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-   -   Boundary conditions for k and epsilon on a periodic channel flow (https://www.cfd-online.com/Forums/openfoam/220080-boundary-conditions-k-epsilon-periodic-channel-flow.html)

Maveryck August 22, 2019 17:02
Boundary conditions for k and epsilon on a periodic channel flow
 
Hi everyone,

I'm simulating a flow through a channel. I impose a mean velocity which traduces in a pressure gradient calculated by openFoam.


Here are boundary and initial conditions for Pressure:

Code:
boundaryField
{
    inlet
    {
        type            cyclic;
    }

    outlet
    {
        type            cyclic;
    }

    FixedWalls
    {
        type            zeroGradient;
    }

    frontAndBack
    {
        type            zeroGradient;
    }
}
Velocity Conditions:
Code:
boundaryField
{
    inlet
    {
        type            cyclic;
    }

    outlet
    {
        type            cyclic;
    }

    FixedWalls
    {
        type            noSlip;
    }

    frontAndBack
    {
        type            noSlip;
    }
}
But it is not clear to me how to determine which boundary conditions are suitable for k and epsilon in this simulation. Until now I have tried the following:

Boundary conditions for k:

Code:
boundaryField
{
    inlet
    {
        type            cyclic;
    }

    outlet
    {
        type            cyclic;
    }

    FixedWalls
    {
        type            kLowReWallFunction;
        value          uniform 0.75;
    }

    frontAndBack
    {
        type            kLowReWallFunction;
        value          uniform 0.75;
    }
}
B.C. for epsilon
Code:
boundaryField
{
    inlet
    {
        type            cyclic;
    }

    outlet
    {
        type            cyclic;
    }

    FixedWalls
    {
        type            epsilonWallFunction;
        value          uniform 14;
    }

    frontAndBack
    {
        type            epsilonWallFunction;
        value          uniform 14;
    }
}
I'm getting small values for nut at the walls, which means I get higher WallShearStress values than expected and thus I don't get proper uPlus-yPlus profiles (which is what I'm looking for).

Has anyone worked with similar cases? any suggestion will be wellcome

Thanks, Maveryck.

Note: y+ at the walls have values around 5-8.

C-L August 26, 2019 02:58
Your boundary conditions are fine for those y+ values.
I expect this is because of the turbulence model you are using - k-epsilon is unsuitable for these flows unless you use highRe wall functions (i.e y+ levels are in the log-region).

Try v2-f if you want to use an epsilon-based model, or use an omega-based model if you don't want to use the four-equation one.

pconen August 26, 2019 05:08
When you have problems finding suitable values you can try out the cfd-online tools.
https://www.cfd-online.com/Tools/turbulence.php


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