An experimental investigation of full-scale sail aerodynamics using pressures, shapes and forces

Abstract

Sail aerodynamics is an active field of study in the research community. The basic behaviour of sails in steady conditions is well known due to numerical computations and wind tunnel testing, but there are several unanswered questions about the sail aerodynamics in actual, dynamic, sailing conditions. The unsteady aerodynamics of upwind sails has been addressed very few times in the past, both by numerical and experimental methods, and there is still a lack of knowledge on the unsteady aerodynamics of downwind sails. At the present time only full-scale testing can give some of the answers to the questions of what is really happening to downwind sails on the water. This project aims at improving the understanding of the unsteady aerodynamics of downwind sails at full-scale. This is done by measuring instantaneous pressures and sail shapes at full-scale, and combining them to obtain the aerodynamic forces developed from the sails. The measuring system, developed at the University of Auckland and named FEPV (`Force Evaluation via Pressures and VSPARS'), is validated for both upwind and downwind sailing by comparing its predictions with wind tunnel measurements carried out at the Twisted Flow Wind Tunnel of the University of Auckland. The steady aerodynamics of upwind and downwind sails at full-scale are investigated, at first, in this project, by averaging the measured pressures, sail shapes and yacht motions over chosen recording times. The averaged differential pressure distributions across the sail sections are compared with the expected and measured pressure fields from the literature. The calculated aerodynamic forces are compared with wind tunnel measurements, and a good agreement is found. The unsteady aerodynamics of downwind sails is then investigated by combining the knowledge of the instantaneous values of the differential pressures across the sails, of the sail shapes and of the aerodynamic forces. The results of two different tests, carried out on a J80 and a Stewart 34 class yachts respectively, are presented in this thesis. In either case, the yacht is equipped with an asymmetric spinnaker and it is found that, even in relatively steady wind conditions and limited yacht motions, the spinnaker has a very dynamic behaviour. The periodic folding and filling of the spinnaker luff influences the differential pressure distributions, in particular near the luff , and the aerodynamic forces, which have large variations from their average values during the recording time. The recovery of the sail shape is found to produce very large suctions near the luff and an increase in the drive force that would not be otherwise achieved.