Airfoilprep extrapolation of CM

Discuss acquisition, use, and manipulation of airfoil data.

Moderator: Bonnie.Jonkman

Claudio.Pedrazzi
Posts: 47
Joined: Wed Nov 09, 2005 12:05 am
Location: Bologna, Italy
Contact:

Airfoilprep extrapolation of CM

Postby Claudio.Pedrazzi » Mon Jan 22, 2007 5:00 am

Hi everybody,
... hoping not to abuse of your patience :) I will ask a new question, probably the 10th in theese days.

Here is a sample list of CL, CD, CM as input to Airfoilprep (in the TableExtrap sheet):

Alpha CL CD CM (not reqd.)
-2 0.0964 0.0110 -0.051
-1 0.2144 0.0110 -0.052
0 0.3321 0.0112 -0.0531
1 0.4491 0.0115 -0.0542
2 0.5651 0.0119 -0.0553
3 0.6799 0.0125 -0.0564
4 0.7932 0.0131 -0.0574
5 0.9043 0.0139 -0.0582
6 1.0127 0.0148 -0.0588
7 1.1168 0.0160 -0.0591
8 1.2144 0.0174 -0.0588
9 1.3002 0.0192 -0.0577
10 1.3607 0.0224 -0.0561
11 1.4113 0.0269 -0.0549
12 1.4460 0.0323 -0.0533
13 1.4657 0.0388 -0.0516
14 1.4742 0.0462 -0.0503
15 1.4735 0.0549 -0.0499

And here is the Extrapolated list that I get:
Alpha CL CD CM
-180 0.0000 0.0100 0.0000
-170 0.6876 0.0067 0.4000
-160 0.8672 0.1202 0.0062
-150 0.7263 0.2942 0.0537
-140 0.6487 0.5080 0.0899
-130 0.5652 0.7360 0.1294
-120 0.4555 0.9512 0.1720
-110 0.3180 1.1279 0.2135
-100 0.1613 1.2454 0.2486
-90 0.0000 1.2900 0.2723
-80 -0.1613 1.2454 0.2783
-70 -0.3180 1.1279 0.2705
-60 -0.4555 0.9512 0.2534
-50 -0.5652 0.7360 0.2337
-40 -0.6487 0.5080 0.2215
-30 -0.7263 0.2942 0.2339
-20 -0.8672 0.1202 0.3215
-10 -0.6088 0.0380 0.3318
-2 0.0964 0.0110 -0.0510
-1 0.2144 0.0110 -0.0520
0 0.3321 0.0112 -0.0531
1 0.4491 0.0115 -0.0542
2 0.5651 0.0119 -0.0553
3 0.6799 0.0125 -0.0564
4 0.7932 0.0131 -0.0574
5 0.9043 0.0139 -0.0582
6 1.0127 0.0148 -0.0588
7 1.1168 0.0160 -0.0591
8 1.2144 0.0174 -0.0588
9 1.3002 0.0192 -0.0577
10 1.3607 0.0224 -0.0561
11 1.4113 0.0269 -0.0549
12 1.4460 0.0323 -0.0533
13 1.4657 0.0388 -0.0516
14 1.4742 0.0462 -0.0503
15 1.4735 0.0549 -0.0499
20 1.2389 0.1202 -0.1295
30 1.0375 0.2942 -0.1902
40 0.9267 0.5080 -0.2265
50 0.8074 0.7360 -0.2600
60 0.6508 0.9512 -0.2930
70 0.4543 1.1279 -0.3244
80 0.2304 1.2454 -0.3518
90 0.0000 1.2900 -0.3727
100 -0.1613 1.2454 -0.3786
110 -0.3180 1.1279 -0.3709
120 -0.4555 0.9512 -0.3538
130 -0.5652 0.7360 -0.3341
140 -0.6487 0.5080 -0.3218
150 -0.7263 0.2942 -0.3343
160 -0.8672 0.1202 -0.4219
170 -0.6876 0.0067 -0.5000
180 0.0000 0.0100 0.0000

I am a bit perplexed about the CM=0.0062 that is computed for ALFA=-160, and from the jump to 0.4 after only 10 degrees (ALFA=-170). Is this behavior physically correct? I would have thought that the CM curve should be about symmetric to the positive side, for so high values of ALFA...

