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EXPERIMENTAL AND NUMERICAL INVESTIGATION OF THE FLOW AROUND THE APPENDICES OF A WHITBREAD 60 485
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SAILING YACHT
Experimental And Numerical Investigation Of The Flow Around
The Appendices Of A Whitbread 60 Sailing Yacht
P.Planquart, M.Riethmuller (Von Karman Institute for Fluid Dynamics, Belgium)
ABSTRACT
In this paper, we present results obtained experimentally and numerically during the study of the flow around the
appendices of a Whitbread 60 sailing yacht. Experiments are carried out in a wind tunnel on a model of reduced scale
(1/15). Force measurements are made using a three component balance. Two different bulbs are tested for different angle
of attack. Velocity measurements are made using the P.I.V. technique and a hot wire anemometer. Vortex shedding
frequencies are computed from the P.I.V. results and drag values are obtained from the velocity measurements in a plane
behind the bulb (wake survey analysis). The experimental results are used to validate the numerical simulations
performed using the commercial code FLUENT 5.2. Different turbulence models are compared and a good agreement
between the experimental and numerical results was found with the use of the standard k-ε turbulence model and wall-
functions.
INTRODUCTION
An experimental and numerical study is actually performed, at the von Karman Institute, on the investigation of the
flow around the appendices and the hull of a Withbread 60 (now called VOR 60—in reference to the Volvo Ocean Race).
The study combines experiments on a scaled model in a wind tunnel and numerical simulations using the
commercial code FLUENT. Wind tunnel experiments have already been used extensively in the past (Tinoco et al 1993,
Caponnetto 1993) for the study of IACC appendages and have provided useful information regarding the effect of bulb
and winglets on the performance (Claughton et al 1998) of IACC keels. The aim of this research is to validate CFD tools
for the design of appendices for a racing sailing yacht. The validation of the numerical simulations is made using the
following data:
1. Velocity measurements around the model (mean and instantaneous).
2. Measurement of drag and lift of the appendices using a three-component balance.
Two different geometries of bulb have been tested. The first one corresponds to the shape of an existing bulb
(Swarbrick 1997) and the second one was designed at the von Karman Institute using simple geometrical shapes.
WIND TUNNEL TESTS
Description of the facility
The experimental investigation is carried out using the scaled model (scale 1/15) represented in figure 1. This model
is placed in the wind tunnel L2B of the von Karman Institute (Figure 2).
Figure 1—Scaled model
The section of the wind tunnel is 35*35 cm2 and the maximum velocity that can be reached in the test section is 35
m/s.
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Figure 2—L2 wind tunnel
The Reynolds number, when performing wind tunnel tests, is lower than in the real case, but the main
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SAILING YACHT
objective of the research is to have a database for validation, and therefore the Reynolds number is not the key parameter.
Also, wind tunnel tests allow easy optical access which is an important point when using an advanced optical
measurement technique like the Particle Image Velocimetry (P.I.V.).
Description of the two geomletries
1. The first bulb, that will be referred as the “existing” bulb has been tested in three different configurations
represented in the Figure 4. A side view of the “existing” bulb is represented in Figure 3.
Figure 3—side view of the “existing” bulb
Figure 4—configuration for the “existing” bulb
2. The second bulb, designed at the von Karman Institute, consists of a body of revolution created by the
combination of an ellipse and a cone. This bulb will be referred as the “new” bulb.
The comparison between the shape of the “existing” bulb and “new” bulb is shown in the following figures.
Figure 5—comparison of the two bulbs
Figure 6—comparison of the two bulbs
Flow visualization
Surface flow patterns observed using oil visualization are footprints of the outer field (Delery 1992). When using the
surface oil flow technique, we must first coat the hull and the appendices with a special prepared paint. The air flowing
over the surface will carry the oil with it and a streaky deposit of the powder remains to mark the direction of the flow.
Using this technique, we are able to analyze the local direction of the flow at the surface and by implication, the general
flow structure in the three-dimensional field above it. This information is also used for the validation of skin friction
computation. The technique allows determining region of separation and attachment of the flow on an obstacle. This
technique has been applied to study the “existing” bulb and one result is given in Figure 7 for an angle of attack of 6°.
