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Old   November 30, 2010, 12:09
Default solve equation
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Diego Villa
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Hi all,
I have a stupid question but I don't find the solution.
I must solve the following equation:
U & grad(eta) - Uz=0
where eta is a scalar field and U is velocity vector and Uz is the z-component of U.

How can I write the scalar matrix etaEqn and solve it?

Thanks.
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Old   November 30, 2010, 13:08
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Santiago Marquez Damian
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Hi, Is your U field divergence free [div(U)==0]???, in this case you have

U & grad(eta) - Uz=0 -> div(U eta)-Uz=0

so that in FOAM

solve(fvm::div(phi,eta)-U.component(vector::Z)==0);

where phi is previously defined as phi=U&Sf, like is scalarTransportFoam, icoFoam, etc.

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Old   November 30, 2010, 13:15
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Santiago Marquez Damian
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One more thing, Is your equation dimensionally consistent?

Best.
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Old   November 30, 2010, 14:45
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Diego Villa
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Thank you Santiago Marquez Damian
I solve like you suggest and it works.
Info<<"Inizio calcolo Matrice Eta"<<endl;
faScalarMatrix etaEqn
(
fam::div(phis,eta)
);
Info<<"Inizio Relax"<<endl;
etaEqn.relax();
Info<<"Inizio Solve"<<endl;
solve(etaEqn == (Us & aMesh.faceAreaNormals()));

Dimensionally is consistent because eta is a areaSaclarField and U is a areaVectorField.
But now I have an other problem, I use as linear solver the
eta smoothSolver
{
smoother DILU;
tolerance 1e-06;
relTol 0;
};
and as boundary condition:
in
{
type fixedValue;
value uniform 0;
}
out
{
type zeroGradient;
}
carena
{
type zeroGradient;
}
far
{
type zeroGradient;
}
but after few iteration the solution explode, do you think I make some macroscopical error? I use also a relax factor of 0.3 for the etaEqn.

Thanks.
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Old   November 30, 2010, 14:59
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Santiago Marquez Damian
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I don't know, may be you can post the console output at the exploding moment and some data about the physical problem.

Regards.
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Old   November 30, 2010, 16:46
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I find the problem is the div shemes, I use a Gauss Linear and all explode, than I change in a Upwind and all works fine!!!

Thank for the help.
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