[hcl734]

Two basic question:

1)
Why do we have to assign a zeroGradient-BC for pressure when we use a fixedValue-BC for velocity?

2)
Why do we have to assign zeroGradient for the outlet when using fixedValue for the inlet?

Sorry if asked before, couldn't find it.

Thx for help!

[nimasam]

for incompressible flow pressure and velocity is directly related to each other, so it is not right to constrain both variables

[hcl734]

So the reason is basicly that I only solve one equation (momentum balance/NS) to obtain pressure and velocity. When I fix pressure and velocity it can not be solved, right?

Just another question about pressure-BC.
What is the difference between setting a fixedValue for pressure and a totalPressure-BC?
I understand the difference between pressure and total pressure but I don't understand the implications entirely.

Let me give an example:
Impinging Jet on a plate
totalPressure for outlet results in



fixedValue pressure

FoamFile
{
    version     2.0;
    format      ascii;
    class       volScalarField;
    object      p_rgh;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

dimensions      [0 2 -2 0 0 0 0];

internalField   uniform 101325;

boundaryField
{    

    
    AXIS
    {
        type            zeroGradient;
    }
    INLET
    {
        type            zeroGradient;
    }
    OPEN
    {

        type            fixedValue;
        value           uniform 101325;

/*
        type            totalPressure;
        U               U;
        phi             phi;
        rho             none;
        psi             none;
        gamma           1.4;
        p0              uniform 101325;
*/

    }
    OPENUPPER
    {

        type            fixedValue;
        value           uniform 101325;

/*
        type            totalPressure;
        U               U;
        phi             phi;
        rho             none;
        psi             none;
        gamma           1.4;
        p0              uniform 101325;
*/    
    
//        Property     | Description             | Required    | Default value
//         U            | velocity field name     | no          | U
//         phi          | flux field name         | no          | phi
//         rho          | density field name      | no          | none
//         psi          | compressibility field name | no       | none
//         gamma        | ratio of specific heats (Cp/Cv) | yes |
//         p0           | static pressure reference | yes       |
    }
    PIPEWALL
    {
        type            zeroGradient;
    }
    SIDE1
    {
        type            wedge;

    }
    SIDE2
    {
        type            wedge;

    }
    WALL
    {
        type            zeroGradient;
    }
}