Close to the tip of the bulb, painting is accumulating, meaning that the “existing” bulb has not an optimal aerodynamic
shape.
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Figure 7—Oil visualization on the “existing” bulb
A vortex tube attached at the tip of the bulb has also been observed when the keel is in incidence. This vortex tube
has been put in evidence using a small piece of wool that has been translated inside the flow field. By recording the X-Y
coordinates of the tube,
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SAILING YACHT
we can find the exact position of the vortex tube. Figure 8 shows the rotating wool and we have plot on Figure 9 the
position of the vortex tube with respect to the hull, keel and rudder. It is worth mentioning that the vortex tube does not
reach the rudder.
Figure 9—position of the vortex tube for an angle of
Figure 8—vortex tube—visualization.
attack of 6°
P.I.V. measurements
The principle of P.I.V. (Lourenzo 1999) is to illuminate particles injected in the flow by a Laser sheet and to observe
the scattered light. In order to perform velocity measurements, two Laser pulses, separated by a short and know time
interval ∆t, are emitted to provide two images recorded on the same photographic plate or by a CCD camera providing the
image in a numerical form. By measuring the displacement of the particles, the velocity components contained in the
plane of the image is deduced in a straightforward manner. P.I.V. is very precious for the study of unsteady phenomena
since it allows to freeze the velocity field at a given instant.
Mean and instantaneous velocity fields have been recorded in the three planes represented in Figure 10.
Figure 10—measurement plane for the PIV
The measurements have been taken for the three different keel configurations of the “existing” bulb. Using the
instantaneous data from the Particle Image Velocimetry measurements, we were able to compute the vortex shedding
frequencies that characterize the detachment of vortices at the keel. Shedding frequencies for the “existing” bulb are given
in Table 1. These frequencies should be divided by 45 for the extrapolation to the real case.
The frequencies were computed using the distance measured between a negative and a positive peak of vorticity.
Knowing the mean velocity, we can compute a first value for the shedding frequency by taking the consecutive distance
between two peaks. However, a statistical approach has been used to compute all the possible combination of peaks and
different frequencies have been obtained. This approach has been used because two consecutive peaks of vorticity, as
shown on the right of Figure 11, might not been the result of two independent vortex shedding.
Table 1—frequencies computed from the PIV results
Frequency 1 Frequency 2 Frequency 3
T negative 8070 4040 2630
T positive 9390 4000 2550
Z 7450 3980 2470
Figure 11 shows a typical P.I.V. results measured behind the keel in the plane A represented in Figure 10. On the left
side, we see the instantaneous velocity field and on the right side a contour map of vorticity extracted from the
instantaneous velocity field using relation (1):
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(1)
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SAILING YACHT
Figure 11—Instantaneous velocity field and vorticity
Drag and lift measurements
Forces on the keel and the bulb (drag and lift) are measured using an in-house three-component balance represented
in Figure 12. The keel and the bulb mounted on the balance are represented on Figure 13.
Figure 12—Schemativ view of the balance
Figure 13—picture of the experimental set-up
(balance+model)
Drag curves for the existing bulb as a function of the angle of attack is represented in Figure 14 for the “existing”
bulb and the new one. For the “existing” bulb, the best results are obtained with the configuration T-negative. The worst
results are obtained with the configuration T-positive. Comparing the new bulb with the existing one, we observe a
decrease in the drag for all angle of attack with the new bulb.
Figure 14—Drag value as function of attack Figure 15—Lift value as function of angle of attack
Lift curves are represented in Figure 15. We observe a rapid increase of the lift for small angles of attack, but starting
at 3° to 5° of angle of attack, depending on the shape of the bulb and the configuration, the lift does not increase anymore
due to separation. The flow around the existing keel in configuration T-negative separates for the highest value of angle
of attack. The flow around the new bulb starts to separate for 3° of angle of attack. These low values of angle of
separation can be explained by the high aspect ratio of the keel (Marchaj 1991).
Drag has also been computed by measuring the velocity field in two planes located before and behind the keel as
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represented in Figure 16. By computing
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SAILING YACHT
the balance of momentum between these two planes, we can deduce the drag using relation (2). The velocity
measurements have been performed using a hot wire anemometer mounted on a displacement table. Mean values have
been recorded, but also R.M.S. value in order to compute the second term of the right side of equation (2).
