EnginSoft - Conference Abstracts

EnginSoft Users' Meeting 2008
Le Tecnologie CAE nell'Industria

Sailing Yacht Computational Aerodynamics: an Investigation in RANS capabilities with large computational resources

Viola Ignazio Maria - Aerodynamicist
Ponzini Raffaele - CILEA

Abstract

The America’s Cup is the oldest trophy in the sport history and it is sailed between two yachts belonging to different countries. It represents the state of the art of the marine tech-nologies. In the present work an aerodynamic investigation of a specific sailing configuration of the last America’s Cup Italian challenger Lunarossa is presented. The boat sailing the downwind curse leg at 45° apparent wind angle with mainsail and asymmetrical spinnaker is considered. The Navier-Stokes well-known solver Fluent is adopted and several steady com-putations are performed with four different grids of 60.000, 600.000, 6.500.000 and 66.000.000 of cells, respectively. These increasing numbers of cells are representative of the increasing computational resources available. In 1996 the first Navier-Stokes application to downwind sails has been performed by Hedges et al. with a number of cells of O(103), six years later Masuyama et al. adopted O(104) cells. In the last ten years, CFD codes and hardware became able to manage more than several millions of cells.

The aerodynamic forces computed with the four meshes are compared with experimental data. A good agreement between the numerical computed aerodynamic forces and the ex-perimentally measured force is shown. Differences between the achievable accuracy with in-creasing mesh size are discussed. Wind tunnel tests have been previously performed at the Politecnico di Milano Wind Tunnel in the 4x14m boundary layer test section, with a 1:12.5 scale model. The model presents flexible sails that can be remotely trimmed operating in-dependently onto 6 drums connected to the sails sheets. A 6- component dynamometer is fixed inside the boat model and the measured forces are displayed in real time to allow the operator to trim the sail optimizing the trust force. When the optimum trim is achieved, forces are re-corded at 100Hz for 20 seconds and the mean value is computed. Without changing the trim, pictures of the sail shapes are recorded from several points of view to allow the photogram-metric reconstruction of the sail shapes. The reconstructed geometry is used in the present work.

The numerical/experimental comparison shows differences in both lift and drag force components smaller than 9% for the more coarse mesh demonstrating that - if skillfully man-aged - Navier-Stokes solver can be a powerful design tool being able to achieve solid results with light computational resources. The most refined mesh shows differences in both lift and drag force components smaller than 4%, indicating the increasing accuracy achievable with a larger computational effort.