# psi in compressibleInterFoam (OpenFoam)

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 April 4, 2013, 11:36 psi in compressibleInterFoam (OpenFoam) #1 New Member   Clement Olivier Join Date: Mar 2013 Posts: 7 Rep Power: 6 Hi everyone, I am currently using the solver compressibleInterFoam (of OpenFoam). I am trying to change the equation of state perfectFluid and therefore, I need to understand what is psi. It seems to be the compressibility defined by But I am not sure at all. Moreover, I do not understand the file pEqn.H, and thus it do not help me to understand how psi is used. If someone could give me some help, I would be very thankful Best regards Clément

 May 28, 2013, 06:44 #2 New Member   Aaron Join Date: Mar 2013 Posts: 10 Rep Power: 6 Hi Clement, Have you managed to figure this out? I am also looking to implement an equation of state and am not sure what to make of the psi term. Referring to the documentation for totalPressure boundary condition (which is more in line with my own interests), psi is defined as compressibility and has units m^2/s^2. Beyond that though, I have not seen any documentation about what it actually is ... Edit: Continuing with the dimensional line of thought, so the units work out in that regard. Regards, Aaron Last edited by dalaron; May 28, 2013 at 06:57. Reason: Forgot to add something

 April 24, 2014, 09:08 #3 Member   Rohith Join Date: Oct 2012 Location: Germany Posts: 57 Rep Power: 6 Hi I think the compressibility is actually the reverse that is being defined above. It is psi = rho/p which makes it a isothermal compressibility Thanks RR musabai likes this.

 February 9, 2016, 07:52 totalPressure definition #4 New Member   Join Date: Feb 2016 Posts: 4 Rep Power: 3 Hi, In totalPressureFvPatchScalarField.H we can read that for compressible supersonic flow the total pressure is defined as : \f[ p_p = \frac{p_0}{(1 + 0.5 \psi G |U|^2)^{\frac{1}{G}}} \f] where \vartable p_p | pressure at patch [Pa] p_0 | total pressure [Pa] \gamma | ratio of specific heats (Cp/Cv) \psi | compressibility [m2/s2] G | coefficient given by \f$\frac{\gamma}{1-\gamma}\f$ \endvartable What compressibility is ? is its dimension really m2/s2 ? and really not s2/m2 ??. I would expect s2/m2 in order to obtain psi*U^2 as an unidimensional number .. I am also surprised by the G definition. It is really \frac{\gamma}{1-\gamma} ? I would expect the inverse frac{1-\gamma}{\gamma} ? Does someone has an opinion on it? Regards Fanny

 February 12, 2016, 04:51 #5 Member   Rohith Join Date: Oct 2012 Location: Germany Posts: 57 Rep Power: 6 Hi I see it is s2/m2 because in the above equation it is multiplied by U2 to get a scalar value. Best Regards, RRK

 November 21, 2016, 10:17 from the Programmer's Guide v 3.0.1 #6 New Member   Join Date: Apr 2016 Posts: 4 Rep Power: 3 Hi, everyone, If it still can help anybody, in studying the Programmer's Guide OF v. 3.0.1, I,ve found, in section 3.4, the definition of psi (): which is called the barotropic relationship (compressibility), and its unit is s²/m² (at least for sonicLiquidFoam).

 Tags compressibility, compressibleinterfoam, openfoam, perfectfluid, psi

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