(2)
Figure 16—plane of measurement
A typical velocity map (mean value) in the downstream plane for configuration T-negative of the existing bulb is
shown Figure 17. This map has been recorded 5 cm behind the tip of the bulb. The free stream velocity is around 34 m/s
and the minimum velocity measured behind the bulb is equal to 30 m/s. The R.M.S value of the velocity, in the same
plane and for the same configuration, is represented in Figure 18.
Figure 18—R.M.S. value—configuration T-negative
Figure 17—velocity magnitude—configuration T-
negative
The following table shows the value of drag measured by the balance compared with the value computed using the
velocity map (wake survey analysis).
Table 2—comparison of drag values for two angle of attack
configuration Angle of attack Balance Wake survey analysis
DRAG ° N N
T-negative 6 0.54 0.543
T-negative 0 0.436 0.444
T-positive 6 0.636 0.63
T-positive 0 0.466 0.456
Z 6 0.551 0.543
Z 0 0.462 0.438
New bulb 6 0.49 0.506
New bulb 0 0.414 0.398
NUMERICAL SIMULATION
Description of the simulations
The numerical simulations are performed using the commercial code FLUENT™ 5.2, which solves the Reynolds
Averaged Navier-Stokes equations using a finite volume technique. The following turbulence models have been tested in
order to determine their influence on the computation of drag and lift: k-ε standard model, k-ε realizable model and
Spalart-Allmaras model. Wall functions are used with the k-ε turbulence models. The Spalart-Allmaras turbulence model
is a relatively simple one-equation model that solves a modeled transport equation for the kinematic eddy (turbulent)
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viscosity and is well suited for the computational of external flow around aerodynamic shape (Spalart et al, 1994).
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SAILING YACHT
The geometry of the “existing” bulb has been created using the same CAD file as for the construction of the
experimental model in order to avoid geometrical differences. The computational domain is shown in Figure 19. For an
angle of attack of 0°, a symmetry plane is used to reduce the number of cells. The grid is generated using the commercial
grid generator GAMBIT.
Figure 20—surface grid of the keel and the bulb
Figure 19—computational domain for the numerical
simulations
A typical surface grid on the keel and the bulb for the T-negative configuration is shown in Figure 20. Triangular
cells are used for the discretisation of the surface of the bulb. Special attention has been taken for the discretisation near
the wall when generating the volumetric grid. Tetrahedral cells are used for the volumetric grid. Due to the limited
computational resources available for this study, the total amount of discretisation cells was always below 500.000. All
the simulations are performed using a second-order discretisation scheme (second-order upwind).
Results—Angle of attack=0°
The influence of the turbulence model on the value of drag for the configuration T-negative of the “existing” bulb is
shown in the Table 3. A value of 0.44 N has been measured experimentally. A good agreement is found when using a
standard k-ε turbulence model (within 2% of the experimental value).
Table 3—Drag for the T-negative configuration
Drag
Angle of attack=0° N
k-ε standard turbulence model 0.430
k-ε realizable turbulence model 0.321
Spalart-Allmaras turbulence model 0.363
Using the same turbulence model, the values of drag for the three configurations of the “existing” bulb are compared
in Table 4. The relative variation of the drag agrees well with the experimental observations, but only the k-ε turbulence
model gives quantitative data close to the experimental values (within 2%).
Table 4—Drag value for the different configurations
Numerical Simulation Numerical Simulation Experi. Results
k-ε std. Spalart-Al.
Configuration Drag—N Drag—N Drag—N
T-negative 0.430 0.363 0.436
T-positive 0.478 0.380 0.466
Z 0.462 0.376 0.462
Results—Angle of attack=6°
One simulation (configuration T positive) has been performed for an angle of attack of 6°, using the standard k-ε
turbulence model and wall functions. Values of drag and lift are given in Table 5. We observe that the agreement with the
experimental values is not as good as for an angle of attack of 0°. It should be mentioned that the discretisation is worst
than in the previous cases, because the computational domain has doubled (no symmetry plane). An analysis of the
sensitivity of the results with the grid size must still be performed for this case.
