5th High Performance Yacht Design Conference Auckland, 10-12 March, 2015

EVALUATION OF MULTI-ELEMENT WING SAIL AERODYNAMICS FROM TWO-DIMENSIONAL WIND TUNNEL INVESTIGATIONS

Alexander W. Blakeley1, abla089@aucklanduni.ac.nz Richard G.J. Flay2, r.flay@auckland.ac.nz

Hiroyuki Furukawa3, furukawa@meijo-u.ac.jp Peter J. Richards4, pj.richards@auckland.ac.nz

Abstract. 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 unprecedented 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 aircraft 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 minimising 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 at the Yacht Research Unit at the University of Auckland. Two wings with different symmetrical profiles have been tested at low Reynolds numbers 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.

1 PhD Student, Yacht Research Unit, Department of Mechanical Engineering, University of Auckland, NZ 2 Professor, Director Yacht Research Unit, Department of Mechanical Engineering, University of Auckland, NZ 3 Associate Professor, Department of Mechanical Engineering, Meijo University, Japan 4 Associate Professor, Department of Mechanical Engineering, University of Auckland, NZ

NOMENCLATURE AWA Apparent wind angle c Chord Cl Lift coefficient Cd Drag coefficient D Drag on wing and any sails g Gap L Lift on wing and any sails R Resistance of hull and appendages S Side force on hull and appendages TWA True wind angle V

A Apparent wind velocity V

mg Velocity made good V

S Yacht velocity V

T True wind velocity Angle of attack of front element Angle between rear and front elements Apparent wind angle True wind angle

A Aerodynamic drag angle

H Hydrodynamic drag angle Leeway angle

1. INTRODUCTION

Following the 33rd America's Cup in 2010 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 unprecedented 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. Hence a wind tunnel study of multi-element wing sails was carried out by the University of Auckland. Some preliminary results of this research are available in [1]. Solid wing sails offer several advantages over flexible sails for high speed sailing. One advantage is that the wing has an internal structure which is used to give the wing its shape. The shape is not dependent on the tension in the lines at the sail corners. The shape of a flexible sail cloth mainsail is highly dependent on the tension in the mainsheet, particular the vertical component which is required to keep the leech from twisting excessively. In a wing sail, this vertical component is eliminated completely from the mainsheet, which is used simply to alter the angle of attack using a horizontal force. The twist in the solid sail is controlled by internal control lines which are under considerably less tension than a conventional mainsheet. Thus the angle of attack can be changed relatively easily and quickly, and the power of the sail can be controlled much more easily than in a conventional soft sail. This makes sailing a high performance catamaran much easier with a wing sail than a soft flexible sail.

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Multi-hull yachts have considerably less hydrodynamic drag than mono-hulls since they have much lower displacements than monohulls of similar length. This is because they derive their righting moment from their wide beam, and not from having a heavy keel. Furthermore they heel only a small amount. A dangerous feature of multi-hulls is that once the overturning moment exceeds their maximum righting moment, they will overturn unless the overturning moment is reduced quickly. When coupled with a high performing solid sail or wing, this means that they are able to sail with small apparent wind angles both upwind and downwind as illustrated in Error! Reference source not found.. It was shown by Lanchester [2] that the minimum angle (A) at which a boat can shape its course relative to the wind is the sum of the under and above water drag angles, namely H and A. A wing sail operating at low angles of attack has a low aerodynamic drag angle, and so the result of this is that high performance multi-hulls with wing sails operate at low apparent wind angles in both upwind and downwind sailing. These angles are illustrated in Error! Reference source not found.. This means that it is only necessary to have aerodynamic data for wing sails for low angles of attack.

Figure 1 Velocity triangles for up and downwind sailing showing the low apparent wind angle A which occurs for fast multi-hulls with wing sails.

