Yacht Sails 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

Performance Rating and Flow Analysis of an Experimental Airborne Drag-Type VAWT Employing Rotating Mesh

This paper presents the results of a performance analysis conducted on an experimental airborne vertical axis wind turbine (VAWT), specifically focusing on the MAGENN Air Rotor System (MARS) project. During its development phase, the company claimed that MARS could generate a power output of 100 kW under wind velocities of 12 m/s. However, no further information or numerical models supporting this claim were found in the literature. Extending our prior conference work, the main objective of our study is to assess the accuracy of the stated rated power output and to develop a comprehensive numerical model to analyze the airflow dynamics around this unique airborne rotor configuration. The innovative design of the solid model, resembling yacht sails, was developed using images in the related web pages and literature, announcing the power coefficient (Cp) as 0.21. In this study, results cover 12 m/s wind and flat terrain wind velocities (3, 5, 6, and 9 m/s) with varying rotational velocities. Through meticulous calculations for the atypical blade design, optimal rotational velocities and an expected Tip Speed Ratio (TSR) of around 1.0 were determined. Introducing the Centroid Speed Ratio (CSR), which is the ratio of the sail blade centroid and the superficial wind velocities for varied wind speeds, the findings indicate an average power generation potential of 90 kW at 1.4 rad/s for 12 m/s and approximately 16 kW at a 300 m altitude for a 6 m/s wind velocity.

Water tunnel testing of downwind yacht sails

Downwind yacht sails, such as spinnakers, are low-aspect-ratio highly cambered wings with a sharp leading edge. They are characterised by substantial three-dimensional flow separation and are thus modelled with difficulty with numerical simulations. Furthermore, accurate full-scale validation data are not available. The first quantitative flow measurements have only recently been achieved in water tunnels. In this study, we aim to provide guidelines on this emerging sail testing methodology. We consider six model-scale rigid models at average-chord-based Reynolds numbers ranging from 5 870 to 61 870. A critical Reynolds number is identified, below which relaminarisation of the reattached boundary layer downstream of the leading edge separation bubble occurs. Both lift and drag increase monotonically at subcritical Reynolds numbers while remaining about constant at transcritical Reynolds numbers. The critical Reynolds number decreases with increasing incidence and is insensitive to the blockage ratio. Spinnakers are normally sailed in front of the mainsail, whose circulation is found to generate an approximately 3∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$3^{\\circ }$$\\end{document} upwash on the spinnaker and higher flow velocity on both sides of it. These findings provide guidelines for the experimental testing of spinnaker-like wings in water tunnels and provide new insights into the flow and experimental testing of highly cambered wings with massive flow separation at low Reynolds numbers.

The Radiocommunication Support Decision System to Use in Distress Situations for Captains of Small Non-conventional Vessels Operating in the Caribbean Sea Area

The paper presents the concept of a Radio Communication System to support the decision of the captain of a small non-conventional vessel including a yacht sailing in the waters of the Caribbean Sea in a dangerous situation requiring a call for RCC assistance. This is important to enhance boating safety in the area, where, due to the reduced and non-overlapping accessible areas of the A1 and A2 communications zones, calling for RCC assistance using common VHF/MF radio equipment is limited. In the event of danger to life and property on a yacht, the ability to contact Marine Rescue Centers is a primary concern. This paper presents a model of an intelligent system for supporting the captain in making the right navigation decision, in a dangerous situation, based on the method of artificial neural networks ANN. The article presents an example of numerical simulation of operation of the discussed system of Radio Communication decision support, assuming a specific position of the small non-conventional vessel in distress, with reference to the existing communication areas A1, A2 in the Caribbean Sea. The result of the simulation is related to the calculation of the distance to the nearest zone, enabling the establishment of communication with the marine Rescue Center to distress call, the value of the new corrected course of the small non-conventional vessel and the time of arrival at the above-mentioned zone.

Vortex flow of downwind sails

This paper sets out to investigate the vortex flow of spinnaker yacht sails, which are low-aspect-ratio highly cambered wings used to sail downwind. We tested three model-scale sails with the same sections but different twists over a range of angles of attack in a water tunnel at a Reynolds number of 21 000. We measured the forces with a balance and the velocity field with particle image velocimetry. The sails experience massively separated three-dimensional flow and leading-edge vortices convect at half of the free-stream velocity in a turbulent shear layer. Despite the massive flow separation, the twist of the sail does not change the lift curve slope, in agreement with strip theory. As the angle of attack and the twist vary, flow reattachment might occur in the time-average sense, but this does not necessarily result in a higher lift to drag ratio as the vorticity field is marginally affected. Finally, we investigated the effect of secondary vorticity, vortex stretching and diffusion on the vorticity fluxes. Overall, these results provide new insights into the vortex flow and associated force generation mechanism of wings with massively separated flow.

