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25.22.1 Judging Convergence

There are no universal metrics for judging convergence. Residual definitions that are useful for one class of problem are sometimes misleading for other classes of problems. Therefore it is a good idea to judge convergence not only by examining residual levels, but also by monitoring relevant integrated quantities such as drag or heat transfer coefficient.

For most problems, the default convergence criterion in FLUENT is sufficient. This criterion requires that the scaled residuals defined by Equation  25.18-4 or 25.18-9 decrease to $10^{-3}$ for all equations except the energy and P-1 equations, for which the criterion is $10^{-6}$.

Sometimes, however, this criterion may not be appropriate. Typical situations are listed below.

Another popular approach to judging convergence is to require that the unscaled residuals drop by three orders of magnitude. FLUENT provides residual normalization for this purpose, as discussed in Section  25.18.1, where residuals are defined for both the pressure-based solver and the density-based solver. In this approach the convergence criterion is that the normalized unscaled residuals should drop to $10^{-3}$. However, this requirement may not be appropriate in many cases:

In such cases, it is wise to monitor integrated quantities, such as drag or overall heat transfer coefficient, before concluding that the solution has converged. It may also be useful to examine the un-normalized unscaled residual, and determine if the residual is small compared to some appropriate scale. Alternatively, the scaled residual defined by Equation  25.18-4 or 25.18-9 (the default) may be considered.

Conversely, it is possible that if the initial guess is very bad, the initial residuals are so large that a three-order drop in residual does not guarantee convergence. This is specially true for $k$ and $\epsilon$ equations where good initial guesses are difficult. Here again it is useful to examine overall integrated quantities that you are particularly interested in. If the solution is unconverged, you may drop the convergence tolerance, as described in Section  25.18.1.

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