Understanding the performance of multi-hulls can be enhanced by analysing a free-body diagram of the yacht and considering the forces in the horizontal plane. This is close to reality, since they do not heel much. The aerodynamic and hydrodynamic forces on a catamaran are shown in Error! Reference source not found.. Sail forces are lift (L) and (D) being normal and parallel to the apparent wind direction, and hull/appendage forces are sideforce (S) and resistance (R) which are normal and parallel to the yacht’s course through the water. There may be a small leeway angle (). Note that for equilibrium, the hydrodynamic and aerodynamic forces in the plane of the water must be equal and opposite, so

. Furthermore, as noted above, . Reducing D and R lead to reductions in

and and thus to a reduction in AWA, which will lead

to increased velocity made good (Vmg) to the top or bottom mark.

Error! Reference source not found.Figure 2 Sketch of horizontal forces acting on a catamaran showing the important directions.

Ignoring the structural aspects and considering primarily the aerodynamic aspects, high speed multi-hull yachts with wing sails need wings that are symmetrical so that they can tack and gybe at will. They need to be able to develop high lift and have low drag. They need to be able to twist so that the height of the thrust can be lowered when required to reduce the overturning moment. From observations of existing vessels, it is apparent that severe twist is important so that the direction of the lift force can be reversed at the top of the wing to provide a righting moment. Generally it is apparent from the 2013 America’s Cup yachts that for a two-element wing, the chords of the main wing and flap are usually of similar length, and the gap between them is relatively small. The first element is thick so that it can take the required structural loads, while the flap is thin. Typical thickness ratios are 25% and 9% for the first and second elements respectively.

2. SOME THEORETICAL CONSIDERATIONS

By applying the sine rule to the velocity triangles in Figure 1 it is evident that , so (1)

(2)

The velocity made good is given by

(3)

Thus to improve performance (i.e. increase the ratios

and ) at a fixed heading (i.e. fixed ) it is necessary to

reduce . Figure 3 shows curves of the ratio for

constant and for various , which are evidently circular arcs. Figure 3 shows clearly how smaller

VT

VA

VS

Upwind

VT

VS

VA

A

A

Downwind

T

T

Vmg

Vmg

S

RD

L A

H

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gives more speed. Maximum yacht speed occurs when and it is the diameter of the circule. The

maximum velocity made good is determined from the horizontal tangents to the circle, and occurs for the

angles and for upwind and

downwind respectively. Thus the importance of being able to determine and minimise the aerodynamic drag angle has been demonstrated. Wing performance between candidate options can therefore be compared on the basis of the aerodynamic drag angle as it influences directly.

‐2

‐1.5

‐1

‐0.5

0

0.5

1

0 0.5 1 1.5 2 2.5

bA =  25 deg

bA =  45 deg

bA =  90 deg

bA =  150 deg

VS

VA

VT

Figure 3 Velocity triangles (speed polars) for fixed values of A for a range of T.

3. LITERATURE REVIEW

The limited available literature on wing sails focuses primarily on their structure and control, rather than on their aerodynamic design. While there is substantial available literature on the aerodynamic properties of aircraft 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. A brief review of some of the literature on wing sails is presented below. Elkaim [3] at the University of California undertook a large amount of work on wing sails while involved in the Atlantis Project, which focused on designing an Autonomous Marine Surface Vehicle for a wide range of applications. A rigid wing sail was used on this vehicle because it was regarded as more aerodynamically efficient than a soft sail, by pivoting it near the centre of

