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June 17, 2011, 07:43 
Naca 0012 (compressible and inviscid) flow convergence problem

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Bipul Saha
Join Date: Jun 2011
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I am currently modelling Flow around a 2D airfoil using Fluent 6.3.I have created 3 mesh geometries(coarse,medium,fine) in gambit and i am modelling it for 3 cases subsonic ,transonic and supersonic.I have gone through the fluent tutorials and have read a lot on the web on how to perform such simulations.But the problem is although i am getting almost accurate coefficient of lift,drag and pressure but my results are not converging.
I am using a density based solver and the flow is compressible and inviscid.Material is ideal gas and the cfl number i am using is .1. Can someone please help me out.It will be a great help. 

July 6, 2011, 07:51 

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about convergence in Fluent...
At convergence, the following should be satisfied:
In addition to residuals, you can alsomonitor lift, drag and moment coefficients. Relevant variables or functions (e.g. surface integrals) at a boundary or any defined surface. In addition to monitoring residual and variable histories, you should also check for overall heat and mass balances. The net flux imbalance (shown in the GUI as Net Results) should be less than 1% of the smallest flux through the domain boundary If solution monitors indicate that the solution is converged, but the solution is still changing or has a large mass/heat imbalance, this clearly indicates the solution is not yet converged. In this case, you need to:
Selecting None under Convergence Criterion disables convergence checking for all equations. Numerical instabilities can arise with an illposed problem, poorquality mesh and/or inappropriate solver settings.
Troubleshooting
Underrelaxation factor, α, is included to stabilize the iterative process for the pressurebased solver
Decreasing underrelaxation for momentum often aids convergence. Default settings are suitable for a wide range of problems, you can reduce the values when necessary. Appropriate settings are best learned from experience! For the densitybased solver, underrelaxation factors for equations outside the coupled set are modified as in the pressurebased solver. A transient term is included in the densitybased solver even for steady state problems. The Courant number defines thetime step size. For densitybased explicit solver:
Reduce the Courant number whenhaving difficulty converging. For densitybased implicit solver:
Convergence can be accelerated by:
A converged solution is not necessarily a correct one!
If flow features do not seem reasonable:
Numerical errors are associated with calculation of cell gradients and cell face interpolations. Ways to contain the numerical errors:
A gridindependent solution exists when the solution does not change when the mesh is refined. Below is a systematic procedure for obtaining a gridindependent solution:
To use a different mesh on a single problem, use the TUI commands file/writebc and file/readbc to facilitate the setup of a new problem. Better initialization can be obtained via interpolation from existing case/data by using solution data interpolation A webbased training module is available to train users in replication of case setup and solution data interpolation. Sumary: Solution procedure for both the pressurebased and densitybased solvers is identical.
All solvers provide tools for judging and improving convergence and ensuring stability. All solvers provide tools for checking and improving accuracy. Solution accuracy will depend on the appropriateness of the physical models that you choose and the boundary conditions that you specify. sorry for lengthy post... sometimes people are lazy when theory is involved... 

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