Visual observations of the flow around the sails of a model 12 metre yacht

(1)

16 NOV. 1P6

_Lab.

v.

_{,Scheepsbouwkunde}

ARCHIEF

Technische Hogeschool

S.U.Y.R.,

N6040

-University off Southampton

1111

i'll'ADVISORY

_{COMMITTEE FOR YACHT RESEARCH}

-VISUAL OBSERVATIONS

OF THE FLOW

AROUND_ THE SAILS

OF A__110DEL_ 12 METRE YACHT

by

C A oMarchaj

A .Q.Chapleo

Sestenill 600

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DEPARTMENT OF AERONAUTICS & ASTRONAUTICS

(2)

VISUAL OBSERVATIONS OF THE FLOW AROUND THE SAILS

OF A MODEL

12 METRE, YACHT,

by

C.A.Marchaj

A Q Chapl eo

Se tel_thlei- 12600

(3)

VISUAL OBSERVATIONS OF THE FLOW AROUND THE SAILS

OF A MODEL

12 METRE YACHT

SUMMARY

Observations were carried out onz-the main sail only

the main and fore sails.

The flow was visualised using tufts of artificial silk of approximately

in0 in length, attached to the main sail in rows, parallel to the mast,

and on the fore sail parallel to the leading edge, at a pitch of 2 in.

The rows were 1 in0 apart and were staggered.

Tunnel speed 50 ft./sec,

Model Reynolds _{.5.222_2L19_2L19.±}

No. _1.57 = 2.3 x 105 based on Rated

Sail Area and mast height.

Mast Reynolds

₁

_{52_a10_}

4

4 based on mast

No. 32 x 1.57 = 1.0 x 10

(4)

RESULTS

For convenience in

disoussion9

the mainsail setting angles in the mainsail only cases have been made independent of the

angle of the model

centre line to the wind by defining an effective angle of attack

Illustrated in Fig. 1.

Although somewhat misleading the main

fore sail cases this

quantity has also been quoted for comparison with the previous cases.

The various combinations

ofe(F and Ociy

over the full range of angles are given in Table 1.

Plots of CL9 and 0

0(1.1 are shown in Fig.'.

The various types of flow. described in the figures conform to the

following key. This is simplified in the discussion. Figures 2 9

and Plates 1 18 show the result of visual observAtions of both sides

of the sails.

A

vs.

)0

for relevant values of 0(_F and

steady flow (streamline flow)

transition

from steady to (transition)

unsteady flow

unsteady

(turbulence)

Steady

reversed flow (separated) 4

(5)

DISCUSSION

When interpreting the results of an investigation using tufts, it has to be borne in mind that their behaviour is restricted by their length,

and by their attachment to the surface. Thus in the case of our

"steady flow, Al', the tufts show little inclination to lie along the

local streamlines, since they are largely in the boundary layer, and require

some help in alignment. The "unsteady flow, C" indicates the presence of

large scale random variations in velocity. The

*unsteady

reversed flow D"

is found under conditions of

large scale

separation, and corresponds to the mechanism of stalling.

(a) Mainsail only

At a low angle of attack %, 09 it is clearly seen that th

mast has a great influenc on the character of the flow over both sides of

the sail (F1g029 Plate 1). The areas of the sail immediately Abaft

the mast, which on an ordinary aerofoil correspond to the regions of pressure

maxima and minima, are most affected, lying as they do in the strongest

part of the wake from the mast. The upper parts of the sail, where the ration, mast diameter/sail chord, is greatest (Fig.') are most affected.

With increasing angle of attack the region of disturbance on the

leeward side progresses downwards. At a typical practical angle of attack (C)ef 100) the upper half of the sail consists of

unsteady

flow

(6)

than cif =

15°, the whole of the leeward side of the sail consists

of steady reversed flow, D (Fig049 Plates 3,4). The downward progression

of the stall is only to be expected, with the highly tapered. plan

and zero spanuise twist, but its development from the mast, rather than frog the trailing edge, can only be attributed to the mast (Figs03 and

7)0

A reverse situation exists on the windward side of the sail, when an increasing angle of attack reduces the areas of the sail affected by the mast (Fig. 4 5 Plates 169 179 18).

