# basic question about the pressure

 Register Blogs Members List Search Today's Posts Mark Forums Read

 December 1, 2009, 09:18 basic question about the pressure #1 Member   Join Date: Sep 2009 Posts: 45 Rep Power: 7 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.

 December 2, 2009, 06:19 #2 Senior Member     Jose Luis Santos Join Date: Mar 2009 Location: Portugal Posts: 213 Rep Power: 9 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 Member   Join Date: Sep 2009 Posts: 45 Rep Power: 7 Thanks Jose.

 July 29, 2010, 11:02 #4 New Member   Fernando Join Date: Feb 2010 Posts: 28 Rep Power: 7 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?

 July 29, 2010, 11:25 #5 Senior Member     Jose Luis Santos Join Date: Mar 2009 Location: Portugal Posts: 213 Rep Power: 9 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: 7
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
{
}
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);
}

{

}
salida
{
}

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
Attached Images
 U_out.jpg (71.9 KB, 9 views)

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post pierresandre FLUENT 24 November 8, 2011 15:32 Giuki FLUENT 1 July 19, 2011 11:35 Graham Main CFD Forum 4 October 31, 2006 18:52 Barry Main CFD Forum 1 February 5, 2003 01:27 Dan Moskal Main CFD Forum 0 October 24, 2002 22:02

All times are GMT -4. The time now is 23:07.