Sail analyses

Updated November 23, 2020

------------------ WORK IN PROGRESS ------------------


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The sail analysis module implemented in flow5 uses the same panel code solver as the airplane module, while its user interface is similar to that of the sail7 prototype. It is recommended to experiment with sail7 before purchasing flow5 to understand the philosophy and general mechanismn of the sail analysis module.
The design intent of the sail module is to

Only the triangular panel methods have been activated in the sail module. The quad panel method and the VLM have been disabled due to their lesser versatililty, and because they do not offer any signficant advantage over the triangular panel methods. This in return allows for a more simple and robust code, without any functional restriction.

The ground effect due to the presence of the water's surface is activated by default.

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Sail modelling

flow5 proposes several internal representions to model sails, and offers the option to import the sail definitions from external CAD and STL files. The intent is to enable quick sail design and modifications for parametric studies and sensitivity analyses.

Nurbs type sails

In this representation, the sail is defined by a NURBS surface.
The NURBS geometry is defined by sets of control points which can be arranged manually in the graphical user interface, or imported from an XML file.
The main advantage of the NURBS model is its versatility and the C2 continuity of the surface.

Spline type sails

As an alternative to NURBS, the sails may be modelled by interpolation of stremawise spline sections. This modelling method has the advantage over NURBS to provide slightly more control over the surface's geometry, with the disadvantage of C0 only continuity at the sections.

Wing type sails

This modelling method follows closely the representation of wings in both xflr5 and flow5. Sections are defined by foils.

Sails from CAD files

Sails can be imported from CAD files. The recognized formats are STEP, IGES and BREP.
Once imported, the imported geometry are converted to NURBS and stored as NURBS sails.

Sails from STL files

The preferred way to import external sails is using the STL format. Once imported, the sails are stored as a dead mesh which cannot be modified.
Since the STL format does not contain any information other than the set of triangles, manual reconstruction of the luff and of the trailing edge are required to be done manually in the flow5 editor.
See the detailed step-by-step guide for more explanations.

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The analyses are intended to provide the forces and moments acting on the sails and the boat as a function of design parameters. The design parameters are:

  • The wind's velocity (Apparent Wind Speed, AWS)
  • The headwind angle (Apparent Wind Angle, AWA)
  • The boat's heeling angle
  • Each of the sails angle w.r.t. the boat
Any combination of the design parameters is possible. The design values are set by interpolation between a minimum and maximum value proportionnaly to a control parameter.
  • In the case of the analysis illustrated in the image on the right, the wind speed and directions are set constant equal to 10 m/s and 10° respectively. The heeling angle is constant equal to -15°. The variables are the rotation angles of the sails around their luff, in this case from 5 to 35° for the main sail and 10 to 45° for the jib.

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    Scripting is done using xml files to define both the sails and the analyses to run. It operates in a similar way to the scripts for plane analyses.
    An example of a script file is available here: boat_script.xml.

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