Maximum time step

Theory

The adaptive step follows the maximum Courant number (Maximum Courant number) and is clipped at $\Delta t_{max}$ , equation (1) (OpenFOAM user guide, 2026). In a muffler the Courant limit of the boundary-layer cells keeps the step at about $10^{-5}$ s, far below the default $10^{-2}$ s, so this limit normally never acts.

It matters for a slow flow on a coarse mesh, where the Courant number alone would allow steps that skip over the time scales of interest, such as the shedding of a jet or the save interval.

Equations

\Delta t^{n+1} \le \Delta t_{max}

(1)

where	meaning	unit
$\Delta t^{n+1}$	Next time step	s
$\Delta t_{max}$	Maximum time step (this setting)	s

Default and rule

Default: 1e-2 s
Rule: $10^{-2}$ s.
Source: This is the program's default. The clipping is OpenFOAM's maxDeltaT (OpenFOAM user guide, 2026).

Valid range > 0

Values

Above 0. Keep it to a fraction of the period of any fluctuation that is to be resolved.

References 1

OpenCFD Ltd. (2026). OpenFOAM user guide. openfoam.com. Web page