# units in OpenFOAM

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 July 15, 2011, 05:44 units in OpenFOAM #1 Member   Tibo Join Date: Jun 2011 Posts: 68 Rep Power: 7 Hi, I am getting confused about the units used in OpenFOAM for some parameters, especially p and phi. 1. p has to be given with units m2/s2 in simpleFoam (which actually are the units of p/rho) but with the usual units kg/m/s2 in reactingFoam for example. I´m pretty sure this has something to do with incompressibility but I am not quite sure and I don´t get why it is programmed this way. 2. phi is rho.U (e.g. User Guide 1.7.1 p.115 or 2.0.0 p.118), right? It thus should have units kg/m2/s but has units m3/s in the time step folders. why is that so? This question might result from a bad understanding of what phi is. Complementary information about it too would be appreciated. Thanks for your help. Tibo rafa13 likes this.

 July 15, 2011, 07:14 #2 Senior Member     Anton Kidess Join Date: May 2009 Location: Delft, Netherlands Posts: 1,139 Rep Power: 20 1. With constant density, you can divide the entire equation with rho and you will end up with a modified pressure, and thus the "uncommon" units. 2. phi is U * Sf (velocity times surface area) in all the solvers I worked with. rafa13 likes this.

 July 18, 2011, 05:37 #3 Member   Tibo Join Date: Jun 2011 Posts: 68 Rep Power: 7 Hi Anton, Thanks for your answer. I´m a bit confused though. As I mentioned, phi is defined as rho * U in the User Guide (p.115 or p.118). Does that mean the definition varies depending on the situation studied or the solver used? Best regards. Tibo

 July 19, 2011, 05:44 Phi units #4 Senior Member   Fabian Roesler Join Date: Mar 2009 Location: Germany Posts: 210 Rep Power: 10 Hi, in simpleFoam you assume constant density. phi = rho*U*Sf [kg/s] in the convective term as rho is constant you can divide the whole equation by rho as akidess posted before. And so you get phi = U*Sf [m³/s] Regards Fabian rafa13 likes this.

 July 19, 2011, 08:04 #5 Member   Tibo Join Date: Jun 2011 Posts: 68 Rep Power: 7 All right! Crystal clear. thx

 Tags phi, pressure, units

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