aerodynamic optimisation study on a rigid...

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

AERODYNAMIC OPTIMISATION STUDY ON A RIGID 2D WING SAIL IN A WIND TUNNEL

Frederic DANBON1, [email protected] Dimitri VOISIN2, [email protected]

Michel DESJOYEAUX3, [email protected] Abstract. A series of studies has been carried out in partnership between Mer Agitée and the CSTB. These tests concerned the aerodynamic optimization of a 2D rigid wing sail in the atmospheric boundary layer wind tunnel of the CSTB in Nantes, France. The principal aim was to study the aerodynamic performance of a rigid wing profile consisting of 2 distinct profiles with their incidence and spacing being independently adjustable. The Main wing sail and Flaps are trimmed separately and this precise control is the rig’s main advantage over a conventional sail. The tests consisted of a wide-ranging parametric study to assess the impact of these adjustments on the performance of the two profiles, evaluated by calculation of the instantaneous resultant lift and drag force, and also to ensure a good mechanical performance of the rigid wing sails in the wind and therefore validate the design decisions. A specifically developed MATLAB program was used to give real-time visualisation of the results during testing, helping in selection of the tested configurations based upon the observed performance. Analysis of these results should help in decision making, helping to choose the type of wing sail for a specific boat and serve as input data for a Velocity Prediction Program (VPP).

1 Scientific Engineer, CSTB Nantes, FRANCE 2 PhD, R&D chief at Mer Agitée - FRANCE 3 Skipper, CEO, Mer Agitée - FRANCE

NOMENCLATURE α angle of incidence of the main profile relative to

the mean wind direction (degree) β angle of incidence of the flap profile relative to

the chord of the main profile (degree) CD drag coefficient expressed in the flow-axis

system aligned with the flow direction CL lift coefficient perpendicular to CD CP pressure coefficient S slot; minimum distance between the trailing

edge of the main profile and the leading edge of the flap profile measured tangentially (m)

integrated axial pressure force acting along the chord of the main profile (N/m)

integrated lateral pressure force perpendicular to the chord of the main profile (N/m)

instantaneous static pressure measured on the pressure tap i (N)

elementary integrated surface of the pressure tap i, projected in the body axis system Ox, Oy (m²)

1 INTRODUCTION

Rigid wing sails appeared gradually over time: first in 1974 on the class-C Miss Nylex, and later in 1988 on the Stars & Stripes H3. In 2010 the BMW Oracle on the 33rd America’s Cup; in 2011 AC45 are used in training for the 34th America's Cup and in 2013 a multihull craft equipped with AC72. Since 2010, rigid wing sails have rapidly grown in popularity.

Due to specific constraints of the class-C, notably because the surface area of the sail is limited to 28 m², rigid wing sails dominated the discipline, and also in particular because of an important weight reduction compared to classic sails. The design of rigid wing sails in the class-C raises aerodynamic questions such as: the best distribution of the available sail surface, adaptation to winds from 0 to 20 kt, effectiveness under headwinds and tailwinds and efficiency under low-wind conditions. Wind tunnel investigations help to validate technical choices, in particular to make the best compromise between aerodynamic performance, structural lightness and simplicity of the control systems. The regulation of the wing involves major uncertainties: angle of attack of the wing sail, the curvature, the twist of the frontal element and the flap (second element), and the slot between both elements. The main purpose of this research is to improve this technology by the design and the manufacturing of a class-C rigid wing sail.

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Figure 1 : Class-C Groupama C winner in 2013 built by Groupama Team (photo by Yvan Zedda).

2 THE WIND TUNNEL AND FLOW CONDITIONS

A Boundary Layer Wind Tunnel (BLWT) is used for measurements of two-dimensional wing sail models. It is a closed-circuit return tunnel driven by an alternating current motor of 200 kW, which allows a maximum flow velocity of 30 m/s. The blades of the wind tunnel fan can be adjusted, allowing an optimal adaptation of the drive to each desired flow velocity. The rotational speed of the fan is computer controlled, therefore the flow velocity can be adjusted depending on the environmental parameters such as temperature, total air pressure and humidity: This allows long measurements with constant dynamic pressure or Re-number, even if the blockage of the test section changes. The rectangular test section is 10m in length and the cross-section is 4m x 2m. The boundary layer wind tunnel facility simulates the mean speed profile and turbulence profile of the natural wind approaching the modelled area by having a long working section with a roughened floor and specially designed turbulence generators, or spires, at the upwind end. In the present study, a minimum floor roughness corresponding to an open water terrain associated with a very low longitudinal turbulence intensity of about 0.3% was used to simulate a uniform 2D wind.

