Convergence for a transonic 2D airfoil

kncujo · January 27, 2020, 07:20

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 · January 28, 2020, 12:02

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 · February 6, 2020, 19:51

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 · February 7, 2020, 04:52

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

Sifislev17 · December 10, 2024, 08:20

Which solver did you use and what boundary conditions?

January 27, 2020, 07:20	Convergence for a transonic 2D airfoil	#1
kncujo New Member giuseppe Join Date: Jan 2020 Posts: 7 Rep Power: 6	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:

January 28, 2020, 12:02	Look for fields	#2
vinerm Senior Member Vinerm Join Date: Jun 2009 Location: Nederland Posts: 2,946 Blog Entries: 1 Rep Power: 36	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. __________________ Regards, Vinerm PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.

February 7, 2020, 04:52	Convergence	#4
vinerm Senior Member Vinerm Join Date: Jun 2009 Location: Nederland Posts: 2,946 Blog Entries: 1 Rep Power: 36	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 __________________ Regards, Vinerm PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.

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February 6, 2020, 19:51		#3
kncujo New Member giuseppe Join Date: Jan 2020 Posts: 7 Rep Power: 6	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?.

December 10, 2024, 08:20		#5
Sifislev17 New Member Iosif Leventakis Join Date: Oct 2024 Posts: 6 Rep Power: 2	Which solver did you use and what boundary conditions?