
[Sponsors] 
Which pressure OpenFOAM use for incompressible flow? P/rho or (P101325)/rho ? 

LinkBack  Thread Tools  Display Modes 
December 15, 2009, 04:59 
Which pressure OpenFOAM use for incompressible flow? P/rho or (P101325)/rho ?

#1 
Senior Member
Jiang
Join Date: Oct 2009
Location: Japan
Posts: 186
Rep Power: 6 
Dear Foamers:
I am a little confused about pressure for imcompressible flow. I want to know which pressure OpenFOAM uses. Because in outflow boundary, pressure value is fixed. So if you give zero, the pressure result will be very small. If you give 101325/rho, the pressure result will be large. in all the tutorials of OpenFOAM , in case/0 ,pressure is set to zero. If that means , the pressure set in P file is (Real Pressure  101325)/rho ? Thank you very much! 

December 15, 2009, 15:23 

#2 
New Member
Join Date: Mar 2009
Posts: 27
Rep Power: 7 
Isn't the pressure already divided by rho?
So the real pressure would be p*rho and then if you have 0 at outlet add atmospheric pressure so: p*rho + 101325 But you should see my post about pressure and knowbody has answered yet. Pressure varies with cell size!! I suspect pressure is not being calculated correctly because it varies too much with cell size. If so you can't use the pressure value. 

October 11, 2013, 11:37 

#3  
Member
Join Date: Aug 2013
Posts: 50
Rep Power: 2 
Quote:
thanks 

October 11, 2013, 22:00 

#4 
Senior Member
Fumiya Nozaki
Join Date: Jun 2010
Location: Yokohama, Japan
Posts: 159
Rep Power: 6 
You can get the real pressure value (that is not divided by rho) using the formula
that jugghead wrote and you can find the rho value by consulting the physical property books at your simulation condition. Hope this helps, Fumiya 

October 12, 2013, 03:53 

#5  
Member
Join Date: Aug 2013
Posts: 50
Rep Power: 2 
Quote:
nu=mu/rho So how can i just get the value from the book. Isnt the simplefoam or other incompressible solver default value used? So what is the default value then? i have done the solver, with nu 1.5exp5 (from motorbike tutorial) Now i need the rho value based on that nu or based on the motorbike tutorial. Thanks 

October 12, 2013, 05:22 

#6 
Senior Member
Fumiya Nozaki
Join Date: Jun 2010
Location: Yokohama, Japan
Posts: 159
Rep Power: 6 
Hi nash,
If you look at the table(http://www.engineeringtoolbox.com/ai...iesd_156.html), you can find that the kinematic viscosity(nu) value is nearly equal to 1.5e5 at 20 degrees Celsius and the density(rho) value is 1.205 kg/m^3 at this temperature. So, the motor bike tutorial solves the flow on these conditions if the working fluid is air. If you try to do another simulation at different condition(different temperature or fluid etc.), you can find the nu and rho value from books and set nu value in the transportProperties dictionary. When your simulation finishes, you can get the real pressure value using the formula that jugghead wrote and the density value you will have found. 

October 12, 2013, 05:46 

#7  
Member
Join Date: Aug 2013
Posts: 50
Rep Power: 2 
Quote:
Now i would like to ask, if i want to get the exact pressure direct from the simulation, i plan to set the rho to 1. So i need to set nu. But i dont know the mu. Any idea? Temperature is at 20 degree celcius. Isnt okay if i do so? Thanks again for your help 

October 12, 2013, 10:26 

#8 
Senior Member
Fumiya Nozaki
Join Date: Jun 2010
Location: Yokohama, Japan
Posts: 159
Rep Power: 6 
I think it's not possible and it is easier to multiply the result by rho after calculation.
Fumiya 

Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Pressure Drop  Please Help  Simple Pipe Flow  Joe A.  FLUENT  2  April 23, 2007 07:50 
FLOW AROUND A PLATE_NEGATIVE ABSOLUTE PRESSURE????  tania  FLUENT  11  March 23, 2004 08:51 
what the result is negatif pressure at inlet  chong chee nan  FLUENT  0  December 29, 2001 05:13 
mass flow inlet  Denis Tschumperle  FLUENT  7  August 9, 2000 02:19 
Hydrostatic pressure in 2phase flow modeling (CFX4.2)  HB &DS  CFX  0  January 9, 2000 13:19 