Super Yachts: The Azimut-Benetti R&D Centre & the CFD design of luxury yachts

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dynamics Magazine V2.02

Azimut-Benetti has been at the top of the world "mega-yacht" industry for over ten-years: owning over 10% of the global market, and nearly 30% of the domestic market; the Italian company is the world boating industry’s largest private group, producing over €800 million worth of yachts every year.

Dedicated to delivering the highest standards in both performance and quality, Azimut-Benetti have adopted STAR-CCM+ as a standard CFD tool for the aerodynamic, hydrodynamic, and thermodynamic design of their yachts. Using STAR-CCM+ Azimut-Benetti have managed to optimize the performance of their yachts, while simultaneously reducing development costs and time to market.

Azimut-Benetti conducts structural and hydrodynamic research and development at a dedicated facility in Varazze (SV), where innovative solutions are developed and later deployed across the entire Azimut-Benetti group, which includes the brands Azimut Yachts, Atlantis and Benetti Yachts – each aimed at a distinct segment of the boating market – as well as Fraser Yachts, a leading brand in the service sector. Within the Azimut Benetti group Computational Fluid Dynamics simulation is deployed for many purposes:

  • internal environment conditioning; 
  • aerodynamics;
  • thermal analysis of engine rooms;
  • exhaust gas flow analysis;
  • resistance calculation for fixed trim hulls;
  • calculating the hydrodynamic characteristics, including flow resistance, of planing hulls, with fixed and free trim (2 degrees of freedom).

Before purchasing CFD software Azimut-Benetti undertook an exhaustive evaluation process. For hydrodynamic simulation, we chose STAR-CCM+ because of its ability to:

1. perform bi-phase calculations in air and water

2. automatically construct a calculation grid that adapts to the physics of the problem

3. calculate the equilibrium position of a planing hull.

For aerodynamic simulation, we needed to be able to work effectively with CAD data to extract a single wetted surface, often starting from a CAD that contains superfluous details that interfere with the fluid volume meshing software; a final requirement therefore is:

4. the ability to automatically clean CAD surfaces (surface wrapper)

Of these calculations the most complex and valuable is the hydrodynamic analysis of planing hulls. It is necessary to consider the hull as a three-dimensional body that interacts with two immiscible fluids (air and water), simultaneously accounting for the manner in which the fluids interact with each other at the free-surface and for the way in which the trim of the boat is influenced through the interaction with both mediums. Simple static simulations (in which the boats running attitude is not determined by the simulation) are not generally sufficient for our purpose. Similarly, although CFD simulations with a single-phase fluid model have been used elsewhere to provide information on the resistance of immersed parts, this does not represent an acceptable approximation for solving our issues.

Historically the process of hydrodynamic design relied heavily on the analytical data provided by multiple towing tank tests, which are both expensive and time consuming to perform: a single tank test typically costs many thousands of Euros. Nowadays we adopt a “Virtual Prototyping” approach in which we try to reduce physical hydrodynamics testing to just a single model in a tank; validating the design that we have found to be the best following numerous "virtual" tank tests using STAR-CCM+.

The Azimut-Benetti Group develops and produces about five new yachts a year. Using STAR-CCM+ we can carry out very many tests on different hulls configurations under a wide variety of operating conditions. We can compare numerous tests even with designs that are not finalized. Thus we are able to virtually optimize the hull in all its details. The final production vessel is therefore fully optimized, reducing experimentation costs and time to market and providing a better overall product.

In 2007 Azimut-Benetti developed, in partnership with CD-adapco, a proprietary method that uses STAR-CCM+ as a fluid dynamic simulation "engine". This method calculates the running attitude of the vessel in 2 degrees of freedom ("sink" and "trim"). In order to represent the full complex geometry of the hull, and to provide appropriate resolution at the free-surface waterline, Azimut-Benetti opted for the trimmed cell calculation method available in STAR-CCM+. The method prescribes a Cartesian base grid with perfect hexahedrons that are subsequently “trimmed” to the CAD surfaces. A very important detail is the treatment of the layer of cells near the solid walls: layers of prismatic cells are used that automatically fit the CAD geometry.

http://www.cd-adapco.com/press_room/dynamics/v202/images/azimut1.jpg http://www.cd-adapco.com/press_room/dynamics/v202/images/azimut2.jpg
Fig 01 Fig 02

Fig 01, illustrates the emerged part of a yacht and the relevant aerodynamic calculation. In this case, the use of the surface wrapper outlined above was fundamental - the task of the wrapper is to produce a surface totally free of connectivity defects, which can then be used as an input datum for the volume mesher. In the case of external aerodynamics, given the large quantity of objects present in the original CAD, the surface wrapper has allowed entire days to be saved that would otherwise have been dedicated to an activity (CAD cleaning) with no added value for an R&D Centre.

Fig 02, finally, illustrates the calculation grid for the external aerodynamics. In this case, Azimut-Benetti has opted for polyhedral grids which guarantee automatic setting but perform better than the tetrahedrons.

In conclusion
Azimut-Benetti maintains its market leadership thanks to a technological leadership such as, for example, the use of accurate and dedicated CFD software. This is put into practice by working alongside key players on the CAE scene such as CD-adapco and by making significant contributions to improving the products, with a long term spin-off for the entire nautical sector.

REFERENCES
Brizzolara S., Serra F., Accuracy of CFD Codes in the Prediction of Planing Surfaces Hydrodynamic Characteristics, Proceedings of the 2nd International Conference on Marine Research and Transportation (ICM RT 2007), Ischia 28-30 June 2007
Caponnetto M. (2001), Practical CFD Simulations for Planing Hulls, Proceedings of the Second International EuroConference on High Performance Marine Vehicles, HIPER’01, Hamburg. V. Bertram ed., pp 128-138
Savitsky D. (1964), Hydrodynamic Design of Planing Hulls, Marine Technology, Vol. 1. No. 1. October 1964.


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