pressure it required less force to actuate than a regular soft sail, and it could be designed to be self-trimming. Marchaj [4] reviewed some research on wing sails, pointing out their advantages in some situations. In discussing the 1976 C-Class race between Aquarius V with a soft sail, and Miss Nylex with an advanced wing sail, he pointed out the advantage of the soft sail with the lighter rig in light conditions, compared to the more aerodynamically efficient but heavier wing sail rig. Some wing sails have been made of double-surface soft sails, and are a good idea for cruising as they can be reefed and dropped when wind conditions are unfavourable, e.g. the Wally and Omer wing sail design [5]. They do not have all the benefits of rigid wing sails, e.g. the leech tension needs to be maintained, requiring a sophisticated boom and powerful mainsheet. C-Class catamaran racing has often produced very innovative ideas regarding wing sails. Examples are the slot gap on the yacht Cogito, described by MacLane [6], which could be changed onshore, but was fixed once racing. Killing and Clarke [7] describe further enhancements of C-Class wings for the yachts Alpha and Rocker, which had more advanced cross-sectional shapes than Cogito which were developed using software developed by Drela at MIT [8,9]. There are maximum heeling and pitching moments that can be applied to catamarans before they tip over. Thus in strong wind conditions it is advantageous to lower the height of the line of action of the side and thrust forces. Thus it can be shown that the optimum lift distribution for a catamaran is such that it changes sign at the top of the wing, requiring large amounts of wing twist [10]. Wing sails can be designed with mechanisms for producing such large amounts of twist, which is impossible for soft sails, and so this is another significant difference between wing and soft sails. One of the commonest areas of wing sail use is for high-speed land yacht sailing, more specifically ice yachts. These yachts skate on ice and are sometimes powered by large wing sails. They are capable of very high speeds. The Greenbird project is arguably one of the most successful versions of wind powered land/ice yachts. On land, the Greenbird holds the land speed record of 126.2 mph, while on ice, they hope to beat the current record of 84 mph by setting a speed equal or higher than the 126.2mph that they set on land [11]. Recently relatively small yachts have been experimenting with wing sails. The X-Wing sail project [12] is a 2-element wing designed for small sailing dinghies, specifically the “Sunfish” class. The designer was inspired by the 2010 America’s Cup and has designed a low-cost wing for those wanting to experiment with the technology.

A

T

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The literature cited above shows that work on wing sail development has taken place for a variety of reasons, but mainly to obtain high speeds or to improve control, and that developments have occurred in both small as well as larger craft. However, it is evident that there is a paucity of multi-element wing aerofoil data obtained specifically for yacht sail design. This paper discusses the results of wind tunnel testing at the Yacht Research Unit at the University of Auckland to help provide this information.

4. CFD OPTIONS

To analyse the flow around upwind sails, the potential flow assumption and the panel method are basic methods, but they cannot model viscous effects which are dominant in locations of flow separation. The Navier-Stokes equations can be solved directly (DNS) in cases with flow separation phenomena. When the Reynolds number is high and the flow becomes turbulent, it is difficult to use DNS because it needs an enormous number of grid points. The turbulence must be modelled with turbulence or sub-grid models. Reynolds Average Navier Stokes (RANS) turbulence models obtain a stationary solution that is representative of the mean flow. They are robust and easy to use, but cannot be adopted to unsteady flow. Large-eddy simulation (LES) is used to model the larger, three-dimensional unsteady turbulent motions. In LES, the smaller scale motions are filtered and subsequently modelled. LES can be expected to be more accurate and reliable than RANS models for flows where large-scale unsteadiness is significant. The program MSES [8] is a coupled viscid/inviscid Euler method. It solves the Euler equations on a discrete two dimensional grid, coupled with an integral boundary layer formation. MSES was created to analyse the flow around an aerofoil more accurately than inviscid codes by incorporating viscous effects and a boundary layer formation into its solutions. Therefore it theoretically offers more realistic results than a vortex lattice method (VLM), although it possibly requires more computational power. A derivation of MSES is the MISES [9] code, which has been developed to determine the flow around multi-element aerofoils. While CFD analysis of wing sails is highly advanced and capable of accurate predictions, it is not easy to predict sail performance if there is flow separation. As mentioned in the present paper, it is important for designers of wing sails to know where flow separation occurs, and physical testing such as wind tunnel testing can help in this regard. Wind tunnel testing can also give good information for checking the output from CFD codes, although they may be unable to give results for the very high Reynolds numbers appropriate for large yachts sailing very fast, e.g. America’s Cup AC72 yachts. However, the Reynolds numbers from wind tunnels would be commensurate to those for smaller C-class yacht wing sails.

CFD analysis of wing sails is not considered further in the present paper.

