[sreehahaha]

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!
sreehahaha

[agustinvo]

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

[sreehahaha]

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!

[agustinvo]

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.

[sreehahaha]

Hi agustinvo,

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

        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:

       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.

[agustinvo]

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?