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July 6, 2011, 07:54 
Convergence

#1 
Member

I see all kind of mistakes on these forums when dealing with convergence, so I will give brief review of methods...
At convergence, the following should be satisfied:
In addition to residuals, you can also monitor 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 the time step size. For densitybased explicit solver:
Reduce the Courant number when having 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. Summary: 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... 

January 13, 2012, 11:18 

#2  
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Charles de Luzan
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Quote:
But what is this webbased training module you are talking about please? Thank you in advance. 

January 18, 2012, 02:31 

#3 
Member

There are training modules for Ansys, very useful. You can find them all over.
for example: http://avaxhome.ws/software/software..._training.html 

January 19, 2012, 11:53 

#4 
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Charles de Luzan
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Thank you very much!
I will spend some time on this. 

February 14, 2012, 09:16 
train moving in tunnel help

#6 
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Join Date: Jan 2012
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hello
i am working on train moving in tunnel. i am getting floating error:invalid number. can any one help me. my email is sheikhnasir39@gmail.com plz help me thanks 

October 23, 2013, 22:33 

#7 
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Chng
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Hey centurion, any idea why for energy residual it only decreases to 10e6 but not less than that even though i used double precision?


October 24, 2013, 03:56 

#9 
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Chng
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actually I'm doing a very precise and micro scale calculation, and the double precision calculation is giving me slightly different results compared to single precision. This is why I'm trying to reduce the energy residual beyond 10e6 during a double precision calculation.


October 24, 2013, 04:13 

#11 
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Chng
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I am doing a study on heat transfer characteristics in a microchannel (fluid channels in the order of 10e3 meters). The delta T for single precision calculation is ~13C. By using double precision instead, I get a delta T of ~14C. As the delta T is only 13C, a difference of 1C is pretty big to be ignored. This is why I am trying to reduce the energy residual. Hope it helps you understand my situation better. =)


October 24, 2013, 04:25 

#12 
Member

double precision is always better, but did you try to make smaller mesh, or different types of meshes. It seems to me that mesh is the issue here
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October 24, 2013, 04:34 

#13 
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Chng
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that is possible, I am still amidst doing the grid independency study so I can't know for sure. thanks for your help, I will look at the results again once a have generated a series of meshes.


November 30, 2013, 11:50 

#15 
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Ricardo Pereira
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Hello Centurion2011,
Your post is so important/informative that you've made me come online just only to say that you've made a good work on compiling all those informations tips regarding the structured way for achieving a successful converged solution. That information should be accessible and visible to everyone before creating a new post in the forum regarding this topic. If someone as enough time (sorry...), I would suggest to include the following information and quick recommendations that I think to be also important:  Mesh Strategy, Quality and Topology tips/recommendations and their impact (some already mentioned);  Domain reordering (Accelerating Convergence);  Information regarding Initialization Methods : Standard, Hybrid, FMG, Interpolation (already mentioned);  Tips for the Near Wall Resolution: Wall functions, Enhanced Wall Treatment and related Yplus, Cell Topology;  Tips/recommendations for Turbulence Models (with examples)  Boundary Conditions recommendations (location, geometry,...)  Transient time step recommendations Some of the above available tips are also acquirable in other thread from Centurion2011. I would like to recommend to the moderators to include all this as a: 'QUICK CFD Best Practices Guide Information' just side by side with the Best Practices at the CFD Online WIKI. What do you think about Centurio2011? Best Regards Ricardo 

January 29, 2014, 01:36 

#17 
New Member

hello..Centurion2011..
the post was really helpful.. i would like to know one thing..What did you mean by ' “stuck” residuals.? is it when the residuals are stuck at a particular value and not changing from it for a number of iterations..? and when my lift shows a straight line.. is my solution converged..?.. 

January 29, 2014, 11:17 
question

#18 
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adnan
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Dear friends, I have question in fluent please.
I used ICEM for simulate heat transfer in kiln, then export to fluent, actually in this time i run my program without combustion. can get converge at residual e2 but with not good report about mass net, as same time reasonable result. so take more for residual until e4 , also get converge but not reasonable result and in this case report mass excepted? any suggest, thanks in advance 

January 29, 2014, 14:32 

#19 
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AA Azarafza
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Hi friend,
Obtaining a value of e2 or even e4 for residuals doesn't necessarily mean that your simulation is converged. However, a value of e3 is acceptable for continuity, for other equations, it depends on your problem. For instance, in my recently work, a value of residual about e8 is not enough to get convergence for a user_defined scalar! So, my suggestion is first, keep doing iteration until you don't observe any big change or oscillation in residuals. Secondly, in some cases, you need to monitor some effective parameters in your model beside the usual residuals monitoring. For example, depending on your work, you can monitor some surface or volume characteristics of flow. Only one more thing, in some cases you may not receive a very low residuals but the values will not change any more longer. In that case, your simulation is converged too. So, don't worry and be patient when you grip into a CFD problem. Hope it helps.
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Regard yours 

February 3, 2014, 12:07 

#20 
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adnan
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Thank you very much Mr.Azarafza,
but could also consider mass flux reports( mass flue rate) explained, how the project near from reasonable solution. because some time good result but minus () mass flux reports (example 3.112*e5) thank you again Mr.Azarafza Last edited by adnanghreeb; February 4, 2014 at 09:34. 

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convergence, fluent, mesh 
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