[xucong1228]

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

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

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

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!

[Tobi]

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

• http://www.cfd-online.com/Forums/ope...tml#post285912

• maybe my small description will help you (it's not 100% correct, too lazy to make a new one )

inlet :: fixedFluxPressure
all walls :: fixedFluxPressure
outlet :: zeroGradient
atmosphere :: depend what you want, I always prefer to set the prghPressureBC

[xucong1228]

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.

[xucong1228]

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?

[Tobi]

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?

[xucong1228]

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.