Interfacing statistical turbulence closures with large-eddy simulation

Progress toward a general purpose hybrid Reynolds-averaged Navier-Stokes (RANS)/large-eddy simulation (LES) framework is described, in which large-scale, statistically represented turbulence kinetic energy is converted automatically into resolved-scale velocity fluctuations wherever the local mesh resolution is sufficient to support them. Existing hybrid RANS/LES approaches alter the nature of the local partial differential equations according to the local mesh resolution, but they do not alter the nature of the data on which these equations operate. The implications of this are discussed. Subsequently, a simple mechanism is introduced to transfer statistical kinetic energy into resolved-scale fluctuations in a manner that preserves a given set of space/time correlations and set of second moments. This process,,which can appropriately be termed Large-Eddy STimulation (LEST), generates the large-scale eddies needed to form the unsteady boundary conditions at RANS interfaces to LES regions, into which turbulence energy can be deposited either through mean convection or through turbulent transport. The proposed approach is designed to work on general meshes with arbitrary clusterings and does not require the user to specify internal boundary surfaces separating RANS and LES regions. Results on a plane channel flow show that the approach helps to preserve the shear stress across regions where turbulence is transported by mean convection and also helps to sustain the fluctuations in the outer, unsteady, portion of the boundary layer by reconstructing the resolved-scale energy that is generated in the statistically modeled near-wall layer and transported across the boundary layer via turbulent mixing.