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  #16  
Old 01-07-2013, 06:36 PM
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daiquiri daiquiri is offline
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No no, we are talking about the vertical variation of the vertical component of the undisturbed (freestream) flow.
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  #17  
Old 01-07-2013, 06:43 PM
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daiquiri daiquiri is offline
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Originally Posted by daiquiri View Post
No no, we are talking about the vertical variation of the vertical component of the undisturbed (freestream) flow.
Errr... no, we don't.

I have understood the windshear in the aeronautical sense. It is not. Found the discussion about it: Asymmetrical Speed Polars
A vertical variation in the freestream wind direction (horizontal) due to some geographical features around the sailing field.
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  #18  
Old 01-07-2013, 07:30 PM
quequen quequen is offline
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Originally Posted by daiquiri View Post
...A vertical variation in the freestream wind direction (horizontal) due to some geographical features around the sailing field.
That's the way I've understood Mikko's comment (not shure if right). It is a common phenomenon in our lakes (surrounded by mountains) . The most visible consecuence is that a perfectly balanced, well trimed boat, will go to windward at slightly different angles/velocities when in different sides (and will require different trimming configurations also). This situation appears and disappears sometimes in the same regata. I never undestood it well...
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  #19  
Old 01-07-2013, 07:42 PM
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daiquiri daiquiri is offline
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I never undestood it well...
Check the link in my previous post.
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  #20  
Old 01-07-2013, 07:58 PM
quequen quequen is offline
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I'm reading, I'm reading...
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  #21  
Old 01-08-2013, 02:35 AM
Mikko Brummer Mikko Brummer is offline
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Originally Posted by quequen View Post
Daikiri, Mikko, thanks for posting. André is waiting for some feedback, sail7 has a Sourceforge Discussion forum for questions and suggestions:
Sail7 at Sourceforge:
http://sourceforge.net/projects/sail7/

Discussion Forum:
http://sourceforge.net/p/sail7/discu...?source=navbar
Mikko:
-Sail7 allows for a vertical velocity gradient of true wind, that results in a more uniform distribution of pressures when the sail has twist. Last release don't allow for a vertical direction variation of True Wind, however.
-Sailcut limitations can't be solved in Sail7, but perhaps with some help Andre could add a .dxf import capability to manage other professional sail-design software's outputs.
-The mast and hull modeling is still a problem, a .dxf import capability coud be very useful at both this points.
-I'm not shure where is the usefullness of including the hull, given the mentioned limitatios of the method.
-To my knowledge, there is no mirror plane nor waterplane consideration. Sails float in the middle of the wind (as I understand it). Inclusion of a waterplane shouldn't be so hard to do, I guess ?
- Is input truewind with a gradient + boat velocity? That would be nice.

- The mirror plane is necessary for meaningful results. The hull (deck) would help a little, as an endplate to the jib, but without the mirror image the CLalpha will be all wrong, and consequently lift & induced drag, too. In my MacSail I found that a mirror plane at about 50% freeboard height gave best results compared to windtunnel tests (this was for dinghys with low freeboard, Europe & 470, and without the hull modeled).
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  #22  
Old 01-08-2013, 04:42 AM
Mikko Brummer Mikko Brummer is offline
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Quote:
Originally Posted by quequen View Post
That's the way I've understood Mikko's comment (not shure if right). It is a common phenomenon in our lakes (surrounded by mountains) . The most visible consecuence is that a perfectly balanced, well trimed boat, will go to windward at slightly different angles/velocities when in different sides (and will require different trimming configurations also). This situation appears and disappears sometimes in the same regata. I never undestood it well...
The wind shear you (and Tom) are speaking about is rarely a geographic feature, but rather a meteorological one. But forget about that for saildesign, just a combination of the true wind with a velocity gradient and the boat motion creates a twist in the apparent wind - that's interesting for saildesign. The asymmetrical twist Tom was referring to in his polar post is not relevant, since it's a special case, and you have to design the sails for both tacks anyway.

Still about the mirror image: You need to have the sailplan mirrored under the waterplane (or some other plane between the sail foot and the sea), if your boat with sails is just "floating in the air", the result will be wrong. It's like calculating an airplane with one wing only - the lift on the wing will be quite different when you add the other wing on the other side of the fuselage.
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  #23  
Old 01-08-2013, 04:48 AM
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daiquiri daiquiri is offline
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Quote:
Originally Posted by Mikko Brummer View Post
The wind shear you (and Tom) are speaking about is rarely a geographic feature, but rather a meteorological one. But forget about that for saildesign, just a combination of the true wind with a velocity gradient and the boat motion creates a twist in the apparent wind - that's interesting for saildesign. The asymmetrical twist Tom was referring to in his polar post is not relevant, since it's a special case, and you have to design the sails for both tacks anyway.
That's exactly what I was suspecting. Thanks for clarifying that one.
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  #24  
Old 01-09-2013, 03:09 PM
WestWindSoarer WestWindSoarer is offline
 
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Hi all,
All in all, I agree with what is said above about the VLM and it's limitations. The method is valid essentially at low a.o.a., and does not account for flow separation.
For wings operating at low Re, this is compensated in the original xflr5 by interpolating viscous drag on pre-calculated values issued from XFoil which is a 2d viscous solver. This is clearly an approximation, and I have nothing similar to propose for sails.
I derived sail7 from xflr5 because it required limited development work, and because it may in the future be used as a basis for more complex models which could include simulation of flow separation. There is little chance that I will do this work myself because my primary interest is in (identified) flying objects, but if somebody is interested, they are welcome to it and I will help as I may.

Last, to answer more specific points : yes, there is an option in the polar definition dialog box to include symmetrical reflection about the z=0 plane, and yes the hull representation is clearly a bad approximation which is best left out of the model. Also, the wing gradient model is very crude, and could easily be improved.

André
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  #25  
Old 01-09-2013, 04:57 PM
quequen quequen is offline
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André, thanks for posting!
Quote:
Originally Posted by WestWindSoarer View Post
... yes, there is an option in the polar definition dialog box to include symmetrical reflection about the z=0 plane...
Ok, that's the "ground effect" option, found at "Polars-Define an analysis" and "Polars-Current Polar-Edit", see attached image, it shows also the wind gradient definition menu.
Attached Thumbnails
Sail7: sail's performance-sail7-groundeffect.jpg  
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