When solving CFD models using Ansys products, there are a number of ways to determine model convergence. DRD recommends monitoring a combination of holistic (like individual equation residual values) and local quantities (like surface and point monitors) to ensure a stable, converged solution. Residuals are representative of the average error across all control volumes in the model, while local quantities can help you better focus on key parameters in your model.
Sometimes you might encounter a situation where one or more residual does not reach its convergence target, but important characteristic features being monitored have steadied to a particular value. Often this is sufficient for approximating the behavior you are simulating, but other times you may need to strive for more precision. To do so, it is helpful to be able to determine where in the model there are locally high residuals that are preventing the overall residual value from reaching its target.
By default, Fluent only exposes mass-imbalance (related to continuity residual) to the user. When post-processing, this can be found in the Residuals group of variables.
To access the other residuals, you need to enable an expert parameter using the console prior to solving. Expert parameters can be accessed using the TUI command ‘/solve/set/expert’. Beyond that, the prompts you receive will depend on your Fluent environment, so we can’t dictate a specific list of responses. However, to enable the remaining residuals you will need to answer “yes” to the prompt “Save cell residuals for post-processing?”.
After solving you should then see more options for plotting residuals, which will allow you to better evaluate where local error is preventing the overall residual from reaching its convergence target.
Often you will find this is associated with a mesh quality issue that requires resolution via remeshing, or even a geometric modification in order to improve mesh quality.
