[MastaMinds]

Hello everyone,

I have been trying to run a 'compressible' simulation using rhoSimpleFoam, and I am still struggling with boundary conditions implementation. In my head 'compressible' means the velocity field has non-zero divergence, it means 'volume' is not constant. But apparently, rhoSimpleFoam is a pressure-based solver, and it uses SIMPLE algorithm for pressure velocity coupling. I think this means it solves an incompressible problem with varying density, am I correct?
And therefore, it cannot capture flow discontinuities.

From my understanding, compressible implies a density-based solver that obtains the pressure field from the thermodynamic model of the fluid.

Moreover, I wanted to use free stream boundary conditions for both velocity and pressure, but I think they wouldn't work as they do in a density-based solver.

Thanks!

[Tobermory]

rhoSimpleFoam is indeed a compressible solver (see discussion below) - it solves a continuity equation and allows for gas mixtures with differing densities, so the dilatation term (divU) can indeed be non-zero. You are right to note that it is a pressure based solver, though, and so is probably better targeted at lower Mach number, variable density flows, since I guess that density based solvers will be more efficient at high Mach#.

As for the definition of "incompressible" - this seems to be used ambiguously. Many treat it as just a measure of flow compressibility effects or Mach number, i.e. whether flow/pressure perturbations result in significant density perturbations (When choose sensibleInternalEnergy or sensibleEnthalpy in thermophysicalProperties). Others use it to mean that divU = 0, i.e. constant density. OpenFOAM uses the latter, when classifying its solvers.