Efficient jib-mainsail fluid-structure interaction modelling – Validations with semi-rigid sails experiments

The present work takes place in the framework of the “Jib Sea” project. The main purpose of the project is to develop a new sail design made of articulated composite panels, for large merchant ships. The French laboratory “ENSTA Bretagne, IRDL” is involved in this industrial project for its expertise on fluid-structure interactions modelling. A fast and robust approach to model fluid-structure interactions for yacht sails is presented. Specifically, interaction effects between the jib and the mainsail are considered in the flow model presented. This is achieved using the lifting-line theory combined with a discrete vortex method, involving distributions of lumped-vortex elements along sail sections. The flow model is coupled with a finite element analysis of the structure, using shell elements for the modelling of sail membranes, beam stringers for battens modelling and a quasi-static resolution based on a dynamic backward Euler scheme. The flow model based on the Lifting-Line Theory is presently validated by a commercial software tools using the Unsteady Reynolds-Average Navier-Stokes equations. Numerical comparisons with experiments are conducted on a 50 m2 composite mainsail prototype and a conventional jib, built and hoisted on an onshore balestron rig. Measurements, such as strain gauges or cable tensions, are synchronized with a wind sensor. These data collected together enable both global and local numerical-experimental comparisons for forces and moments, providing a validation of the proposed fluid-structure interactions modelling of ship sails. A good match between experimental and numerical modelling is observed on local comparisons as relative differences are all less than 25% for TWA∈[−20;20] and TWS<10 kn. Global comparison results exhibit validations with experiments for |TWA|<10 deg and TWS<10 kn, where numerical-experimental relative differences are less than 10%.

Improving the Downwind Sail Design Process by Means of a Novel FSI Approach

The process of designing a sail can be a challenging task because of the difficulties in predicting the real aerodynamic performance. This is especially true in the case of downwind sails, where the evaluation of the real shapes and aerodynamic forces can be very complex because of turbulent and detached flows and the high-deformable behavior of structures. Of course, numerical methods are very useful and reliable tools to investigate sail performances, and their use, also as a result of the exponential growth of computational resources at a very low cost, is spreading more and more, even in not highly competitive fields. This paper presents a new methodology to support sail designers in evaluating and optimizing downwind sail performance and manufacturing. A new weakly coupled fluid–structure interaction (FSI) procedure has been developed to study downwind sails. The proposed method is parametric and automated and allows for investigating multiple kinds of sails under different sailing conditions. The study of a gennaker of a small sailing yacht is presented as a case study. Based on the numerical results obtained, an analytical formulation for calculating the sail corner loads has been also proposed. The novel proposed methodology could represent a promising approach to allow for the widespread and effective use of numerical methods in the design and manufacturing of yacht sails.

The scales of the leading-edge separation bubble

The leading-edge separation bubble is a flow feature occurring on the suction side of thin foils as a result of separation at the sharp leading-edge followed by reattachment downstream along the chord. For a flat plate at zero angle of attack, the reattachment length of the bubble depends on the plate thickness t. At a nonzero incidence, instead, the underlying scale governing bubble length is not clear. To investigate, we undertake a critical review of experimental and theoretical studies and develop an analytical formulation to predict the reattachment length of plates both at zero and at small incidences. We focus on conditions where the bubble is turbulent, i.e., when transition occurs at a negligible distance from the point of separation. This occurs at thickness- and chord-based Reynolds numbers Ret≳104, Rec≳105. At angle of attack α = 0, we find that the reattachment length is xR≈4.8t when the chord-to-thickness ratio is c/t&amp;gt;12. At α&amp;gt;0, we find that xR/c=πσα2, where σ≈7.9 is the inverse of the growth rate of a turbulent free shear layer. These results allow estimating xR on the thin wings of, for example, aerial vehicles and yacht sails.

A fast and robust approach to compute nonlinear Fluid-Structure Interactions on yacht sails – Application to a semi-rigid composite mainsail

A numerical method to compute the nonlinear Fluid-Structure Interactions (FSI) of yacht sails is presented in this paper. The inviscid lifting-line flow model is used by including a quadrature method to efficiently compute sail pressure loads. The structural calculation is performed with a quasi-static resolution by using a dynamic backward Euler scheme, in order to improve the computation convergence. A specific thickness sail approach is also proposed to make the FSI solving easier. The assembly of these flow and structural methods leads to a fast and robust strategy to compute nonlinear FSI on yacht sails, and the proposed approach is applied on a complex semi-rigid composite mainsail.

