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Old   June 1, 2009, 02:23
Default Improve Drag
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Tom
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Is there anyway to reduce the drag i have further. At 0 deg AOA my drag obtained is 0.012, actual 0.008. The solution is no longer dependent on the number of elements on the airfoil or the mesh. Changing the airfoil size to increase the inlet velocity speeds does not change much too. Transient modeling gives the same results.

Turbulence intensity is below 0.1%, 0.001 according to the limited data which i have to verify. Length scale. Any suggestions for it? Reynolds number is 300k.

The SST model seems to be giving me an early separation of the flow. Which i think is a problem. As i have tried the transitional model with zero AOA (Gamma model) i can get a good result but the problem lies in different AOA which i do not have the different transition reynolds number. Could anyone suggest a way to improve the drag result to slightly below 0.01.

Thanks.

Last edited by Tom; June 1, 2009 at 03:55.
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Old   June 1, 2009, 11:39
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Noone? Or has anyone successfully use the gamma theta transition model? What criterias should i have.
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Old   June 1, 2009, 18:32
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Glenn Horrocks
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Hi,

Accurate drag numbers are tricky to get.

Are you sure your experimental results are accurate? It is difficult to get accurate drag numbers experimentally also. I would speak to the experimentalists if possible, they usually have some ideas about issues they had to address to get a reliable drag number. Surface roughness and incoming turbulence levels are the main issues which cause issues here for airfoils.

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Old   June 1, 2009, 21:12
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And i thought i would be able to fine tune it slightly to obtain a better approximate. I dont think im able to get hold of the ones doing the experiment. The turbulent intensity given covers quite a range.
Guess i have to play ard with variables. No luck so far.
Are there any guides on tuning coefficients. Thats the last thing i want to touch if i cant get it right. and how should i plot a contour of where flow separates.
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Old   June 2, 2009, 01:01
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Hi,

Playing with model constants is the last resort - and generally a futile one. 99% of the time the error is elsewhere in the simulation. But if you still want to give it a try I recommend you talk to your CFX support person and get hold of the papers published on the turbulence transition model by Menter and (can't remember the second author). They published a few papers on the validation of the model so you should look at the work they have already done before starting your own tweaks.

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