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-   -   buoyant flow in a horizontal duct (http://www.cfd-online.com/Forums/openfoam-solving/93547-buoyant-flow-horizontal-duct.html)

 ehsanshams October 18, 2011 19:14

buoyant flow in a horizontal duct

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.

my inlet BCs:

U:
inlet
{
type surfaceNormalFixedValue;
refValue uniform -0.01;
}

T:
inlet
{
}

p:
inlet
{
type calculated;
value \$internalField;
}

p_rgh:
inlet
{
type fixedValue;
value uniform 1e5;
}

 romant October 19, 2011 05:05

Quote:
 Originally Posted by ehsanshams (Post 328497) 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, }```

 ehsanshams October 19, 2011 11:57

Roman,
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,

 romant October 20, 2011 02:53

inlet conditions and outlet conditions

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
{
}

 ehsanshams October 20, 2011 14:19

thanks Roman,
I tried this and worked well:

p_rgh:
-----------
inlet
{
}

outlet
{
type fixedValue;
value uniform 1e5;
}

U:
--------

inlet
{
type surfaceNormalFixedValue;
refValue uniform -0.01;
}

outlet
{
}

what does rho rhok; mean?

does it matter if use fixedValue instead of buoyantPressure for p_rgh at outlet?

thank you

 romant October 21, 2011 02:03

Quote:
 Originally Posted by ehsanshams (Post 328787) 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.```

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