[ArathoN]

hi foamers,

My friend told me that in Fluent there is an option to initialize the flow properties then run the desired the simulation. I searched for such an option on openfoam but i found nothing.

So i was thinking if i use a 1ep.-model like spalart-allmaras to initialize the flow then i swith to my desired RAS model, Does this improve my solution? and more importantly does the solution converge faster?

[ArathoN]

Resolved.

First before running any simulation use:

refineMesh

It will help the first steps of the simulation.

Then if you use kEpsilon, it's better to run potentialFoam to initialize the U & p fields, you need to add this in the fvsolution file

potentialFlow
{
    nNonOrthogonalCorrectors 10;
}

then run

potentialFoam

Finally if you'll use the kOmegaSST initialize the fields with the data from the kEpsilon, the same with every other model derived from the kEpsilon. In genereal the kEpsilon is the fastest one so it's better to use it then feed the data to the others and the gain in cpu time reduction is huge.

That's all i learned. If some one has more advice i'll gladly hear him.

[RodriguezFatz]

Originally Posted by ArathoN

Then if you use kEpsilon, it's better to run potentialFoam to initialize the U & p fields, you need to add this in the fvsolution file

potentialFlow
{
    nNonOrthogonalCorrectors 10;
}

Hi, i think this is exactly what Fluent does when using "Hybrid Initialization".

[ArathoN]

Originally Posted by RodriguezFatz

Hi, i think this is exactly what Fluent does when using "Hybrid Initialization".

Exactly I have a friend using fluent and old me about this initialization so after some test and research i found out how to do it in openFoam. In the case of kEpsilon running potentialFoam before the main solver has little effect (at least for the cases i studied), but in case of different models (LaunderSharmaLowRe, kOmegaSST,....) the initialization with kEpsilon helps really in reducing the time needed for the convergence.

[RodriguezFatz]

Aditionally to the speedup, I also had a case that diverged with a simple constant initialization but not with the potentialFoam init.

[ArathoN]

Originally Posted by RodriguezFatz

Aditionally to the speedup, I also had a case that diverged with a simple constant initialization but not with the potentialFoam init.

If you use very accurate schemes, sometimes the case will diverge. It's better to start the simulation with a diffusive scheme but robust, then after some iterations switch to the accurate one. That's what i do when i need to simulate complex cases.

[shipman]

Dear Arathon,

I also try to initialize my flow field using potentialflow before running my multiphase calculation according to your above explanation, but i couldnt succeed. in multiphase pressure dimension is kgm/s^2 however in potetialflow it is m^2/s^2. i set my cases as follows and but giving following error (is it error or not no idea). also how can i initialize the value of k and epsilon?? could you help to me pls...thanks

==>0/U

dimensions      [0 1 -1 0 0];
internalField   uniform (0 0 0);
boundaryField
{
    inlet
    {
        type            fixedValue;
        value           uniform (0 -3.104 0);
    }
    outlet
    {
        type            zeroGradient;
    }
    wall
    {
        type            fixedValue;
        value           uniform (0 0 0);
    }

=>0/P
dimensions [0 2 -2 0 0 0 0];

internalField   uniform 0;
boundaryField
{
    inlet
    {
         type           zeroGradient;
    }
    outlet
    {
        type            fixedValue;
        value           $internalField;;
    }
    wall
    {
         type            zeroGradient;
    }

==> fvSchemes:

ddtSchemes
{
    default              steadyState;
}

interpolationSchemes
{
   default         linear;
    interpolate(U)  linear;
}

divSchemes
{
    default              none;
   div(phi,U)      Gauss upwind;
    div(phi,k)      Gauss upwind;
    div(phi,epsilon) Gauss upwind;
    div(phi,nuTilda) Gauss upwind;
    div((nuEff*dev(T(grad(U))))) Gauss linear;
}
gradSchemes
{
    default              Gauss linear;
    grad(p)         Gauss linear;
    grad(U)         Gauss linear;
}

laplacianSchemes
{
    default         Gauss linear corrected;
    laplacian(nuEff,U) Gauss linear corrected;
     laplacian(1,p)  Gauss linear corrected;
    laplacian(DkEff,k) Gauss linear corrected;
    laplacian(DepsilonEff,epsilon) Gauss linear corrected;
    laplacian(DnuTildaEff,nuTilda) Gauss linear corrected;
}

snGradSchemes
{
 default         corrected;
}

fluxRequired
{
    default         no;
    p               ;
}

==>fvsolution:

solvers
{
   "(U|k|omega)"
    {
        solver          PBiCG;
        preconditioner  DILU;
        tolerance       1e-06;
        relTol          0;
    }
   p
    {
        solver          PCG;
        preconditioner  DIC;
        tolerance       1e-06;
        relTol          0;
    }
}

potentialFlow
{
nNonOrthogonalCorrectors   10;
}

sorry i forgat to paste the error:

DICPCG: Solving for p, Initial residual = 1, Final residual = 8.80882e-07, No Iterations 180
DICPCG: Solving for p, Initial residual = 0.168621, Final residual = 9.37342e-07, No Iterations 131
DICPCG: Solving for p, Initial residual = 0.00789625, Final residual = 9.67837e-07, No Iterations 115
DICPCG: Solving for p, Initial residual = 0.000384399, Final residual = 8.94174e-07, No Iterations 101
DICPCG: Solving for p, Initial residual = 2.60676e-05, Final residual = 8.91063e-07, No Iterations 3
DICPCG: Solving for p, Initial residual = 1.92316e-06, Final residual = 5.88908e-07, No Iterations 1
DICPCG: Solving for p, Initial residual = 7.12976e-07, Final residual = 7.12976e-07, No Iterations 0
DICPCG: Solving for p, Initial residual = 7.12976e-07, Final residual = 7.12976e-07, No Iterations 0
DICPCG: Solving for p, Initial residual = 7.12976e-07, Final residual = 7.12976e-07, No Iterations 0
DICPCG: Solving for p, Initial residual = 7.12976e-07, Final residual = 7.12976e-07, No Iterations 0
DICPCG: Solving for p, Initial residual = 7.12976e-07, Final residual = 7.12976e-07, No Iterations 0
continuity error = 0.0244929
Interpolated U error = 0.000203337
ExecutionTime = 6.04 s ClockTime = 6 s