Left: you may consider yourselves a highly polished race crew but as far as the CFD is concerned you’re just a bunch of cylinders. Above left: a render of the Morrelli & Melvin Superfoiler that is currently in development, with rigid platform plus sails, rig and crew. Centre/right: the same boat with the pressure distributions across the sail surfaces plotted, both rig only and then with the addition of the platform. Including the platform not only changes the flow around the platform, it also changes the loadings over the sail surfaces
at yacht hull performance only by focusing on what was in the water are largely in the past. More recently, as sailing speeds have increased dramatically the aerodynamics of the hull and its effect on overall performance of the craft have started to grab the attention of yacht designers. And now in the ‘flying’ age, one might say that the performance and sophistication of our latest tools are at last enabling hull aerodynamics to take their rightful place in the global design equation.
The start of the current evolution in integrating aerodynamics
into yacht design occurred in the early years of 3DL moulded sails in the 1990s, with North’s concept of the ‘engine above the deck’. We came to understand that to make the very best sails the mast and mast design needed to be integrated with the sails and the rig analysed as a whole. These were the early days of aerodynamic models that first took advantage of Finite Element Analysis (FEA), which integrated sail and mast deformation through knowing both the material components of the mast and the aerodynamic pressure loadings generated from wind flow. Analysing the two as a package in turn produced more realistic flying shapes for the sails. The process today starts with the North Design Suite (NDS), proprietary sail design and analysis software that has been developed in-house over many decades and is largely unequalled in its sophistication and wide user base. Having a broad user base and applying the tools to a wide variety of projects over many years are critical to both developing the software itself and also the knowledge and experience to be able to get the best results from the tools.
NDS is an integrated family of specialised software programs
that brings unprecedented power and flexibility to sail and rig analysis. The suite includes boat and sail geometry (Desman), sail design and 3D sail modelling (Spiral), FEA (Membrane), CFD (Flow) and a North velocity prediction program. Each specialised module interfaces with others within the suite, allowing our sail designers and engineers to virtually sail and test a boat in an infinite variety of conditions before finalising design and beginning construction. NDS significantly expands a designer’s ability to test different sail and rig combinations and accurately measure results. Equally as important, NDS significantly reduces tuning and development time once afloat.
As mentioned above, aero models can now take advantage of Finite Element Analysis in integrating both sail and mast deformation to produce real sail flying shapes. These deformed sail shapes are critical to the accuracy of the boat’s global aero model. We then incorporate the platform and/or hull with different aerodynamic loadings, which after coupling with the effect of a deformed rig/sail produces an improved and more realistic aerodynamic model.
In summary, NDS is used to create the project’s base geometry which is then modelled with the boat’s onset flow conditions. We
then combine this with the platform geometry, which in turn is modelled in OpenFOAM (open source RANS CFD software) to produce the final data sets.
The use of OpenFOAM for aerodynamics has been an ongoing collaboration between North Sails and the Wolfson Unit since 2010, initially as a tool for high-end projects (America’s Cup, Volvo Ocean Race and so on), but in the past few years more lofts are using it over a wider range of projects and for different aims (performance prediction, sail pressure distribution, windage calculations, flow visualisation and so on). This collaboration allows the sail designer to access high-end CFD resources, both in terms of computer power and software, while not requiring exten- sive specialist CFD knowledge themselves. The system uses the University of Southampton’s own supercomputer, Iridis4, running the OpenFOAM solver which is controlled via a dedicated interface that enables a variety of automated processes.
As both sail and yacht designers have come to better understand the importance of the aero-hydro balance in design, it becomes less of an afterthought and more an integral part of improving overall performance.
These types of analysis techniques are still more easily applied individually; the total effect is more difficult to integrate into a complete, all singing, all dancing aero package that a VPP could then use to predict changes in performance from changes in overall boat parameters.
Our process for this project has taken a part of what was used
in the last AC and added to it, advancing the art and the science. The theory
Until recently most VPPs modelled windage (platform, hull, rigging, crew and all other items disturbing the wind flow) as an after- thought. Quite often the primary use of the windage module was to adjust the VPP results to match observed and/or assumed per- formance (not always true, but that really was quite often the case in practice). Windage of the rigging is fairly straightforward and well documented for the portion of the rigging in the free stream, but when it gets closer to the hull and sails life’s not that simple. The hull and/or platform become much more complicated when the mast and sails are added, so too are the computations as the matrices grow larger to accommodate all possible combinations of input parameters. Hence many VPPs adopted basic geometric drag features to predict the influence of each component. These were modelled as such, meaning they were considered individual pieces alone, once again not being referenced as part of the global picture.
The true picture of their influence on overall performance is of course not so simple. The air turbulence, velocity and angle of the wind over each component are influenced by its surroundings and when everything is combined, in one model, there can be significant
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