3 MODEL AND MEASURMENT TECHNIQUES

3.1 The wing sail model

The rigid wing sail model consists of two distinct profiles; the main wing sail and the flap, with their incidence and spacing being independently adjustable. This precise control is the wing’s main advantage over a conventional sail. The global wing sail has a total chord of 1.33m (sum of the 2 profiles, with the respective chords of 0.833m to 0.5m for the main and for the flap),

is 2m high and is mounted vertically between two rotary tables in the tunnel (floor and top), as shown in Figure 2

Figure 2: Schematic view of the supporting system The prototype prepared by the team MerAgitée and MerForte corresponds to a sectional model (extruded 2D section) representing various elements of a wing sail at two-thirds of a C-class full scale. The model consists of a flexible skin of carbon fibre (3 layers), stretched over cross-sections to ensure a rigid assembly. The carbon fibre frame is covered with shrinkable aeronautical fabric. A further important difference between the main and flap profiles, beyond the ability to independently regulate their incidence angle and their chord length (distance between the leading and the trailing edge) comes from the geometric shape of the leading and trailing edges. The geometry is fixed for the flap but for the main, two leading edge shapes (symmetric and asymmetric) and one deformable trailing edge were used. The deformable trimmer could be more or less curved in order to adjust the distance (Slot, denoted s) between the trailing edge of the trimmer and the leading edge of the flap, as shown in Figure 3.

Figure 3: View of the deformable trailing edge of the main profile, the Trimmer

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3.2 Airfoil features

2D shapes have been modelled to understand the effect of different design parameters: flap deflection angle, slot between main element and flap and effect of leading edge droop. The symmetric and asymmetric main elements are modeled by a two-dimensional NACA-0016 airfoil. The asymmetric main element has a leading edge droop on 25% of the chord, without modifying the total chord length. The geometry of the flap element is a two-dimensional NACA-0012 airfoil.

Figure 4: Symmetric versus asymmetric wing sail

When the incidence angles of the two profiles are increased, the blockage area ratio of the test section also increases. In order to minimise its effect on the pressure distribution on the upwind surface, the wing sail was slightly moved and not placed in the center of the wind tunnel. No blockage correction was made.

3.3 The pressure measurement and integration techniques

Experimental integration of pressure distribution on the model was done with a PSI pressure scanning system. For a clear understanding of the behavior of the pressure distribution along the wing sail at the various configurations of incidence angle and assemblies of the profiles, 192 static pressure taps, 0.8mm in diameter, were positioned along the circumference of the wing sail’s mid-span (128 for the main profile and 64 for the flap). The numbers of taps is considered sufficient to fully describe the overall pressure loading at any instant in time. Figure 5 shows the pressure tap distribution used.

Figure 5: Schematic pressure tap distribution along the symmetric and asymmetric wing sail

The pressure taps on the model are connected to 4 independent 32 channel pressure scanners consisting of piezoresistive sensors ESP32HD via vinyl tubes which are 1.6 mm in diameter and 600 mm in total length with a damping restriction placed 250 mm from the sensor. This restrictor acts as a low-pass filter at 100 Hz, before the

signal is amplified, and is used to increase the attenuation for higher frequencies.

Figure 6: View of the pressure system embedded in the wing sail

Each PSI transducer had a pressure range of 2.5kPa with a static accuracy of ±0.03%. The pressure measurements are sampled at 200 Hz by a digital computer. Record lengths of one minute were sampled. A classical statistical treatment was done for each input, computing the maximum, minimum, mean and root mean square values. The reference dynamic pressure was measured using the 1rst PSI channel, via a Pitot static tube located 300 mm below the ceiling of the BLWT in the free stream above the boundary layer. At the end of the sampling period the measured pressures, consisting of the maximum, the minimum, and the mean values for each channel, were converted to pressure coefficients (CP) by dividing each by the reference dynamic pressure. CP was plotted, using a specific Matlab program, around the profile, using a normal vector representation. Figure 7 illustrates this.

