Hi Hongtao, The simulation becomes unsteady because the steady state is unstable. You can add initial disturbance to speed up the transition. The timestep can estimated from the maximum CFL number. But the exact time step need to be found by trail and error. Cheers Ankang -----Original Messages----- From:🌸那慢慢🌸 <2644878715@qq.com> Send time:Sunday, 07/07/2024 18:06:57 To: nektar-users <nektar-users@imperial.ac.uk> Subject: [Nektar-users] finaltime and timestep | This email from 2644878715@qq.com originates from outside Imperial. Do not click on links and attachments unless you recognise the sender. If you trust the sender, add them to your safe senders list to disable email stamping for this address. | Hello everyone， I am a student who has just been exposed to fluid simulation and am using the base in flow-stability-cylinder to calculate the Re of vortex shedding in the flow around a square cylinder. I found that when the simulation time was extended, cases that did not originally experience vortex shedding experienced vortex shedding. I am confused why vortex shedding is related to the length of the simulation time. In addition, I understand that the finaltime should be set to 20-40 times the lift pulsation cycle, but I am not sure how to determine the timestep. I sincerely hope that you can give valuable suggestions, thank you very much! Hongtao Cheng