[dl6tud]

I tried to make my own boundary condition, and discovered, that the mixed boundary condition does not behave as expected. With the following set, I thought it would work like 'zeroGradient' or 'fixedGradient' with gradient=0, but it does not. The Poisson equation for p does not converge ...

type mixed;
gradient 0;
value 0;
valueFraction 0;

Any hints? Mixed boundary condition is a mix of Dirichlet and a normal gradient?

[dl6tud]

I found the solution: As implemented now, the mixed boundary condition can not be used as purely gradient boundary condition (valueFraction = 0).

//- Return true if this patch field fixes a value.
// Needed to check if a level has to be specified while solving
// Poissons equations.
virtual bool fixesValue() const
{
return true;
}

[malv83]

Originally Posted by dl6tud

I found the solution: As implemented now, the mixed boundary condition can not be used as purely gradient boundary condition (valueFraction = 0).

Hello, I know it is a very old post but still useful. I have one question though, does this mean that we solve the problem changing true by false?

In my current implementation (OpenFOAM 5.x) the default value is true and valueFraction = 0 does not behave as zeroGradient BC