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airfoil optimizer chain with drag problem

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Old   October 18, 2009, 05:38
Default airfoil optimizer chain with drag problem
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Matthias Arnold
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hi,

to say it first: I'm a beginner in OF (vers 1.6).

Now my Problem: My aim ist to build a small optimizer chain to find the optimum low drag airfoil for casing of struts with a constant angle of attack of 0°. To reach this aim every optimizer iteration is done by following steps:
perlscript write_blockmesh_edgegrading.pl -> move blockMeshDict to /constant/polymesh/ -> blockMesh -> simpleFoam -> read out cd_value
This steps work automatic, but the result is not so nice
for example for a NACA0018 airfoil to case a diameter of 0.18m (=> chord 1m) the result of the simulation is approx: cd = 0,0155 with a look to the measured data of this airfoil it should be around 0.0075
So the result is with a faktor of 2 to high.

As far as I analysed the Problem the friction drag looks good, but the pressure part is to high. Resulting from this the optimizer converges to a unphysical result (NACA0004 looks a bit wrong. expected result around 0028), because it has a to high attitude to search for a low pressure drag and not for the combinition of pressure drag and friction drag.

what i already tried to solve the problem:
checking convergence: the residuals are <= 10^-6 and the drag value ist at it's final value after 400 of the 1000 iterations
changing turbulencemodell: i found in the forum that the SpalartAllmaras modell is used frequently for airfoil calculation. with this modell the cd converged to 0.04. with the RNGkEpsilon modell i had the above result of 0.0155. with all other modells i had worse results.
changing mesh resolution: there was no big change in results on rising the number of meshcells. (approx +- 2%)
searching the forum to find a solution: I found many hints, but they didn't give me a solution for my problem.
changing velocity: with the higher Re the cd value got a little bit smaller, but not in a significant size.

So my question is: can you please take a look on my casefiles attached?
As i worte above I'm a beginner with OF, so i fear there could be some "stupid" mistakes.

thank you very much for your help.
Attached Files
File Type: gz NACA00_optimierer_k_epsilon.tar.gz (6.8 KB, 3 views)
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Old   October 18, 2009, 05:47
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Matthias Arnold
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Sorry, made a mistake with the uploaded case files. They had a mixed up in turbulence modell.
Sorry for that.
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File Type: gz NACA00_optimierer_k_epsilon.tar.gz (6.7 KB, 12 views)
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Old   October 18, 2009, 13:43
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Simon Lapointe
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These results don't seem so wrong to me. You will rarely obtain the good drag values using RANS models.

On an airfoil at a low angle, an important part of the boundary layer is laminar, hence the friction drag is smaller than in a turbulent boundary layer. Traditional RANS models (SA, k-epsilon...) predict a fully turbulent boundary layer resulting in a higher drag than experimental data.

Concerning the bad result with the SpalartAllmaras model, did you have a mesh with y+ around 1 ? Unless you use wall functions (which are optional with SpalartAllmaras) you need a good resolution to resolve the near-wall region.

In my experience, near wall resolution gives a little better drag prediction than wall functions.

Hope this helps.
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