[alimea]

​Dear foamers

I have read a paper by the title of:
Method for time-dependent simulations of viscoelastic flows: vortex shedding behind cylinder
​I want to validate my research with this paper, but a problem have been occurred!
When I want to investigate time step independency, I will earn the attached results. it means that for dt=0.001 I can't get good in-dependency from time step in comparison with dt=0.0005. I'm working on viscoelastic fluid with these non-dimension numbers:
Re=100 - El=1 - Betta(etta_p/etta_0)=0.9 - epsilon=0.1 - zetta = 0.01 (EPTT model)
time derivative discretization scheme: CrankNicolson 0.9
In addition I added the energy eqn to my equations (Is it possible that solving this eqn need a very small time step?)
I guess it may because of high betta(the ratio of polymeric viscosity to total viscosity).

Could you please give me your idea?


http://uupload.ir/files/s243_dt_study_cl.jpg

[piu58]

As far as i understand is quite normal what you observe.
You get fluctuations of the drag because of the Strouhal number. This number is more or less time step independent, but the phase changes.

If you lower the time step the drag increases up to dt=0.001. This is the point where your solution gets time step independent.

[alimea]

Originally Posted by piu58

As far as i understand is quite normal what you observe.
You get fluctuations of the drag because of the Strouhal number. This number is more or less time step independent, but the phase changes.

If you lower the time step the drag increases up to dt=0.001. This is the point where your solution gets time step independent.

thanks Uwe
if you see the results, if I decrease the dt from 0.001 to 0.0005 I will have 9% change in CD! is it acceptable? is my solution dt independent now?

note: by dt=0.001 I have CFL=0.1 !!

[piu58]

Time step independent doesn't mean time independent. You have vortex shedding in your simulation which changes the CD periodically. The CD values of 0.001 and 0.0005 are very comparable.

[alimea]

Originally Posted by piu58

Time step independent doesn't mean time independent. You have vortex shedding in your simulation which changes the CD periodically. The CD values of 0.001 and 0.0005 are very comparable.

Yes, those are different and I agree that the problem is time dependent. Changing CD with time is showing this dependency.
But when I decrease dt from 0.001 to 0.0005 and I see these 9% change, can I conclude that the problem is dt independent? I think No.
however, thank you for your reply

[piu58]

I don't understand what you mean. The difference in maximum between dT=0.001 and 0.0005 is around 0.005. Compare this with the 1.4: That is around 0.4%, not more.

[alimea]

Originally Posted by piu58

I don't understand what you mean. The difference in maximum between dT=0.001 and 0.0005 is around 0.005. Compare this with the 1.4: That is around 0.4%, not more.

I'm sorry, but I think sth is wrong in your comparison. the difference between dt=0.001 and 0.0005 in CD should be compare with the amplitude that is calculated as:

amplitude of oscillation = (CD_max -CD_ Average) = (1.4 - 1.34) = 0.06

so the difference will be:

0.005 / 0.06 *100 = 8.5%

Do you agree with me?