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Old   December 1, 2009, 09:18
Question basic question about the pressure
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Hi all,

This is very basic I think but I'd like to be sure about this:
In the pressure file 0/p, we set the dimensions of the pressure such as we need to multiply the pressure by the fluid density to get "Pascal" as units.
So, during the post-process, when we visualize the pressure, basically we need to multiply it by the fluid density to get the "real" pressure field.
Am I right?

Thanks.
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Old   December 2, 2009, 06:19
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Jose Luis Santos
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Hi,

Yes you are correct: [p]=m^2/s^2; P=p*rho -> [P]=kg/(ms^2)=Pa

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Jose Santos
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Old   December 2, 2009, 06:46
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Thanks Jose.
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Old   July 29, 2010, 12:02
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So, another question related. What about "nu" units?
Im running a very simple case, flow through a pipe, imposing p = 0.026 at inlet and 0 at outlet. (p dimensions are [0 2 -2 0 0 0 0], that is p is divided by rho).
In transportProperties i set:

nu nu [0 2 -1 0 0 0 0] 3.047e-6;

Since the fluid im trying to simulate has a dinamic viscosity etha= 0.0032 Pa*s and densisty rho=1050 kg/m3. What i did was nu = 0.0032/1050 = 3.047e-6.

When i try to compare results with analytical solution, Poiseuille´s flow, vmax (of the parabola developed) should be:

vmax=(0.026 * r^2)/(4*L*nu)

But vmax in the results is around half the expected. Should there be a mistake considering nu units? Which are the units of 3.047e-6?
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Old   July 29, 2010, 12:25
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Fernando,

[0 2 -1 0 0 0 0]
declares the nu dimensions as m^2 s^-1.

I would say that your problem lies somewhere else, probably in the boundary conditions.

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Jose Santos
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Old   July 29, 2010, 13:07
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Jose, thanks for the reply. Lets see, the mesh consists of a pipe, with an inlet, the wall, and the outlet. The boundary conditios are:

0/p:

dimensions [0 2 -2 0 0 0 0];
internalField uniform 0;
boundaryField
{
wall
{
type zeroGradient;
}
inlet
{
type fixedValue;
value uniform 0.026;
}
outlet
{
type fixedValue;
value uniform 0;
}

}

0/U:

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

internalField uniform (0 0 0);

boundaryField
{

pared
{
type fixedValue;
value uniform (0 0 0);
}

entrada
{
type zeroGradient;

}
salida
{
type zeroGradient;
}

I cannot find an error. I attach the velocity profile at the outlet, it seems preetty parabolic, but the analitical solution predicts a parabolic profile with maximmum velocity of 0.47 m/s.

Any idea? Im using simpleFoam, laminar, nu = 3.047e-6.

Thanks
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