Mobile Geographic Visualization Service for Marine Safety Information based on Geo-Location Based Augmented Reality

Abstract. Marine leisure sports activities such as sea fishing, skin scuba diving and yacht sailing are affected by sea conditions such as waves and tides, and weather conditions, as well as by marine traffic. Timely provision of such high accuracy maritime information is very important because that affects the safety of people engaged in leisure activities. Safety information for marine leisure activities includes obstacle information such as the position of other ships and sea reefs and rocks in the vicinity, weather conditions. If marine leisure activists can identify and use various safety-related information around them, it will be a great help to carry out safe marine leisure activities. The geographic visualization environment provides users with the ability to intuitively and easily convey complex information. Therefore, mobile location-based services based on geographical visualization utilizing complex maritime safety information have the advantage of effectively providing the information necessary for people engaged in safe marine leisure activities.In this study, we developed a location-based mobile application based on geographic visualization that support safe marine leisure activities by integrating ship navigation information, marine chart(map), and ocean weather information. The mobile geographical visualization service has a function to effectively integrate various type of data related to marine safety and transmit the fused information to users in order to support safe leisure activities. First, location of ships around the user's location is transmitted from AIS server and displayed on the map. Since the ship type, speed, and direction information are displayed on the map, the user can check the position of the ships operating in the vicinity. Second, since the ocean weather information such as wave, current, temperature, wind speed is received from the server and displayed on the map, the safety can be grasped by using the ocean condition and weather information in real time. The integrated marine safety information is visualized in mobile devicess in two ways. The first method is in which graphs, icons, and images are displayed on a two-dimensional marine chart(map). The second scheme utilizes a location based Augmented Reality(AR) technique. If the user directs the camera of the mobile device to a certain direction or point, safety information of the point can be expressed on the device. For example, when the user directs the camera of the mobile phone to a ship in the vicinity, information on the speed, moving direction, and type of the ship can be displayed on the mobile phone.The contributions of this study are as follows. First, we proposed a novel method to visualize various location-based maritime safety information on mobile devices. Map information of marine charts, weather information expressed in graphic format, and vessel information represented by icons are effectively fused and visualized on the limited display of mobile devices. Second, information transmission channel is diversified by integrating marine safety information by 2D maps and location based AR. The user can utilize either of the 2D maps or 3D AR by the purpose and the kind of the safety information. To build an environment that can consistently convey the same marine safety information in 2D maps and 3D AR remains as a challenge to extend the application of this study for the future.

FSI analysis on the sail performance of a yacht with rig deformation

Thin fabric-based yacht sails have a cambered shape to generate lift force; however, their shape can be easily deformed by wind pressure and also affected by the deformation of the mast. These deformations can change the airflow characteristics over the sail. Therefore, Fluid-Structure Interaction (FSI) analysis is needed to evaluate the sail force precisely. In this study, airflow over the deformed sail and rig was studied using FSI. Elastic deformation of the sail and rig was obtained by an aerodynamic calculation under dynamic pressure loading on the sail surface. The effects of rig deformation on the aerodynamic performance of the sail were examined according to the rig type and mast flexibilities. As a result, the changes of lift force for a fractional type rig with a thin mast section were more significant than with a masthead rig.

The leading-edge vortex of yacht sails

It has been suggested that a stable Leading Edge Vortex (LEV) can be formed from the sharp leading edge of asymmetric spinnakers, which are high-lift sails used by yachts to sail downwind. If the LEV remains stably attached to the leading edge, it provides an increase in the thrust force. Until now, however, the existence of a stable and attached LEV has only been shown by numerical simulations. In the present work we experimentally verify, for the first time, that a stable LEV can be formed on an asymmetric spinnaker. We tested a 3D printed rigid sail in a water flume at a chord-based Reynolds number of ca. 104. The sail was tested in isolation without hull and rigging. The flow field was measured with Particle Image Velocimetry (PIV) over horizontal cross sections. We found that on the leeward side of the sail (the suction side), the flow separates at the leading edge reattaching further downstream and forming a stable LEV. The LEV grows in diameter from the root to the tip of the sail, where it merges with the tip vortex. We detected the LEV using the γ criterion, and we verified its stability over time. The lift contribution provided by the LEV was computed solving a complex potential model of each sail section. This analysis indicated that the LEV provides more than 20% of the total sail's lift. These findings suggest that the maximum lift of low-aspect-ratio wings with a sharp leading edge, such as spinnakers, can be enhanced by promoting the formation of a stable LEV.

