# Detached eddy simulation

The Detached Eddy Simulation (DES) is a method for calculating turbulent flows with the help of computers. Since turbulence occurs both spatially and temporally on very different and, above all, very small scales, extremely fine computing grids and time steps are required for the correct resolution of all phenomena ( direct numerical simulation ). In practice, these conditions can often not be met due to limited computing power. Turbulence models like DES serve to reduce the computational effort.

DES is a combination of the common Large Eddy (LES) and Reynolds Averaged Navier Stokes (RANS) models, with dynamic switching between the two models depending on the grid resolution and the distance from walls. The motivation for this lies in the lower computational effort of RANS and the higher accuracy of LES. Walls, i.e. surfaces on which the flow velocity is zero due to the sticking condition, play a special role, as very thin boundary layers occur here , which have a great influence on the turbulent flow.

The Detached Eddy Simulation (DES) was first published in 1997 by Philippe Spalart . In its original form , it is based on the Spalart-Allmaras turbulence model (a transport equation ), but research is also being carried out into its application in conjunction with other models.

The DES replaces the wall distance, which occurs as a variable in the Spalart-Allmaras model , in areas far from the wall by the largest width of a grid cell. With this formulation, an LES-like behavior of the calculation can be achieved in the areas remote from the wall. In fact, you get a RANS formulation in the boundary layer on walls and an LES formulation in the free flow, i.e. the most suitable method in the respective area (in terms of accuracy and computational effort).

Since RANS and LES have different requirements for the grid, the creation of a suitable grid divided into appropriate zones has a major influence on the success of the calculation. The same applies to the numerical methods used. However, these are mostly necessarily the same in the entire computation area, which sometimes leads to compromises in terms of accuracy.

## literature

• Philippe R. Spalart: Detached-eddy simulation. In: Annual review of fluid mechanics 41 (2009)

## Individual evidence

1. Comments on the Feasibility of LES for Wings, and on a Hybrid RANS / LES Approach. Spalart PR, Jou WH, Strelets M, Allmaras SR. Advances in DNS / LES, 1997.