[knuckles]

OpenFOAM thermodynamics packages are specified by combining 7 parameters: type, mixture, transport, thermo, equationOfState, specie, energy. As far as I can tell, the last 6 parameters collectively define the property models to be used, and thus determine the answer to any given thermodynamic question (up to an arbitrary constant). If this is the case, then the only degree of freedom left for the “type” parameter to constrain is how the model is implemented. Is this what “type” represents?

For example, I have two different combustion solvers. Solver A allows the thermo model parameters to take on the values

hePsiThermo, reactingMixture, const, hConst, perfectGas, specie, sensibleEnthalpy

while Solver B allows a combination in which only the first parameter is different:

heRhoThermo, reactingMixture, const, hConst, perfectGas, specie, sensibleEnthalpy

According to my understanding, both of these models will accept the same inputs and produce the same outputs, although they may implement things differently “under the hood.” I know that Solver A won't accept heRhoThermo, and Solver B won't accept hePsiThermo. What is so fundamental about the rho/psi distinction that these two externally-identical thermo models are not interchangeable?