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-   -   InletOutlet boundary condition in motorBike tutorial (http://www.cfd-online.com/Forums/openfoam/70548-inletoutlet-boundary-condition-motorbike-tutorial.html)

madad2005 November 27, 2009 06:52

InletOutlet boundary condition in motorBike tutorial
 
In the motorbike tutorial, the boundary condition for velocity at the pressure outlet is set to:

type inletOutlet
inlet uniform (0 0 0)
outlet uniform (20 0 0)

Can I ask why the inlet value has been set to (0 0 0)? I'm expecting this to be the same as what has been defined at the inlet itself, which was (20 0 0). All other quantities that use inletOutlet are set as I'd expect too.

olesen November 27, 2009 08:45

Quote:

Originally Posted by madad2005 (Post 237932)
In the motorbike tutorial, the boundary condition for velocity at the pressure outlet is set to:

type inletOutlet
inlet uniform (0 0 0)
outlet uniform (20 0 0)

Can I ask why the inlet value has been set to (0 0 0)? I'm expecting this to be the same as what has been defined at the inlet itself, which was (20 0 0). All other quantities that use inletOutlet are set as I'd expect too.

I'm not sure which directories you have or which OpenFOAM version, but on my OpenFOAM-1.6.x tutorials/incompressible/simpleFoam/motorBike/0/U contains this

Code:

    outlet
    {
        type            inletOutlet;
        inletValue      uniform (0 0 0);
        value          $internalField;
    }

whereas the 0/k contains this:

Code:

    outlet
    {
        type            inletOutlet;
        inletValue      $internalField;
        value          $internalField;
    }

There are no 'inlet' or 'outlet' keywords, but rather 'inletValue' and 'value'.

The 'value' is needed to initialize the boundary condition class (also supplies a value for post-processing etc) and will be overwritten in subsequent time steps with the current values for those faces.

The 'inletValue' is the value that should be used if there are any flow reversals in the system. If there is somehow a flow reversal at the outlet boundary, a zero-velocity should be a reasonable approximation of the entrainment of a quiescent fluid from teh surrounding environment.
For turbulence quantities, using a zero-value for an entrainment condition when there is backflow might not be a great idea (eg, when epsilon -> zero, the equations are ill behaved). Using the same turbulence quantities as the normal inlet has can't really be exactly correct either, but probably not that bad. Even if it is a somewhat incorrect value, the flow reversal velocities are probably fairly short lived and not that strong. I don't think it should normally affect your solution.

madad2005 November 27, 2009 09:15

Thanks for the reply.

Yes, those are the entries I am talking about. My mistake was due to the fact I wasn't refering directly to the code when I posted. But, thank you for taking the time to clarify exactly was it is doing.


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