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multi wing element - negative coefficient of drag?

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Old   March 9, 2010, 10:05
Default multi wing element - negative coefficient of drag?
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smith
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Hello everybody!

I'm simulating a two element wing with fluent.
The szenario is the following:

i have the main wing (the big one) and the flap wing (the smaller one). The main wing remains at its position but the flap wing gets turned within a angle of 0°-40°.

I simulated already all 5 cases and i realized that when the flap wing is turned at 30 degree as well as at 40 degree, the main wing becomes a negative coefficient of drag.

I ask my self how this is possible and i dont find the solution/error in my fluent simulation. The velocity inlet vector direction is okay...


i added a picture of my pressure contours, maybe there is a logical explanation for this problem? i could also attach my case file if this would help to help me ^^


Kind regards,

Zweeper
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Old   March 9, 2010, 19:20
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What viscous model and wall treatment are you using?

What is the mesh like close to the boundaries?
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Old   March 10, 2010, 05:36
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i am using the RNG k-epsilon model with standard wall treatment.
I have a boundary layer around my wings with a calculated grid spacing so that my y+ is between 50 and 150. I thought that for this model the y+ should be between 50 and 500, is this correct?
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Old   March 10, 2010, 14:11
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well, i also attached now my fluent file. this helps to get a faster idea of my problem. as i mentioned, the main wing has an negative coefficient of drag.

i have still no clue how this is possible. i hope someone can help me with this problem.

http://rapidshare.com/files/361592259/40_degree.rar


regards,

zweeper
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Old   March 11, 2010, 06:51
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No ideas?
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Old   March 11, 2010, 07:32
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Sebastian Gatzka
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I know this sucks, but have you tried a mesh containing hex elements?
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Old   March 11, 2010, 07:36
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no, i just tried the tri/pave one.
which kind of mesh is a mesh containing hex elements in gambit?


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Old   March 11, 2010, 07:48
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Just had a look into your case file.

Hex elements refer to hexahedronal elemtens - I'm talking about cubes (3D), squares (2D)... I'm not sure this is the right terminology.

As I see, you used these elements in your boundary layer.
I'm no expert in such things but maybe some more would do good?
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Old   March 11, 2010, 07:51
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hm im not sure if the problem depends on the mesh size/amount/elements?

as i mentioned, i turned the flap behind the wing from 0 degree up to 40 degree and this negative drag coefficient just occurred at 30 and 40 degree.
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Old   March 11, 2010, 08:36
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What I am thinking about is the scewness of the mesh if the calculation is with triangle-cells.

I have dicussed the problem when dealing with scewed (non-orthogonal) meshes in this topic: http://www.cfd-online.com/Forums/ope...tml#post216549
(It's in the OpenFOAM forum but that doesn't matter at this stage).

So MAYBE your mesh produces some "bad" solution because the gradients are not calculated correct.

I'm fairly new to FLUENT and I'm not sure what's behind the gradient discretization sheme you are using (Green-Gauss cell based).

Well, tet-meshes are not that seldom, so I hope the code has some good non-orthogonal correction procedures ...

Just to make sure: I'm guessing that this may lead to your problem.
I have experienced severe problems from inside OpenFOAM when dealing with non-orthogonal meshes ...
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Old   March 11, 2010, 13:20
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hm okay thanks for your help but my mesh is in the acceptable range of skewness. i still don't think that it will affect my solution that big way.

maybe im wrong... i wanted to test it but i cant create a quad map mesh around my airfoil
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