Thread: Discussion fuselages
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Old Jul 17, 2012, 01:36 PM
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Cape Town, South Africa
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Gerald, when it comes to simulating low Reynolds number flows I would rather trust the results of a wind tunnel over CFD. I am not up to speed with the latest research in this area, but from what I remember from 2/3 years back was that a LOT of effort was being put into this area. Most of the interest came from the need to study low Re flows found in micro air vehicles. Researchers were using fairly complex CFD models, but even these had limitations when it came to accurately modelling general flows at low Reynolds numbers.


Most decent commercial CFD packages (read Fluent/CFX/Star-CCM+/Star-CD) have some ability to model laminar to turbulent transitions. WIthout getting into too much technical detail, they typically use either a Spalart-Almaras or a variant of the k-omega model to account for the effect of turbulence. The problem with both these models is that while they can predict laminar-turbulent transitions they cannot model relaminarization adequately (if at all).

While you may get reasonable accuracy at low AOA, you will not be so lucky at higher AOA as the turbulence model cannot resolve the flow details adequately. Even at low AOA you will have to be careful in setting up the CFD model. This raises another question: how do you know that you have an adequate CFD model in the first place? Normally this is done by comparing the simulated results to reliable experimental data obtained from wind-tunnel measurements. The bottom line is that you have to tread VERY carefully if you want to get meaningful results.

CFD is one of the better GIGO (Garbage In Garbage Out) machines out there. I also find it amazing how people will readily believe the colorful plots this method produces without questioning the results. I am not saying that you cannot get some use out of CFD for our problems, just that you have to do so knowing that the models used are currently far from adequate (at least the ones we have access to).

Anyway, just my 2 cents. What was this thread about again?

Regards,
Imraan (who likes calling himself the PPE - Pretty Picture Engineer
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