[liviaadi]

Dear Foamers,

I am a new user of CFD and I am struggling to understand how to impose boundary conditions.
For example I am not understanding why I have to specify simultaneously conditions on pressure and velocity. I was expecting that I have to impose one and have the other as output.

Thanks !

[piu58]

In principle it is possible to have a flow both, pressure and velocity driven. I know such boundary conditions are rare, but physical possible. To specify the system you have to give all the physics.

If you want to have a velocity determined inflow or outflow, the pressure gradient is zero and this has to be set. It is best using this b.c if possible. Somewhere in the field you have to give a pressure reference point.

If you want to drive your flow by a pressure difference, you have to set the pressure profile at the boundary. You have a free outflow or inflow then and have to set this: Gradient of velocity is zero.

[liviaadi]

Thank you very much for the answer.
I have understood that I can consider a flow driven by a pressure difference at inlet and outlet and in this case I have to set the condition zeroGradient for the velocity.
If I am imposing a velocity profile I have to consider zeroGradient for pressure. Is that correct ?

If this is true, I still do not understand why.
In fact, if I am imposing a velocity profile it is not true that the pressure gradient is zero. So this is what I am not understanding.

Thanks !

[piu58]

> In fact, if I am imposing a velocity profile it is not true that the pressure gradient is zero.

Yes, that may be at your problem. In that case (that you know the pressure gradient for you case) simply set it.