Trans RINA, Vol 152, Part B1, Intl J Small Craft Tech, 2010 Jan-Jun PRESSURE DISTRIBUTIONS ON MODERN ASYMMETRIC SPINNAKERS


I M Viola and R G J Flay, Yacht Research Unit, The University of Auckland, New Zealand (DOI No: 10.3940/rina.ijsct.2010.b1.103) SUMMARY


An electronically scanned multi-channel pressure system was used at the Yacht Research Unit’s Twisted Flow Wind Tunnel (University of Auckland) to test 3 asymmetric spinnakers. The sails were designed for the most recent America’s Cup Rule (AC33) and tested on a large-scale model. The present paper focuses on pressure measurements on three asymmetric spinnakers, which were measured on 5 chord-wise sections with 11 pressure taps on each section. All the 3 sails were tested at apparent wind angles of 40°, 55° and 70° and at heel angles of 0°, 10° and 20°. The 3 sails were firstly tested in uniform flow and then one of the sails was re-tested in twisted flow conditions. It was found that the suction peak on the A2 sail increased when the model was heeled to 10°, and then decreased when the heel angle was increased further to 20°, which agrees with the similar observed behaviour in the drive force variation with heel angle. Measurements in straight and twisted flow showed that all sail’s sections gave higher suctions in the twisted flow and a corresponding increase in the drive force


NOMENCLATURE AC


AC33 33rd America’s Cup Class (2nd hypoth.) AC90 33rd America’s Cup Class (1st hypoth.) AWA CFD Cp Cx


America’s Cup


DOG h


IACC Mitre q SA


YRU z


1.


Apparent wind angle (deg) Computational fluid dynamics Pressure coefficient (-) Drive force coefficient (-) Deed of Gift


Model height (m) International America’s Cup Class


Mid-section Horizontal section of the sails at ½ of the mitre height


Line made up of the points on the sail surface equally far from the leech and the luff


Dynamic pressure coefficient (Pa) Sail area (m2)


Yacht Research Unit Vertical height (m)


INTRODUCTION


In a previous paper by the same authors [1], aerodynamic forces and the pressure measurements on large-scale models tested in the Yacht Research Unit’s Twisted Flow Wind Tunnel at the University of Auckland were presented.


In particular, 3 asymmetric spinnakers


coupled with the same mainsail were measured at 3 apparent wind angles and 3 heel angles. The force measurements were used to correlate the aerodynamic forces with the optimum sailing condition of each sail. The general


pressure distribution measured on the


asymmetrics was also presented and correlated with the flow field around the sail. The present work describes in detail the pressure measurements recorded at the same time as the force measurements. They show how the pressure distribution changes when there are changes in boat heel, apparent wind angle, sail shape, and finally, due to testing in uniform flow and twisted flow.


In the previous paper [1], a brief review of downwind sail wind tunnel measurements was presented, noting the increasing interest in asymmetric spinnakers.


In


particular, the evolution of the America’s Cup and Volvo Ocean Race class rules reflects the increasing desire to sail with asymmetric spinnakers tacked onto bowsprits instead of symmetric spinnakers tacked onto poles.


To give a complementary review to [1], in the following section the state of the art of pressure measurements on sails is presented, both at full-scale and through wind- tunnel experiments.


2. STATE OF THE ART


Warner and Ober measured pressure distributions on a mainsail and jib in 1923 as reported by Marchaj [2]. Manometers were used to measure 28 points on the leeward side of the sails of the yacht Papoose in a full- scale experiment. Since then, for many years there is no knowledge of pressure measurements on sails from the published literature. Half a century later, in the 1970s, Gentry was interested in the slot


effect due to the


jib/mainsail interaction and investigated their pressure distributions with an Analogue Field Plotter on a 2D model [3]. He was involved in the design of the masts for the America’s Cup (AC) defenders Courageous, Freedom and Liberty in the 1974 and 1977, 1980, 1983 AC defences respectively [4]. These were investigated with the Computational Fluid Dynamic (CFD) code of the Boeing Commercial Airplane Company. In the 1980s, Wilkinson studied the mast-sail interaction on a 2D model-scale section during his PhD [5] and also in successive research [6], [7]. After these few authors, pressure measurements published.


on sails have rarely been


Thanks to the authors above, CFD codes were tuned and validated, for instance [8], which allowed a deeper understanding of the pressure


importantly, the correlation of the pressure with the velocity field. However, the complexity of


distribution and, very the


©2010: The Royal Institution of Naval Architects


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