[ngj]

Hi Arne

I have two possible explanations, which depends slightly on the details, and you will be able to check the correctness using a laminar solution:

1. If you are using U from the large to the small domain: In that case you are using a velocity field, which does not conserve mass! On collocated grids as in OpenFoam, mass conservation is enforced through the face fluxes, so if you prescribe a boundary value all the way around for U, then mass conservation fails.

2. If you are using face fluxes between the two grids: In that case you might have forgotten to take the direction of the normal vectors into account, i.e. you are not conserving mass.

Furthermore, if you are also pre-scribing values for the pressure on the boundaries, then you have an extremely stiff system, hence I would suggest pressure dirichlet when the flux is outward and neumann when inward. Virsa virsa for the velocities.

Hope it helps

/ Niels