Figure 7: Typical view of the pressure distribution around the wing sail (line=mean, dashed=min or max)

This representation allows a rapid view and understanding of the mean and fluctuating pressure distributions around the tested model and the resulting loads in each configuration. This practically real-time visualisation of the results during testing helped in

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selection of the tested configurations based upon the observed performance. The wing pressure measurements, taken simultaneously over the surface of the sail, can also be integrated to determine the wind-induced mean and dynamic loads which can be expected to occur under given wind conditions. This treatment program also indicates the individual statistical values of the efforts on each profile and on the whole wing sail. Statistical values of the pressure coefficients CP are also displayed for each profile considering only the corresponding pressure taps on each element.

3.4 Axis Systems and Force Coefficients

Two systems of axes are widely used in fluid-loading studies; a flow-axis system 0u, 0v, fixed in relation to the mean flow and with 0u aligned along the mean flow direction and a body-axis system, fixed in relation to the structure and with 0x commonly aligned with a principal axes of the body studied, here the chord of the main profile. Wind pressure measurements, taken simultaneously over the surface of the wing sail, are summed (or integrated) to determine the wind-induced mean and dynamic loads (expressed in Newton per meter N/m), using the following relationships:

Axial force:

192

1

. )()(i

xii

sqx StptF (1)

Lateral force:

192

1

. )()(i

yii

sqy StptF

(2)

Each integration surface Si is projected in the main body-axis system and then transferred to the flow-axis system to evaluate the drag and lift coefficient by dividing each component by the reference dynamic pressure and reference length. In this study the force coefficient data are presented in terms of the drag coefficient, CD, acting along the 0u axis of the flow and the lift (lateral force) coefficient, CL acting along the 0v axis (perpendicular to the flow). Using the flow-axis system, CD and CL are defined by the following classic formulations :

CD = CX cos α + CY sin α (3)

CL = -CX sin α + CY cos α (4) The center of rotation of the flap is located at 80mm before the trailing edge of the main profile, see Figure 8.

Figure 8: Axis systems and notations used

4 WIND TUNNEL TESTS

4.1 Experimental configuration

As shown in Figure 9, the wing sail is also equipped with classical tell-tales to allow visualisation of key flow features such as attachment or flow separation.

Figure 9: 2/3 of C-Class wing sail model in wind tunnel

4.2 Results and Discussion: Aerodynamic comparison between symmetric and asymmetric nose wing sail

The tests consisted of a wide-ranging parametric study to assess the impact of these adjustments on the performance of the two profiles and also to ensure a good mechanical performance of the rigid wing sails in the wind and therefore to validate the design decisions. For upwind conditions, low angles of incidence of the Main element (10° to 15°) and low camber (10° to 15°) of the flap lead to a lift to drag ratio, which is approximately 50% higher for the asymmetric wing sail compared to the symmetric one. In the wind tunnel velocity range of 0 to 20 m/s, the aerodynamic coefficients of the model proved to be constant, no Reynolds number effect were observed. Reynolds number of results is 1.76.106

s

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To illustrate upwind conditions we compare asymmetric vs. symmetric airfoils with an angle of incidence α of 8° (for the main), a camber β of 15° (for the flap) and a slot of 20mm between main trailing edge and flap leading edge (here called M08C15S20). The pressure fields and stagnation points are different but lift coefficients are equivalent (CL sym=1.712 versus CL asy=1.716) (see Table 1 and Table 2). Drag forces are 62% higher for the symmetric wing sail (CD sym=0.16 vs. CD asy=0.10) and the pressure field mainly differs on the leading edge because the lift force is directed forward for the asymmetric airfoil. Consequently the lift to drag ratio is 62% higher for the asymmetric wing sail. Note that the pressure distribution on the flap is mainly equivalent. In terms of dynamic loads on the wing, the instantaneous CP for the flap element is almost equal for the symmetric and asymmetric profiles but much higher for the symmetric Main element (CPsym (Instan.) = -6.59 vs. CPasy (Instan.) =-4.3). Hence aerodynamic efficiency and lift stability is greatly improved for the asymmetric wing sail.

