THE USE OF COMPUTATIONAL MODELLING IN A STUDY OF

DOWNWIND SAILS

Peter Richards and Gordon Mallinson

University of Auckland

New Zealand

Auckland - The City of Sails

Home of

The America’s Cup

The Yacht Research Unit

The primary function of the YRU is to co-ordinate and promote research and engineering activities within the University of Auckland that are related to yachts.

Images created by

Gordon Mallinson’s

package SeeFD

(Son of GRAFFIC)

Wind direction

America’s Cup Course

Wind Speed (m/s)

0

1

2

3

4

5

6

70

1020

30

40

50

60

70

80

90

100

110

120

130

140

150160

170180

190200

210

220

230

240

250

260

270

280

290

300

310

320

330340

350

34567

VMG maximum with a wind speed of 5 m/s

True wind direction

Wind direction

Spinnaker

Mainsail

Spinnaker pole

Boom

Spreaders

Masthead

Velocity Triangles

A

VT

VA

VB VT

VA

VB

A

(a) Boom Height (b) Masthead

Apparent Wind Twist

Test Section of Tamaki Twisted Flow Wind Tunnel

Schematic of Tamaki Twisted Flow Wind Tunnel

Test section

Honeycomb and mesh

25m

3m

Wind FlowFans

Wooden blocks

Pitot tube

Flow twisting vanes

(Not to scale)

Twisted flow?

Interaction Test Scenarios

Mainsail only Spinnaker only Combined sails

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

80 90 100 110 120 130 140 150 160 170 180 190

AWA (deg)

forc

e c

oe

ffic

ien

t

Av Cl Combined Av Cd Combined Av Cl Summed Av Cd Summed

Lift and Drag Coefficients - Wind Tunnel

-0.3

0

0.3

0.6

0.9

1.2

1.5

1.8

90 120 150 180

Apparent Wind Direction (degrees)

Fo

rce

Co

eff

ice

int

CD Spinnaker Both

CL Spinnaker Both

CD Spinnaker Only

CL Spinnaker Only

Lift and Drag on the Spinnaker - CFD

-0.3

0

0.3

0.6

0.9

1.2

1.5

1.8

90 120 150 180Apparent Wind Angle (degrees)

Fo

rce

Co

eff

icie

nt

CD Main Both

CL Main Both

CD Main Only

CL Main Only

Lift and Drag on the Mainsail - CFD

-0.3

0

0.3

0.6

0.9

1.2

1.5

1.8

60 90 120 150

Apparent Wind Angle (degrees)

Fo

rce

Co

effic

ein

t

CT Spinnaker Both

CS Spinnaker Both

CT Main Both

CS Main Both

CT Combined Both

CS Combined Both

Thrust and Side Forces: Spinnaker, Main and Both

-0.3

0

0.3

0.6

0.9

1.2

1.5

1.8

60 90 120 150

Apparent Wind Angle (degrees)

Fo

rce

Co

eff

icie

nt

CT Combined Summed

CS Combined Summed

CT Combined Both

CS Combined Both

Thrust and Side Force: Summed and Both

Twisted Onset Flow

-0.3

0

0.3

0.6

0.9

1.2

1.5

1.8

60 90 120 150

Apparent Wind Angle (degrees)

Fo

rce

Co

eff

icie

nt

CT Combined SummedCS Combined SummedCT Combined BothCS Combined BothCT Combined TwistedCS Combined Twisted

Thrust and Side Force: Summed, Both and Twisted

CONCLUSIONS• Both straight and twisted flow around a pair of

downwind sails have been modelled. • Under some sailing conditions there is significant

interactions between the sails, however under normal sailing situations this interaction is quite small.

• With twisted flow the thrust and side force coefficients are similar to those for straight flow if referenced to the masthead apparent wind speed and direction.