[kncujo]

Hi everybody,
i'm new to CFD simulation and i'm trying to simulate a 2D flow over an airfoil (st genese 29) at Mach number 0.8.
I have to use an unstructured mesh.

I obtain a solution with this residuals (in the figure). The oscillation of the residuals is normal? Could i say that it's converged?

thanks for answer.
Figure:

[vinerm]

Hi

Residuals are not good indicators of convergence. They are useful to say whether a solution is converging or diverging but whether the solution has converged or not, it is difficult to say by just looking at the residuals, except for very simple problems.

Recommendation would be to define a few point, line, or surface averaged field values, such as pressure at inlet, velocity at outlet, shear stress at the wall, etc. and observe if those have become stagnant with iterations. Either they should become constant or settle down in a periodic manner, i.e., statistically steady manner. Then you may say that the solution is converged.

[kncujo]

I think I understand what you mean.
I can therefore also consider the force on a surface and see if its value stabilizes after a certain number of iterations. It's correct?.
For example, now I am trying to simulate the motion field on the same airfoil in supersonic regime, this time the residues do not reach a low enough value, but the lift coefficient seems to have stabilized.
Can I interpret a convergence?.

[vinerm]

There are three points to be considered

1. Stability of parameters, such as monitor points, point toward the numerical convergence. If those are stable, then numerically you have convergence
2. However, that doesn't mean that the results are good. If the residuals are below 1, then the results are most likely plausible and even within a certain error from the expected values but may not be right there
3. Conservation of mass, momentum, and energy is another aspect that you need to look at. At the end of simulation, do you have these fields conserved or not points to whether the result is accurate or not