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Appropriate pressure boundary condition in incompressible flow

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Old   September 15, 2020, 05:32
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Hey there,

why are you using freestream conditions. As far as I know, you should use this condition if you have e.g., an airfoil and you set the velocity far away from the location of the wing. The header files of the boundaries give you an idea for which purpose to use it or how it works:

Code:
Description
    This boundary condition provides a free-stream condition for velocity.
    It is an inlet-outlet condition that uses the velocity orientation to
    continuously blend between fixed value for normal inlet and zero gradient
    for normal outlet flow.
Usage
    \table
        Property        | Description             | Required    | Default value
        freestreamValue | freestream velocity     | yes         |
    \endtable
    Example of the boundary condition specification:
    \verbatim
    <patchName>
    {
        type            freestreamVelocity;
        freestreamValue uniform (300 0 0);
    }
    \endverbatim
Note
    This condition is designed to operate with the freestreamPressure condition
For incompressible flows - e.g., a pipe flow - you commonly set the inlet velocity and the pressure outlet to fixed values. However, this always depends on the data you have available.

The totalPressure at an inlet + an fixedValue at the outlet does make sense for pressure based flows (here a gradient will force the flow). A totalPressure at the outlet will be a fixedValue if you do have only outflow. The condition does change only if you have inflow, hence the pressure of the face of your outlet patch that has an inflow will reduced by the kinematic pressure (1/2 rho UČ).

@Bodo, as you do have the inlet velocity, I would specify the outlets with a fixedValue for the pressure at the same value (e.g., 0) and an zeroGradient for the velocity or an inletOutlet for the velocity while setting the inletValue to (0 0 0) which will add artificial walls to the faces which will have an inflow for some reasons.


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Tobi
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Old   June 6, 2022, 09:44
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Hi All,

I get confused on this matter as well.
When we look at the SIMPLE algorithm, the pressure is found in a way that satisfies the continuity BC. So, it should not be priori. Now the question is that why do we actually set the pressure BC?

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incompressible flow, pressure bc, simplefoam not converged

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