# Pressure units in incompressible solvers

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 March 7, 2012, 05:17 Pressure units in incompressible solvers #1 New Member   Per Christian Endresen Join Date: Feb 2012 Location: Trondheim, Norway Posts: 13 Rep Power: 6 Hi I am quite new to OpenFOAM, and have some basic questions about units. From the tutorial cases I see that the units for pressure in incompressible solvers (e.g. simpleFoam) are m^2/s^2. Which make sense since pressure is constant. I guess I then have to scale (divide) my pressure initial and boundary conditions with rho in order to get a correct solution? My real question is: can I define my pressure units to be kg/ms^2 and define density rho in the transportProperties file and get the same result? I want to be able to do this in order to avoid having to scale my pressure. Thanks in advance for replies. Per

 March 7, 2012, 08:46 #3 New Member   Per Christian Endresen Join Date: Feb 2012 Location: Trondheim, Norway Posts: 13 Rep Power: 6 Thanks for the reply Felix Especially the fact that a set pressure just is a reference is useful to know. And I should have figured that out from the NS equation. Regarding the pressure gradient. Forgive me if this is a stupid question. If one wants a pressure gradient at for instance the outlet of a pipe section (due to a propeller), is it correct that this must be scaled with the density? Since (just an example) the incompressible NS equation for 2D pressure driven flow between two plates is reduced to nu*(ddu/du^2) = (1/rho)*(dp/dx) where (1/rho)*(dp/dx) = d(p/rho)/dx = dp'/dx since rho = const. (p' = p/rho). Is it correct to assume that p' is the pressure OpenFoam Calculates, and thus one must scale the gradient when setting the boundary condition? Or am I missing something? Regards Per

 March 8, 2012, 12:40 #4 Senior Member   Felix L. Join Date: Feb 2010 Location: Hamburg Posts: 165 Rep Power: 10 Hello, Per, short answer: you are correct. Greetings, Felix.

 Tags density, pressure, rho, simplefoam, units

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