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megacrout July 15, 2011 05:44

units in OpenFOAM

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.
Im pretty sure this has something to do with incompressibility but I am not quite sure and I dont 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.


akidess July 15, 2011 07:14

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.

megacrout July 18, 2011 05:37

Hi Anton,

Thanks for your answer.
Im 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.


fabian_roesler July 19, 2011 05:44

Phi units

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 [m/s]



megacrout July 19, 2011 08:04

All right!
Crystal clear.

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