Extrapolating drag coefficient

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Irfan.Zafar
Posts: 14
Joined: Mon Jun 18, 2012 1:27 pm
Organization: Global Blade Technology
Location: Netherlands

Extrapolating drag coefficient

Postby Irfan.Zafar » Thu Jun 28, 2012 3:12 am

Hello,
I have a question regarding extrapolating drag coefficient. I have Cd data of an airfoil up to 12 deg, lift data is available up to 20 deg and the stall angle of the airfoil is about 15 deg. I want to ask if I extrapolate Cd from 12 deg using flat plate theory or some other method then would it cause any significant error? Thanks in advance,

With kind regards,

Irfan

Khanh.Nguyen
Posts: 8
Joined: Wed Mar 14, 2012 2:14 pm
Organization: NWTC
Location: Boulder, CO, USA

Re: Extrapolating drag coefficient

Postby Khanh.Nguyen » Fri Jun 29, 2012 10:58 am

Irfan,

There is no reliable method to extrapolate airfoil data around the stall region, where the flow is dominated by viscous effects and the types of stall (leading edge stall, trailing edge stall, and thin airfoil stall). Thus, the extrapolation results, regardless of the methods used, would incur some errors.

Note that the extrapolation method used in AirfoilPrep is developed for post-stall data.

Khanh

Irfan.Zafar
Posts: 14
Joined: Mon Jun 18, 2012 1:27 pm
Organization: Global Blade Technology
Location: Netherlands

Re: Extrapolating drag coefficient

Postby Irfan.Zafar » Tue Jul 10, 2012 12:58 am

Dear Nguygen,
Thank you for your reply.
I have data up to about 15 deg AOA for an airfoil. To reduce uncertainty is it better to first use CFD to get data up to e.g. 27 deg AOA and then using some extrapolation method I can extrapolate data to 180 deg AOA or there is any other option? Can you please give your opinion? Thanks,

Kind regards,
Irfan

Khanh.Nguyen
Posts: 8
Joined: Wed Mar 14, 2012 2:14 pm
Organization: NWTC
Location: Boulder, CO, USA

Re: Extrapolating drag coefficient

Postby Khanh.Nguyen » Tue Jul 10, 2012 10:27 am

Irfan,

I agree. In your case, CFD is the best option. I suggest that you check the CFD results over the angle of attack range with the wind tunnel data.

Khanh


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