[alberto]

Originally Posted by jorkolino

Hi there,

What is the correct precedence in the code (cause I've seen it both ways): first solve then relax or vice versa?

Relaxation is applied implicitly before solving if you do:

yourEqn = ...;
yourEqn.relax();
yourEqn.solve();

If you see the relax() method applied to a field (like p in simple algorithm), you are explicitly relaxing the variable.

You typically relax the equation before solving the equation. In the SIMPLE algorithm, you explicitly relax the pressure after the solution of the pressure equation and the flux correction, but before the velocity correction.

I noticed relaxation factor of unity has huge impact on results when linear div scheme is used, in comparison to when no relaxation is used. I would expect relaxation factor of 1 to have no impact on the solution. That happens to a well defined parameter: fixed inlet value and zerogradient outlet value in 1D.
It is almost certain that there is a problem with Foam not only with certain BC, but also with the precedence of relaxation. URF of 1 should not change the behavior at all.

There is no problem in the relaxation itself, which, if done properly, should not change the converged solution at all in general (if you exclude URF = 0). There is some sensitivity to under-relaxation in OpenFOAM due to the co-located grid arrangement, however, as discussed in another thread it is usually smaller than the discretization error of the corresponding schemes (see also Peric's book for the theory).
What you see is due to something else (can your solution converge with URF = 1?)

About the directioMixed BC, it is basically an extended Robin condition (a*U + b*dU = c), which allows you to specify a different "fraction" of fixedGradient / fixedValue in each direction (Sorry for the horrible wording ).

There are a couple of BC's to look at, which are derived from the directionMixed BC:

- outletStabilised, which applies upwind differencing to outlets
- pressureInletOutletVelocity (see header for description)

Best,