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yPlus Calculation for Spalart Allmaras RANS Simulation

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Old   October 3, 2017, 04:01
Default yPlus Calculation for Spalart Allmaras RANS Simulation
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Hello All,

I've been trying to use the y+ calculation option on a transient external flow case using pimpleFoam -postProcess to make sure that my mesh resolution is sufficient closer to the flow surfaces.

Unfortunately, I get zeros for all my surfaces as my min, max and average. I've found that if I run an LES on the same case, I'm able to get some numbers. Similarly, if after I run the RAS simulation, I change up my turbulence model to LES before I run the yPlus utility, I'm able to get non-zero numbers for the RAS. I'm uncertain how trustworthy this number is. But, more fundamentally, I don't understand what I'm doing wrong with the model.

Please let me know what data needs to be analyzed to figure this out.
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Old   November 19, 2017, 15:28
Default Having a similar issue!
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Hey,

I think I'm dealing with a similar issue!
Are you also using OF version 5?
Do you mean with 'zeros' a very low number or actually zero?

I'm having the problem where even for very coarse meshes my y+ is always smaller than 1. As in your case, when I change to LES before I postprocess yPlus, I get numbers above 1, but I'm also not sure these are trustworthy.

Maybe you or someone else has found a solution already?
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Old   November 19, 2017, 21:40
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Quote:
Originally Posted by vdeschuy View Post
Hey,

I think I'm dealing with a similar issue!
Are you also using OF version 5?
Do you mean with 'zeros' a very low number or actually zero?

I'm having the problem where even for very coarse meshes my y+ is always smaller than 1. As in your case, when I change to LES before I postprocess yPlus, I get numbers above 1, but I'm also not sure these are trustworthy.

Maybe you or someone else has found a solution already?
Hello, could you paste the contents of your nut file here, please?
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Old   November 20, 2017, 04:43
Default nut file
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Here you go!

This is for a case were I used a very coarse mesh, wall functions, and here I expected to see a high y+ value.

Code:
/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  5.0                                   |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       volScalarField;
    location    "0";
    object      nut;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

dimensions      [0 2 -1 0 0 0 0];

internalField   uniform 0;

boundaryField
{
    inlet
    {
        type            calculated;
        value           uniform 0;
    }
    outlet
    {
        type            calculated;
        value           uniform 0;
    }
    top
    {
        type            slip;
    }
    bottom
    {
        type            slip;
    }
    cylinder
    {
        type            nutkWallFunction;
        value           uniform 0;
    }
    side1
    {
        type            empty;
    }
    side2
    {
        type            empty;
    }
}

// ************************************************************************* //
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Old   November 20, 2017, 09:41
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The nutkWallFunction requires k to compute y+. Checkout out the source code for that BC:

https://github.com/OpenFOAM/OpenFOAM...arField.C#L157

The SA turbulence model has no ability to compute k so it returns a value of zero for k. Checkout the source code:

https://github.com/OpenFOAM/OpenFOAM...llmaras.C#L302

Instead use the nutUWallFunction which uses a velocity instead of k to compute y+.

-Chris
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