### 12.6.4. Appendix 3: Models

| Applications | Comments
--- | --- | ---

Scale-Adaptive Simulation (SAS)

- Use for globally unstable flows

- Use CFL~1 for best results (higher CFL possible but less resolution)

- Avoid PRESTO scheme

- Check Q-criterion carefully during run time to ensure SRS structures

- Safest SRS model, as it has URANS fallback position on coarse grids/time steps

- Danger of falling into URANS mode if flow instability is not strong

Detached Eddy Simulation (DES)

- Use for globally unstable flows and with care also for locally unstable flows

- Always use DDES over DES to reduce the impact of the DES limiter on attached boundary layers, and use the DDES shielding function

- Grid in SRS region must be of LES quality, no RANS fallback position

- Use CFL~1

- Avoid PRESTO scheme

- Check -criterion carefully during run time to ensure SRS structures

- DDES is not available for CFX

- More aggressive than SAS in terms of unsteadiness

- Careful grid generation important, otherwise danger of gray zones or grid-induced separation

Shielded Detached Eddy Simulation (SDES)

- Improved version of DDES. It provides better shielding of the RANS boundary layers and a more aggressive definition of the grid length scale – leading to more rapid transition from RANS to LES.

- SDES is not available for CFX

- Similar to DDES but improved shielding and reduction of gray area zone and grid-induced separation.

Stress-Blended Eddy Simulation (SBES)

- Optimal model that features asymptotic shielding of the RANS boundary layer as well as explicit switch to existing algebraic LES model. Can also be run in WMLES mode.

- This is the model of choice for globally and locally unstable flows.

- Can also be used in WMLES mode, once LES content is introduce upstream. This can be achieved by synthetic turbulence or by a previous RANS-LES transition upstream (for example backstep).

Large Eddy Simulation (LES)

- Use for free shear flows

- Use if boundary layers are laminar

- Use for turbulent boundary layers only with high grid resolution at low Reynolds numbers

- Use CFL~1

- Apply synthetic turbulence at inlets

- Check -criterion carefully during run time to ensure SRS structures

- Typically too expensive for wall-bounded flows

Wall Modeled LES (WMLES)

- Use for wall boundary layers at moderate and high Reynolds numbers

- Resolve boundary layer volume () by cells

- Use CFL~1

- Apply synthetic turbulence at inlets

- Check -criterion carefully during run time to ensure SRS structures

- Scales much more favorably with Reynolds number than standard LES but still very expensive

- Limit wall region to a small portion of flow domain (ELES)

- Note that the SBES model (SST+WALE) can also provide WMLES capabilities.

Embedded LES (ELES) Zonal Forced LES (ZFLES)

- Use for wall boundary layers at moderate and high Reynolds numbers

- Resolve boundary layer volume () by cells

- Use CFL~1

- Apply synthetic turbulence at RANS-LES interface

- Check -criterion carefully during run time to ensure SRS structures

- Allows flexible combination of models in different parts of the domain.

- •If wall boundary layers in LES domain, consider using WMLES (default in CFX)

Vortex Method (VM) – Fluent

- Use to generate synthetic turbulence at RANS-LES interface or LES (WMLES) inlet

- Restrict interface zone to the minimal section where turbulence must be converted (do not extend LES zone far into the freestream)

- If large RANS-LES interface cannot be avoided increase (and check) the number of vortices specified. Can be as high as 104.

- Use CFL~1

- Check Q-criterion carefully during run time to ensure SRS structures

- Grid in LES region of interface must be of LES quality

Harmonic Turbulence Generator (HTG) - CFX

- Restrict inlet zone to the LES minimal section where turbulence must be converted (do not extend LES zone far into the freestream)

- Grid in LES region of interface must be of LES quality

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