flow around cylinder in Re=40
 

Hi all
I want to solve the flow around the cylinder in viscoelastic fluid. At first I did it for Newtonian fluid in Re= 10 , 40 , 100 and for viscoelastic fluid in Re= 10 , 100 and obtained exact results. But when I change the Re to 40 for viscoelastic fluid, it became diverged!
I changed many parameters of solution:
urf = from 0.1 to 0.9 --> diverged!
corunt Nu. = from 0.1 to 0.9 --> diverged!
changing the kind and size of mesh --> diverged!
changing the kind and size of mesh --> diverged!
increasing the number of solving pressure correction Eqn from 2 to 20 --> diverged!
decreasing the min residuals to 1e-8 --> diverged!
changing discretization schemes of div terms: Gauss linear (central), Gauss upwind(1st), Gauss upwind(2nd), limitedLinear , QUICK --> diverged!

But I have done some ways to solve that:
1- I saw in this page that sb proposed to the other that decrease the residuals to 1e-19 !! I did it and my solution became converged! I can't analyze that! Is it possible?
2- In the other way I increased the number of solving pressure correction Eqn to 20, decreasing the min residuals to 1e-8
3- setting the time step to 0.001 instead of setting Cr=0.3

Now I don't know that my results are reliable or not!
Could you please tell me what happend that these solutions are appropriate for solve it?

Thanks

Using double precision you can set the residual to 10^-12, however I suggest to you a small dt, the numerical stability properties depends strongly on the viscous part.

Thank you
Could you please explain more?
Do you want to say that my 3rd way(decreasing dt to 0.001) is the best?
So why the other ways give me good results?
Why could I solve the problem of divergence with decreasing residuals?

I didn't get this sentence and its relation with my problem:
"the numerical stability properties depends strongly on the viscous part."

There is more than a single answer to your problem and that should require a deeper debug.
I can suppose that the you are solving a non-Newtonian fluid where the viscosity depends on the stress. An eccessive error in the tolerance could affect the evaluation of the viscosity and if your dt is close to the boundary of the stability region you could see the onset of instability and then a divergence in the solution

OK. So I have to decrease my dt to 1e-4 (for example)

Thanks