Theory
The convergence rule (Δp mean-stability tolerance) needs a window long enough for its mean to be meaningful. No verdict is given until samples exist past the start point, equation (1); before that the status is "insufficient", which is not a failure. The most recent samples are searched for a dominant oscillation period ; the window actually used is , equation (2), capped at the samples available. The same window is what the reported pressure drop averages, and the live chart shades exactly the samples the rule used.
A longer window reduces the artificial drift a straight-line fit shows on an oscillating signal (about 1.5 % of the mean at 20 samples, 0.06 % at 500) and the statistical uncertainty of the mean; it also lengthens every run by at least that many iterations.
Equations
| where | meaning | unit |
|---|---|---|
| Δp window (this setting) | iterations | |
| Upwind start-up iterations (later if the solver was restarted for a perforate update) | iterations | |
| Settle iterations after a restart, found from the data (MSER-5, see Upwind iterations before the switch) | iterations | |
| Samples after the start point | - | |
| Dominant oscillation period found in the last samples (0 if none) | iterations | |
| Window used | iterations |
Default and rule
- Default
- 500
- Rule
500 iterations.
- Source
The window of 500 iterations limits the finite-sample drift of an exactly periodic signal (a square wave: about 1.5 % at , 0.06 % at ), which makes shorter windows misread stable limit cycles as drifting. Settle iterations: the restart between the two solver phases disturbs the solution (pressure drop 17 % off for about 40 iterations on the two-baffle test muffler), and the disturbance must have died out before samples count; the point where it has is found from the pressure drop itself (White, 1997).
Valid range whole number > 0
Values
Whole numbers above 0. The automatic maximum number of iterations grows with it.
Short windows
A window much shorter than 500 can call an oscillating but stable mean "drifting", or accept a mean that is still wandering.