Sail Aerodynamics Research Articles

Unsteady Upwind Aerodynamics of a Sailing Yacht

_ Sail aerodynamics has been a researched topic since the start of competitive racing, such as the America's Cup and Whitbread Round the World Race. Recently, with restrictions on the use of wind tunnels and towing tanks in the America's Cup, there has been an increase in computational fluid dynamics (CFD) to study the aerodynamics and performance of yacht sails. These studies typically model steady-state conditions, but in contrast, this paper demonstrates the implementation of dynamic wind conditions in Reynolds Averaged Navier Stokes CFD and the effects it has on the aerodynamics of a yacht sailing upwind. A user-defined function was used to give a velocity profile and vary the wind direction with time to simulate a gust condition with a period of 10 s. In addition to the gusting model, steady models were run for the apparent wind angles (AWA) experienced in the gust to see if the dynamic wind conditions could be modelled by a series of steady state snapshots. The results show minor differences in the streamlines of two models and the pressure distributions show no significant difference. There is only a slight difference in sail forces at high AWAs. Although further research would be required this information could be valuable to anyone using CFD to test sail designs. Keywords CFD; transient; steady; upwind; UDF; RANS

Aerodynamic Analysis of Rigid Wing Sail Based on CFD Simulation for the Design of High-Performance Unmanned Sailboats

The utilization of unmanned sailboats as a burgeoning instrument for ocean exploration and monitoring is steadily rising. In this study, a dual sail configuration is put forth to augment the sailboats’ proficiency in its wind-catching ability and adapt to the harsh environment of the sea. This proposition is based on a thorough investigation of sail aerodynamics. The symmetric rigid wing sails NACA 0020 and NACA 0016 are selected for use as the mainsail and trailing wing sail, respectively, after considering the operational environment of unmanned sailboats. The wing sail structure is modeled using SolidWorks, and computational fluid dynamics (CFD) simulations are conducted using ANSYS Fluent 2022R1 software to evaluate the aerodynamic performance of the sails. Key aerodynamic parameters, including lift, drag, lift coefficient, drag coefficient, and thrust coefficient, are obtained under different angles of attack. Furthermore, the effects of mainsail aspect ratios, mainsail taper ratios, and the positional relationship between the mainsail and trailing sail on performance are analyzed to determine the optimal structure. The thrust provided by the sail to the boat is mainly generated by the decomposition of lift, and the lift coefficient is used to measure the efficiency of an object in generating lift in the air. The proposed sail structure demonstrates a 37.1% improvement in the peak lift coefficient compared to traditional flexible sails and exhibits strong propulsion capability, indicating its potential for widespread application in the marine field.

Numerical simulation of windsurfing sail aerodynamics: Insights into forces and flow characteristics

In windsurfing, the sail is the primary source of propulsive force. Due to the unique shape and structure, the aerodynamic performance of a windsurfing sail is not well understood. This study aims to investigate the aerodynamic performance of a windsurfing sail under various angles of attack and sail heights using steady Reynolds-averaged Navier–Stokes simulations. The simulations were validated with wind tunnel measurements. The study finds that the local maximum lift coefficient and stall angle are different at different sections along the sail height. Further investigations on the pressure distributions and sectional flow fields indicate that the different sectional lift coefficients and stall are significantly influenced by the geometric profiles and orientations of the luff pocket. The sectional geometric profile consists of a reduced camber near the sail tip, which is prone to flow separation. Thus, flow separation first occurred at the sail tip with increasing angle of attack, then extended towards the sail foot. During flow separation, a recirculation region was found on the suction side of the sail, leading to a reduction in lift and increase in drag. The findings of this study provide insights into the aerodynamic forces and flow characteristics of windsurfing sails, which can be applied to improve sail design and performance.

Recent Advances in Numerical and Experimental Downwind Sail Aerodynamics

Abstract. Over the past two decades, the numerical and experimental progresses made in the field of downwind sail aerodynamics have contributed to a new understanding of their behaviour and improved designs. Contemporary advances include the numerical and experimental evidence of the leading-edge vortex, as well as greater correlation between model and full-scale testing. Nevertheless, much remains to be understood on the aerodynamics of downwind sails and their flow structures. In this paper, a detailed review of the different flow features of downwind sails, including the effect of separation bubbles and leading-edge vortices will be discussed. New experimental measurements of the flow field around a highly cambered thin circular arc geometry, representative of a bi-dimensional section of a spinnaker, will also be presented here for the first time. These results allow interpretation of some inconsistent data from past experiments and simulations, and to provide guidance for future model testing and sail design.

