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

ρ = P/RT + ρ

_{o}, where ρ

_{o}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 ρ

_{o}= 1027 kg m

^{-3}.

Also for mercury

R = 6818 with ρ

_{o}= 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.