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CFD simulations for the Alinghi Team, Defender of the 32nd America's Cup
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After the first successful partnership
between Alinghi and EPFL ended with the conquest of the 31st America's
Cup in March 2003, the scientific collaboration has been further developed
in different research domains.
The Chair of Modelling and Scientific Computing has been confirmed as
research partner for the numerical simulation of different design aspects
involving fluid-dynamic phenomena, ranging from the transitional
(laminar-to-turbulent) flow around appendages to the fluid-structure interaction between
wind and sails and between waves and hull.
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Download Information Notice: images subject to copyright;
use of images under permission only.
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The numerical simulation of the flow around an America's Cup sailing yacht has achieved a fundamental role in the design process [1,2]. Indeed, Computational Fluid Dynamics (CFD) allows the designers to test several different boat configurations and sailing conditions in a timeframe much shorter than the one required by experimental campaigns (in wind tunnel and towing tank) and full scale testing.
In order to guarantee the required accuracy of the numerical results, a great effort was devoted to the validation and calibration of the numerical models versus available experimental measures, with particular attention on what concerns turbulence and (laminar-to-turbulent) transition models [3]. Indeed, a precise prediction of the transition location and turbulence development on, e.g., boat appendages is crucial to obtain an accurate estimate of the forces acting on the different boat components.
The global capability of our computational tools to reproduce accurately the flow physics has benefited, on one hand, of the recent improvements of the physical and numerical models; on the other hand, the increasing computational power available on EPFL supercomputer facilities allowed us to simulate a larger number of configurations on finer computational grids when compared to the simulations carried on only three years ago. As a matter of fact, if during the 2003 America's Cup campaign we simulated around 100 boat configurations on computational grids of about 6 million elements (corresponding to 30 million unknowns), for the current campaign we were able to analyse more than 400 different configurations on grids with up to 20 millions elements (resulting in algebraic systems of 160 millions of unknowns).
Many design aspects have been considered in our numerical simulations: the shape of every appendage element (keel, bulb, winglets, rudder) has been the subject of specific numerical simulation studies in order to reach an optimal configuration
[4].
Streamlines around yacht appendages
Important advances have also been obtained in the modelling of free-surface flows where the evolution of the water-air interface is accounted for in order to predict the correct wave pattern around the hull. In particular, we have recently included the possibility to simulate the boat dynamics, coupling the flow solver with a module for the rigid body motion, which can be used to predict the hydrodynamic equilibrium attitude in different sailing conditions and, in perspective, to analyse the dynamical behaviour of the boat in waves and during maneuvering
[5].
Free-surface elevation in the bow region
Moreover, the fluid-structure interaction between sails and wind has been studied to better understand the behavior of downwind sails, their dynamic and stability properties as functions of wind conditions and sail trimmings, even when dealing with large force oscillations due to fluid separations and huge turbulent wakes behind sails. This allowed the designers to evaluate the downwind sail performances in a very large variety of cases, thus helping to find the optimum conditions to maximize the driving force. A dedicated software, called Virtual Wind Tunnel (VWT), has been developed to provide the Alinghi designers with valuable tools that could ensure adequate numerical prediction of real physical phenomena.
Velocity field around downwind sails
References
[1] N. Parolini and A. Quarteroni,
Mathematical Models and Numerical Simulations for the America's Cup,
Comp. Meth. Appl. Mech. Eng., 194, pp. 1001-1026 (2005)
[2] N. Parolini,
Computational Fluid Dynamics for naval engineering applications,
EPFL, PhD Thesis n. 3138 (2004).
[3] Ansys CFX Documentation, ANSYS, Inc.
[4] N. Parolini and A. Quarteroni,
Modelling and numerical simulation for yacht design. To appear in Proc.
of the 26th Symposium of Naval Hydrodynamics (2007)
[5] L. Formaggia, E. Miglio, A. Mola and N. Parolini,
Fluid-structure interaction problems in free-surface flows:
application to boat dynamics. To appear in Int. J. Num. Meth. Fluids (2007).
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