# Which pressure OpenFOAM use for incompressible flow? P/rho or (P-101325)/rho ?

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 December 15, 2009, 05:59 Which pressure OpenFOAM use for incompressible flow? P/rho or (P-101325)/rho ? #1 Senior Member   Jiang Join Date: Oct 2009 Location: Japan Posts: 186 Rep Power: 8 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! rv82 likes this.

 December 15, 2009, 16:23 #2 New Member   Join Date: Mar 2009 Posts: 27 Rep Power: 9 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. http://www.cfd-online.com/Forums/ope...cell-size.html 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. vbnhfylbh likes this.

October 11, 2013, 11:37
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Quote:
 Originally Posted by jugghead 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. http://www.cfd-online.com/Forums/ope...cell-size.html 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.
may i know which default value of rho is used by simpleFoam (incompressible solver)? Because i need to have the right value for my pressure since i need to do performance curve of my fan simulation.

thanks

 October 11, 2013, 22:00 #4 Senior Member   Fumiya Nozaki Join Date: Jun 2010 Location: Yokohama, Japan Posts: 194 Rep Power: 9 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
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 Originally Posted by fumiya 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
basically one defines the nu value in transport properties. The nu value is given by this equation

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 1.5exp-5 (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: 194 Rep Power: 9 Hi nash, If you look at the table(http://www.engineeringtoolbox.com/ai...ies-d_156.html), you can find that the kinematic viscosity(nu) value is nearly equal to 1.5e-5 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. nash likes this.

October 12, 2013, 05:46
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Quote:
 Originally Posted by fumiya Hi nash, If you look at the table(http://www.engineeringtoolbox.com/ai...ies-d_156.html), you can find that the kinematic viscosity(nu) value is nearly equal to 1.5e-5 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.
Thanks for the explanation.

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?

 October 12, 2013, 10:26 #8 Senior Member   Fumiya Nozaki Join Date: Jun 2010 Location: Yokohama, Japan Posts: 194 Rep Power: 9 I think it's not possible and it is easier to multiply the result by rho after calculation. Fumiya

 October 29, 2014, 07:56 #9 Member   Join Date: Aug 2011 Posts: 83 Rep Power: 7 Hello together, I know this thread is older but I am a little confused at the moment concerning the pressure. If I use the incompressible solver interFoam and set the pressure to 0 in 0/p, I sometimes get a negative pressure p. Is it also true in this case that I calculate the "real" static pressure with p + 101325? Thanks a lot for your help idefix

 November 21, 2014, 02:26 #10 Member   Artem Shaklein Join Date: Feb 2010 Location: Russia, Izhevsk Posts: 43 Rep Power: 8 Hello, idefix. In incompressible limit , so only affects a solution. It doesn't matter whether absolute static pressure p_abs[0] = 1e5 or p_abs[0] = 2e5 (by the way, p_abs[0] = 0 can be used as well, but it is nonsense, because in vacuum just small amount of matter is presented and it can't be modeled by continuum mechanics theory). If density is constant, it's useful to divide all equations by density, so to recover abs pressure one has to do the math . In this case pressure (0/p) has dimensions [Pa/(kg/m^3)]. But this approach can be used in general case as well (just consider a number of digits to store: 1.013250001e5 vs 1.0e-4). To recover pressure one needs next . In this case pressure (0/p) has dimensions [Pa]. I looked inside interFoam case and found that 0/p has [Pa] dimensions. So you should go with .

 February 5, 2015, 11:53 #11 Senior Member   M. C. Join Date: May 2013 Location: Italy Posts: 122 Rep Power: 5 Just a question for confirmation, as I'm getting confuse with my results, in order to see how to change my BC. In 0/U I defined pressureInletvelocity and in 0/p I defined total pressure for inlet and set it equal to 0, so I defined: total = static + dynamic --> 0 = 0 + rho*U^2/2 (value for U= (0 0 0) --> all is zero) When I plot a slice on paraview, what pressure I get? static pressure divided by rho? total pressure divided by rho? or in other way to ask, is pressure calculated by openfoam the static one or the total pressure? thanks a lot. Bye

January 5, 2016, 12:17
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 Originally Posted by student666 Just a question for confirmation, as I'm getting confuse with my results, in order to see how to change my BC. In 0/U I defined pressureInletvelocity and in 0/p I defined total pressure for inlet and set it equal to 0, so I defined: total = static + dynamic --> 0 = 0 + rho*U^2/2 (value for U= (0 0 0) --> all is zero) When I plot a slice on paraview, what pressure I get? static pressure divided by rho? total pressure divided by rho? or in other way to ask, is pressure calculated by openfoam the static one or the total pressure? thanks a lot. Bye
Did you find the answer for your question as I'm troubling in this, too?

Thanks!

April 22, 2016, 04:38
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Jigar Parekh
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Quote:
 Originally Posted by student666 Just a question for confirmation, as I'm getting confuse with my results, in order to see how to change my BC. In 0/U I defined pressureInletvelocity and in 0/p I defined total pressure for inlet and set it equal to 0, so I defined: total = static + dynamic --> 0 = 0 + rho*U^2/2 (value for U= (0 0 0) --> all is zero) When I plot a slice on paraview, what pressure I get? static pressure divided by rho? total pressure divided by rho? or in other way to ask, is pressure calculated by openfoam the static one or the total pressure? thanks a lot. Bye
Hi student666,

Notations -
p = static pressure
ptot = total pressure
rho = density

to my knowledge, for incompressible flows, OF solves for p/rho. You can find total pressure (ptot = p + 1/2 * rho *U^2) using the ptot utility.

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