behavior, whereas the nonlinear model reproduces the correct trends, but overestimates the level of scalar dissipation anisotropy. The results provide some support for mixed models. Applications of dynamic models are also discussed. Initial tests on non-buoyant jet flows show that the constant-coefficient Smagorinsky model generates an entirely laminar jet, whereas the Lagrangian dynamic model yields very good results without the need to tune the model coefficient. In the context of applying the dynamic model to premixed combustion, or other physical processes with large scale-separations, we show that the dynamic determination of unknown scaling exponents is a more promising approach than dynamic evaluation of model coefficients.