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July 15, 2011, 06:44 |
units in OpenFOAM
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#1 |
Member
Tibo
Join Date: Jun 2011
Posts: 68
Rep Power: 15 |
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 |
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July 15, 2011, 08:14 |
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#2 |
Senior Member
Anton Kidess
Join Date: May 2009
Location: Germany
Posts: 1,377
Rep Power: 30 |
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. |
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July 18, 2011, 06:37 |
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#3 |
Member
Tibo
Join Date: Jun 2011
Posts: 68
Rep Power: 15 |
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 |
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July 19, 2011, 06:44 |
Phi units
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#4 |
Senior Member
Fabian Roesler
Join Date: Mar 2009
Location: Germany
Posts: 213
Rep Power: 18 |
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 |
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July 19, 2011, 09:04 |
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#5 |
Member
Tibo
Join Date: Jun 2011
Posts: 68
Rep Power: 15 |
All right!
Crystal clear. thx |
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Tags |
phi, pressure, units |
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