The Improved Delayed Detached Eddy Simulation (IDDES) model ([592], [219]) is a hybrid RANS-LES model (consisting of a combination of various new and existing techniques) that provides a more flexible and convenient scale-resolving simulation (SRS) model for high Reynolds number flows. Since the model formulation is relatively complex and the application of the model is non-trivial, it is recommended that you consult the original publications of Shur et al. [592] and Gritskevich et al. [219].
The IDDES model has the following goals in addition to the formulation of the standard DES model:
Provide shielding against Grid Induced Separation (GIS), similar to the DDES model ([622]).
Allow the model to run in Wall-Modeled LES (WMLES) mode in case unsteady inlet conditions are provided to simulate wall boundary layers in unsteady mode. The IDDES model is designed to allow the LES simulation of wall boundary layers at much higher Reynolds numbers than standard LES models. As an alternative to unsteady inlet conditions, unsteadiness could also be triggered by an obstacle (such as a backward facing step, or a rib inside or upstream of the boundary layer).
The IDDES model implemented in Ansys Fluent is based on the BSL / SST model ([428]) with the application of IDDES modifications as given in [219]. Similar to DES, the k-equation of the BSL / SST model is modified to include information on the local grid spacing. In case the grid resolution is sufficiently fine, the model will switch to LES mode. However, the goal is to cover stable boundary layers (meaning no unsteady inlet conditions and no upstream obstacles generating unsteadiness) in RANS mode. In order to avoid affecting the BSL / SST model under such conditions, the IDDES function provides shielding similar to the DDES model, meaning it attempts to keep the boundary layer in steady RANS mode even under grid refinement.
If you want to resolve the wall boundary layer in WMLES mode, unsteady inlet conditions need to be provided (see Vortex Method or Spectral Synthesizer). The model can also be run with the Embedded LES (ELES) option (see Embedded Large Eddy Simulation (ELES)) and the IDDES option (see Detached Eddy Simulation (DES) in the User's Guide) in Ansys Fluent.
The IDDES-BSL / SST model is based on modifying the sink term in the k-equation of the BSL
/ SST model. (The 
-equation remains unmodified.)
 | (4–286) |
 | (4–287) |
where 
.
In Equation 4–287, 
is based on the RANS turbulent length scale and the LES grid length scales.
Note that the sub-grid length scale 
differs from the DES formulation. In the IDDES formulation, a more general
formulation for 
is used that combines local grid scales and the wall distance 
.
The complete formulation is relatively complex and has been implemented as published by
[219], except for the value of 
; while this variable is defined as the maximum edge length in the case of a
rectilinear hexahedral cell in Ansys Fluent, for other cell types and/or conditions an extension of
this concept is used.