Tutorial – Converting RANS to DES
Converting a RANS case to DES requires changes to the both the chosen Workflow and the Solution settings in the template case. To begin with, create a child run of the RANS case you wish to run in DES. Go into the ‘CFD Setup’ tab for this newly created run and then press Edit to alter the run’s workflow. Select a ‘DES’ workflow from the available choices. If no DES options are available, contact Bramble support as and they will be able to add it to your account.
Next create a copy of the RANS template case and open the editor. First, update the name and description for this new template case to something meaningful for you. Then, from the Solutions tab, update the Solution Stage, Average Window and Convergence Criteria settings.
For the Solution Stage, select ‘Transient’ from the ‘Solution’ drop-down. This will expose the ‘DES Solution Time’ and ‘DES Time Step settings’. Update these, and the number of ‘RANS Iterations’ as desired. For example: RANS its = 500, DES Solution = 2s, DES Time Step = 0.0002s.
The Averaging Window is the number of time-steps you wish to average the solution over. For example, if you wish to average for 0.5s of physical time and are using a time-step of 0.0002s, then the window should be 0.5 / 0.0002 = 2500.
RANS simulations will typically use Convergence Criteria to automatically stop the simulation once convergence is detected. This isn’t used in a DES simulation as it is preferred to capture a certain amount of physical time (controlled by the averaging window). Therefore, it is necessary to delete any defined convergence criteria. Note, when all of these have been deleted, the ‘Start Time’ and ‘Number of times unconverged’ settings will disappear.
With these changes made, the template case can be saved and the simulation launched as normal.