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Old   May 10, 2000, 04:18
Default Fluent's drawback?
  #1
Donald Ohtsuka
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My boss is considering to buy some CFD code. He want me to check fluent's drawback. Can anybody answer me?
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Old   May 10, 2000, 06:49
Default Re: Fluent's drawback?
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andy
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Without knowing the purpose for which FLUENT is being considered it is hard, if not impossible, to answer your question. However, ignoring that minor difficulty...

FLUENT is a general purpose code and can address a wide range of problems in fluid mechanics. Although this generality can be what is needed it is also the key to FLUENT's main failing: it cannot predict particular classes of flows as well as CFD codes written solely for that class of flow. That is, if you are only interested in hypersonic flows then a CFD code written only to solve hypersonic flows can be substantially more accurate and more efficient. However, the costs of working effectively with specialised codes are different to those of working with general purpose codes. It can be higher or lower - the key is usually access to educated people who know what they are doing. Perhaps using a bigger computer, waiting longer for the answers and accepting a higher risk the answers are misleading is perfectly acceptable. It depends on what you are using the code to achieve.

A personal example, I recently considered proposing FLUENT for a set of LES simulation (it has an LES button and could be driven by relatively inexperienced people). After a phone call with FLUENT it became clear that:

- The code is very inefficient requiring around 30 times more computing time per time step than a purpose written code. This figure is very approximate but was based on a real prediction running within FLUENT at the time - I could not believe the computing resource it required.

- The low accuracy of the convection terms is a poor choice for LES. i.e. requiring much too fine a grid to match a scheme which uses a more accurate differencing scheme.

- It was not possible to generate curvilinear grids of a high enough quality. Related to accuracy of convection terms (or perhaps not given the low accuracy differencing!).

- The computer memory requirement was not investigated but it looked to be around 10 times more.

- I was not convinced that FLUENT were on top of the problems of near wall treatment for this type of flow since the parameters to control it were not in evidence.

To be fair, I think this should be recognised as something of an extreme example caused by FLUENT adopting low accuracy differencing, unstructured grids and general purpose implicit solvers for a class of flows which are best served by high accuracy differencing, structured grids and very simple efficient solvers.

To conclude, I believe codes like FLUENT can help address a significant range of engineering problems. However, they are best used by people who understand their strengths and weaknesses and they are best not used in isolation.

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Old   May 10, 2000, 11:39
Default Re: Fluent's drawback?
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Alton Reich, PE
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Donald,

I'm going to preface my remarks by stating that I work for a competitor of Fluent. It may may me biased, but I'm going to give you what I think is sound advice.

The best way to determine which code is best suited for your application is to try it. In fact, you should try it with more than one vendor's code. You may find that several will solve the problem, but that one performs better, or is easier to use. Most vendors will be happy to set you up with a trial license for that purpose.

Regards, Alton

BTW: If you have any questions about our code, CFD-ACE+, I'd be happy to answer them.
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Old   May 10, 2000, 21:47
Default Re: Fluent's drawback?
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John C. Chien
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(1). Define your problem, or problems to be solved first. (2). Check each cfd code to see if similar problem or problems have been solved previously using the code. (3). Get the documents about the results in terms of internal reports or published papers, and check if you are satisfied with the results. (4). If the answers to these questions are yes, then move on to explore the possibility of doing the computation in-house. In this area, you need hardware, system software, the code, and the experienced user. (5). If you can not find the published results, then each problem of cfd is a PhD dissertation subject. On the other hand, if it has been done, then it may be very simple to run a similar case on your own.
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Old   May 11, 2000, 14:47
Default Re: Fluent's drawback?
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Chris
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Although it is right in principle to test various codes it is rather impractical. If a person is a first-time user of a CFD-code he is regarded as a novice. How can a novice testing a code do justice to that particular code.

Recently I helped on a project using a CFD-code totally unknown to me. Even after the succesfull completion of this particular project I would not consider myself to be in a position to give a clearcut awnser to a question like the one under discussion. I tend to agree with John. It is better to first try and find out who has numerically investigated problems similar to your own and with what code and how well did the results compare with experiment.
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