BC, which allows entrainment in a particular direction
 

Hi
I am looking for a BC in openFOAM, where the boundary allows entrainment in a direction, which is perpendicular to the axis of the jet flow. Previously I've used cylindrical domain, where the zero Gradient BC was applied on the lateral surface, which satisfied my requirements.

But now, I'm trying to use a conical frustum domain, where the normal of the lateral surface is not perpendicular to the jet axis.

Any comments would be really helpful.