oscillation in residuals and force coefficient plots

- May 30, 2020 at 4:07 pm
  
  shayanmehr
  Subscriber
  
  I am simulating flow around 2D hydrofoil in ansys fluent.
  
  I am doing this for various angle of attacks between 0 to 20 degrees. the results are ok until 12 degree but after that residuals and lift and drag coefficients plots became oscillating. how can I solve this problem?
  
  for more information:
  
  the model is k-epsilon (realizable and standard wall functions). scheme is coupled and gradient option is green-gauss cell based. pressure, momentum, turbulent kinetic energy and turbulent dissipation rate are chosen second order.
  
  screen shots of oscillating plots are uploaded below
- May 31, 2020 at 3:55 am
  
  Kalyan Goparaju
  Ansys Employee
  
  Hello,
  
  It may be that beyond 12 degrees AOA, the flow is separating and could also be shedding. That may be the cause of the unstable/oscillatory behavior of the residuals and the force monitors. Such a condition is heavily transient and you should consider switching over to an unsteady simulation.
  
  Thanks,
  
  Kalyan
- May 31, 2020 at 1:04 pm
  
  shayanmehr
  Subscriber
  
  thank you for your response.
  
  what is proper time step size and number of time steps for transient case? I didn't do transient simulation before. Is there other option which I must consider in transient simulation?
- June 1, 2020 at 4:06 am
  
  Kalyan Goparaju
  Ansys Employee
  
  Hello,
  
  Please read the following chapter on the help manual for details regarding setting up transient problems in Fluent.
  
  https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v201/en/flu_ug/flu_ug_sec_solve_calc_time.html
  
  Please mark the appropriate answer as 'Is Solution' so that it can help others.
  
  Thanks,
  
  Kalyan