## 2.6. Detached Eddy Simulation Theory

This section outlines the theoretical details of the DES model in ANSYS CFX. Details on setting up a DES simulation and modeling advice are available. For details, see The Detached Eddy Simulation Model (DES) in the CFX-Solver Modeling Guide.

Experience has shown that the use of LES in boundary layer flows at high Re numbers is prohibitively expensive [57] and therefore not useful for many industrial flow simulations. On the other hand, turbulent structures can be resolved in massively separated regions, where the large turbulent scales are of the same dimension as the geometrical structure generating them (airfoil flaps, buildings, blunt trailing edges on turbine blades). DES is an attempt to combine elements of RANS and LES formulations in order to arrive at a hybrid formulation, where RANS is used inside attached and mildly separated boundary layers. Additionally, LES is applied in massively separated regions. While this approach offers many advantages, it is clearly not without problems, as the model has to identify the different regions automatically. DES models require a detailed understanding of the method and the mesh generation requirements and should not be used as “black-box.” You are advised to read the ANSYS CFX technical report [55] on the subject, which explains all details on mesh generation requirements, zonal formulation and boundary conditions.

The ANSYS CFX version of the DES model is based on the SST formulation. The advantage of this combination is that the accurate prediction of turbulent boundary layers up to separation and in mildly separated regions carries over from the SST model. In addition, the SST model supports the formulation of a zonal DES formulation [56], which is less sensitive to mesh resolution restrictions than the standard DES formulation, as proposed by Strelets [58]. Refer to the ANSYS CFX technical report [55] for details.

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