Experimental and numerical wind tunnel investigation of the aerodynamics of upwind soft sails

The present paper reports an experimental/numerical study about the aerodynamics of sails in upwind conditions. This work is part of a wider research frame carried out in the recent years at Politecnico di Milano, involving numerical, wind tunnel and full-scale studies on the aero-hydrodynamics of sailing yachts. The experimental apparatus adopted in this work is the scaled-version of the full-scale setup adopted in the Sailing Yacht Laboratory (SYL) at the same institutions. More specifically, soft sails were tested in wind tunnel in close hauled conditions, properly equipped with thin and soft pressure strips at different sections of head and main sail. Therefore, simultaneous data about distributed pressure, aerodynamic forces and 3D flying shapes were gathered. Fine trimming were investigated during the tests to study the different effect on pressure distribution, performance and sail shapes. The measured sails geometry about the same conditions, fed the steady numerical simulations, based both on Vortex Lattice Method and Reynolds Averaged Navier Stokes equations, for comparison and validation. With regard to the latter, a sensitivity analysis on the simulation parameters was also reported in comparison to experimental results. Aim of this work was to improve the state-of-the-art with an extensive set of combined measurements and computations of different nature and objectives in order to provide a holistic view on the upwind sails aerodynamics.

Design of a Foiling Optimist

Because of the successful application of hydrofoils on the America's Cup catamarans in the past two campaigns the interest in foiling sailing craft has boosted. Foils have been fitted to a large number of yachts with great success, ranging from dinghies to ocean racers. An interesting question is whether one of the slowest racing boats in the world, the Optimist dinghy, can foil, and if so, at what minimum wind speed. The present paper presents a comprehensive design campaign to answer the two questions. The campaign includes a newly developed Velocity Prediction Program (VPP) for foiling/non-foiling conditions, a wind tunnel test of sail aerodynamics, a towing tank test of hull hydrodynamics and a large number of numerical predictions of foil characteristics. An optimum foil configuration is developed and towing tank tested with satisfactory results. The final proof of the concept is a successful on the water test with stable foiling at a speed of 12 knots.

Experimental Study of the Aerodynamics of Sail in Natural Wind

Abstract In order to evaluate the impacts of a motor vessel after installing wind sails, the aerodynamics of the sail should be accurately calculated. However most of the research on sails are based on stable wind instead of natural wind which is changing horizontally and vertically. In this paper wind tunnel tests are carried out based on stable wind field and simulated natural wind field, the results shown that there are 16–44% decrease in natural wind in terms of lifting coefficient and 11–42% decrease for drag coefficient. This would provide a valuable reference to the effectiveness evaluation of the impact of sails for sail assisted ships.

CFD prediction of steady and unsteady upwind sail aerodynamics

In this paper, RANS CFD simulations of steady and unsteady upwind sail aerodynamics are verified and validated against wind tunnel experiments previously performed at Politecnico di Milano. Experimental data is presented, and the experimental results are analysed to estimate repeatability. Blockage effects caused by tunnel boundary layers and measurement equipment are estimated using detailed CFD simulations, allowing such effects to be accounted for. A verification procedure is performed, resulting in very low numerical uncertainties. The force predictions are then validated for both steady and unsteady conditions. The steady results show very good agreement, with a maximum error of 3.1%. For the unsteady conditions, good agreement is found for the force mean values, whilst the prediction of force amplitudes is less satisfactory.

Wind Tunnel Investigation of Dynamic Trimming on Upwind Sail Aerodynamics

An experiment was performed in the Yacht Research Unit's Twisted Flow Wind Tunnel (University of Auckland) to test the effect of dynamic trimming on three IMOCA 60 inspired mainsail models in an upwind (apparent wind angle βAW = 60°) unheeled configuration. This study presents dynamic fluid structure interaction results in well controlled conditions (wind, sheet length) with a dynamic trimming system. Trimming oscillations are done around an optimum value of the optimization target coefficient CFobj previously found with a static trim. Different oscillation amplitudes and frequencies of trimming are investigated. Measurements are done with a 6 component force balance and a load sensor giving access to the unsteady mainsail sheet load. The driving force coefficient CFx and CFobj first decrease at low reduced frequency fr for quasi-steady state then increase, becoming higher than the static state situation. CFx and CFobj show an optimum for the three different design sail shapes located at fr = 0.255. This optimum is linked to the power transmitted to the rig and sail system by the trimming device. The effect of the camber of the design shape is also investigated. The flat mainsail design benefits more than the other mainsail designs from the dynamic trimming compared to their respective static situation. This study presents dynamic results that cannot be accurately predicted with a quasi-static approach. These results are therefore valuable for future fluid-structure interaction numerical tools validations in unsteady conditions.