5. EXPERIMENTAL SETUP

5.1 Wind Tunnel

The experiments were performed in the University of Auckland open-return wind tunnel. The wind tunnel has been specifically designed for testing yacht sails and has a standard operating cross section of 7 m wide by 3.5 m high. The flow is produced by two 3-m diameter 4-bladed fans and then driven through a 1 m thick honeycomb screen and two tight mesh screens to remove swirl and to give a uniform velocity profile and relatively low turbulence flow. Permanent pitot-static probes set up well upstream of the wing recorded the dynamic and static pressures whilst another probe measured the atmospheric pressure outside the working section. The wind tunnel walls were brought inwards to give a 2.5m wide by 3.5m high test section and the wing models were located near the outlet of the nozzle as shown in Figure 4.

Figure 4 Photograph of wing model in wind tunnel showing base of support struts and endplate marking for angle fixing.

5.2 Multi-element Wing Construction

The multi-element wing used in the wind tunnel comprised a NACA 0025 main element and a NACA 0009 aft element with a 50:50 chord ratio. The wing spans 2.5 m across the width of the wind tunnel and has a chord length of 1 m (with zero aft element deflection and no gap). 20 mm thick MDF boards at each end of the model contained the placement locations for the elements and tested configurations. Once attached to the wind tunnel walls the model can be rotated and then securely fixed to achieve various angles of attack. By constructing the model in this fashion the trailing vortices as experienced by a finite spanned wing are virtually eliminated.

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Each element has 64 independent pressure taps which measure pressure differentials through 1 mm diameter holes placed around the centreline of the windward and leeward surface of each wing, sufficiently far away from the wind tunnel side-wall boundary layers. They are dispersed from the leading to the trailing edge with a higher concentration around the leading edges of the elements. The pressure taps can be seen in Figure 5. They are slightly off-set from the stream-wise direction to avoid interference from wakes of upstream holes interfering with downstream taps. Tubes connecting to each individual pressure tap feed out an end of each wing to a box containing pressure transducers. The pressures were logged at a rate of 200 Hz over a 30-s period. The pressure transducer box was located downstream of the wing out of the free-stream.

Figure 5 Close up photograph of leading edge of a wing model showing the 1 mm diameter pressure tap holes.

The pressure measurements, along with the locations of the pressure taps can be used to obtain a pressure distribution across the surface of each element. Aerodynamic forces result from these pressure distributions acting over the surface of the wings. By integrating the pressure distributions in the vertical and horizontal directions, the vertical and horizontal forces can be computed, respectively. Since pressure acts normal to a surface, the surface curvature of the aerofoil between two points should not be neglected. However, the pressure taps were distributed closely together around regions of high curvature on the wing elements and therefore a straight line approximation of the surface between any two pressure taps was regarded as sufficiently accurate for the present work. The integrated x and y forces were firstly resolved into directions relative to the chord of the aerofoil. Therefore their resultants were then decomposed appropriately to find the lift and drag relative to the free-stream velocity. Finally non-dimensional representations of the coefficients of pressure (Cp), lift (Cl), and drag (Cd) were computed.

5.3 Experimentation Variables

The variables altered during the wind tunnel testing were as follows:

• Front wing model angle of attack, ��relative to main element chord

• Aft element chord-line deflection, �, relative to main element chord

• Gap between main and aft element, g, defined at zero aft deflection angle �, as a % of the main wing chord

• Aft element pivot point position, as a % of the main wing chord.

• Free stream flow velocity, V.

Figure 6 shows these variables with respect to the model. It also defines the pivot location for the aft element and defines the gap between the foils.

Figure 6 Wing variables altered in experiment.

Reynolds number Re is defined by the reference length 2c (=1.0 m), the reference velocity V and the kinematic viscosity for air.

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6. RESULTS AND DISCUSSION

6.1 Pressure coefficients

Figure 7 Example of pressure coefficient distribution. = 10°, g = 1%, = 2°, Re = 800,000.

