Outlet temperature too low, bc problem

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January 10, 2020, 20:22
Outlet temperature too low, bc problem
#1
Member

Alex
Join Date: May 2019
Posts: 35
Rep Power: 3
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.
Code:
```dimensions      [0 0 0 1 0 0 0];

internalField   uniform 300;

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

sideWallsNotheated
{
}

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

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

Code:
```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);
}```
Attached Images
 Screenshot from 2020-01-11 02-06-24.png (57.2 KB, 12 views) Screenshot from 2020-01-11 02-04-56.jpg (63.5 KB, 9 views)

 January 11, 2020, 01:09 #2 Member   Geir Karlsen Join Date: Nov 2013 Location: Norway Posts: 59 Rep Power: 9 Outlet BC for T should be inletOutlet if you want zeroGradient as flow goes out of the domain?

January 11, 2020, 13:36
#3
Member

Alex
Join Date: May 2019
Posts: 35
Rep Power: 3
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?
Attached Images
 Screenshot from 2020-01-11 19-25-03.png (36.0 KB, 4 views) Screenshot from 2020-01-11 19-24-39.png (47.7 KB, 2 views) Screenshot from 2020-01-11 19-24-45.png (38.6 KB, 3 views) Screenshot from 2020-01-11 19-24-53.png (48.1 KB, 1 views)

 January 11, 2020, 14:21 #4 Member   Geir Karlsen Join Date: Nov 2013 Location: Norway Posts: 59 Rep Power: 9 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

 January 13, 2020, 19:03 #5 Senior Member   Peter Hess Join Date: Apr 2011 Location: Austria Posts: 220 Rep Power: 12 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

 January 13, 2020, 20:21 #6 Member   Alex Join Date: May 2019 Posts: 35 Rep Power: 3 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 Code: ```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" }``` Code: ```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" }```

 January 13, 2020, 21:00 #7 Senior Member   Peter Hess Join Date: Apr 2011 Location: Austria Posts: 220 Rep Power: 12 Upload the case please! Last edited by peterhess; January 13, 2020 at 22:03.

January 13, 2020, 21:44
#8
Member

Alex
Join Date: May 2019
Posts: 35
Rep Power: 3
I uploaded the case, thanks for the help!
Attached Files
 Project_wavy.zip (11.2 KB, 4 views)

 Tags boundaries condition, temperature bc, velocity bc