[giack]

Hi to all
Someone know how velocity is obtained from pressure in pressureInletOutletVelocity?

thank to all

[immortality]

its like zeroGradient when fluid flows out of domain and is the inletValue specified when fluid flows into the domain.it only says that we have a specified pressure in its folder and doesn't use it to calculate velocity.

[giack]

I think you are wrong because for example I set this boundary condition and for inflow I have a U different from the fixed value that I set ( (0,0,0) in my case). In some forum I find that the velocity of the flow inflow into the domain is calculated from the flux, but I don't understand how...Any suggestion?

[immortality]

see description in pressureInletOutletVelocityFvPatchVectorField.H
in opt folder->src->finiteVolume->fields->fvPatchFields_>derived:

Description
    This velocity inlet/outlet boundary condition is applied to pressure
    boundaries where the pressure is specified.  A zero-gradient condtion is
    applied for outflow (as defined by the flux); for inflow, the velocity is
    obtained from the patch-face normal component of the internal-cell value.

    The tangential patch velocity can be optionally specified.

    \heading Patch usage

    \table
        Property     | Description             | Required    | Default value
        phi          | flux field name         | no          | phi
        tangentialVelocity | tangential velocity field | no  |
    \endtable

    Example of the boundary condition specification:
    \verbatim
    myPatch
    {
        type            pressureInletOutletVelocity;
        phi             phi;
        tangentialVelocity uniform (0 0 0);
        value           uniform 0;
    }
    \endverbatim

Note
    Sign conventions:
    - positive flux (out of domain): apply zero-gradient condition
    - negative flux (into of domain): derive from the flux in the patch-normal
      direction

and see .C file of it.
this of code I found in pressureDirectedInletOutletVelocity.C that is like pressureInletOutletVelocity:

refValue() = inletDir_*phip/(rhop*ndmagS);

that is phi/(rho*area_of_cellface) that is same internalField(U) normal to ilet patch and is what I said you.

[giack]

I see the file .H e .C but I not familiar with C language so it's difficult for me understand these files. My BC is:
pressureInletOutletVelocity
value uniform (0,0,0)
so U must be zero in portion of patch in which there is inflow? But my U is different from zero.Why?
For this reason i previously give reason to the user that say that U is calculated from the flux for inflow.

thank you very much for you replies

[immortality]

no.it seems that both inflow and outflow uses internalField value of velocity even in inflow situation.I correct myself your value is not important in inflow case.as I saw in the code phi/(rho*area) is used as you know phi=rho*patch_ area*U_normal then above expression is equal to U_normal at neghbour patch.
it seems correct because when you specify a value for pressure you shouln't specify velocity concurrent with that for well-posedness of cfd problem.
(p and T or U and T)

[giack]

I didn't find the code lines that you write in my .C file, probably because the implementation of the BC is changed from two different version but I think that meaning is the same.
thank you very much for your help

[immortality]

for completeness it seems that in outlet velocity is implicitly calculated and in inlet just explicitly.
I think its because outflow is more likely to be unstable and inflow is more stable(from my experiences and what have read so far.somebody help me why is it so?)
And in inflow because maybe pressure is set such that involves velocity(like totalPressure) it should use velocity at that same time(explicit) not at later time after matrix calculation (implicit)
I think its the cause or soneone correct me.