Understanding The Detached Eddy Simulation From DES to IDDES
 

One of the most popular hybrid RANS-LES models is Detached Eddy Simulation (DES), devised originally by Philippe Spalart. The term DES is based on the Idea of covering the boundary layer by RANS model and switching the model to LES mode in detached regions thereby cutting the computational cost significantly yet still offering some of the advantages of an LES method in separated regions.

The SST IDDES (Improved DES) turbulence model in #STARCCM+ is more ambitious, also non zonal and aims at resolving log-layer mismatch in addition to MSD, The modification tends to depress δ near the wall and give it a steep variation, which stimulates instabilities, boosting the resolved Reynolds stress, and new empirical functions, some involving the cell Reynolds number, which address log-layer mismatch and the bridge between wall-resolved and wall-modeled DES (grids with moderate values of the spacing in wall units), yet IDDES should be chosen only for flows where LES content of the boundary layer is of high priority otherwise the computational cost shall rise sharply.

https://allaboutcfd-tomersblog.com/2...-des-to-iddes/

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