Reynolds-Averaged Navier-Stokes (RANS) and Large-Eddy Simulation (LES) are two distinct mathematical formulations for the prediction of turbulent flows. While LES resolves part of the turbulent scales, RANS models the influence of the entire turbulent field on the mean flow, computing only average (time or ensemble) quantities. This results into a substantial reduction of computational demands for RANS models in exchange of some accuracy. For statistically periodic flows (ex. flows over bluff bodies) RANS with the most common turbulence models available tends to give poor predictions of the mean flow. On the other hand, LES models seem to improve the accuracy of the predictions, but at a computational cost that is usually unaffordable for most engineering applications. Therefore, a new class of mathematical models, called hybrid or bridging methods, was developed to combine the advantages of LES and RANS. Detached-Eddy Simulation (DES), eXtra Large-Eddy Simulation (XLES), and Partially-Averaged Navier-Stokes (PANS) are examples of such modelling philosophies. All these techniques, LES, DES, XLES and PANS are resolving/simulating parts of the turbulence scales and are therefore denoted generally as Scales-Resolving Simulations (SRS).
The movies below illustrate the application of DES to a backward-facing step at Reynolds=37500 (top) and XLES to the cylinder flow at Reynolds=3900 (bottom). Some resolved instantaneous non-isotropic small turbulent scales are visible.