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Wind tunnel and CFD investigation of unconventional rigs
Abstract
This paper presents research activities carried out by the authors to investigate aerodynamic behaviour of several unconventional saiplans in comparison to the sloop traditional solution. In particular an "A - shaped" mast, placed in the stern area of the yacht has been considered in single-jib and double-jib configurations. Wind Tunnel tests and performance prediction analyses have been performed in order to compare different configurations.
Most of the modern sailboats are armed with a sloop sail plan. This type of plan has some aerodynamic and operational drawbacks. The central mast position and its equipment create vortices on the sails (decreasing the efficiency) and, in particular, considerably reduce the habitability of both the deck and interior. In this paper we present a solution to this problems through the concept of a 35 ft yacht armed with a “simplified sail plan”. It is an “A-shaped” mast placed in the stern area of the yacht that has been considered in single jib and double jib configuration. This sail plan represents an opportunity of great interest for a Design for All approach, because it follows some of it fundamental principles, for example: to enable prehensility, to require minimal efforts and ability to the users, to use of technological solution (also innovative) to meet the requirements and skills of the user, etc.
US Sailing, the Offshore Racing Council (ORC), the Glenn L. Martin Wind Tunnel (GLMWT), Quantum Sail Design Group (QSDG), the Wolfson Unit and North Sails have collaborated on a series of wind tunnel test programs to investigate the performance of both upwind and offwind sails. These programs were initiated in response to perceived inequities in the ratings of boats of various rig designs sailing under the International Measurement System (IMS).
Observations of on-the-water performance have lead to the conclusion that there are biases within the rule with respect to rig planform design. Specifically, it has been concluded that large spinnakers are penalized so that a fractional rig, with its small spinnaker, is favored when sailing offwind, that there are un-rated benefits to a masthead rig upwind, and that there are errors in the relative handicapping of overlapping and non-overlapping jibs. The IMS Rule uses a Velocity Prediction Program (VPP) in which sail forces are represented by algorithms that are based on a combination of science and reverse engineering from the measured sailing performance of real boats. The results of investigations at both GLMWT and Wolfson have been used to modify this IMS aerodynamic model, thereby reducing the pre-existing biases.
Aim of the present paper is to present some results concerning the relationship between upwind flying shapes geometry and sail plan aerodynamic performance and to provide an experimental database to the scientific community for numerical simulation benchmarking activities concerning upwind sails aerodynamics. In particular experimental data are available from wind tunnel tests performed by the authors in the Politecnico di Milano twisted flow wind tunnel using a typical IMS cruiser-racer 1:10 scaled model. Aerodynamic forces and three-dimensional sail shapes have been measured in upwind conditions at different apparent wind angles and sail trim settings. The measured shapes have been obtained using a computer based in house developed IR-camera system and processed in terms of global parameters (i.e. sail twist, camber and maximum draft position). Flying sails shapes at different height are provided at close hauled conditions together with aerodynamic coefficients and centre of effort position with reference to different sails trimming.
This paper discusses the formulation of models for sail forces and moments as needed for the prediction of performance of a given yacht. It is shown that the effects of heel angle may be accounted for accurately using a simple geometric construction, with excellent agreement with wind-tunnel tests. With the aid of lifting-line theory the usual Kerwin model for accounting for the effects of sail trim is extended to include both sail twist and the effects of nonuniformity in the incident flow. The penalty in increased induced drag resulting from a constraint on heel moment is derived. Some earlier results regarding optimum loading are generalised for nonuniform flow, in which it is shown that optimum loading requires the induced downwash to vary linearly with height.
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