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buoyant flow in a horizontal duct

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Old   October 18, 2011, 19:14
Default buoyant flow in a horizontal duct
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Ehsan
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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;
}
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Old   October 19, 2011, 05:05
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Quote:
Originally Posted by ehsanshams View Post
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, 
}
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Old   October 19, 2011, 11:57
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Ehsan
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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,
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Old   October 20, 2011, 02:53
Default inlet conditions and outlet conditions
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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;
}
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Old   October 20, 2011, 14:19
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Ehsan
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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
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Old   October 21, 2011, 02:03
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Quote:
Originally Posted by ehsanshams View Post
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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