Sail7: sail's performance

[quequen]

André Deperrois has recently launched a new free development tool, Sail7 v0.03alpha, it's a branching of XFLR5 applied specifically to sailboats. It applies VLM to the aerodynamic analysis of hulls and sails. And can manage Sailcut files!.
Very promising!

download from:
http://www.xflr5.com/sail7/sail7.html

 

[quequen]

It deals with multiple sails, vertical wind gradient, streamlines at LE and TE, can import Sailcut files, generates customizable sequences, export data to .txt files ... really nice!

 

[quequen]

New release, useful adds and some bugs corrected:

sail7 v0.04 alpha, December 8th, 2012

Added display of Luff angle to NURBS section view
Added a 3D display of lift and drag alongside with body forces
Improved the calculation and display of NURBS sections
Corrected the rotation of the NURBS type sails around the sail's leading edge
Improved the formatting of polar data points in the polar properties window
Added lift and drag to the graphic results
Corrected a bug in the display of bodies
Corrected the style of lines
Improved the selection of points in the sail section view

http://www.xflr5.com/sail7/sail7.html

 

[kaminskas]

very interesting

Hi,

it's very interesting project! I do have a question, is it possible to add 3rd sail: jib on bowsprit, to see it's effect on staysail?

best regards,
Tomas

 

[quequen]

Hi Tomas
You can add as many sails as you want, it's pretty easy. And you can import sails from sailcut, that ensures you your sails are buildable.
Anyway, VLM method has many limitations (we need an aero/hydrodynamicist opinion at this point, hope we'll see some good post here).
I.e., VLM will show a constant growup of Lift while increasing Angle of Attack, not reflecting leeward Boundary Layer separation nor Stall effects.

(by the way, there is a new alpha release v0.05 with some minimal bug corrections)

 

[Andy]

This looks very promising! Shouldn't be too difficult to implement an empirical limit on the boundary layer separation (maybe with in degrees of probability) and the same for the onset of stall? Keep up the good work, we need more projects like this!

 

[rapscallion]

This looks great! I use sailcutCAD and I was trying to study the sail shape in OpenFoam, but this looks so much easier!

 

[daiquiri]

This looks great! I use sailcutCAD and I was trying to study the sail shape in OpenFoam, but this looks so much easier!

OpenFoam is a full-NS (Navier-Stokes) solver, while Sail7 uses the VLM (Vortex Lattice Method). Hence very big difference in output capabilities.

Details about VLM can be found in many websites, like here: http://www-mdp.eng.cam.ac.uk/web/library/enginfo/aerothermal_dvd_only/aero/vlm/vlm.html
The graphical illustration in the above website shows well the assumptions which are beneath the method. The main assumptions are:
- potential flow
- small angles of attack (AoA)
- a straight horse-shoe vortex associated to every panel
There are methods to account for some degree of flow separation form the surface, but they are empirical and hence not universally valid. Hence, VLM works well for linear part of the lift curve, which means - for small angles of attack. At high AoAs, as the separation and 3D turbulence become more important, the error of the Sail7 will likely become progressively bigger.

All this is to say that a VLM method is a very useful tool for the analysis of aircraft wings working in the small to moderate AoA range, but is rather dubious (IMO) when it comes to sails. Ok for small AoA's, but there are points of sailing (close hauled, broad reach, running) when the main will work very close to stall, hence with a large portion of separated flow. It means that large 3D turbulence effects will be heavily influencing the aerodynamic lift. Same consideration for the separation behind the mast at small AoA's. VLM has no means for accounting for that phenomena, which can be only corrected empirically. Does Sail7 have such corrective tools?

In other words, though easy to use and producing some visually attractive flow lines, you won't be able to simulate anything like this with a VLM software like Sail7: http://www.boatdesign.net/forums/hy...-underwater-hull-form-41874-9.html#post541652 (see the post #135 by Mikko Brummer).