Pressure Measurements on Yacht Sails: Development of a New System For Wind Tunnel and Full Scale Testing

The paper presents an overview of a joint project including Politecnico di Milano, CSEM and North Sails, aiming at developing a new sail pressure measurement system based on MEMS sensors (an excellent compromise between size, performance, costs and operational conditions) and pressure strips and pads technology. These devices were designed and produced to give differential measurement between the leeward and windward side of the sails. The research has been developed for the final employment on the Lecco Innovation Hub Sailing Yacht Lab, a 10 m length sailing dynamometer which aims at being the reference contemporary full scale measurement device in the sailing yacht engineering research field, to enhance the insight of sail steady and unsteady aerodynamics. The pressure system is described in details, the data acquisition process and system metrological validation are reported; furthermore, some results obtained during a wind tunnel campaign carried out at Politecnico di Milano Wind Tunnel, as a benchmark of the whole measuring system for future full scale application, are reported and discussed in details. Moreover, the system configuration for full scale testing, which is being finalized at the time of this paper, is also described

Modelling leading edge separation on a flat plate and yacht sails using LES

This work presents an investigation into the aerodynamics of upwind sailing using different methods of modelling turbulence, comparing Large Eddy Simulation (LES) and Reynolds-Averaged Navier-Stokes (RANS) methods. A preliminary simulation of a flat plate at an angle of attack was performed in order to investigate the potential for LES in modelling flows around a sharp leading edge. Comparison with existing experimental data highlighted the formation of a double recirculation bubble structure at the leading edge that only LES was able to predict. However, a fine mesh and small timestep are required to correctly reproduce the pressure distribution on the flat plate. The RANS and LES models were subsequently used to model an upwind sail geometry tested in the wind tunnel. The CFD results showed good agreement with the experimental data and in particular highlighted the superior performance of the LES model in predicting the leading edge recirculation structure compared to RANS. The solution was sensitive to the mesh and timestep used. A mesh size of order y+∼1 and a Courant number ≤ 1 provided results with RMS errors less than 0.1 between the simulation and the experimental values of the coefficients of pressure at the top of the mainsail.

A THREE-DIMENSIONAL INVERSE METHOD FOR THE DESIGN OF SAILS

This paper investigates an inverse process for the design of yacht sails. The method is described and then applied to the design of optimal sails for a specific yacht. The proposed inverse method generates the three-dimensional shapes of a headsail and mainsail from prescribed loading (i.e. differential pressure) distributions, accounts for the effect of the sea surface, and also simulates the twist and shear of the incoming flow. The uncoupled iterative routine solves a sequence of analysis steps so that the sail shapes are deformed in such a way that their updated loading distributions converge to the specified target distributions. During each iteration equations derived from two-dimensional Thin Aerofoil Theory, calculate a geometry correction from the difference between the current and target loading distributions. This correction is applied to the sail geometry, and a vortex lattice method code calculates the updated three-dimensional differential pressure distributions, which are again compared to the target distributions. Usually only five iterations are required to converge to sail shapes that have the target loading distributions. The inverse method has been validated by inverting the traditional way of analysing sails, i.e. a set of sails with known geometry has been analysed and the loading distributions on the headsail and mainsail were calculated. These distributions were then used as an input for the inverse code. It was found that the difference in camber between the original sails and the calculated geometry is less than 0.01% of camber at the mid-span of the sails. The second part of the paper presents two methods for the design of optimal sails for a yacht. One of the methods uses the more traditional analysis approach, while the other employs the inverse method described in this paper. The optimisation is performed for a Transpac 52 yacht in 12 knots (6.5 m/s) of true wind speed to obtain the best velocity made good. Results from both methods are presented and discussed and it is found that the results in terms of boat speed are similar although the trims differ slightly. However, the new inverse method is approximately nine times faster than the traditional analysis approach.

