The idea behind the DES model of Strelets [58] is to switch from the SST-RANS model to an LES model in regions where the turbulent length, L_t, predicted by the RANS model is larger than the local mesh spacing. In this case, the length scale used in the computation of the dissipation rate in the equation for the turbulent kinetic energy is replaced by the local mesh spacing, .

(2–193)

The practical reason for choosing the maximum edge length in the DES formulation is that the model should return the RANS formulation in attached boundary layers. The maximum edge length is therefore the safest estimate to ensure that demand.

The DES modification of Strelets can be formulated as a multiplier to the destruction term in the k-equation:

(2–194)

with C_DES equal to 0.61, as the limiter should only be active in the model region. The numerical formulation is also switched between an upwind biased and a central difference scheme in the RANS and DES regions respectively.