View Full Version : Windsurf Sail Wind tunnel data Cl Cd

7th February 2009, 11:28 PM
Does anyone have any windsurf sail lift/drag vs alpha data?
Maybe Jim Drake. I don't know how to get in touch with him.
I'd prefer not to get into deriving cl and cd purely from theory as
this is difficult in 3-d without some type of check.
My math model is severely limited using generic sail data from Marchaj.

8th February 2009, 12:05 AM
I kind of doubt that...wind tunnel testing would be theoretically possible...but the problem is dynamic behaviour of the sail (no) real wind is stready one.

In car industry or turbine theory it is not an issue, because the wind gusts are still negligible compared to speed generated flow.

this is a bit tricky, even if there would be a measurement in wind tunnel, personally I think the actually values looks a bit different.
The biggest problem generally I see in the model it self...if you assume only a steady flow, the model will be very bad. If there is a stochastic part of wind (flow) the model will not be good either (because the values the coefficients will be again for a steady state).

Hope this ain't that messy, and you could understand what i am trying to say.

Hang loose,
Ciao Michal.

8th February 2009, 01:09 AM
I understand what you are saying. Point taken. I take issue with my model will be very bad. Simplified mathematical models are used in all engineering disciplines to predict physical behavior. Even though they may be incomplete, these simplified models can give much insight. For example I'm not going to solve unsteady navier-stokes to figure out what the head loss is in 100 ft of 1/2 inch pipe with three 90s.
In fact, I'm pretty surprised at how reasonable my existing model is at predicting velocities, planing, optimal sheeting angle and sail heel. Yes the model is steady state. The model uses generic sail data from Marchaj. Just looking for some more sail data that is closer to the windsurf sail plan form. If there is some test data taken at the proper reynold number it would be very useful. Solving for lift and drag of the windsurf plan form purely from theory is daunting no matter the method (including CFD). But, besides your words of encouragement got any data?

8th February 2009, 01:34 AM
sorry i have nothing, I understand flow physics till a certain level (deeper I could go if your questions concern plasma physics and molecular flow of ionised fluid in E-B field...), but I definetely have no data sails aerodimac coeff.

But you´ve got my attention...would be intereted for your present work...any chance of sharing?

Ciao M.

8th February 2009, 09:44 PM
But, besides your words of encouragement got any data?

Gastra (http://www.gaastra.com/) years ago had a set up where they put a sail on top of a truck with a bunch of sensors to simulate and measure different conditions. Maybe you could contact them ...

8th February 2009, 11:01 PM
that definetely is pretty close to the reality...

9th February 2009, 12:29 AM
Relative to davide's post where a sail was mounted on a truck to study design and performance related issues, it's my recollection that Bruce Peterson (now of Sailworks) was one of the principals involved in the testing. I think that he would be the best source to contact.

9th February 2009, 02:01 AM
Hi Windsomniac, 25 years ago I tested a wing in a wind tunnel at school. This wing's shape was close to an old fashioned windsurf sail but with two masts (I can't describe it easily). Would it be of interest for you? The results are still at home somewhere, but I won't be back before next Sunday.

9th February 2009, 03:18 AM
Farlo, i'd like to see your data. Yes that would be terrific!
Davide and SteveC, I'll try to get in touch /w gaastra and Bruce, thanks!
mim, i don't think my model is user friendly enough at this stage to share, but I'll keep you posted. i will eventually need some peer review.

9th February 2009, 03:40 AM
Thinking out loud here, I guess I might be able to back out lift and drag coeffcients out my model by analyzing really good time stamped video. I would need to know the sailor's weight, equipment size, and the wind speed in the video. This would probably about as accurate as full CFD, but maybe more work.

9th February 2009, 04:30 PM
hi windsomniac,

We are planning at my school (Technical University of Budapest) some windsurf sail measurements in the wind tunnel. It's gonna take place in april-may, and I'm responsible for it, so I think there is no problem of generating a sail lift/drag vs angle diagram:) And if you have any other ideas to measure, just share it!
I read the comment of steady state wind, and that it's not gonna give good results. I agree, that you almost never find steady states in nature, but you have to start somewhere...

9th February 2009, 04:36 PM
yeah, you are right, you must start somewhere...

But the idea with the truck, is very close to nature, this I would consider as a second step, after steady state measuremetns.

Good luck, with the experiment...are you going to publish that, or put on some webpage...would be nice to have to possibility to take a look.


10th February 2009, 12:22 AM
Hi sztér,
Thats great! I'm really excited. Yes I have lots of ideas for you. In order of priority I would test the sails for:
1. Lift and drag of sail at various angles of attack (alpha) from 0 to 90 degrees with some special attention at the lower values for alpha 0,2,5,10,15,20,30,60, & 90
2. Variation in % camber on results of 1. a. 5%, b. 10%, c. 15%
3. Variaration of 1. with sail rake (with wings this is called sweep)
a. upright, b. locked down , c. somewhere in between
locking down the sail greatly reduces induced drag under the sail
4. Variation of 1. with negtive heel of the sail with
a. heel at zero, b. heel at 22 deg, c. heel at 45 deg.
When I think of all the permutations of these variations on the experiment I don't think there would be any time for mim's unsteady analysis.
I will get in touch with you about ideas for taking the measurements. For now take a look at the books from author C. A. Marchaj

10th February 2009, 04:10 PM
Hey guys,
that is gonna be a nice experiement...have fun!

Ciao M.

PS: ok unsteady would definetely be too much, if not impossible!

10th February 2009, 09:31 PM
Hi guys,

I got pretty excited too about the experiment! Dan, we should definitely get in contact, especially because I'm not always sure about what negative heel and these things mean... (I'm Hungarian and we use a mixture of English, German and Hungarian terms to describe different things in windsurfing). I don't know if you can see my e-mail address, so I just write it here: lukacseszter1983@yahoo.com.
To mim: I don't think the results are gonna be published, but who knows... Anyways, if you are interested I can send the results to you after the measurements.
Oh, and another thing I almost forgot. The diameter of the wind tunnel is 4 meters, but we can only use 2 meters to measure. So, we need a small sail or a model. I was looking for some prokid riggs, so that the have more or less the same properties as the adult ones. What do you think about this? Would it be better to make a model?

And finally for the unsteady case: actually that would be a nice phd topic, in fluid-structure interaction!


10th February 2009, 11:54 PM
Hi Guys, I had a much smaller wind tunnel and my model was ~40 cm high. It was made of aluminium masts and thick plastic (~1 mm) for the sail. 25 years ago cambers were not that popular. Windsomniac can you give me an email address where I will post the results, not before next weeek end I'm afraid.