[Sedullo]

Hello to everyone, I am simulating a natural convection problem, where only the wavy part of the right side wall is heated, the heated air is flowing upward. The temperature of the air entering the channel is set to 300 K and the temperature of the heated wall to 340 K.

I do not understand why the temperature at the outlet is so low, as you can see in the pictures attached. In the second picture the velocity vectors are shown.

I guess the problem is in the bc. Here I copy the bc of the velocity and the temperature.

dimensions      [0 0 0 1 0 0 0];

internalField   uniform 300;

boundaryField
{
    sideWallheated
    {
        type            fixedValue;
        value           uniform 340;
    }
    
    sideWallsNotheated
    {
        type            zeroGradient;
    }

    inlet
    {
        type            inletOutlet;
        inletValue      uniform 300;
        value           uniform 0;
    }

    outlet
    { 
        type            outletInlet;
        outletValue     uniform 300;
        value           uniform 0;
    }

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

internalField   uniform (0 0 0);

boundaryField
{
    sideWallheated
    {
        type            noSlip;
    }

    sideWallsNotheated
    {
        type            slip;
    }

    inlet
    {
        type            outletInlet;
        outletValue     uniform (0 0 0);
        value           uniform (0 0 0);
    }


    outlet
    {
        type            inletOutlet;
        inletValue      uniform (0 0 0);
        value           uniform (0 0 0);
    }

[gkarlsen]

Outlet BC for T should be inletOutlet if you want zeroGradient as flow goes out of the domain?

[Sedullo]

Hi gkarlsen, thanks for the reply.

I did as you said and actually that problem seems to be fixed, the temperature is high also at the outlet.
But now I have a new problem, I observe a strange phenomena, after 140 seconds I can see a sudden lowering of the temperature which lasts for about 10 seconds and afterwards it disappears.

I attach a picture of the steady situation (everything red at the outlet) and three pictures of the sudden lowering of temperature phenomena happening at the outlet.

What do you think could be the reason?

[gkarlsen]

Looks like reverse flow, which you can confirm by looking at U in the direction of your outlet. If this is the problem it can be mitigated partly by using totalpressure BC instead of fixedvalue. Also you could take a look at:

https://www.cfd-online.com/Forums/op...ow-outlet.html

[peterhess]

Hello!

Why the temperatures value in both inlet and outlet is 0?

That do not make much sense.

Increase (replace) those with 300...

---------------------------

At inlet use outletInlet

and

at outlet use inletOutlet

You have those reversed!

Regards

Peter

[Sedullo]

Hi Peter,

thanks for the suggestion, but actually nothing is changed, it is like that variable did not affect the final result.
Still have this backflow phenomena.

I attach also the bc for p and p_rgh to see if you can see here some problems

FoamFile
{
    version     2.0;
    format      ascii;
    class       volScalarField;
    object      p;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

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

internalField   uniform 0;

boundaryField
{
    wall
    {
        type            calculated;
        value           $internalField;
    }

    #includeEtc "caseDicts/setConstraintTypes"
}

FoamFile
{
    version     2.0;
    format      ascii;
    class       volScalarField;
    object      p_rgh;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

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

internalField   uniform 0;

boundaryField
{
    sideWallheated
    {
        type            fixedFluxPressure;
        rho             rhok;
        value           $internalField;
    }

    sideWallsNotheated
    {
        type            fixedFluxPressure;
        rho             rhok;
        value           $internalField;
    }


    inlet
    {
        type            totalPressure;
        p0              uniform 0;        
    }

    outlet
    {
        type            totalPressure;
        p0              uniform 0;        
    }

    #includeEtc "caseDicts/setConstraintTypes"
}

[peterhess]

Upload the case please!

[Sedullo]

I uploaded the case, thanks for the help!