Normal AND tangential at the Outflow??

carlos

(This is rather a FEM question)

I have listened several times that, for the use of outflow BC there must be imposed the normal component of the stress tensor AND their tangential component.

In the FEM variational formulation normal component appears in a natural way (divergence theorem). In consequence, this is really a "do nothing" BC. But how about the tangential BC? I mean, how deal with it in the Finite Element framework?

I am interested in this question because I do not have Boundary conditions for the pressure (but now I begin to wonder if this ir really validid or not...)

Any help is grateful.

Carlos.