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LTSInterFoam BC problem

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Old   June 16, 2015, 10:38
Default LTSInterFoam BC problem
  #1
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xu cong
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Hello Foamers
I am new to Openfoam and I am doing a simulation about drainage,you can see model I use from the attachment

length=5.5m
width=3m
depth=0.1m

The left side is inlet (Q=20 L/s)
The right side is outlet and there is a gully on the bottom.(we can assume the gully is the second outlet)
The top floor is atmosphere.

I used the LTSInterFoam for simulation because I heard this Foam is faster than interFoam.However, I found some very strange result after simulation:

1.As you can see in the attachment, the water cannot get into the gully.
2.The flow rate of inlet is not equal to the flow rate of two outlets.


I think it should be the problem of boundary condition, but I don't know which one it is.
So I listed my files U/P_pgh/alpha, I hope someone could help me.

P_pgh
Code:
dimensions      [1 -1 -2 0 0 0 0];

internalField   uniform 0;

boundaryField
{
    inlet
    {
        type           buoyantPressure;
        value          uniform 0;
        
    }

    outlet
    {
        type           zeroGradient;
    }

    outlet2
    {
        type           zeroGradient;
    }

    sides
    {
        type           buoyantPressure;
        value          uniform 0;
    }

    atmosphere
    {
        type           totalPressure;
        p0              uniform 0;
        U               U;
        phi             phi;
        rho             rho;
        psi             none;
        gamma           1;
        value           uniform 0;
    }

    lowerwall
    {
        type            buoyantPressure;
        value          uniform 0;
    }
}
U
Code:
dimensions      [0 1 -1 0 0 0 0];

internalField   uniform (0 0 0);

boundaryField
{
    inlet
    {
        type            flowRateInletVelocity;
        volumetricFlowRate  constant 20e-3;           
        rhoInlet            1000;
    }

    outlet
    {
        type            zeroGradient;
    }

    outlet2
    {
        type            zeroGradient;                  
    }

    sides
    {
        type            fixedValue;
        value           uniform (0 0 0);
    }

    atmosphere
    {
        type            pressureInletOutletVelocity;
        value           uniform (0 0 0);
    }

    lowerwall
    {
        type            fixedValue;
        value           uniform (0 0 0);
    }
}
alpha
Code:
dimensions      [0 0 0 0 0 0 0];

internalField   uniform 0;

boundaryField
{
    inlet
    {
        type            inletOutlet;
        inletValue     uniform 1;
        value           uniform 1;
    }

    outlet
    {
        type            zeroGradient;
    }

    sides
    {
        type            zeroGradient;
    }

    atmosphere
    {
        type            inletOutlet;
        inletValue     uniform 0;
        value           uniform 0;
    }

    lowerwall
    {
        type            zeroGradient;
    }

    outlet2
    {
       type            zeroGradient;
    }
}
Lastly,can anyone tell me how to calculate the flow rate of water in this multiphase flow.I used the 'calculator' and 'integrate variable' from paraview.However i don't think it is precise.
Any help would be great, thank you so much!
Attached Images
File Type: jpg model.jpg (8.7 KB, 28 views)
File Type: jpg gully problem.jpg (14.7 KB, 27 views)
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Old   June 24, 2015, 09:02
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  #2
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Tobias Holzmann
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Hi,

Boundary Condition for U
  • for the outlet you can use inletOutlet and set inletValue to zero to provide no backflow
  • the rest is okay


Boundary Condition for alpha
  • all boundary conditions are okay. Due to U you can set different BC for alpha.
  • here for example you can use zeroGradient for outlets, if you have determined that there is no backflow
Boundary Condition for p_rgh
  • the old game (: p_rgh is some imaginary pressure
  • maybe my small description will help you (it's not 100% correct, too lazy to make a new one )
Code:
inlet :: fixedFluxPressure
all walls :: fixedFluxPressure
outlet :: zeroGradient
atmosphere :: depend what you want, I always prefer to set the prghPressureBC
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Old   June 26, 2015, 04:20
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Thank you so much for your kind reply, my model seems better now, but there is still a problem about the velocity in the vertical exit.

The exit is just some fast air flow, the velocity is more than 1000m/s which is obviously wrong, and the pressure is very low.
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Old   June 26, 2015, 04:35
Default velocity problem
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The simulation is better
but there is a problem with the vertical exit
The air speed is super high with a low pressure.

Why is that? is it because of the pressure of atmosphere?
Attached Images
File Type: png vitesse problem.png (18.2 KB, 17 views)
File Type: png vitesse problem2.png (30.3 KB, 18 views)
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Old   June 29, 2015, 07:48
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Hi,

normally you should not ask "why this happens" (:
Check your results, and then think about it. For me it seems that you choose a BC configuration that allows you to create a pressure gradient. This gradient will not be compensated with flux, so that a small gradient will always accelerate your flow till infinity or till your solver blow up.

Can you share you case?
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OpenFOAM Beginners should check out the new wiki on wiki.openfoam.com
A list of some active OpenFOAM contributers can be found »here«
A book about the basics of »Mathematics, Numerics, Derivations and OpenFOAM« can be found on www.Holzmann-cfd.de
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Old   June 29, 2015, 08:51
Default case
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Here is my case.
Thank you for your reply and sorry about my way of asking.
It has been over two weeks and I can't solve it so I was a little bit frustrated, sorry about that.
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