Theory
The absolute target (Residual target) depends on how OpenFOAM normalises the residual (OpenFOAM user guide, 2026) and on the problem. A relative criterion asks instead how much the residual has dropped since the start: a fall of 3 decades means the equation imbalance is a thousandth of what it was in the first iterations. Gate 2 compares the largest initial residual of the first 10 iterations with the largest of the last 50, equation (1); the field passes when the absolute OR the relative criterion is met.
This exists because the steady solution of a muffler with jets keeps a residual floor of to while its pressure drop is stable: the absolute target alone would rarely be met.
Equations
| where | meaning | unit |
|---|---|---|
| Decades the initial residual of field has fallen | - | |
| Initial residual of field at iteration | - | |
| Residual drop (this setting) | decades |
Default and rule
- Default
- 3
- Rule
3 decades. Blank switches the relative criterion off (the absolute target alone decides).
- Source
The relative form and the value 3 follow from the program's residual-sensitivity study: OpenFOAM's initial residual is normalised so that a typical first iteration reads of order 1, so a 3-decade fall corresponds roughly to the absolute target reached from a typical initial field. A relative-reduction criterion is common CFD practice.
Valid range blank = off; > 0
Values
Blank (off) or a positive number of decades.
Start of the run
The reference is the first 10 iterations of the whole run (start-up phase, final phase and any continuation are judged as one log), i.e. the residuals of the crude initial field.