# buoyant flow in a horizontal duct

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 October 18, 2011, 19:14 buoyant flow in a horizontal duct #1 New Member   Ehsan Join Date: Jan 2010 Posts: 7 Rep Power: 9 Sponsored Links hi, I am trying to solve flow in a horizontal duct with inflow/outflow and constant temperature walls (top and bottom). I tried to solve it using buoyantPimpleFoam. I am not sure what I am doing wrong in my boundary conditions that give me unreasonable results. I will appreciate any helpful comments. my inlet BCs: U: inlet { type surfaceNormalFixedValue; refValue uniform -0.01; } T: inlet { type zeroGradient; } p: inlet { type calculated; value \$internalField; } p_rgh: inlet { type fixedValue; value uniform 1e5; }

October 19, 2011, 05:05
#2
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Roman Thiele
Join Date: Aug 2009
Location: London, UK
Posts: 368
Rep Power: 14
Quote:
 Originally Posted by ehsanshams hi, I am trying to solve flow in a horizontal duct with inflow/outflow and constant temperature walls (top and bottom). I tried to solve it using buoyantPimpleFoam. I am not sure what I am doing wrong in my boundary conditions that give me unreasonable results. I will appreciate any helpful comments. my inlet BCs: U: inlet { type surfaceNormalFixedValue; refValue uniform -0.01; } T: inlet { type zeroGradient; } p: inlet { type calculated; value \$internalField; } p_rgh: inlet { type fixedValue; value uniform 1e5; }
For you temperature you should define an inlet temperature, you do have an temperature of the flow that comes in, don't you?

so for T inlet BC, I would suggest
Code:
```inlet
{
type fixedValue;
value uniform 293; // in case it is room temperature,
}```
__________________
~roman

 October 19, 2011, 11:57 #3 New Member   Ehsan Join Date: Jan 2010 Posts: 7 Rep Power: 9 Roman, Thank you for reply. I had already tested it with defined temperature and it would not help. The problem is I get reverse flow at inlet and also very large pressure inside the domain. Now, I define p_rgh, T, and U as a constant value (at inlet), and still get a wrong answer. Does anyone know what excatly the surfaceNormalFixedValue does at the inlet? How can I learn the way openfoam handles boundary conditions? Thank you,

 October 20, 2011, 02:53 inlet conditions and outlet conditions #4 Senior Member     Roman Thiele Join Date: Aug 2009 Location: London, UK Posts: 368 Rep Power: 14 Inlet conditions U: inlet { type fixedValue; value uniform (1 0 0); } p_rgh: inlet { type buoyantPressure; rho rhok; value uniform 1e5; } T: inlet { type fixedValue; value uniform 293; } outlet conditions U: outlet { type pressureInletOutletVelocity; value uniform (0 0 0); } p_rgh: { type fixedValue; rho rhok; value uniform 1e5; } T: outlet { type zeroGradient; } __________________ ~roman

 October 20, 2011, 14:19 #5 New Member   Ehsan Join Date: Jan 2010 Posts: 7 Rep Power: 9 thanks Roman, I tried this and worked well: p_rgh: ----------- inlet { type zeroGradient; } outlet { type fixedValue; value uniform 1e5; } U: -------- inlet { type surfaceNormalFixedValue; refValue uniform -0.01; } outlet { type zeroGradient; } what does rho rhok; mean? does it matter if use fixedValue instead of buoyantPressure for p_rgh at outlet? thank you

October 21, 2011, 02:03
#6
Senior Member

Roman Thiele
Join Date: Aug 2009
Location: London, UK
Posts: 368
Rep Power: 14
Quote:
 Originally Posted by ehsanshams thanks Roman, what does rho rhok; mean? does it matter if use fixedValue instead of buoyantPressure for p_rgh at outlet?
buoyantPressure for p_rgh is also a zeroGradient boundary condition, but especially designed for buoyantFlows.

The description from OF for buoyantPressure is
Code:
```Description
Set the pressure gradient boundary condition appropriately for buoyant flow.

If the variable name is "pd" assume it is p - rho*g.h and set the gradient
appropriately.  Otherwise assume the variable is the static pressure.```
__________________
~roman

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