Research on sail aerodynamics performance and sail-assisted ship stability

Through the use of wind energy the goals of energy conservation and emission reduction could be realized for ocean-going ships. In this paper the performance coefficients of a sail model and the optimal sail attack angle are determined based on wind tunnel test results. The stability criteria calculation method of sail-assisted ships is proposed on the basis of current literature covering the stability criteria regulations of ocean-going ships. Calculation method for the stability parameter is introduced to investigate the reasonableness of the sail models. Finally a sail model was made and its stability is demonstrated using the proposed stability criteria calculation method of sail-assisted ships.

Recent Advances in Sailing Yacht Aerodynamics

The analysis of sailing yacht aerodynamics has changed dramatically over the last 15 years and has enabled significant advances in performance prediction. For instance, the growth of computational fluid dynamics has significantly changed the way high-performance sails are designed. Three-dimensional mathematical models of fully rigged sail plans and the visualization of the turbulent unsteady flow pattern around them are now quite common, whereas ten years ago such a simulation would have been very rare and 20 years ago it would have been impossible. The parallel development of optimization techniques has resulted in new sail and yacht design methods. Changes in the experimental techniques have been as dramatic as in the numerical techniques. The introduction of the twisted flow device, the real-time velocity prediction program, and the most recent pressure measurements have allowed a step change in the potentialities of experimental sail aerodynamics. This paper aims to review the recent advances in sail aerodynamics and to highlight potential research areas for future work.

On the uncertainty of CFD in sail aerodynamics

SUMMARYA verification and validation procedure for yacht sail aerodynamics is presented. Guidelines and an example of application are provided. The grid uncertainty for the aerodynamic lift, drag and pressure distributions for the sails is computed. The pressures are validated against experimental measurements, showing that the validation procedure may allow the identification of modelling errors. Lift, drag and L2 norm of the pressures were computed with uncertainties of the order of 1%. Convergence uncertainty and round‐off uncertainty are several orders of magnitude smaller than the grid uncertainty. The uncertainty due to the dimension of the computational domain is computed for a flat plate at incidence and is found to be significant compared with the other uncertainties. Finally, it is shown how the probability that the ranking between different geometries is correct can be estimated knowing the uncertainty in the computation of the value used to rank. Copyright © 2013 John Wiley & Sons, Ltd.

Upwind sail aerodynamics: A RANS numerical investigation validated with wind tunnel pressure measurements

The aerodynamics of a sailing yacht with different sail trims are presented, derived from simulations performed using computational fluid dynamics. A Reynolds-averaged Navier–Stokes approach was used to model sixteen sail trims first tested in a wind tunnel, where the pressure distributions on the sails were measured. An original approach was employed by using two successive simulations: the first one on a large domain to model the blockage due to the wind tunnel walls and the sails model, and a second one on a smaller domain to model the flow around the sails model. A verification and validation of the computed aerodynamic forces and pressure distributions was performed. The computed pressure distribution is shown to agree well with the measured pressures. The sail surface pressure was correlated with the increase of turbulent viscosity in the laminar separation bubble, the flow reattachment and the trailing edge separation. The drive force distribution on both sails showed that the fore part of the genoa (fore sail) provides the majority of the drive force and that the effect of the aft sail is mostly to produce an upwash effect on the genoa. An aerodynamic model based on potential flow theory and a viscous correction is proposed. This model, with one free parameter to be determined, is shown to fit the results better than the usual form drag and induced drag only, even if no friction drag is explicitly considered.

Sail aerodynamics: Understanding pressure distributions on upwind sails

The pressure distributions on upwind sails is discussed and related to the flow field around the headsail and the mast/mainsail. Pressures measured on several horizontal sections of model-scale and full-scale sails are used to provide examples. On the leeward side of the sails, leading edge separation and turbulent reattachment occurs, sometimes followed by trailing edge separation. On the windward side, leading edge separation occurs on the mast/mainsail and, at low angles of attack, it can also occur on the headsail. Differences were found between the leading edge bubbles on the two sails. Pressure trends for different angles of attack are presented, and these can be explained in terms of standard aerodynamic theory, particularly in terms of short and long leading edge separation bubble types. It was found that the pressure distributions measured on mainsails at full- and model-scale showed good agreement on both the windward and leeward sides.

