[cryabroad]

Hello Foamers,

This has been a question that bothers me, why do we need to solve for density in the rhoPimpleFoam solver? I searched a lot on this forum but the threads I found are not that clear.

For the rhoPimpleFoam solver, here's what I understand. The solver first solves the momentum equation (UEqn.H), then energy equation (EEqn.H), then the pressure equation (pEqn.H) and additional equations like turbulent ones. At the end of the EEqn.H, there's thermo.correct(), which means properties like viscosity, thermal diffusivity and temperature are updated. At the beginning of the pEqn.H, there's rho = thermo.rho(), so density is updated before the pressure is solved.

However, after the pressure is solved, there's #include "rhoEqn.H" (line 84 of pEqn.H, OpenFOAM-4.x), then there's another rho = thermo.rho() line (line 91 of pEqn.H, OpenFOAM-4.x). Why do we need to solve an equation for density when density has already been (and should be) determined by the equation of state? Won't this give you a density field that is not consistent with the equation of state? Also, why do we need to update the density twice in the pressure equation? In rhoSimpleFoam the rho = thermo.rho() line only appears once at the end of the pEqn.H.

I feel like this can be a problem related to compressible flows. I've mainly dealt with incompressible flows so it's hard for me to go through these compressible solvers, although they use almost the same pressure-velocity coupling strategy.

Thanks in advance,

Ruiyan