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-   -   troubled by my airfoil design (http://www.cfd-online.com/Forums/fluent/50113-troubled-my-airfoil-design.html)

radix December 23, 2008 05:47

troubled by my airfoil design
 
Hello everyone... I'm comparing the result (Cl and Cd) from my NACA 9410 in Fluent and in a software named Designfoil. The result difference between these two software (especially the Cl) is about 1/3. For instance, in Fluent, the Cl is about 3,18 but in Designfoil it's only 1,06. Please,can anyone help me with this problem? What makes these two results being so far away...? Thanks.

zongtwi December 23, 2008 21:46

Re: troubled by my airfoil design
 
I can't comment on Designfoil, I have no experiece in using that software. But with respect to FLUENT, there are many things that will affect the accuracy of your results.

Your physical models, especially turbulence, and which wall funtion you use will have a significant effect on your results. The k-w model should give high quality results, but it is quite expensive in terms of computational costs.

Your mesh needs to match which turbulence model and wall function you are using as well. If you are using the low Re approach, then your Y+ values should be very small, in the region of around 1. Otherwise, for high Re approach, Y+ should be around 150.

Have you checked for grid independencies for your results? You might want to control the mesh sizing around your wake area. Try to use grid adaption for pressure gradients, for a few runs. Bear in mind, for 3D cases, this might significantly increase your mesh count, so ensure that you have enough RAM for your simulation.

Have you also tried using higher order discretization schemes?

How do you calculate the Cd and Cl values? Do you ask FLUENT to calculate and plot these values out? Then please ensure that you are using the correct reference area for the calculation.

Hope that helps.

radix December 25, 2008 08:07

Re: troubled by my airfoil design
 
"Your mesh needs to match which turbulence model and wall function you are using as well. If you are using the low Re approach, then your Y+ values should be very small, in the region of around 1. Otherwise, for high Re approach, Y+ should be around 150." Thanks for the reply. Honestly,i'm still new with Fluent. Would you mind explaining more about the quote sentences from your message above? Thank you.

Alagesan December 25, 2008 09:18

Re: troubled by my airfoil design
 
Hi Radix,

I am also comparing an airfoil Cl and Cd results using fluent and xfoil. Can you give further details such as your mesh, what turbulence model and discretization schemes you are using.

Read through fluent user guide chapter 12 - Modeling turbulence. In that all wall functions are explained, it will help you.

Just go through the below link it gives some details.

http://www.cfd-online.com/Forum/flue...cgi/read/43947

You can use the search function of this forum, which is very helpful.

Cheers, Alagesan


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