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-   -   convergence criteria in Fluent (http://www.cfd-online.com/Forums/fluent/121111-convergence-criteria-fluent.html)

 star July 22, 2013 02:16

convergence criteria in Fluent

Hello friends. I just started to work on Fluent 14.0. I am learning it to simulate flow around a 2D airfoil. I want to ask a basic question.
How will we know that solution has converged? Is it depend on the value of Residuals which we set? and if it converges, it means the solution is correct?
I will thankful if somebody help me.

 flotus1 July 22, 2013 04:46

In the case of an airfoil, it might be reasonable to judge convergence rather on lift and drag coefficients than on the residuals alone.

And of course, a conveged solution is not necessarily a correct one due to the multitude of possible error sources in a simulation.

 star July 22, 2013 05:55

Thanks Alex for your kind reply. So you mean that I should check the stability of lift and drag coefficients? In the case of converged solution, could you please tell me some of the error sources or how to make sure the converged solution is correct?
I know about one thing, i.e to change the grid size and check the result with previous grid size. If the difference is very small so it means it is correct? Is it correct way or there are others?

 flotus1 July 22, 2013 08:39

http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

 star July 22, 2013 12:48

Thanks Alex.. i got another question out of it if you could help.
it says" Usage errors are caused by user specifications. For example, the user may run a turbulent simulation as laminar or an unsteady simulation as steady. These errors can be monitored via a convergence study."
What I know is laminar and turbulent flow depends on Reynolds no. but how will we know about steady and unsteady flow? how can we know from convergence?

 flotus1 July 22, 2013 13:59

This point might be a little confusing in the FAQ. I actually think the conclusion that user errors can be monitored with a convergence study is questionable.

But let us first review the terminology: steady/unsteady is not equivalent to laminar/turbulent.
Turbulent flows are always unsteady.
Laminar flows can be steady or unsteady.

The most important thing to tell if a flow is turbulent or not: Technical flows are mostly turbulent flows. Apart from this, the exact regions of laminar and turbulent flow regime are only known for very simple flows.
It might be a good idea to start off a simulation with a turbulence model and monitor turbulent quantities like the turbulent kinetic energy k. If it is very low throughout the computational domain (provided low values at the inlet) you might run a laminar computation. If this converges well, the flow might actually be laminar.
But again, other factors like numerical viscosity introduced by a first order upwind scheme might force the flow to be laminar in the computation while it is actually turbulent...

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