Table 5—lift/drag—angle of attack=6°
Numerical Simulation Experimental Results
Configuration Drag—N Drag—N
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T-positive 0.54 0.63
Lift—N Lift—N
T-positive 2.89 3.5
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SAILING YACHT
Contours of static pressure on the bulb and on the keel are shown in Figure 21. The contour of velocity magnitude in
a vertical plane located 5 cm behind the tip of the bulb is represented in Figure 22. The deficit of velocity due to the bulb
is clearly shown but not the one due to the keel. The velocity map confirms the lower value of drag obtained by numerical
simulations.
Figure 21—contours of pressure
Figure 22—velocity magnitude 5 cm behind the tip of
the bulb
Validation
The numerical results for drag and lift generated by the keel are validated with the data obtained using the balance,
only when the simulations are performed for no incidence. In these cases, a very good agreement is found when using the
k-ε standard turbulence model with wall functions. The use of wall function substantially saves computational resources,
because the viscosity-affected is not resolved and is well suited for this type of computation. Results with the Spalart-
Allmaras, which is specifically designed for aerospace applications, did not agree much with the experimental data, but
this is mainly due to the insufficient grid resolution near the wall. Also this model is not performing very well when there
is a change in length scale, such as when the flow changes abruptly from a wall-bounded flow to a free shear flow.
Simulations performed with an angle of attack of 6° have not been validated by the experimental results. Grid is now
being modified in order to check the sensitivity of the results with the grid size.
Contours of skin friction on the surface of the “existing” bulb are validated with the visualization using the surface
oil technique. Near the tip of the bulb, the skin friction reaches zero (Figure 23), and this explains the accumulation of oil
observed experimentally. As mentioned already before, the shape of the “existing” bulb is not an optimal aerodynamic
shape.
Figure 23—contours of skin friction
CONCLUSIONS
Results of a detailed experimental investigation of the aerodynamic performances of two bulbs of different geometry
have been presented. Experimental results include visualization of separation zone, the determination of vortex shedding
frequencies and the measurement of values of drag and lift for different angles of attack. Two different methods are used
for the determination of drag and similar results are obtained. One geometry has also been investigated using numerical
simulations with the commercial code FLUENT 5.2. A very good validation is found when computing the drag value for
no incidence. This good agreement has only been found when using the standard k-ε turbulence model with wall
functions. For an angle of attack of 6°, the comparison between the experimental and numerical results is not so good.
This might be due to the poor grid resolution. Having now more computational resources, the numerical simulations will
continue on more refined grids. The “new” bulb will also be investigated numerically.
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ACKNOWLEDGEMENTS
The authors would like to thank the following persons for their help during the experiments or the numerical
simulations: F.Helleput, R.De Voghel, T.André and D.Dupont
REFERENCES
J.Swarbrick: Drawing of WOR 60—private communication, 1997
E.N.Tinoco: ‘IACC Appendages studies', The Eleventh Chesapeake Sailing Yacht Symposium, January 1993
M.Caponnetto: ‘A review on Il Moro di Venezia Design', The Eleventh Chesapeake Sailing Yacht Symposium, January 1993
A.R.Claughton, R.A.Shenoi, J.F.Wellicome: ‘Sailing Yacht desing: Theory', Addsion Wesley Longman Limited, 333 pages, 1998
J.M.Délery: ‘Physics of Vortical Flows', Journal of Aircraft, Vol. 29, N°5, sept-oct. 1992
S.F.Hoerner: ‘Fluid-Dynamic Drag, Practical information on aerodynamic drag and hydrodynamic resistance', published by the Author, 1958
P.R.Spalart, S.RT.Allmaras: ‘A one-equation turbulence model for aerodynamic flows', La Recherche Aérospatiale, n°1, 5–21, 1994
L.M.Lourenço: ‘Particulate image and tracking velocimetry', von Karman Institute Lecture Series, course note 148, 1999
C.A.Marchaj: ‘Aero-hydrodynamics of sailing', Adlard Coles Nautical, 1991
FLUENT 5 User's Guide: Fluent is a registered trademark of Fluent Incorporated
GAMBIT User's Guide: Gambit is a registered trademark of Fluent Incorporated
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Representative terms from entire chapter:
turbulence model
