Les or no-model Les (UDNs). Does it make a difference?

The ever increasing computing power has encouraged the CFD community to venture into large-scale simulations on very high resolution meshes that were hard to imagine about a decade ago. As a result, Large Eddy Simulation (LES) methods can now in principle be afforded on high resolution meshes for specific nuclear reactor safety applications. LES for industrial type applications with complex geometries is mostly characterized by: a) a finite volume CFD method using a non-staggered arrangement of the flow variables and second order accurate spatial and temporal discretisation schemes, b) an implicit top-hat filter, where the filter length is equal to the local computational cell size, and c) eddy viscosity type LES models. An LES based on these three characteristics is indicated as an industrial LES in this paper. It has become increasingly clear that the numerical dissipation in CFD codes, typically used in industrial applications, may inhibit the predictive capabilities of an explicit LES. In practice, this may result in a situation where the differences between the predictive capabilities of an explicit LES and the simulations performed without any explicit use of a subgrid scale closure model are practically negligible. In this paper, the latter one is referred as an Under-resolved Direct Numerical Simulation (UDNS) or No-Model LES. In order to better understand the aforementioned situation, there is a need to quantify the numerical dissipation rate, especially in industrial type CFD codes. In this paper, we quantify the numerical dissipation rate in physical space based on an analysis of the transport equation for the mean turbulent kinetic energy. Using this method, we quantify the numerical dissipation rate for the first time in industrial LES of, as a first basic demonstration case, fully-developed turbulent channel flow at Re_τ = 395. Furthermore, we present an explanation of the observed flow trends using industrial LES. This explanation is based on the analysis of the turbulent kinetic energy budgets. In addition, we present preliminary results for a backward-facing step configuration.