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Can't get Uy component convergence - Couette flow

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Old   September 28, 2017, 13:59
Default Can't get Uy component convergence - Couette flow
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Lisandro Maders
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Hello,

I am trying to make a tutorial case for students of a Couette flow between 2 parallel plates, with one of them at a constant velocity.

The domain is 2D:

Length (x direction) = 3 [mm]
Height (y direction) = 0.03 [mm] (yes, that small)

Boundary conditions I am using for velocity:

Code:
dimensions      [0 1 -1 0 0 0 0];

internalField   uniform (0 0 0);

boundaryField
{
    movingWall
    {
        type            fixedValue;
        value           uniform (14 0 0);
    }

    fixedWalls
    {
        //type            noSlip;
        type            fixedValue;
        value           uniform (0 0 0);
    }

    inlet
    {
        type            zeroGradient;
        //type            cyclic;
    }

    outlet
    {
        type            zeroGradient;
        //type            cyclic;
    }

    frontAndBack
    {
        type            empty;
    }
}
and for pressure:

Code:
dimensions      [0 2 -2 0 0 0 0];

internalField   uniform 0;

boundaryField
{
    movingWall
    {
        type            zeroGradient;
    }

    fixedWalls
    {
        type            zeroGradient;
    }

    inlet
    {
        type            fixedValue;
        value           uniform 0;
    }

    outlet
    {
        type            fixedValue;
        value           uniform 0;
    }

    frontAndBack
    {
        type            empty;
    }
}
The case is laminar, 2D, steady-state.

My mesh files and case setup files are attached.

The problem is that I am having ok results for Ux, but weird behavior for Uy component. The convergence for Uy do not get smaller than 0.01, even though the Ux convergence is around e-16 !!

Looking at the results (see Figure attached), I have a weird field of Uy. However, all the values are close to e-12 (pretty much zero). Why is it happening?

Best regards,

Lisandro
Attached Images
File Type: jpg problem_Uy.jpg (28.1 KB, 9 views)
Attached Files
File Type: zip constant.zip (912 Bytes, 0 views)
File Type: zip placas_paralelas.zip (4.8 KB, 0 views)
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Old   September 28, 2017, 14:01
Default Showing the OK results for Ux components
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Lisandro Maders
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Just attaching the Ux components results..
Attached Images
File Type: jpg Ux_ok.jpg (29.6 KB, 7 views)
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Old   September 28, 2017, 15:09
Default Residuals calculation
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Lisandro Maders
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Actually, I think there is a problem in the way I am interpreting the residuals in OpenFOAM. I run same case right now in Fluent, it converges in 10 seconds, and if I scale the Uy results to see between -1e-12 and 1e-12 I also see some numerical errors like the ones in the Figure attached.

I know this is actually zero for the computer, however, why isn't the Uy residuals converging? Why is it considering such small values in residuals calculation?

Am I missing something here?

Regards,

Lisandro
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convergence, couette flow, uy component

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