Boundary condition. How to get the normal gradient of the patchField U
 

Dear Foam,

I have a home-made solver, which has the volVectorField named Vs aside the classic U and p field. In a custom inlet boundary condition of U (named "customBC"), I need to access Vs, which is quite easy with a lookupObject. I also need the normal gradient of Vs on my inlet BC. I have implemented several possibilities, which work fine, but they are either costly or not flexible.

1. First possibility: get snGard(Vs) in the BC and extract the right patchField from snGard(Vs) with such code:
   Code:

   ---

   // in the custom boundary condition of U
const volVectorField& Vs = this->db().lookupObject<volVectorField>("Vs");
const surfaceVectorField snGradVs = fvc::snGrad(Vs);
const label& patchIndex(this->patch().index());
const vectorField& patchSnGradVs = snGradVs.boundaryField()[patchIndex];

   ---

   This works well, but if I have N "customBC", snGrad(Vs) is calculated N time, which is costly. It is also not very efficient to calculate snGrad on the entire field and keeping only the patch field...
2. Second possibility: calculate snGrad(Vs) in the solver directly. The calculation is done only one, even with multiple "customBC". But it is not very flexible. If I don't need "customBC", snGradVs is calculated for nothing...

Is there a why to calculate the snGrad of a boundary field more efficiently?

Cheers,
Marcel

From a boundary field you have access to the face value ("psif"), the value at the cell center ("psic") and the delta coeffient ("delta"), which is 1/(distance from center to face).
From these values you can calculate the normal gradient sngrad=delta*(psif-psic).

I don't remember the functions to access psif,psic and delta, but you'll find them if you browse the fvPatchField documentation.

Martin