Theory
The flow-through time is the time the mean flow needs to replace the gas in the muffler, equation (1): the fluid volume enclosed by the exported surfaces divided by the total volumetric inlet flow. Each inlet's flow is converted to m³/s, whatever its boundary-condition mode.
A transient run starts from the initial (potential-flow) field, and the first flow-through carries that impulsive start out of the domain. The auto end time, equation (2), adds four more flow-through times, so that the last two averaging windows (Averaging window) lie in established flow.
Quality gate 2 of a transient run checks that the run reached its end time and that the mean pressure drop of the last window agrees with that of the window before it. The check needs at least two averaging windows of simulated time.
Equations
| where | meaning | unit |
|---|---|---|
| Flow-through time | s | |
| Fluid volume enclosed by the exported surfaces | m³ | |
| Sum of the volumetric inflow of all inlets | m³/s | |
| End time (auto) | s |
Default and rule
- Default
- auto: 5 flow-through times (0.5 s until the flow-through time is known)Computed by the program
- Rule
Auto: . Before the geometry is exported, or while no inlet flow is set, the end time is 0.5 s. A typed value shorter than one flow-through time triggers a warning.
- Source
This is the program's rule; it replaces a fixed end time of 0.5 s. It has been verified against synthetic flow-through times only; no transient run with an end time other than 0.5 s has been validated yet. The time-step cost of a transient run is set by the smallest cells (Maximum Courant number).
Valid range > 0; a warning below one flow-through time
Values
Above 0. Long end times are expensive: every time step costs about one steady iteration, and the step size follows the smallest cell.
Too short
Below one flow-through time the initial transient has not left the domain; below two averaging windows the stationarity check cannot be made and gate 2 fails.