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December 1, 2009, 09:18 
basic question about the pressure

#1 
Member
Join Date: Sep 2009
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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 postprocess, 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. 

December 2, 2009, 06:19 

#2 
Senior Member

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

December 2, 2009, 06:46 

#3 
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Thanks Jose.


July 29, 2010, 11:02 

#4 
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Fernando
Join Date: Feb 2010
Posts: 28
Rep Power: 8 
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] 3.047e6; 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 = 3.047e6. 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 3.047e6? 

July 29, 2010, 11:25 

#5 
Senior Member

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. Regards, Jose Santos 

July 29, 2010, 12:07 

#6 
New Member
Fernando
Join Date: Feb 2010
Posts: 28
Rep Power: 8 
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 = 3.047e6. Thanks 

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