Figure 10: Asymmetric wing sail M08C15S20 ASY

Table 1. Asymmetric wing sail α=8°, β=15°, Slot=20mm

CD CL Lift / Drag

MAIN -0.04 2.28 -59.99

FLAP 0.32 0.76 2.34

MAIN + FLAP 0.10 1.71 17.38

Mean Minimum Maximum

Cp Main (Mean) -0.81 -3.92 0.99

Cp Main (Instan.) -4.30 1.14

Cp Flap (Mean) -0.28 -1.48 0.61

Cp Flap (Instan.) -1.64 0.68

Figure 11: Symmetric wing sail M08C15S20 SYM Table 2. Symmetric wing sail α=8°, β=15°, Slot =20mm CD CL Lift / Drag

MAIN 0.07 2.31 33.75

FLAP 0.31 0.72 2.30

MAIN + FLAP 0.16 1.71 10.72


Cp Main (Mean) -0.82 -6.09 0.99


Cp Flap (Mean) -0.28 -1.50 0.60


For downwind conditions, with a high incidence of the main Element (15° to 30°) and high camber (25° to 40°) the lift to drag ratio is 20% greater for the asymmetric airfoil as can be seen in Table 3 and 4 (Figure 12). The asymmetric leading edge generates a powerful suction on the main element leeward side (Figure 12). This results in CL sym=3.66 versus CL asy=3.83 with less drag force CD sym=0.26 vs. CD asy=0.08. Consequently the lift to drag ratio is 32% higher for the asymmetric wing sail. The results in Table 3 and 4 also show that the instantaneous to mean CP ratio is higher on the symmetric airfoil. The asymmetric wing sail allows better trimming control facilities. Hence it appears that the asymmetric wing sail is more efficient in downwind conditions.

CP

CP

CP

CP

Direction vector of the total resulting Force (green)

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Figure 12: Asymmetric wing sail M10C40S20 ASY

Table 3. Asymmetric wing sail α=10°, β=40°, Slot=20mm

CD CL Lift / Drag

MAIN 0.08 3.83 48.49

FLAP 1.00 0.89 0.89

MAIN + FLAP 0.42 2.72 6.43


Cp Main (Mean) -1.52 -8.68 0.99


Cp Flap (Mean) -0.26 -2.99 0.99


Figure 13: Symmetric wing sail M10C40S20 SYM

Table 4. Symmetric wing sail α=10°, β=40°, Slot=20mm

CD CL Lift / Drag

MAIN 0.26 3.66 14.26

FLAP 1.00 0.87 0.87

MAIN + FLAP 0.54 2.61 4.87


Cp Main (Mean) -1.35 -10.42 0.98


Cp Flap (Mean) -0.31 -2.80 0.97


5 CONCLUSIONS

A method for determining mean and unsteady aerodynamic forces and moments produced by a 2-element wing sail in a wind tunnel is investigated in this work. This allows parametric comparisons between two shapes. The asymmetric and the symmetric wing sail lift coefficients are roughly equivalent at an incidence of 8°. Mean lift coefficients for small angles of incidence for the overall wing are minimally affected by these geometric nose changes, thus the impact of the nose geometry is mainly seen in the drag and in the performance at higher angles of incidence. Analysis of the instantaneous values indicates a difference of 10% between the mean and the instantaneous forces. This difference leads to an important change in the mechanical sizing of the structure of the wing. The lift to drag ratio must be optimised to increase the boat speed. It is clear that the asymmetric airfoil is better than the symmetric airfoil, but to manage leading edge droop on both sides it is necessary to use a mechanical control system. These systems cost in terms of additional weight. Finally, wind tunnel data are necessary as an input to VPP software to estimate whether an asymmetric wing is beneficial or not in comparison to a symmetric one.

Acknowledgements

The authors would like to thank Mer Forte design team for their contribution in the study.

References

1. Bot, P., Viola, I.M., Flay, R.G.J., (2013) “Wind-tunnel pressure measurements on model-scale rigid downwind sails” Proc. 3rd International Conference

CP

CP

CP

CP

Direction vector of the total resulting Force (green)

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on Innovations in High Performance Sailing Yachts, June 26th-29th, Lorient, France

2. Viola, I.M., Flay, R.G.J., (2011) “Sail pressures from full-scale, wind tunnel and numerical investigations”, Ocean Engineering, 38(16), 1733-1743

3. R.G.J. Flay (1996), “A twisted flow wind tunnel next term for testing yacht sails”, Journal of Wind Engineering and Industrial Aerodynamics, 63 (1996), 171:182.

4. Yutaka Masuyama et. al (2009). “Database of sail shapes versus sail performance and validation of numerical calculations for the upwind condition”, Journal of Marine Science and Technology, 14, 137:160.

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