Unsteady RANS with no vortex shedding why?

- July 2, 2019 at 11:35 pm
  
  sharm2a2
  Subscriber
  
  Hello, I am simulating flow over hemisphere by using unsteady RANS but at the end no vortex shedding appear. The experimental and LES results showed that vortex shedding or kelvin Helmholtz instability is present for that Reynolds number. Can anybody please explain why URANS ( Unsteady RSM model) is not giving this instability ? Thanks and Regards Amit
- July 3, 2019 at 6:15 am
  
  Amine Ben Hadj Ali
  Ansys Employee
  
  The URANS simulation should produce only the large-scale unsteadiness. If you know the response time or the Strouhal number you can have an etimate for period and you need to resolve that period by not less than 50-60 time steps!
- July 3, 2019 at 6:23 am
  
  Amine Ben Hadj Ali
  Ansys Employee
  
  Moving to Fluids
- July 3, 2019 at 11:20 pm
  
  sharm2a2
  Subscriber
  
  Hello Abenhadj
  Thank you for your reply but can you please elaborate. I did not understand “ you need to resolve that period by not less than 50-60 time steps“
  
  Thanks and Regards
  Amit
- July 4, 2019 at 7:16 am
  
  Amine Ben Hadj Ali
  Ansys Employee
  
  50-60 time steps per period. Period is 1/frequency. Frequency from Strouhal number.