Thanks a lot for your answers!
Best regards
Claudio
Claudio Pedrazzi

Senior Project Engineer
Technical Dept.
TEROM S.p.A.
Bologna, Italy
www.terom.it
http://www.terom.it/ASP/Pages/eng/News.asp

Pat.Moriarty
Posts: 40
Joined: Mon Nov 07, 2005 4:59 am
Location: NWTC/NREL

AirfoilPrep CMs

Postby Pat.Moriarty » Mon Jan 22, 2007 4:46 pm

Hi Claudio,

Your correct, these numbers are a little suspect. The explanation of where these come from can be found in Appendix D of the AeroDyn User's manual (AirfoilPrep used to be known as FoilCheck, just to make things even more confusing) http://wind/designcodes/simulators/aerodyn/AeroDyn.pdf

It turns out that when you have reverse flow on an airfoil you can get some big moment coefficients. And apparently this has led to pitch system failures of turbines in the past. Therefore when AirfoilPrep was written, the writers hard coded large values at alpha = -170 Cm = 0.4 and alpha = 170 Cm = -0.5 (both of which are in your dataset). These values are based on some wind tunnel measurements of airfoils done at OSU - see report for actual papers if you're interested.

I'm not sure how accurate these numbers are for any airfoil including the one you are examining, as these data are hard to come by. If you can find some more accurate results I would suggest using those.
Big whirls have little whirls, That feed on their velocity; And little whirls have lesser whirls, And so on to viscosity - Lewis Richardson.

Claudio.Pedrazzi
Posts: 47
Joined: Wed Nov 09, 2005 12:05 am
Location: Bologna, Italy
Contact:

Postby Claudio.Pedrazzi » Tue Jan 23, 2007 12:35 am

Hi Pmoriart, thanks for your answer.

I am :-) old enough for having used Foilcheck... but my question is more about the LOW values before the "fixed ones" at big angles.

I copy here, for easy reference, the phrase from the Foilcheck manual, pagg 44:
Fixed values are used at angles near 180. The pitching moment can be large for reversed flow over the
airfoil. Some wind turbines have experienced structural failures in their pitch control system as a result
of this type of flow in very high winds. We therefore elected to use large and constant values as follows:
(-170, 0.40), (180, 0.0), (170, -0.50). These are the largest (absolute) values seen in limited
data that are available from the Ohio State University reports listed at the end of this appendix. Of
course, if you have data for your airfoil, we suggest using that data instead of FoilCheck.


It is clear to me why the values are big, what I would like to understand is why the CM goes to such a SMALL value immediatly before. In the positive region of ALFA, the behaviour is relatively regular, in the negative it is not.

Unfortunately I have no better data available, we are a small startup company, I have to do with this. My intention would be to correct manually the data in order to have a regular curve. What do you think about this idea?

Best regards & thanks again for your patience and help!

Claudio
Claudio Pedrazzi



Senior Project Engineer

Technical Dept.

TEROM S.p.A.

Bologna, Italy

www.terom.it

http://www.terom.it/ASP/Pages/eng/News.asp

Marshall.Buhl
Posts: 437
Joined: Fri Oct 21, 2005 10:22 am
Organization: NREL/NWTC
Location: Boulder, CO
Location: Boulder, CO
Contact:

Postby Marshall.Buhl » Tue Jan 23, 2007 9:19 am

cpedrazzi wrote:My intention would be to correct manually the data in order to have a regular curve. What do you think about this idea?

I'd be inclined to do a literature search for pitching-moment data for high angles of attack and use it. I'd manually fair it in with the data you started with. You can plot the data in Excel and then click on individual points and drag them up and down to get a smooth curve.