The shapes of the main and aft elements are NACA0025 and NACA0009, respectively [13]. The chord of the main element c1 is 500 mm, and it is same as the chord of the aft element c2. The total wing chord c = c1 + c2 = 1 m. Figure 7 shows an example of the pressure coefficient variation over the foils. In Figure 7 the deflection angle of the aft element is 10°, the gap between main and aft elements is 1% referenced to the main element chord c1, the angle of attack, is 2° and Reynolds number based on the total wing chord c as the reference length is 800,000. The horizontal axis of the graph indicates the dimensionless distance along the chord direction. The vertical axis shows the pressure coefficient. The position x/c = 0 corresponds to the leading edge of the main element, and x/c = 0.5 indicates the trailing edge of the main element and the leading edge of the aft element. The vertical axis has been inverted with negative values at the top. Thus the red line indicates the pressure coefficient at the top of the elements, and those along the bottom are shown by the blue line. Like ordinary wings, the pressure at the top is negative, and lift occurs due to the pressure difference between the top and bottom of elements. The distance between the lines is indicative of the normal force acting on each element.

6.2 The influence of angle of attack

Figure 8 Example of stall. = 10°, g = 1%, Re = 700,000.

Figure 8 shows the distributions of the lift coefficient Cl

and the drag coefficient Cd versus angles of attack Cl increases almost linearly with increase in up to 10° and Cd also increases a little. Cl reaches a maximum value at around = 10°. At larger angles of attack the wing stalls and Cl decreases sharply and Cd increases rapidly, as expected.

6.3 The influence of aft element deflection

Figure 9 Cl and Cd versus for varying aft element deflection. g = 2%, Re = 700,000.

= 0

= 0

= 15

= 15

= 30

= 30

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Figure 10 Pressure coefficient distribution for varying . g = 2%, Re = 700,000, = -1°.

Figure 9 shows Cl and Cd versus for varying aft element deflections. Deflecting the aft element downwards increases Cl substantially, although the values of Cl for = 15° and 30° are almost the same. The gradients for each are nearly identical. The stall angle for at high aft element deflections (15°) is lower than that at = 0°. The drag coefficients are substantially higher when the aft element is deflected at angles of 15° and 30° compared to = 0°. It can be seen that increasing the flap angle from = 15° to = 30° produces almost no increase in lift and a significant increase in the drag. To show why the lift is so low for = 30°, Figure 10 compares the pressure distributions for three different flap deflection angles for = -1°. At = 30°, the pressure coefficients are smaller than for = 0° and 15°, and the difference between the values at the upper and lower surface is small. In particular, it can be seen that the large aft element deflection of = 30° produces virtually no pressure difference across it, as well as virtually no pressure difference across the front element as well.

Figure 11 Cl and Cd versus for varying aft element deflection. g = 2%, Re = 300,000, pivot = 80%.

Figure 11 shows another example when the cambers are varied. Cl at ° is larger than that at ° over whole rangebefore the flow stalls. Cl at ° is larger than that at ° when is small, but the slope of Cl-a curve for ° is lower than the other curves, and Cl at ° becomes larger beyond °This means Cl has a local maximum at a certain camber. 6.4 The influence of gap size Figure 12 shows a comparison of wing performance for different gap sizes. As can be seen in the figure, the distributions of Cl and Cd for each gap are almost the same and the only small difference is that g = 1% gives a slightly higher Cl at = 10°. In other tests in this study it has been observed that the influence of gap size is very small.

= 25 = 30 = 15 = 0

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Figure 12 Cl and Cd versus for varying gap size for = 10° and Re = 700,000.

Figure 13 Cl and Cd versus for varying gap size for = 10°, Re = 300,000.

Figure 13 shows the effect of varying gap size at a smaller Reynolds number. While the flow stalls at ° in Figure 12, Cl begins to decrease at a smaller in Figure 13. Thus a smaller Reynolds number causes stall to occur at a smaller

Figure 14 Cl and Cd versus for varying gap size for negative gaps, = 15°, pivot = 90%, Re = 700,000.

Figure 14 shows lift and drag coefficient result when the gap is negative, i.e. the main and aft elements are partially overlapping. Note that this is only physically possible for non-zero values of . While Cd at g = 0 and at g = -2% are amost identical, Cl at g = 0 is larger than that for g = -2% near the angle where the flow stalls. Thus negative gaps do not seem to be beneficial.

