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A Density of a perfect fluid

  1. May 31, 2018 #1
    During CFD modeling of a gas-solid flow, flow of solid was modelled as a perfect fluid using OpenFOAM.

    The density of the perfect fluid is calculated using the following equation as given in the documentation.

    ρ = P/RT + ρo , where ρo is the density at T = 0 kelvin, ρ is the density of the perfect fluid at pressure P (Pa) and temperature T (K). There is no other mention about this in the documentation of OpenFOAM.

    My struggle is to calculate the R (J kg -1 K-1) for the solid. In the OpenFOAM tutorials for the normal conditions of water R = 3000 with ρo = 1027 kg m-3.

    Also for mercury
    R = 6818 with ρo = 13529 kg m-3.

    I tried to plot the ρ with P/T for water and the linear equation was

    ρ = 0.4321 (P/T) + 848.78, where R = 2.314

    So could anyone please tell me how to calculate R for a certain fluid or solid. I have searched the internet for days and still didn't find any reference to this equation.
  2. jcsd
  3. May 31, 2018 #2


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    Gold Member

    I'm still tryng to figure out how you are applying the ideal gas law to solids and liquids.
  4. May 31, 2018 #3
    Actually this is the equation, OpenFOAM has mentioned in their user guide. Not my idea. I thought this is some physics that I cannot understand.

    You can see the equation in page U-203 of their user guide. Here is the link.

    I have seen the same problem asked in cfd-online.com, but with no answers


    Thanks for your comment anyway. I would be grateful if you could give me a solution.
  5. Jun 1, 2018 #4
    I have no idea too, but the only thing that you can do -i think- is to look in some database for ##\rho_0## and ##\rho## for the solid you are interested in and then calculate an approximation of R.
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