On both sides of the sail, localised areas of unsteady flow can

be observed, close to the mast, and extending aft (Fig02(iii)). A possible explanation for this may be the gap between mast and sail. There was a strong tendency for the windward side tufts to flow through this gap.

As the angle of attack increases, so do the cross-flows around the foot of the sail, and the directions of the tufts on both sides being the

same, indicated that the shed vortex lay off the surface, on the leeward.

side. This illustrated in the sketch below, which shows a view looking forward

on the trailing edge of the sail.

vortex

Leeward Windward

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(b) Main and foresail

For low angles of attack 0<efm O the influence of the mast is similar, on the upper parts of the sail, to the mainsail _{only case.} _{Up to} about 0<efm = 100 the flow around the leeward side of _{the foresail. is} steady, and that round the mainsail largely so.

Once more the flow breakdown with increasing angle of attack progresses

downwards from the mast head (F1g069 Plates 6,7). _{For angles of}

attack 61f

>le (-)0.150

_{the rate of increase of the stalling}

'M

increases rapidly (Figs. 6,7 _{Plates 69 7, 8)9 particularly over the}

foresail. _{Referring to F1g019 it is in this region that 01/01)}

reaches a maximum, and the slope of CL _{vs. (4X) starts to decrease.} _{It is this} region, (/5)k)eN=15°, which is likely to be _{encountered in practical sailing}

to windward.

At Of =

25° ( (/5-10 = 300)9 when OL reaches its maximum,

nearly all of the foresail is stalled, but the mainsail consists largely of the transition flow B9 except for the upper parts outside the influence

of the foresail (F1g089 Plate 10). _{This state persists until at least} 40°.

The effect of varying the foresail anglec, _{for a practical value of}

(A)k.) =

15°, is not very marked on the external flow, _{but some effect was} noted within the gap between, the sails, _{and, to a lesser extent, on the region} of the mainsail, immediately aft of the _gap. _{With the foresail close to}

the mainsail, the tufts _{were inclined slightly upwards.}

As the gap increased,

(8)

Flow around the foot of the foresail is similar to that round the mainsail, the tufts being inclined down and aft on both sides of the

sails, There is no evidence of a leading edge separation on the

fore-sail,

As before, the windward side of both sails is rendered more stream=

line with increasing angle of attack,

CONCLUSIONS

1, It appears that the foresail has a profound effect on the aerodynamic

characteristics of the rigs; in every case changes in

these

characteristics can be related to changes in the flow associated with

the foresail,

2. The effect of the mast cannot be over emphasised., particularly if

tests on models of different aspect ratios are to be performed, It seems

most desirable to construct masts exactly to scale,

modifying the

design,

in general, only to ensure the same Reynolds Number effect as for full

scale, In this particular case this would not be necessary,

since both

model and full scale mast Reynolds Numbers are in the range (102 305)

(9)

,TABLE

Plate

Fig°

4.14

,Side

Rig

_ No. No 1 2. 10 5

Ieed.

'Main 2

3 /5

5 5

4

25

5 5 5

Main &

Fore

10

7

6

7

15

20

10

5 r,

9

25

10

5

1 10 8

35

10 5

4

15

5

₅ 15

121

5

13

8

10

10[

5

Wind. 'ff

14

15

10

5 25

10

4

5 5 V'

Main

17

5

15

5

lO

5

25

3

4

20 10 5

6

5 5 7 9 9 5 5

(10)

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25°

30° 35° FIGURE 1.

(11)

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LEEWARD. 7

V4_

2-exef-= 21/2°

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STEW FLOW.

(STREAMLINE FLOW.)

TRANSITION FROM STEADY

FLOW TO UNSTEADY FLOW.

(TRANSITION).

UNSTEADY.

(TURBULENCE)

frntl

,STEADY REVERSED FLOW

QSEPARATED) I

row,

P&I ill I Ma/741C 41 PLATE II

(12)

FIGURE 3.

LEEWARD.

(13)

FIGURE 4, LEEWARD. -PLATE 16

-- WI -

(14)

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04sF =AO

044,5°

LEEWARD 5.

(15)

LEEWARD

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FIGURE 7.

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FIGURE 8.

.LEEWARD

(18)

FIGURE 9.

WINDWARD

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