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Accelerating Reference Frame

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Old   July 1, 2014, 12:11
Default Accelerating Reference Frame
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Jakob
Join Date: May 2014
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Hello everyone!

my name is Jakob and this is my first thread in the forum. I have been working with OpenFOAM for a couple of months now and so far found the forum to be extremely help full! However I spend multiple days now searching through the forum and the internet to find a solution to my problem and was not able to find an answer to it:

I would like to simulate the free fall of an object in a axisymmetric mesh (it is a wedge mesh). To minimize the mesh motion my idea was to use the 6-dof solver on the object with pimpleDymFoam and have an accelerating reference frame that moves with the object (for now the simulations didn't include the object yet but just show the moving reference frame), at an acceleration of g. My first thought to solve this issue was to just ramp the velocity at the inlet at a constant acceleration of g over my simulation time of 1.6 seconds.
The velocity looks great: Here showing the velocity at time 0 and 1.5 seconds.
Velocity_0_seconds.png Velocity_1.5_seconds.png

However I ran into some issues with the pressure distribution. Namely the pressure should be constant in my whole domain (since the atmospheric pressure is constant), but in my simulations it decreases from the inlet to the outlet, as it can be seen in the picture. This pressure drop is constant over the 1.5 seconds:
Pressure_0_seconds.png

So here is my question: Could this be solved with different boundary conditions or is my approach just completely wrong? If it is wrong what would the solution be?

Velocity BC:
Code:
#include        "include/initialConditions"

dimensions      [0 1 -1 0 0 0 0];

internalField   uniform (0 0 0);

boundaryField
{
    //- Set patchGroups for constraint patches
    #include "${WM_PROJECT_DIR}/etc/caseDicts/setConstraintTypes"

    inlet
    {
    type                uniformFixedValue;
    uniformValue    table
    (
        (0    (0 0 0))
        (1    (0 0 9.81))
        (2    (0 0 19.62))
    );

    }

    outlet
    {
       
    type                inletOutlet;
    inletValue         uniform (0 0 0);
 
    }

    OuterWall
    {
        type            slip;
    }

    front
    {
    type                wedge;
    }

    back
    {
    type                wedge;
    }
}
Pressure BC:
Code:
#include        "include/initialConditions"

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

internalField   uniform $pressure;

boundaryField
{
    //- Set patchGroups for constraint patches
    #include "${WM_PROJECT_DIR}/etc/caseDicts/setConstraintTypes"

    inlet
    {
    type                zeroGradient;
    value              $internalField;
    }

    outlet
    {      
    type            fixedValue;
    value          $internalField;
    }

    OuterWall
    {
        type            slip;
    }

    capsule
    {
        type            zeroGradient;
    }

  
    front
    {
    type         wedge;
    }

    back
    {
    type         wedge;
    }

}
I hope I gave you all the information you need and was able to explain my problem in a somewhat understandable manner....

Thank you for your help!

Jakob
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Old   July 2, 2014, 18:25
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Jakob
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Does nobody have an idea? Is there any additional information I should supply?
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