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Momentum source containing div(tensor) using fvOptions

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Old   August 2, 2022, 07:46
Post Momentum source containing div(tensor) using fvOptions
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Sreehari Perumanath
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Hi Foamers,

I'm fairly new to OpenFOAM and I'm using v6. In my momentum equations, I have a source term that contains a divergence of a random symmetric stress tensor. Is it possible to define this term using fvOptions? If yes, please let me know how.

Many thanks in advance!
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Old   August 2, 2022, 09:50
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Agustín Villa
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How is that tensor defined? If it is already defined in your solver you could add the term directly there.
Otherwise you could try with a codeStream fvOption
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Old   August 2, 2022, 10:25
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Thanks for the reply agustinvo.

I already tried adding this term directly into the UEqn definition within a new solver (myIcoFoam). However, it looks like it has some issues. That is why I thought to use fvOptions instead. But, I'm confused how to define div(tensor) in fvOptions. If possible, could you please elaborate your suggestion on using codeStream? Many thanks!
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Old   August 3, 2022, 18:21
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What are these issues? Can you show your equations?

I am not an expert in codeStream, however I think it could be easy to add it in your solver if the tensor is already created there.
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Old   August 4, 2022, 06:37
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Hi agustinvo,

Sorry for the slight delay in replying. So my UEqn goes like:

Code:
        fvVectorMatrix UEqn
        (
            fvm::ddt(U)
          + fvm::div(phi, U)
          - fvm::laplacian(nu, U)
  	== 
  	    fvc::div(stochTau)/rho
        );
The stochastic stress tensor is defined already in a separate header file as:

Code:
       forAll(stochTau,i)
{
	Tcell=T.internalField()[i]; // local temperature
	rhocell=rho.internalField()[i]; local density
	visc=nu.value()*rhocell; // viscosity=kinematic_viscosity*density
	dv=mesh.V()[i]; //cell volume
	dt=runTime.deltaTValue();

	dummy=vector(rnumxx.scalarNormal(),rnumyy.scalarNormal(),rnumzz.scalarNormal()); //random vector; uncorrleated elements
	diag=dummy&chol; //correlated elements with sum=0. [chol is predefined and this step is correct]
	stochTau[i]=Foam::sqrt(2*visc*kb*Tcell/dv/dt)*symmTensor(diag.x(),rnumxy.scalarNormal(),rnumxz.scalarNormal(),diag.y(),rnumyz.scalarNormal(),diag.z());

}
Looking at its definition, everything seems fine. The issue that I face is that when I look at the variance of the resulting velocity normalised with 3*kb*T/rho/dV, I get 0.25 instead of 1. The normalising factor comes from equipartition theorem. What I noted is that if I multiply the RHS with 2, I get the expected result of 1. I don't understand where I'm missing this factor. Any help is much appreciated.

Last edited by sreehahaha; August 4, 2022 at 08:04.
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Old   August 4, 2022, 09:20
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I would need to see the reference model to check what can be missing. Do you have a test case? Which OpenFOAM version are you using?
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