Second order and PRESTO
 

I get a strange result. When I use Second Order pressure scheme, I get a very good residual convergence, but Streamlines don't look fine. and when I use PRESTO! I have non-converged residuals (converged up to -3 or -4), but streamlines make much more sense. Does anybody have any explanation for this?

Thank you

Residuals are not a measure of accuracy nor are they measures of convergence.

PRESTO! solves for the pressure on cell faces whereas Standard / Second Order interpolates them. PRESTO! gives a more physically accurate representation of the pressure field.

Interpolation naturally smooths the pressure field and can give better numerical stability and is probably the reason for better residuals.

Thank you for your answer.

Then how should I get a more converged (based on residuals) solution with PRESTO!? In what I get, velocities, epsilon and k are fluctuating around <10e-3 , and continuity is around <1e-5. I have 4 points monitoring, and seems that velocities are not changing at those points. but residuals doesn't get smaller! Any idea?

1e-3 is actually pretty good as a residual. It means your solution improved by three orders of magnitude based on the initial conditions. As said earlier, don't base yourself on the residuals to judge proper convergence. Residuals only indicate how much your solution improved compared with the initial conditions. If your initial conditions are already really good, your residuals could be only 0.5. That doesn't mean your results are bad. Look at your monitor points, like the velocity points you have, and see if they still change or not. If not, it could indicate that the solution has converged (they could change if you simulate for longer). Now, whether the results are correct is a whole other story. That's up to you to determine, comparing with experimental/analytical results or other numerical results.