[frubicle93]

I have been attempting to implement "do-nothing" or "traction-free" boundary conditions for outlets in OpenFOAM. This is the condition that

- p n + nu*(n dot grad) u = 0.

I've seen it a lot in theoretical work as it eliminates some terms from the variational formulation of Navier-Stokes (Gresho, 1990), and I think some commercial CFD codes have this as an option.

Has anyone already attempted to implement this BC in OpenFOAM?

It is easy to make this BC starting from the mixed type BC. However, SIMPLE requires more boundary conditions than are required analytically (e.g. zeroGradient pressure at inlets/walls), and I am unsure what to do with the pressure on the outlet patch. If I set p = 0 at the outlet, then the above reduces to zeroGradient. I could set p (or the mean value of p) to any value (which would clearly have a large influence on the result).