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 Lily Kabanj April 15, 2000 10:37

Errors in CFD

I have some questions regarding errors in CFD simulation and is hopeful that someone can point me in the right direction.

(1) What kind of error should one consider when presenting a CFD simulation results and (2) how do one calculate/estimate the error(s) in a 3-D FLUENT simulation ?

 Jonas Larsson April 15, 2000 10:59

Re: Errors in CFD

1. Errors can come from different sources - You can have problems with your physical models - turbulence models that don't work for your type of flows, combustion models that behave badly, ... You can also have numerical problems, which means that you are not solving the equations/models correctly. This can be due to bad convergence, grids that don't resolve the physics, too dissipative schemes, schemes that gives oscillations, ... In the end you can never be sure that your solution is correct - there is no mathematical way to prove that your solution is really a valid solution to RANS. You need experience to be able to tell if your solution is good or not.

2. It is not possible to do a strict error-estimate on a CFD solution. What you can do is to verify that you don't have any of the problems mentioned above - You can look at residuals to make sure that your solution is sufficiently converged. You can look at conservation of different global properties - mass flow, energy, ... to make sure that you are capturing the global physics. You can also try different models to make sure that you get a similar solution with another model. Another thing to check is that your grid is fine enough - try refining the grid to see if you get the same result. Even if you've done all this you can't be sure that your solution is good, or know how big errors you have, but you can be more confident in your solution at least.

 John C. Chien April 15, 2000 13:36

Re: Errors in CFD

(1). Normally, you need a mesh to do CFD calculations. If the solution is a function of the mesh used, then you will have errors. It is a must that when you present the solution, the solution is independent of the particular mesh you created and used. (2). There is no need to address the absolute accuracy of the results, because within CFD one can use linearized potential equation, full potential equation, Euler equation, parabolized equation, Navier-Stokes equations, etc. As long as the solution of the equation is independent of the mesh and is repeatable, then the solution to that equation is accurate. (3). So, the only way to estimate the error in solution of the equation, is to try a few different meshes and check the solution sensitivity vs the meshes. (4). If you are solving the real world problem, then comparison with the test data is probably the only way to go. Otherwise, CFD solutions are just solutions to a set of equations, including some physical models if required. (5). To make it easier to understand, if your fine mesh solution is different from your coarse mesh solution, then you can say that the coarse mesh solution has higher errors in it.( it is an over-simplified example, so a systematic study in mesh independency is essential. Otherwise everyone will be presenting the 20x20x20 solution, which is much easier to obtain than that of 100x100x100 solution.

 Fred Uckfield April 17, 2000 02:45

Re: Errors in CFD

"All models are wrong, some are useful." G. E. Box

Be more productive by looking for the use in models instead of spending time questioning their validility.

OK, OK, I know, if the quatitive accuracy is too bad then the usefulness diminishes quickly. Depending on what industry you are applying CFD in you may well be surprised at how innaccurate you can be whilst still finding use in your models, therefore being more a more productive employee, serving your company better and so being a postivive contributer to the human race.

Fred.

 LW April 17, 2000 11:59

Re: Errors in CFD

I still remeber the paradox quoted in Fluent TG a few years ago: "Nobody belives analysis results except the analyst, everybody belives testing results except the tester" (may not exact). The comments for the Error question have been excellent. I think there are only a few things you can do: 1) Make sure your physical problem is well represented by the mathematical model (equations), 2) Make sure the mathematical model is numerically well converged. 3) Check the model results agains testing data, EXPERIENCE (IMPORTANT)or even the other modeling resutls that you know to be true.

 John C. Chien April 17, 2000 12:08

Re: Errors in CFD, an important issue

 Lily Kabanj April 18, 2000 02:30

Re: Errors in CFD

Thank you for all your comments on my question. I appreciate it very much.

I am the only cfd user in a research group whose backbone work are mainly physical experiments and from time to time, I find myself having to justify tbe validity of the results of the numerical simulation.

The other reason was certain fluid mechanics journal will no receive papers on numerical work unless some "error analysis" has been done, although it wasn't clear what kind of analysis is required.

Once again, thank you for your time in answering my question. Further comments from other gurus and experts are most welcome =).

 Althea April 25, 2000 09:54

Re: Errors in CFD

LW, (or anyone else),

Can you please tell me where you got your quote, I don't know what the Fluent TG is.

Regards Althea

 LW May 1, 2000 08:52

Re: Errors in CFD

It was in either Fluent Tutorial Guide or User's Guide (1995 version). I believe it was quoted from some other source by the Guide. Thx!

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