| mrishi |
August 10, 2020 11:29 |
Problem replacing fvm::laplacian with fvm::div(fvc::grad) in divDevRhoReff
I am working to change the default divDevRhoReff() defined in turbulenceModels/linearViscousStress.C. The change requires replacing the laplacian(U) operation with div(grad(U)). I change the content of divDevRhoReff(U) as follows:
Code:
template<class BasicTurbulenceModel>
Foam::tmp<Foam::fvVectorMatrix>
Foam::linearViscousStress<BasicTurbulenceModel>::divDevRhoReff
(
volVectorField& U
) const
{
/*
//first change
Info<<"broken into two: nu + nut" <<endl;
return
(
- fvc::div((this->alpha_*this->rho_*this->nu())*dev2(T(fvc::grad(U)))) //split
- fvc::div((this->alpha_*this->rho_*this->nut())*dev2(T(fvc::grad(U)))) //split
- fvm::laplacian(this->alpha_*this->rho_*this->nu(), U) //not changed
- fvm::laplacian(this->alpha_*this->rho_*this->nut(), U)
);
//no issues as expected; some overhead due to repeated grad calculations.
*/
//second change
Info<<"changed fvm::laplacian to fvm::div(fvc::grad) for nut term" <<endl;
const volTensorField A = fvc::grad(U); //to create type required by fvm::div
const surfaceScalarField N = fvc::interpolate(this->nut()); //surfacescalarField required by fvm::div, created out of volScalarField nut
return
(
// - fvc::div((this->alpha_*this->rho_*this->nu())*dev2(T(fvc::grad(U)))) //split
// - fvc::div((this->alpha_*this->rho_*this->nut())*dev2(T(fvc::grad(U)))) //split
// - fvm::laplacian(this->alpha_*this->rho_*this->nu(), U) //not changed
- fvm::div(N, fvc::grad(U)()) // changed, removed alpha,rho to remove clutter.
// - fvm::div(this->alpha_*this->rho_*(fvc::interpolate(this->nut())),A) //changed to div(grad(U)) instead of laplacian(U) , same as above
);
}
In the code marked second change, I have commented out the non-problematic components so as to isolate the issue. It generates following error:
Code:
../turbulenceModels/lnInclude/linearViscousStress.C:120:5: error: could not convert ‘Foam::operator-(const Foam::tmp<Foam::fvMatrix<Type> >&) [with Type = Foam::Tensor<double>]()’ from ‘Foam::tmp<Foam::fvMatrix<Foam::Tensor<double> > >’ to ‘Foam::tmp<Foam::fvMatrix<Foam::Vector<double> > >’
The issue is type incompatibility of the fvMatrix thus created, which is of type Tensor instead of Vector. While the div operator is rank-reducing, I suppose doing this in an implicit form using fvm may create such an issue. How can I circumvent this situation while still creating a proper fvVectorMatrix which adds correctly to my UEqn.H?
PS: The reason I need to do this is because I need to carry out some operations on the grad(U) before calculating its final divergence. In other words, my nu cannot be taken out of the second derivative directly, so I cannot use laplacian(nut,U). |
