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Convergence Results and RMS

A measure of how well the solution is converged can be obtained by plotting the residuals for each equation at the end of each timestep. A reasonably converged solution requires a maximum residual level no higher than 5.0E-4. Typically, the RMS residual will be an order of magnitude lower than this.

The RMS (Root Mean Square) residual is obtained by taking all of the residuals throughout the domain, squaring them, taking the mean, and then taking the square root of the mean. This should present an idea of a typical magnitude of the residuals.

The Maximum Residuals and/or the RMS Residuals can be displayed in the convergence history plots by selecting a specific monitor in Monitor Settings. For details, see Monitors Tab.

The increase of a residual after any particular timestep does not imply that the solution is diverging. It is usual for residuals to occasionally get larger, especially at the beginning of a Run.

Note that even though convergence is good, there are still places where the residuals become larger temporarily.

It is also possible to have runs that do not converge at all, but simply deviate around the same values.

If the solution fails to converge, or convergence is only happening very slowly, some tips on how to improve the convergence are available. For details, see Monitoring and Obtaining Convergence.

Tip

If you want to obtain Residual Plots for old Runs, select File > Monitor Finished Run and select a file to view.