What this step does
The flow in a muffler is turbulent, so the program solves the Reynolds-averaged Navier-Stokes equations with a two-equation eddy-viscosity model (Menter, 1994) rather than resolving the turbulence. Three choices define the equations:
- Analysis type: constant density (the incompressible solver / the transient incompressible solver), or density and energy solved with the ideal-gas law (the compressible solver, or the transient compressible solver with local time stepping under the pseudo-transient option of Compressible solver);
- Turbulence model: k-ω SST, fixed;
- Time dependency: steady, solved iteratively with SIMPLE until the pressure-drop mean has converged, or transient, marched in time with PIMPLE and averaged over the last window of simulated time.
The resulting solver:
| analysis | steady | transient |
|---|---|---|
| incompressible | the incompressible solver | the transient incompressible solver |
| compressible | the compressible solver (or the transient compressible solver, pseudo-transient) | not available |
Heat transfer and the acoustic medium "From CFD fields" (Physics step) lock the analysis type to compressible, and both require a steady run.
Source
The solver is chosen once, when the case is built, and written into the case's controlDict; the run, a resumed run and the report read it back from there. Solver documentation: (OpenFOAM user guide, 2026).
Valid range
Steady first
A steady run is far cheaper. Use transient only when the pressure drop of the steady run keeps oscillating strongly and a time-accurate average is wanted.
Fields in this step 3
| Field | Symbol | Unit | Default |
|---|---|---|---|
| Analysis type | incompressible | ||
| Time dependency | steady | ||
| Turbulence model | k-ω SST |