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Old   July 26, 2005, 16:58
Default CFD Software Recommendations
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Hi to you all.

I am new to CFD and am looking for advice from experienced users. I am in the process of selecting a CFD software for a new project and think it's unwise to only trust answers given by sales-people. My project deals with the external aerodynamics of a subsonic airplane and I want to study flow separation and loads. I would appreciate your recommendations for a commercial or noncommercial software (or a combination thereof) that would include a pre- and postprocessor and a solver. I prefer a Windows based systems, but am open to other operating systems. My budget is limited and I am looking for something that does an adequate job, quickly and easily, perhaps on a dual CPU system (larger cases could be outsourced).

Also, I would appreciate if you could tell me what questions you would have liked to ask your vendor when you purchased your CFD package, but didn't.

All help is greatly appreciated.


Mr. Gudmundsson
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Old   July 27, 2005, 03:44
Default Re: CFD Software Recommendations
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Perhaps the trickiest thing to get right for this kind of application is the meshing. For a subsonic airplane wing profile characteristics are important, but it's also likely to have a high aspect ratio wing. This almost inevitably drives you towards a mesh using mostly hexa cells. Try meshing a high aspect ratio wing with sufficiently small triangular surface patches to see how big the grid gets ... So if you want a package that includes meshing, you need to make sure that the hexa meshing capabilities are really good.

If you're going to be chasing flow separation, you will also need to mesh down to very (sub 0.1 mm) fine spacing normal to the surfaces, which is another question to ask about the meshing tools. You also need to ask about the turbulence models, and their abilities to predict separation off curved surfaces. Popular choice for this now is the Menter SST model, although there are probably other suitable models out there.

Your "adequate job, quickly and easily" specification is the tough one to meet, especially considering the amount of time it will take you to learn the software. The loads part is probably not too difficult (and you will be checking these loads with other methods as well, I'm sure), but predicting areas and critical conditions for flow separation on something that is supposed to streamlined is actually very tricky.

As far as suitable software selection is concerned, it makes quite a big difference if you're designing an aerobatic sportsplane or a high subsonic mach number UAV.
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Old   July 27, 2005, 08:59
Default Re: CFD Software Recommendations
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Please see that, may be it will help
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Old   July 27, 2005, 16:58
Default Re: CFD Software Recommendations
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I agree with Charles and I appreciate your dilemma - I was in a similar position a number of years ago. You gotta be clever about meshing - not just the solver; a crappy mesh gives crappy solution. But even before that you also gotta be clever about getting your geometry into the mesher. In my company, we spent weeks just bringing stuff in from Cad through Iges to the mesher - before we even got to the sexy stuff like turbulence models.

To be 'quick and easy' then, you don't want to spend a lot of time playing with geometry translation issues which can be a real pain. This was a big bottleneck in our analysis.

Second thing to look for would be accuracy of the solver (which again goes back to meshing). However, most commercial codes these days offer similar solvers/turb models etc and so it really boils down to how easy it is to integrate the code into your system.

Anyway, questions I would have liked better answers on when I was looking would have been:

Do you integrate into CAD (if at all)? What sort of meshing is included in the package? How well does the solver compare to published data? What support is included?

I hope this helps.

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Old   August 7, 2005, 02:56
Default Re: CFD Software Recommendations
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The real stumbling block in any cfd analysis is the generation of a suitable and refine enough mesh. in other words the translation from your CAD system to the CFD solver, you need to focus on this point, iges and step files formats are good, but in real industrial applications, this is just a myth. If I have to select a cfd programme, I would make sure of its ability to read the native CAD file format directly without any translations, take for instance the ANSYS multi physics programme or the star CD that can read proe and solid works formats (other formats are available too). Most CFD vendors will lend their programmes for a month or two free of charge, so make use of this option and install the programme and test it by yourself.
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