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8.2 Turbulence models
Turbulence modelling is part of general momentum transport which is concerned with models for the viscous stress in a fluid. Momentum transport is configured through the momentumTransport file in the constant directory of a case. The file includes the mandatory simulationType keyword that specifies how turbulence is modelled, which includes the following options:
laminar-
uses no turbulence models;
RAS-
uses Reynolds-averaged simulation (RAS) modelling;
LES-
uses large-eddy simulation (LES) modelling.
The file then includes a sub-dictionary of the
same name as the chosen simulationType which contains the model
selections. A typical example is shown below that uses the
–
(k-epsilon) turbulence model.
16 17simulationType RAS; 18 19RAS 20{ 21 model kEpsilon; 22 23 turbulence on; 24 25 printCoeffs on; 26} 27 28// ************************************************************************* //
The file shows the selected RAS simulation followed by the RAS sub-dictionary containing the model selections, in particular the model which is set to kEpsilon. The choice of RAS models is described in section 8.2.1 and more information can be found in Chapter 7 of Notes on Computational Fluid Dynamics: General Principles. The LES models are listed in section 8.2.3 .
Where the laminar option is selected, the sub-dictionary is optional and will default to a Newtonian model, using the viscosity specified in the physicalProperties file. Other models, including non-Newtonian and visco-elastic models, are described in section 8.3 . Non-Newtonian models can also be combined with turbulence models (whereas visco-elastic models cannot).
For a general introduction to turbulence for CFD, the reader may also wish to consult Chapter 6 of Notes on Computational Fluid Dynamics: General Principles.
8.2.1 Reynolds-averaged simulation (RAS) modelling
If RAS is selected, the choice of RAS modelling is specified in a RAS sub-dictionary which requires the following entries.
-
turbulence: switch to turn the solving of turbulence modelling on/off.
-
printCoeffs: optional switch to print model coeffs to terminal at simulation start up, defaults to false.
-
<model>Coeffs: optional dictionary of coefficients for the respective model, defaults to standard coefficients.
Turbulence models can be listed by running foamToC with a relevant table listed from the RAS or LES tables. For example, the RAS tables are listed by running the following command.
foamToC -table RAS
foamToC -table RASincompressibleMomentumTransportModel
foamSearch $FOAM_TUTORIALS momentumTransport RAS/model
foamInfo buoyantKEpsilon
8.2.2 RAS turbulence models
For incompressible flows, the RAS model can be chosen from the list below.
LRR-
Launder, Reece and Rodi Reynolds-stress turbulence model for incompressible flows.
LamBremhorstKE-
Lam and Bremhorst low-Reynolds number k-epsilon turbulence model for incompressible flows.
LaunderSharmaKE-
Launder and Sharma low-Reynolds k-epsilon turbulence model for incompressible flows.
LienCubicKE-
Lien cubic non-linear low-Reynolds k-epsilon turbulence models for incompressible flows.
LienLeschziner-
Lien and Leschziner low-Reynolds number k-epsilon turbulence model for incompressible flows.
RNGkEpsilon-
Renormalization group k-epsilon turbulence model for incompressible flows.
SSG-
Speziale, Sarkar and Gatski Reynolds-stress turbulence model for incompressible flows.
ShihQuadraticKE-
Shih’s quadratic algebraic Reynolds stress k-epsilon turbulence model for incompressible flows
SpalartAllmaras-
Spalart-Allmaras one-eqn mixing-length model for incompressible external flows.
kEpsilon-
Standard k-epsilon turbulence model for incompressible flows.
kEpsilonLopesdaCosta-
Variant of the standard k-epsilon turbulence model with additional source terms to handle the changes in turbulence in porous regions for atmospheric flows over forested terrain.
kOmega-
Standard high Reynolds-number k-omega turbulence model for incompressible flows.
kOmega2006-
Standard (2006) high Reynolds-number k-omega turbulence model for incompressible flows.
kOmegaSST-
Implementation of the k-omega-SST turbulence model for incompressible flows.
kOmegaSSTLM-
Langtry-Menter 4-equation transitional SST model based on the k-omega-SST RAS model.
kOmegaSSTSAS-
Scale-adaptive URAS model based on the k-omega-SST RAS model.
kkLOmega-
Low Reynolds-number k-kl-omega turbulence model for incompressible flows.
qZeta-
Gibson and Dafa’Alla’s q-zeta two-equation low-Re turbulence model for incompressible flows
realizableKE-
Realizable k-epsilon turbulence model for incompressible flows.
v2f-
Lien and Kalitzin’s v2-f turbulence model for incompressible flows, with a limit imposed on the turbulent viscosity given by Davidson et al.
For compressible flows, the RAS model can be chosen from the list below.
LRR-
Launder, Reece and Rodi Reynolds-stress turbulence model for compressible flows.
LaunderSharmaKE-
Launder and Sharma low-Reynolds k-epsilon turbulence model for compressible and combusting flows including rapid distortion theory (RDT) based compression term.
