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NACA airfoil aerodynamics forces

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Old   August 25, 2012, 22:51
Post NACA airfoil aerodynamics forces
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Jos
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Hello everyone;

I am doing some 2D CFD simulations on a NACA airfoil and trying to estimate the lift and drag forces using Fluent -13......I have managed to set up the domain as per the experiment( obtained from a journal paper) and the lift forces are being predicted accurately but my drag coefficient is nearly 10 times higher than expected.....I am running simulation at low Re and using the k-w SST model ...
I am sure this problem was seen before in FLuent 6.3 , could someone pls tell me whether this is resolved in Fluent -13...

Thanks in advance
Joseph
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Old   August 26, 2012, 02:40
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Originally Posted by joseph@CFD View Post
Hello everyone;

I am doing some 2D CFD simulations on a NACA airfoil and trying to estimate the lift and drag forces using Fluent -13......I have managed to set up the domain as per the experiment( obtained from a journal paper) and the lift forces are being predicted accurately but my drag coefficient is nearly 10 times higher than expected.....I am running simulation at low Re and using the k-w SST model ...
I am sure this problem was seen before in FLuent 6.3 , could someone pls tell me whether this is resolved in Fluent -13...

Thanks in advance
Joseph
The problem you are telling is a typical representation of bad viscous mesh and good in-viscid mesh and generally encourted by the user's who are new in this field(though i don't know about you). In simple words your boundary layer mesh i.e mesh very near to wall is not fine enough to resolve boundary layer and that's why you are getting very high value of drag. By the way which which mesh you are using for your analysis structured, unstructured of hybrid? You should not start with fully unstructured mesh for this problem, start with atleast hybrid or if possible with structured. Hope it helps you. If further guidance is needed then you are welcome. Thanks

Regards
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Old   August 26, 2012, 08:03
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Thanks for the information.....yes earlier I was using an unstructured mesh in the analysis ...I am now trying with a hybrid mesh and now the problem seems to be the exact opposite , I am getting a good result for the drag coefficient but a poor result for the lift coefficient....could this also be due to the mesh...Btw my wall y plus values are between 10-15.....One further question would be the reference values for a 2D simulation....does the length correspond to chord length and area for a 2D case correspond to chord length also...

Thanks
Joseph
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Old   August 26, 2012, 13:21
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could this also be due to the mesh
Yes, reduce the growth rate of mesh away from wall and if possible try with fully unstructured

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Btw my wall y plus values are between 10-15
Wall y+ are ok. SST Kw with wall y+ upto 10 give good results

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does the length correspond to chord length and area for a 2D case correspond to chord length also...
Yes
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Old   August 28, 2012, 06:39
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Thanks a lot for your reply.....the 2D analysis appear to be fine now but just one further question is there any way to keep the wall y-plus value from fluctuating throughtout the iteration process becuase I have noticed that it is within acceptable range at the begining of the calculation but later on increases......Thx
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Old   August 28, 2012, 09:36
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Originally Posted by joseph@CFD View Post
Thanks a lot for your reply.....the 2D analysis appear to be fine now but just one further question is there any way to keep the wall y-plus value from fluctuating throughtout the iteration process becuase I have noticed that it is within acceptable range at the begining of the calculation but later on increases......Thx
Actual wall y+ values are those which you obtain after the converged solution. Wall y+ are solution dependent and solution variables keep on changing during iterations and so the wall y+, until you get a converged solution.
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