Cheers

 

[Mikko Brummer]

Hi Tomas
You can add as many sails as you want, it's pretty easy. And you can import sails from sailcut, that ensures you your sails are buildable.
Anyway, VLM method has many limitations (we need an aero/hydrodynamicist opinion at this point, hope we'll see some good post here).
I.e., VLM will show a constant growup of Lift while increasing Angle of Attack, not reflecting leeward Boundary Layer separation nor Stall effects.

(by the way, there is a new alpha release v0.05 with some minimal bug corrections)

Nice! I haven't had time to look closely, but:
- can you specify a wind shear & twist in the wind?

- Sailcut may produce sails that are "buildable", yes, but that does not mean those shapes can be achieved flying in the real world.

- the lack of modeling the mast is one problem

- VLM results for the hull can be rather nonsense

- where is the reflection plane for the "mirror" model of the sails considered? At waterline - at deck level?

 

[daiquiri]

Does anyone know if any validation work has been performed for that software, and where can the relative paper be downloaded from?

 

[quequen]

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/discussion/?source=navbar

-Regarding the BL separation, perhaps some empirical corrections could be added to show al least the limits of accuracy. I know XFLR6 can do it only in 2D aerofoil analysis, including the BL curvature (althought I don't understand how it works nor how to use it).
-I don't think there are validations performed yet, I can post some sail7 file in .dxf if somone wants to validate it using a NS solver. Or maybe someone has a sailcut file allready analyzed using some other software.

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 ?

 

[daiquiri]

Last release don't allow for a vertical direction variation of True Wind, however.

Frankly, I have never really understood why is it so important to simulate the variation of the vertical wind component (if I understand correctly the usage of the term "wind shear" here).
When does this vertical component of the wind happen in reality, how often, and how much difference does it make to have it included in the simulation? My understanding is that it is geographically a very localized phenomena, caused by the presence of mountains or other high obstructions on the nearby shore. But is the sail science become so precise lately to have a need to include this feature in the simulations too?

 

[Petros]

daiquiri,

all wind have variable speeds as coming up off a surface, it is call the boundary layer. It would be most pronounced in light winds, where the air movement near the surface is zero, and at the top of the mast at free stream speed. the design and twist of the sail will be greatly affected by the vertical distribution of airspeed, it will affect the lift distribution and the heel as well. The affect would be less I suppose on very large sailboats, but small sailboats are greatly affected. You can not optimize a sail or wing without considering this effect. for recreational sailing, there would not be much benefit, but for one design classes, it will be the difference between winning and being an also-ran.

 

[daiquiri]

daiquiri,
all wind have variable speeds as coming up off a surface, it is call the boundary layer. It would be most pronounced in light winds, where the air movement near the surface is zero, and at the top of the mast at free stream speed. the design and twist of the sail will be greatly affected by the vertical distribution of airspeed, it will affect the lift distribution and the heel as well. The affect would be less I suppose on very large sailboats, but small sailboats are greatly affected. You can not optimize a sail or wing without considering this effect. for recreational sailing, there would not be much benefit, but for one design classes, it will be the difference between winning and being an also-ran.

Yes, I understand that very well. I'd say, it's one of the basics of sail design.

But I was referring to the vertical variation of the vertical wind component (not the horizontal one), which seems to be a case here. And also in another thread which I cannot find now, where Tspeer has introduced that matter.

Cheers

 

[Petros]

the vertical component caused perhaps by the up and down motion of the waves? I can not imagine there is much other vertical movement (except perhaps that caused by the sail design). I would guess that on a small boat that could make a difference, on larger yachts (like over 40 ft) there may not be that much of an effect. there was a thread about Olympic sailing medalists using sails that incorporated the pitching movement of the hull into the sail design.

when I worked in aerodynamics where we did three dimensional wing design on aircraft, considering the span-wise flow on the wing did affect the design (shape of the wing). so much so I do not see how you can optimize a wing, or sail design for that matter, without considering the span-wise flow. In the case of a sail, it would be the flow the moves vertically on the sail. this is inevitably caused by the taper, at any given point on the surface, the adjoining point will have a different pressure, and the surface flow will flow in the direction of the lower pressure.