RNGkEpsilon-
Renormalization group k-epsilon turbulence model for compressible flows.
SSG-
Speziale, Sarkar and Gatski Reynolds-stress turbulence model for compressible flows.
SpalartAllmaras-
Spalart-Allmaras one-eqn mixing-length model for compressible external flows.
buoyantKEpsilon-
Additional buoyancy generation/dissipation term applied to the k and epsilon equations of the standard k-epsilon model.
kEpsilon-
Standard k-epsilon turbulence model for compressible flows including rapid distortion theory (RDT) based compression term.
kOmega-
Standard high Reynolds-number k-omega turbulence model for compressible flows.
kOmega2006-
Standard (2006) high Reynolds-number k-omega turbulence model for compressible flows.
kOmegaSST-
Implementation of the k-omega-SST turbulence model for compressible flows.
kOmegaSSTLM-
Langtry-Menter 4-equation transitional SST model based on the k-omega-SST RAS model.
kOmegaSSTSAS-
Scale-adaptive URAS model based on the k-omega-SST RAS model.
realizableKE-
Realizable k-epsilon turbulence model for compressible flows.
v2f-
Lien and Kalitzin’s v2-f turbulence model for compressible flows, with a limit imposed on the turbulent viscosity given by Davidson et al.
8.2.3 Large eddy simulation (LES) modelling
If LES is selected, the choice of LES modelling is specified in a LES sub-dictionary which requires the following entries.
-
turbulence: switch to turn the solving of turbulence modelling on/off.
-
printCoeffs: optional switch to print model coeffs to terminal at simulation start up, defaults to false.
-
<model>Coeffs: optional dictionary of coefficients for the respective model, to override the default coefficients.
-
<delta>Coeffs: dictionary of coefficients for the delta model.
The LES models used in the tutorials can be listed using foamSearch with the following command. The lists of available models are given in the following sections.
foamSearch $FOAM_TUTORIALS momentumTransport LES/model
8.2.4 LES turbulence models
For incompressible and compressible flows, the LES model can be chosen from the list below.
DeardorffDiffStress-
Differential SGS Stress Equation Model for incompressible flows
Smagorinsky-
The Smagorinsky SGS model.
SpalartAllmarasDDES-
SpalartAllmaras DDES turbulence model for incompressible flows
SpalartAllmarasDES-
SpalartAllmarasDES DES turbulence model for incompressible flows
SpalartAllmarasIDDES-
SpalartAllmaras IDDES turbulence model for incompressible flows
WALE-
The Wall-adapting local eddy-viscosity (WALE) SGS model.
dynamicKEqn-
Dynamic one equation eddy-viscosity model
dynamicLagrangian-
Dynamic SGS model with Lagrangian averaging
kEqn-
One equation eddy-viscosity model
kOmegaSSTDES-
Implementation of the k-omega-SST-DES turbulence model for incompressible flows.
8.2.5 Model coefficients
The coefficients for the RAS turbulence models are given default values in their respective source code. If the user wishes to override these default values, then they can do so by adding a sub-dictionary entry to the RAS sub-dictionary file, whose keyword name is that of the model with Coeffs appended, e.g. kEpsilonCoeffs for the kEpsilon model. If the printCoeffs switch is on in the RAS sub-dictionary, an example of the relevant …Coeffs dictionary is printed to standard output when the model is created at the beginning of a run. The user can simply copy this into the RAS sub-dictionary file and edit the entries as required.
8.2.6 Wall functions
A range of wall function models is available in
OpenFOAM that are applied as boundary conditions on individual
patches. This enables different wall function models to be applied
to different wall regions. The choice of wall function model is
specified through the turbulent viscosity field in the 0/nut file. For example, a 0/nut file:
16 17dimensions [0 2 -1 0 0 0 0]; 18 19internalField uniform 0; 20 21boundaryField 22{ 23 inlet 24 { 25 type calculated; 26 value uniform 0; 27 } 28 outlet 29 { 30 type calculated; 31 value uniform 0; 32 } 33 upperWall 34 { 35 type nutkWallFunction; 36 value uniform 0; 37 } 38 lowerWall 39 { 40 type nutkWallFunction; 41 value uniform 0; 42 } 43 frontAndBack 44 { 45 type empty; 46 } 47} 48 49 50// ************************************************************************* //
There are a number of wall function models available in the release, e.g. nutWallFunction, nutRoughWallFunction, nutUSpaldingWallFunction, nutkWallFunction and nutkAtmWallFunction. The user can get the full list of wall function models using foamInfo:
foamToC -scalarBCs | grep nut
![eqn](img/index522x.png)
![eqn](img/index523x.png)
![eqn](img/index524x.png)
Having selected the particular wall functions on various wall patches in the nut file, the user should select the following boundary conditions at wall patches for other turbulence fields.
-
epsilon field: apply the epsilonWallFunction to corresponding patches.
-
omega field: apply the omegaWallFunction to corresponding patches.
-
k, q or R field: apply kqRwallFunction to corresponding patches.