Trans RINA, Vol 152, Part B1, Intl J Small Craft Tech, 2010 Jan-Jun


A COMPARATIVE STUDY BETWEEN WIND-TUNNEL EXPERIMENTS AND RANS SIMULATIONS OF MODERN SQUARE HEADED MAIN SAILS


A B G Quérard and P A Wilson, University of Southampton, UK (DOI No: 10.3940/rina.ijsct.2010.b1.82)


SUMMARY


A commercial code (ANSYS CFX10) which is based upon Reynolds Average Navier Stokes, is used here to compare with wind-tunnel experiments of a modern ORMA60’ rig in an upwind condition. Two mainsails of different tip chord length and a head sail are tested. The flying shapes are acquired by a digital camera to feed the numerical model with the same geometry has used in the experiments. The results of the study underline the need for an extreme accuracy in the acquisition of the flying shapes. It is also noted that modelling the hull in addition to the mast and sails improve the prediction significantly. Presence of a hull tends to tangle the tip vortices generated by the sails’ foot and affect the flow up to the middle of the mast, thereby increasing both lift and drag. The effects of scaling are discussed.


NOMENCLATURE 0


CD = Cl = D


1 2


1 2


1 2


ρSU ² L


ρSU ² 0


cmean : mean chord length, m 0


Cp PP U


ρ = −


P P0 Rn Uc


u* ui


y + = 0 ² : Local pressure, Pa = ν Reynolds Number


: Static pressure, Pa 0 mean


S : Lateral planform area, m² U0


: Axial free-stream velocity, m s-1 : Friction velocity at nearest wall, m s-1 : Velocity components, ms-1


y uy ν


: Distance to nearest wall, m *


ρ : Air density, kg, m-3 ν : Kinematic fluid viscosity of air, m² s-1 τ


: Stress Tensor 1. INTRODUCTION


The design of racing yacht sails follows two main objectives: tending towards an optimal shape and then maintaining this shape


under aerodynamic loading.


Traditionally, wind-tunnels have been used for these purposes. In more recent times potential flow analysis has enabled


estimations of drive force. The first


numerical method for calculating lift and induced drag of sails was performed in 1968 by Milgram [1], [2]. The method involved the representation of the sails by vortex lattices and flat wakes. This was followed in 1989, Greeley et al. [3] who improvement to the


previous method by solving


iteratively the problem with the vortex wakes of the sails convected along the streamlines at each timestep. A further step was made in 1996 by Ramsey [4], by including the aerodynamics of the above-water portion of the hull. To do so, the sails were represented using a similar approach to Greeley et al while the hull was represented by sources panels. Nowadays, inviscid methods are still being used for sail shape optimization in close-hauled conditions.


Since potential flow restricts the fluid to be inviscid and irrotational, this often leads to poor estimates of forces and moments on the sails when vortices or detached flows develop. On the other hand, the trend in modern racing yachts is to have sails with large square heads, as seen


for example on many multihulls, and in the


particular classes IACC or Open60. Linear distribution of twist along the span is a key setting, but to do so, the loading distribution on the square


head has to be


predicted so that reinforcement can be positioned accordingly. Unfortunately, large tip vortices govern the flow on this part of the sail, increasing the inaccuracy of flow predictions obtained using potential flow analysis.


In the mean time, Computational Fluid Dynamics (CFD) tools needed to incorporate viscous effects into design trade-off studies have developed sufficiently to be used within


design cycle turn-around times. Current


techniques are known as Reynolds Averaged Navier Stokes (RANS) solvers, and have seen their first practical application to upwind and downwind sail design during the 30th America’s Cup. Since then, several applications of RANS have been made to the study of sails. In 2D, an example is found in the work of Doyle et al [5], who investigated the sail interactions of the Maltese Falcon, a three masts modern clipper. More recently, Chapin et al [6] combined wind-tunnel experiments with 3D RANS simulations to study the π-sail configuration of the Hydraplaneur, a catamaran designed for offshore speed records with a mast on each hull.


proposed a significant


The present work aims to develop a methodology to study modern square head rigs in close-hauled conditions,


©2010: The Royal Institution of Naval Architects B-1


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