[kk415]

Hi Foamers


I have a very simple problem. I want to access grad P at boundary faces.


Code: Info<< fvc::grad(pf) << endl;


Output:

dimensions [0 1 -2 0 0 0 0];

internalField nonuniform List<vector> 4((1.92787 -1.80556 0) (1.92787 1.80538 0) (5.53881 -1.80556 0) (5.53881 1.80538 0));

boundaryField
{
movingWall
{
type extrapolatedCalculated;
value nonuniform List<vector> 2((5.53881 0 0) (5.53881 0 0));
}
fixedWalls
{
type extrapolatedCalculated;
value nonuniform List<vector> 6((0 -1.80556 0) (0 -1.80556 0) (0 1.80538 0) (0 1.80538 0) (1.92787 0 0) (1.92787 0 0));
}
frontAndback
{
type empty;
}
}



So I can access grad(P) of interior faces like:


Code:
forAll(fluidRegions[0].owner(), faceI)
{
Info<< fvc::grad(pf)()[faceI] <<endl;
}


Output:
(1.92787 -1.80556 0)
(1.92787 1.80538 0)
(5.53881 -1.80556 0)
(5.53881 1.80538 0)



Similarly I want to write a face loop for all the boundary faces and want to print grad(P). How can I do that?

[mitti]

The shitty solution would be to get the patchID for every patch:

label patchLabel1 = mesh.boundaryMesh().findPatchID("boundary1")
label patchLabel2 = mesh.boundaryMesh().findPatchID("boundary2")

and then loop over the faces for every patch:

forAll (U.boundaryField()[patchLabel1], faceI) {
// code
}
forAll (U.boundaryField()[patchLabel2], faceI) {
// code
}

But when you have a lot of patches/boundary: Have fun.

[kk415]

Hey Thanks for Reply


I get how to write the boundary face loop. But I still didn't get how to get grad(p) at boundary.


I try using Info<< fvc::grad(pf).boundaryField()[faceI] <<endl;


But it gives error like boundaryField is not defined in this class. However when I simply give command like Info<< fvc::grad(pf) <<endl;


Then it give output as



dimensions [0 1 -2 0 0 0 0];

internalField nonuniform List<vector> 4((1.92787 -1.80556 0) (1.92787 1.80538 0) (5.53881 -1.80556 0) (5.53881 1.80538 0));

boundaryField
{
movingWall
{
type extrapolatedCalculated;
value nonuniform List<vector> 2((5.53881 0 0) (5.53881 0 0));
}
fixedWalls
{
type extrapolatedCalculated;
value nonuniform List<vector> 6((0 -1.80556 0) (0 -1.80556 0) (0 1.80538 0) (0 1.80538 0) (1.92787 0 0) (1.92787 0 0));
}
frontAndback
{
type empty;
}
}



This means the boundary field information is stored in grad(P). However I dont know how to use it. Can you help me with that?

[Kombinator]

Hey,

If you need grad(p) on the patch, something like this should work (I didn't test it, sorry):

// Find your patch
label patchID = mesh.boundaryMesh().findPatchID("yourPatch");
  
if(patchID == -1)
  
  {
    FatalError << "patch not found!" << exit(FatalError);      
  }
    
// Print p gradient on the patch
forAll(p.boundaryFieldRef()[patchID], faceI)
{
    Info<<p.boundaryFieldRef()[patchID].snGrad()()[faceI]<<endl;
}

Regards,
Artem

[mAlletto]

.snGrad() gives you the surface normal gradient, which is not equal to the gradient. The surface normal gradient at the boundary is defined as the difference between the cell center value and the boundary face center value divided by the distance between the cell center and the face center. It is described in the programmer guide.

[kk415]

With lot of try and error I found this solution,


Code:


Info<< fvc::grad(pf)().boundaryField()[0] << endl;


Output:


type extrapolatedCalculated;
value nonuniform List<vector> 2((5.53881 0 0) (5.53881 0 0));