# Sig Turbulence/Library Added Incompressible RAS Models/kOmegaSSTF

## Contents

## 1 kOmegaSSTF incompressible RAS model

The kOmegaSSTF model was developed by:

Gyllenram, W. and Nilsson, H., *Design and Validation of a Scale-Adaptive Filtering Technique for LRN Turbulence Modeling of Unsteady Flow*, JFE, May 2008, Vol.130.

It sets an upper limit ( or lt below) to the modelled length scale ( or Lt below), applied to :

where

.

should be greater than 1. The value 3 was determined in Gyllenram and Nilssson, *Very Large Eddy Simulation of Draft Tube Flow*, IAHR Symposium, Yokohama, 2006.

and are the discetized time and length scales.

The filter is applied to the turbulent viscosity:

where is the length scale given by the turbulence model.

The implementation of the kOmegaSSTF model is the same as that for the kOmegaSST model, except for the following difference:

**kOmegaSST:**

// Re-calculate viscosity nut_ = a1_*k_/max(a1_*omega_, F2()*sqrt(S2));

**kOmegaSSTF:** (please help me implement this more general!)

// Compute Filter scalar alph = 3.0; //Should be in a dictionary scalarField Lt = sqrt(k_)/(betaStar_*omega_); scalarField lt = alph*Foam::max(Foam::pow(mesh_.V().field(), 1.0/3.0), (mag(U_)*runTime_.deltaT())->internalField()); // Re-calculate viscosity nut_.internalField() = Foam::min(Foam::pow(lt/Lt,4.0/3.0), 1.0)* (a1_*k_/max(a1_*omega_, F2()*sqrt(S2)))->internalField();

### 1.1 Usage

Modify the RASModel in constant/RASproperties to:

RASModel kOmegaSSTF;

and add to the same file:

kOmegaSSTFCoeffs { alphaK1 0.85034; alphaK2 1.0; alphaOmega1 0.5; alphaOmega2 0.85616; gamma1 0.5532; gamma2 0.4403; beta1 0.0750; beta2 0.0828; betaStar 0.09; a1 0.31; c1 10; Cmu 0.09; }

Make sure that the addedIncormpressibleRASModels library is linked to by adding at the end of the system/controlDict file:

libs ("libaddedIncompressibleRASModels.so");

### 1.2 Current limitations

- None :-)
- Note that this is a model for unsteady simulations. Steady simulations will most likely not converge to a steady solution.

### 1.3 Tutorial

In the kOmegaSSTF directory there is a pitzDaily tutorial

Run:

cd $WM_PROJECT_USER_DIR/src/turbulenceModels/RAS/incompressible/kOmegaSSTF/tutorials/pitzDaily blockMesh turbFoam >& log

One of the Dellenback Abrupt Expansion case-study cases also uses kOmegaSSTF by default.

### 1.4 Utility writekOmegaSSTFilter

In the kOmegaSSTF directory there is a utility named writekOmegaSSTFilter, which is used to write out the filter function, , for viewing in paraFoam or other manipulation.

Compile:

cd $WM_PROJECT_USER_DIR/src/turbulenceModels/RAS/incompressible/kOmegaSSTF/utilities/writekOmegaSSTFilter/ wmake

Run:

writekOmegaSSTFilter

## 2 Others

Back to Sig Turbulence