Marshall
Mr. Marshall L. Buhl Jr.
NWTC-3811
National Renewable Energy Laboratory
Golden, CO 80401 USA
Marshall.Buhl@nrel.gov
Voice: +1 (303) 384-6914
Cell: +1 (303) 915-6623
Fax: +1 (303) 384-7079

Pat.Moriarty
Posts: 40
Joined: Mon Nov 07, 2005 4:59 am
Location: NWTC/NREL

AirfoilPrep CMs

Postby Pat.Moriarty » Tue Jan 23, 2007 12:03 pm

Hi Claudio,

I think that the large jump you are seeing between the low values and high values at the angles is not physical. This is highly dependent on the airfoil shape, but for a smoothly varying airfoil (not a lot of camber) I would expect that Cm would also smoothly vary.

As Marshall suggests I would look in the literature and start with the OSU reports listed in the FoilCheck manual, of which most are available on the NREL publications server.

cheers,
Pat
Big whirls have little whirls, That feed on their velocity; And little whirls have lesser whirls, And so on to viscosity - Lewis Richardson.

Marshall.Buhl
Posts: 437
Joined: Fri Oct 21, 2005 10:22 am
Organization: NREL/NWTC
Location: Boulder, CO
Location: Boulder, CO
Contact:

Re: AirfoilPrep CMs

Postby Marshall.Buhl » Wed Jan 24, 2007 9:35 am

pmoriart wrote:As Marshall suggests I would look in the literature and start with the OSU reports listed in the FoilCheck manual, of which most are available on the NREL publications server.t

The OSU data are available here:

http://wind.nrel.gov/OSU_data/


Marshall
Mr. Marshall L. Buhl Jr.
NWTC-3811
National Renewable Energy Laboratory
Golden, CO 80401 USA
Marshall.Buhl@nrel.gov
Voice: +1 (303) 384-6914
Cell: +1 (303) 915-6623
Fax: +1 (303) 384-7079

Scott.Larwood
Posts: 98
Joined: Thu May 17, 2007 9:24 am
Location: Stockton, CA

Re: Airfoilprep extrapolation of CM

Postby Scott.Larwood » Wed Nov 28, 2012 10:49 pm

Hello,

I noticed this jump in the CM for negative values of angle of attack when modeling rotor fragments. I believe this comes from an error in the function GetCM in the table extrapolation for negative angles of attack, which was not following the algorithm mentioned in the AeroDyn manual (Laino 2002). I have added a corrected version and will send to Khahn Nguyen for verification. Note that this error makes all moment coefficient extrapolation suspect for tables developed with AirfoilPrep, which might impact pitch moment calculations for extreme load calculations.
Attachments
AirfoilPrep_v2p2_SML.xlsm
(183.18 KiB) Downloaded 4670 times
--
Scott Larwood
Assistant Professor of Mechanical Engineering
University of the Pacific
3601 Pacific Avenue
Stockton, CA 95211

Email: slarwood@pacific.edu
Phone: 209.946.3080
Website: http://web.pacific.edu/x19551.xml

Marshall.Buhl
Posts: 437
Joined: Fri Oct 21, 2005 10:22 am
Organization: NREL/NWTC
Location: Boulder, CO
Location: Boulder, CO
Contact:

Re: Airfoilprep extrapolation of CM

Postby Marshall.Buhl » Thu Nov 29, 2012 1:41 pm

Scott,

Thanks for pointing this out. Our next version of AeroDyn will replace the functionality of AirfoilPrep, but I'll also put in a bug report for AirfoilPrep.


Marshall
Mr. Marshall L. Buhl Jr.
NWTC-3811
National Renewable Energy Laboratory
Golden, CO 80401 USA
Marshall.Buhl@nrel.gov
Voice: +1 (303) 384-6914
Cell: +1 (303) 915-6623
Fax: +1 (303) 384-7079

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

Re: Airfoilprep extrapolation of CM

Postby Khanh.Nguyen » Thu Dec 13, 2012 10:31 am

Scott Larwood’s version of AirfoilPrep has been verified and this version is released as v2.02.01 (Dec 13, 2012). It includes a correction to the calculation of CM in the negative angle of attack range on the TableExtrap tab.
Regarding the jumps in CM at 170 and -170 angles of attack, these CM values are still hard coded as -0.5 and 0.4, respectively.

Khanh


Return to “Airfoils”

Who is online

Users browsing this forum: No registered users and 1 guest