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-   -   Solution not converging (http://www.cfd-online.com/Forums/fluent/50129-solution-not-converging.html)

Alex December 24, 2008 11:44

Solution not converging
 
Hello Fluent users,

I am analysing 2D airfoil flow at Re = 1 million. When I was using fully turbulent models such as k-e and k-w models the convergence is good but the Cl and Cd results are higher than the experimental results (around 30 % error).

Since I know the transition point I divided the domain into two and assigned fluid zone before the transition as laminar and after the transition point as turbulent.

When I select the laminar zone in the fluid (before transition point) boundary condition my solution is not converging and the residuals are oscillating like small waves especially continuity and epsilon. But the Cd and Cl values are stabilized and close to the experimental values.

Currently I am using realizable k-e model with enhanced wall function and using quick discretization schemes.

I am looking forward for someones advice.

Cheers, Alex


zhaowg December 27, 2008 02:53

re: Solution not converging
 
if the oscillating range is relatively lower and some characteristic value is stabe, u can think u have got a convergence result

zongtwi December 27, 2008 07:28

re: Solution not converging
 
Usually oscillating residuals means the solver is struggling to achieve good convergence...however you are seeing good correlation of the results. I would say that there are some time dependant flow structures due to the turbulant transition that you have included in your simulation. This unsteady flow structures cannot be predicted using the steady state solver, which causes the oscillatory behaviour for the residuals. Quantitatively, some parameters show good results but the overall flow structures may not have converged properly.

I would recommend changing your analysis to unsteady state, and run your case with a low time step. Monitor some parameters and wait until they stabilise. You should get overall good convergence and improved quality for your results.

Hope that helps.

Alex December 27, 2008 09:42

re: Solution not converging
 
Thanks Zhaowg and Zongtwi for your kind reply.

I will try unsteady analysis as suggested by Zongtwi and i'll post the outcome.

Further more I need some information regarding steady state analysis. Is it possible to get a converged solution by changing the turbulence source terms in laminar or turbulence zone boundary condition in the form of UDF or constant value.

I changed the wall boundary condition (laminar zone) from no slip to specified shear stress and input the values obtained from contours of wall shear stress from the non converged solution. In this case the convergence is good but the Cl and Cd results are away form the experimental results. i am not sure I am doing in the right way or not.

Your advice is welcome on this issue.

Cheers, Alex.

Alagesan December 29, 2008 08:29

re: Solution not converging
 
I tried unsteady analysis, in this also I am facing the oscillating residuals. The Cl and Cd results are good (less than 5% error).


rajeshwar April 14, 2015 00:55

Non Convergence of solution
 
I am B.Tech U.G student.As a design project,I am simulating A CD nozzle in Fluent.I did the meshing and designing of CD nozzle in ICEM CFD.The mesh type was tetragonal.After importing it in FLUENT,I gave velocity inlet for inlet and Pressure outlet(for outlet).It is an incompressible flow.I gave pressure based solver.My mesh quality was 40%(which is pretty decent that is what I was told off atleast :P).But I have encountered following problems:
The solution is not converging
The solution(velocity at the exit) varies as the number of iterations are changed.
What is the error??


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