Downwind sail aerodynamics: A CFD investigation with high grid resolution

A computational fluid dynamics (CFD) code was applied to an America's Cup Class Yacht to investigate sailing performance in a downwind configuration. Apparent wind angles at 45°, 105° and 120° are reported, sailed with mainsail and asymmetrical spinnakers. Numerical results are in good agreement with wind tunnel data. A large mesh investigation was performed, ranging from 60,000 elements up to 37 million elements, which shows a converging trend to the experimental values with differences smaller than 3% in both lift and drag. The most commonly used turbulence models in sail applications were tested and the results are presented here in two meshes with 1 million elements and 6.5 millions, respectively. All turbulence models over-estimate forces more than solving the Navier–Stokes system without any additional equations, hence turbulence models do not increase solution accuracy according to these results.

Mathematical models and numerical simulations for the America’s Cup

This paper presents a review of the mathematical models which can be adopted to describe the different physical phenomena characterizing the flow around a sailing yacht. The complete model accounting for laminar–turbulent transition regime, free-surface dynamics and fluid–sails interaction is introduced as long as some simplified models that have been used to reduce the computational complexity. Drawing on the experience of the Ecole Polytechnique Fédérale de Lausanne (EPFL) as Official Scientific Advisor to the Alinghi Team, winner of the 2003 America’s Cup, we discuss the role of Computational Fluid Dynamics simulations based on Reynolds Averages Navier–Stokes (RANS) equations and their integration in standard yacht design process. Numerical results in different areas (appendages design, free-surface flows, aerodynamics of sails) are presented and discussed.

A twisted flow wind tunnel for testing yacht sails

Abstract The paper outlines the requirements for wind tunnel testing model yachts for sail aerodynamics investigations. It is shown that the apparent wind onto a yacht is “twisted” over the mast height, due to the vector addition of the yacht and wind velocities. The design features of a special wind tunnel built in New Zealand which can produce twisted flow in the test section which is suitable for testing model yachts are described. This wind tunnel is basically a conventional open-circuit long test-section boundary-layer wind tunnel, with the addition of some vertical vanes at the outlet. These can be twisted to desired orientations to achieve flow in the model test region which has the velocity and direction varying with height in the required manner. Some commissioning results from one twist configuration are presented which illustrate the success of the method. Some of the drawbacks of this approach which are difficult to overcome are also outlined.

Optimum sail design for small heel and weak wind shear conditions

Aerodynamics of sails in weakly sheared wind has been studied based on vortex mechanics. Assumptions of small disturbance simplify the basic equations, which show that the effective application of the rudder and coupled heeling and yawing motions give directional stability to sailing craft close to wind. Analysis has also revealed how heel-yaw motion and sheared wind affect the optimization of the thrust with constraints. The result has been compared with the present author's previous work that disregards the heel and the wind shear. While the optimum circulation is independent of the wind shear and the heel-yaw motion, the optimum geometry of sails is little affected by the wind shear. Closed-form solutions are given to represent the optimum design.

Flow simulations for wind-tunnel studies of sail aerodynamics

The aerodynamic of yacht sails is described in order to highlight some of the difficulties which arise when wind-tunnel investigations using scale models are carried out. It is shown that exact similitude of full-scale is impossible to achieve in the wind-tunnel due to the complex nature of the full-scale flow and the conflicting requirements of the relevant dimensionless scaling parameters. The main difficulties are that the sails may approximate both streamlined and bluff bodies, depending on whether the yacht is sailing up- or down-wind, and that the relative motion of the yacht and wind introduces a height variation into the apparent wind direction and speed. Various remedies for these difficulties are outlined, together with the important factors related to obtaining useful date from wind-tunnel studies. These factors can be subdivided into test section wind structure, testing procedures and model construction. Recommendations regarding the methods required to achieve the best results are presented.

Balance of Helm and Static Directional Stability of Yachts Sailing Close-Hauled

SummaryAn analysis is presented for equilibrium of yawing moments for a yacht sailing in the displacement mode in smooth water. The geometry of hull form and sail plan are quite unrestricted, although special attention is given throughout to currently popular types.The free-surface flow is simplified by a reflection technique. In cases where the hull form is appropriate, the hull hydrodynamics are treated by lifting-line theory: in other cases slender-body theory is applied. The sail aerodynamics are treated by methods taken from aerofoil theory. Some design charts are given and computation methods are suggested for practical design calculations.Static directional stability is treated theoretically and some general conclusions drawn regarding the influence of various characteristics on stability.Where the theory has been applied, results have agreed with the available experimental data within the limits normally provided for adjusting balance of helm. Changes of balance with angle of heel and static stability have been given by the theory at least qualitatively correctly; experimental results are, however, not available for comparison.