LES and DNS Turbulence Modeling

Abstract

This chapter is divided into two parts, LES and DNS. In the first part, the LES turbulence model is derived from first principles, and its terms are described in detail. The usage of LES filters is described, along with various recommendations. The LIKE algorithm is applied to show how to model large eddies properly by applying the appropriate node-to-node computational distances. LES-specific boundary and initial conditions are described, and dozens of practical recommendations are provided. In the second part, analogous discussions and recommendations for DNS are included as well. 5.1 LES Modeling (and Comparison with RANS and DNS) The larger the eddy, the higher its nonisotropic nature and the more complex its behavior. The larger eddies obtain their kinetic energy from the bulk fluid energy, contain most of the turbulent kinetic energy (~80%), transfer kinetic energy to the smaller eddies by stretching and breaking them up ("cascading"), and are responsible for the majority of the diffusive processes involving mass, momentum, and energy. For these reasons, the simulation of large eddies is highly desirable. On the other hand, the smaller eddies take the kinetic energy from the larger eddies and transfer their energy back to the fluid through viscous shear. For high Re, the small-scale turbulent eddies are statistically isotropic. Therefore, they are "more universal" and more independent of the boundary conditions and the mean flow velocity than the larger eddies. Thus, simulation of the smaller eddies is also desirable. So, why not simulate (resolve) the larger eddies and approximate (model) the behavior of the smaller eddies? Based on this premise, large eddy simulation (LES) models have been developed for several decades to capture these important eddy features