Optimization of a thruster sections by Prandtl's theory

Wind power is now the world's fastest growing energy source. It is well known that energy production depends on the shape of the thruster (sail of yacht or a blade) and on the wind characteristics on a given site. The objective of this study is to maximize the performances of a thruster by optimizing the most important aerodynamic design parameters.One of the most important challenges in aerodynamics is to accurately predict the forces acting on the thruster, where the thruster is modeled by different approaches such as the lifting line theory, where the application of the vortex filament method for the thruster performances is used. To have an optimal thruster, the elliptical lift distribution is the key design principle for the airfoils forming the thruster sections. The main purpose is to optimize the aerodynamic design of a thruster to increase its performances under wind action. The optimization carried out with computer codes for the design and analysis of a thruster, based on the non linear lifting line theory, shows that an optimal thruster is to be formed by airfoils placed along the span in a decreasing law of thickness.

D-4 風洞実験によるウィングセールの各種パラメータに関する空気力学的研究(流体特性)

Following the 33rd America's Cup which featured a trimaran versus a catamaran, and the recent 34th America's Cup in 2013 featuring AC72 catamarans with multi-element wing sail yachts sailing at unprecendented speeds, interest in wing sail technology has increased substantially. Unfortunately there is currently very little open peer-reviewed literature available with a focus on multi-element wing design for yachts. The limited available literature focuses primarily on the structures of wings and their control, rather than on the aerodynamic design. While there is substantial available literature on the aerodynamic properties of aircrafy wings, the differences in the flow domains between aeroplanes and yachts is significant. A yacht sail will operate in a Reynolds number range of 0.2 to 8 million while aircraft operate regularly in excess of 10 million. Furthermore, yachts operate in the turbulent atmospheric boundary layer and require high maximum lift coefficients at many apparent wind angles, and minimizing drag is not so critical. This paper reviews the literature on wing sail design for high performance yachts and discusses the results of wind tunnel testing. Two wings with different symmetrical profiles have been tested at low Reynolds number with surface pressure measurements to measure the effect of gap geometry, angle of attack and camber on a wing sail's performance characteristic. It has been found that for the two element wing studied, the gap size and pivot point of the rear element have only a weak influence on the lift and drag coefficients. Reynolds number has a strong effect on separation for highly cambered foils.

Numerical study of a flexible sail plan submitted to pitching: Hysteresis phenomenon and effect of rig adjustments

A numerical investigation of the dynamic fluid structure interaction (FSI) of a yacht sail plan submitted to harmonic pitching is presented to analyse the system׳s dynamic behaviour and the effects of motion simplifications and rigging adjustments on aerodynamic forces. It is shown that the dynamic behaviour of a sail plan subject to yacht motion clearly deviates from the quasi-steady theory. The aerodynamic forces presented as a function of the instantaneous apparent wind angle show hysteresis loops. It is shown that the hysteresis phenomenon dissipates some energy and that the dissipated energy increases strongly with the pitching reduced frequency and amplitude. The effect of reducing the real pitching motion to a simpler surge motion is investigated. Results show significant discrepancies with underestimated aerodynamic forces and no more hysteresis when a surge motion is considered. However, the superposition assumption consisting in a decomposition of the surge into two translations normal and collinear to the apparent wind is verified. Then, simulations with different dock tunes and backstay loads highlight the importance of rig adjustments on the aerodynamic forces and the dynamic behaviour of a sail plan. The energy dissipated by the hysteresis is higher for looser shrouds and a tighter backstay.

Numerical Simulation Analysis for Influence of Lateral Inclination Angle on the Yacht Resistance

The article analyses the yacht navigation attitude in the navigation process based on ЦАГИ method. It solves RANS equations in the CFD softwareFluent based on VOF method, combined with RNG k model. It calculates numerical simulation on four yachts which have same dimension but different lateral inclination angle and analyses the flow field, then verify the relationship between the yacht sailing resistance and the changes of lateral inclination angle.

Fluid-structure interaction analysis of deformation of sail of 30-foot yacht

Most yacht sails are made of thin fabric, and they have a cambered shape to generate lift force; however, their shape can be easily deformed by wind pressure. Deformation of the sail shape changes the flow characteristics over the sail, which in turn further deforms the sail shape. Therefore, fluid-structure interaction (FSI) analysis is applied for the precise evaluation or optimization of the sail design. In this study, fluid flow analyses are performed for the main sail of a 30-foot yacht, and the results are applied to loading conditions for structural analyses. By applying the supporting forces from the rig, such as the mast and boom-end outhaul, as boundary conditions for structural analysis, the deformed sail shape is identified. Both the flow analyses and the structural analyses are iteratively carried out for the deformed sail shape. A comparison of the flow characteristics and surface pressures over the deformed sail shape with those over the initial shape shows that a considerable difference exists between the two and that FSI analysis is suitable for application to sail design.