Knowledgebase
CFD convergence issues
Posted by Luis Sousa on 29-Jun-20 03:50 PM
One common cause of CFD simulations not converging is when there are insufficient grid cells between an extract and a supply diffuser. When these components are positioned too close together, the flow can become trapped and to avoid this situation, we recommend that there are at least 3 grid cells between the extract and supply objects. You can edit the grid region between these components and manually add the extra cells. An alternative solution is to use a finer grid.

Some steps that can improve CFD calculation time are:

- Keep the model geometry the most orthogonal as possible, eg aligned with XYZ axis;

- Grid merge tolerance should generally not be higher than 10% of grid size.
- Very small dimensions are likely to lead to flow instabilities (eg. mass residual showing a "flat" line);
- Reasonable convergence can be assumed once the variable residual has stabilised (not oscillating) and the value has reached around 0.001 or 10-4.

For more detailed information on improving your CFD simulations see this program help page: https://designbuilder.co.uk/helpv6.0/#Convergence_Difficulties_and.htm