[Wanja]

Hey hey hey
For LES, I want to calculate the gradient of the reynold stress which is volTensorField, it’s gradient should be volVectorField however I receive error due to grad(R) where R is a tensor.
Error is

'no matching function for call to ‘grad(Foam::tmp<Foam::GeometricField<Foam::SymmTens or<double>, Foam::fvPatchField, Foam::volMesh> >)’


How do we calculate gradient of reynolds stress ?

[Ali Amarloo]

Hi Wanja,

did you find a way to calculate gradients of reynolds stress?

[HPE]

- Reynolds stress tensor object is volSymmTensorField, not tensor.
- Its gradient will not produce a vector field. The gradient operator increases the order of the tensor. So you will get instead a third-order tenso, which does not exist in OpenFOAM (or anywhere else I know).

[Ali Amarloo]

Hi HPE,
Thanks for your comment.
that is correct.
by the vector of gradients of Reynolds stress, we (or at least I) meant the final 3 values that are calculated based on Reynolds stress gradients and are inserted into 3 equations of momentum (aka RANS).
I can save the R tensor and maybe calculate the gradients myself and then using the equations, and by them, I can calculate those momentum sources (which are induced by Reynolds stress gradients) but I think it would be more accurate if I can have the exact numbers openFoam inserts into equations by the turbulence model.

Or if I can have the values inside that third-order tensor, the final calculation will be only a couple of sum and subtracts.

cheers,
Ali

[HPE]

I couldn't follow the line of explanations of yours, I'm afraid.

>> by the vector of gradients of Reynolds stress, we (or at least I) meant the final 3 values that are calculated based on Reynolds stress gradients and are inserted into 3 equations of momentum (aka RANS).

There won't be any final 3 values, if anyone takes the gradient of a symmetric tensor.

I kindly suggest you to visit Vector calculus to see the mapping properties of the gradient operator.

[Ali Amarloo]

sorry for my bad english.

let me try it again.

Reynolds stress has 6 values so if we take the gradient it will be even more terms than that.

but when we see the RANS equations, we have temporal part, advection, diffusion, etc. and there is only one term which is calculated based on the gradients of Reynolds stress.

That term is a vector because we have three equations in 3 directions (x, y, z).

for example in simpleFoam:

tmp<fvVectorMatrix> tUEqn
(
fvm::div(phi, U)
+ MRF.DDt(U)
+ turbulence->divDevSigma(U)
==
fvOptions(U)
);

I am talking about "turbulence->divDevSigma(U)", which must be a vector.

I am very basic in c++, so I know that is the term I am looking for, but I don't know how can I save numbers (or the vector) separately.

So instead of that term, If I have access to the gradients of those 6 values (gradient of elements in Reynolds stress), I can calculate this turbulence term myself.

I hope, it is more clear now

[HPE]

>> but when we see the RANS equations, we have temporal part, advection, diffusion, etc. and there is only one term which is calculated based on the gradients of Reynolds stress.

The operator there is the divergence operator, not the gradient operator.