flowfield CFD REFERENCE

DES, DDES and IDDES

How hybrid RANS–LES methods change behavior near the wall and in separated flow.

DES97 · Shielding · Grey area · Hybrid RANS–LES

Detached-eddy simulation (DES), delayed DES (DDES) and improved delayed DES (IDDES) are hybrid methods. The equations below show what changes in the turbulence closure as the modeled length scale changes.

DES97 uses a grid-dependent length scale

For the original Spalart–Allmaras-based formulation, DES replaces the wall-distance scale dd in the model with

ℓDES=min⁡(d,CDESΔ),\ell_{\mathrm{DES}}=\min(d,C_{\mathrm{DES}}\Delta),

where CDESC_{\mathrm{DES}} is a model constant and the original formulation uses the maximum local grid spacing for Δ\Delta. The aim is RANS behavior in attached boundary layers and LES-like resolution in separated regions on an appropriate mesh. A grid that is fine parallel to a thick attached boundary layer can switch too early, depleting modeled stress before enough turbulence is resolved.

DDES adds shielding

A common SA-based representation is

ℓDDES=d−fdmax⁡(0,d−CDESΔ).\ell_{\mathrm{DDES}}=d-f_d\max(0,d-C_{\mathrm{DES}}\Delta).

Here fdf_d approaches zero in a shielded attached boundary layer, keeping the RANS scale. Outside that region, as fdf_d approaches one, the expression recovers the DES limiter. This addresses premature switching. The separated shear layer still needs a grid and time step that can support resolved fluctuations.

IDDES adds a wall-modeled LES branch

The improved delayed formulation combines delayed DES behavior with a wall-modeling branch designed to allow WMLES behavior under appropriate grid and inflow conditions. It uses additional blending and length-scale definitions. Implementations based on SA and SST differ, so cite the solver’s exact variant and its definition of grid scale.

What to inspect in an aero case

Use hybrid methods when separated wakes, unsteady loads or broadband flow features are the quantity of interest and you can afford a suitable three-dimensional mesh. Check wake resolution, model activity, time-step sensitivity and adequate sampling. A long transition region between modeled and resolved turbulence can affect shear-layer development.

Sources