6.5 The influence of aft element pivot point position

Figure 15 Cl and Cd versus for varying pivot points for = 10°, g = 0%, Re = 700,000.

The pivot point is the location about which the aft element rotates as shown in Figure 6. It is defined by the distance aft of the leading edge of the main wing in terms of % main element chord. Thus for example, when the pivot point is 0%, it means that the pivot point is at the leading edge of the main element, and 100% means that

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pivot point is at the trailing edge of the main element. Figure 15 shows results for various pivot points. g and Re are all constant. It can be seen that Cl at pivot = 40% is slightly larger than other results, but otherwise there is little obvious effect, even for the very large changes of position from 40% to 90%.

Figure 16 Cl and Cd versus for varying pivot points for = 10°, g = 2%, Re = 300,000.

Figure 16 shows the results when Re is smaller (300,000). It can be seen that Cl and Cd are almost identical, but that the flow separates at smaller when the pivot point is 40% compared to 90%.

6.6 The influence of Reynolds number

Figure 17 Pressure coefficient variation over the elements for two different Re, for = 10°, g = 1%, = 2°.

Figure 17 shows a comparison of the Cp variation when Re is varied. For Re = 800,000, the pressure coefficient curve is smooth. On the other hand, when Re is 200,000, the pressure coefficient has a step-like distribution at x/c = 0.18. The authors speculate that this step-like distribution at this low Reynolds number may be a laminar separation bubble. Furthermore, there may also be a laminar separation bubble on the pressure surface at x/c = 0.4. When the Reynolds number is larger, flow separation does not occur in these results and the curves are smooth. The pressure coefficient distributions on the aft elements are almost the same for both Reynolds numbers.

Figure 18 Cl and Cd versus Re. = 10°, = 2°, g = 1%.

Figure 19 Cl and Cd versus Re. = 10°, = 10°, g = 1%

Figure 18 and Figure 19 show Cl and Cd versus Reynolds number. In many cases in this study, Cl and Cd do not change significantly when the Reynolds number is varied as shown in Figure 18. On the other hand, when the aft

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element deflection and the angle of attack are large, Cl and Cd change with Re as indicated in Figure 19. Generally, Cd increases with increase in Cl as shown in Figure 11 and Figure 16.But, in Figure 19, Cd becomes small when Cl becomes large for this value of To analyse why this reduction in Cd occurs, the pressure coefficients corresponding to Re = 400,000, 500,000 and 600,000 in Figure 19 are shown in Figure 20.

Figure 20 Pressure coefficient variation over the elements for three different values of Re. = 10°, = 10°, g = 1%.

The pressure coefficient over the top of the wing at Re = 400,000 is almost constant downstream of the leading edge. This indicates that the flow has separated at the leading edge of the main element for this small Reynolds number. The wing is stalled over its entire surface, and thus Cl becomes small and Cd is large. When the Reynolds number is larger (Re = 600,000), no separation, or only a very small amount of flow separation occurs, and Cd is again small. The pressure coefficient near the leading edge of the main element is significantly large, and Cl is large. When the angle of attack and the aft element deflection are large, it is necessary to operate the wing at Reynolds numbers in excess of 600,000 in order to avoid separation and poor performance. Note that the pressure distribution at Re = 600,000 may show small laminar separation bubbles on the suction surfaces of both the fore and aft elements near their leading edges in the strongly adverse pressure gradient regions. The results presented in the paper were obtained with the two-element, 1-m chord wing mounted between the walls of the 2.5-m wide duct near the outlet of the open jet. The turbulence intensity of the flow was about 1%, which is much lower than that in the atmosphere, but is quite large compared to 0.1% in good quality aeronautical wind tunnels. Turbulence in the onset flow

has the same effect as roughening the surface of an aerofoil, and causes early transition, and thus the emulation of higher Reynolds number behaviour. Even the turbulence from a small upstream rod [14] has been found to be sufficient to substantially alter the separation behaviour of bluff bodies. For testing aerofoils like those in the present tests, turbulence in the onset flow will affect the location of transition, as will the presence of the pressure gradients on the top and bottom surfaces. It is expected that the tested wings will simulate the behaviour of C-class catamarans, which operate at similar Reynolds Numbers. It is not known with certainty in the present tests where transition was occurring, and whether or not bypass transition was present. The effect of the transition location on the behaviour of wings can be investigated by tripping the boundary layer artificially using a wire or similar, and it is planned to carry out such tests in the future.

