Reynolds Average Navier-stokes (RANS) approaches is the most widely applied simulation technique for complex flows. This approach can be accurately used thin shear layers. But it can not be used for the flow regimes; flow with separation which is characterized by flow structures atypical compared to that of the thin shear layers predicted by the RANS models, techniques such as LES are attractive. While Large Eddy Simulations (LES) is carries a disadvantage of computational cost for the boundary layer turbulence at high Reynolds numbers. But the technique offers many advantages over RANS methods for predicting separated flows.
The non-affordable computational cost of LES in the attached boundary layers and the inability of the RANS models to provide a reliable prediction of large separation zones make it very attractive to create an approach that combine the power of LES in the separated regions and the fined tuned RANS technology in the attached boundary layer. In this approach, the attached boundary layer, eddies while the larger detached eddies in the separated regions would be simulated, smaller eddies in this region too are modeled but they have relatively less influence. Hence this approach is called Detached-Eddy simulations.
DES is a new approach to the treatment of turbulence aimed at the prediction of separated flows with a low computational costs and the accuracy in predictions. It combines the RANS model in the attached boundary layer and the LES model in the separated regions. It is essentially a three dimensional unsteady approach using a single turbulence model, which functions as a sub grid model in the regions where the grid density if fine enough for an LES, and as a RANS model in regions where it is not.