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.