7. CONCLUSIONS

In this study, the influence of geometric factors such as the aft element deflection, the gap and the aft element pivot point have been investigated in a wind tunnel study. From the study, the following conclusions can be drawn. Cl and Cd are significantly related to the aft element deflection. As the aft element deflection is increased, Cl and Cd also increase. It is the same aerodynamically as adding camber to a single element wing. The addition of aft element deflection increases Cl, but too high a deflection results in an increase in drag. The influences of the gap size and the pivot point location are found to be relatively small in this study. The Reynolds number has a large influence on the sensitivity of the wing to separation. As expected, a smaller Reynolds number caused the flow to separate more easily, especially with high aft element deflections and high angles of attack.

Acknowledgements We would like to recognise the following people who have contributed to this project. We thank Joseph Nihotte and James Turner for carrying out a large amount of the wind tunnel testing, and for analysing some of the results. We also thank David Le Pelley for his help and advice in setting up and overseeing the wind tunnel testing.

References

1. Blakeley, A.W., Flay, R.G.J. and Richards, P.J., Design and optimisation of multi-element wing sails for multihull yachts, Proceedings of the 18th

Re = 0.4 x 106

Re = 0.6 x 106

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Australasian Fluid Mechanics Conference 3rd - 7th December 2012 Edited by P.A. Brandner and B.W. Pearce. Published by the Australasian Fluid Mechanics Society. ISBN: 978-0-646-58373-0.

2. Lanchester, F.W., Aerodynamics, Vol. 1, p 431, A. Constable and Co., London, 1907.

3. Elkaim, G.H., “Autonomous Surface Vehicle Free-Rotating Wingsail Section Design and Configuration Analysis”. Journal of Aircraft, 2008. 45(6): p. 1835-1852.

4. Marchaj, C.A., Sail Performance: Techniques to Maximize Sail Power. 1996: McGraw-Hill.

5. Gonen, I. Omer Wing-Sail Ltd. Available from: www.omerwingsail.com.

6. MacLane, D., “The Cogito Project- Design and Develop of an International C-Class Catamaran and Her Successful Challenge to Regain the Little Americas Cup”. Marine Technology and SNAME News, 2000. 37(4): p. 163-184.

7. Killing, S., “Alpha and Rocker - Two Design Approaches that Led to the Successful Challenge for the 2007 International C Class Catamaran Championship”. Proceedings of the 19th Chesapeake Sailing Yacht Symposium. 2009. Annapolis, Maryland.

8. Drela, M. "Newton Solution of Coupled Viscous/Inviscid Multielement Airfoil Flows,", AIAA paper 90-1470, presented at the AIAA 21st Fluid Dynamics, Plasma Dynamics and Lasers Conference, Seattle Washington, June 1990.

9. Drela, M. and Youngren H., "A User's Guide to MISES 2.53", MIT Computational Sciences Laboratory, December 1998.

10. Peart, C.R.C., “Numerical Modeling of a wing-sail Rigged Class in a Wind Gradient in Both Upwind and Off the Wind Conditions”. MSc Thesis, School of Engineering Science, University of Southampton, UK, 2011.

11. Greenbird. Greenbird supported by ecotricity. Available from: http://www.greenbird.co.uk/.

12. Sails, X.-W. X-Wing Sails - Wing Sails for Sunfish and More. Available from: http://www.solidwingsails.com/.

13. Abbott, I.H. and A.E. Von Doenhoff, Theory of Wing Sections: Including a Summary of Airfoil Data. 1959: Dover Publications.

14. Gartshore, I.S. “Some effects of upstream turbulence on the lift forces imposed on prismatic two dimensional bodies”, J. Fluids Eng. 106(4), 418-424, 1984.

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