[jonhrx71as]

Hi Guys,
i want to compute the normal velocity at the boundary patch outlet.
I have two ways in my mind, both give me different results.

1) Calculate the normal velocity via the flux phi (which is definied at the boundary face)(incompressible case):

const auto& phip = patch().lookupPatchField<surfaceScalarField>("phi" );
scalarField uNormal = phip/patch().magSf();

2) Calculate the normal velocity via the velocity on the boundary patch scalar multiplied with the patch normal vector:

const auto& Up = patch().lookupPatchField<volVectorField>("U");
scalarField uNormal2 = (Up & patch().nf());

I thought Up is the interpolated value on the boundary patch from the velocity known at the cell center.
Why are the results different ?


Thanks in advance!

jonhrx71as