Wind Coefficients for nine ship models

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WLND COEFFICIENTS FOR NINE SEI? MODELS

by

Christian Aage

Aerodynamic s Department

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TABLE OF CONIENTS Page ABSThACT 1 INThODUC'IION i 'lEST EQUIPMENT i COEFFICIENTS 2 BOUNDARY LAYER

SECONDARY WIND EFFECTS

ACKNOWLEDGEMENTS ₅ REFERENCES ₅ LIST OF FIGURES Figtxre Page i Coordinate System 2 2 Velocity Profile ₃

3 CARGO SHIP A. (Full-load) 6

l-t CARGO SHIP A. (Ballast) ₇

5 CARGO SHIP B. (Without Containers) 8

6 CARGO SHIP B. (With Containers) ₉

7 CARGO SHIP C. 10

8 TAM(ER

li

9 PASSJNGER LINER 12

10 FERRY ₁₃

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ABSIRACT

Four-component wind coefficients for nine ship models are presented with a short description of the test equipment and the boundary layers in

model and full scale. Mention is made of two

important secondary wind effects.

INIRODUC1ON

Accurate wind force data are needed in connection with trial trip analyses as well as manoeuvring and stability investigations.

Theoretical calculation of the air flow around the ship is not possible

today, so we have to rely on model tests. In cases where a special

model test with the ship in question cannot be performed for economic or other reasons, one can either use one of the general formulae

proposed by some authors or pick one specific model test with a model

as similar as possible to the ship in question. With the present

state of knowledge the latter method seems to be the most accurate. Therefore, a very large "fund" of published model test results

is needed to cover the wide variety of ship forms. And furthermore,

the constantly changing ship forms and the arrival of new types of'

ships should be followed up by new model tests. As a contribution

to this, a series of nine model tests carried out at RyA during the last few years is presented here.

The test series comprises three careo ships in different condi-tions, a tanker, a passenger liner, a ferry, and a fishing boat.

TEST EQUIPMENT

The tests were performed in the HyA circulation wind tunnel

with a wind speed of' 60 rn/sec. The test section is 1.0 x 0.7 m. The

models were about 0.5 m long, which gives Reynolds' numbers of about

6

1.7 x 10 based on lengih. Blockage corrections have not been made.

A special four-component strain-gauge balance was used to mea-sure the components X, Y, N and K (longitudinal force, transverse force, yawing moment and heeling moment) as functions of wind

direc-tion angle l . The components Z and M (vertical force and pitching

moment) were considered unimportant for the ship models tested. For sign conventions, see Figure 1.

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-2-z

OrigLn at the intersection of Centerline-, Waterline-, and Midship- (Lpp/2) planes.

Figure 1 - Coordinate System.

COEFFICIENTS

The results are presented as the non-dimensional coefficients:

Cx = +

CY=

CN=

CK =

Y

v2 N g V2 As Loa

+V2AsHs

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where

g =

mass

density of

air.

V = wind velocity in the free

'treain.

M' = projected front area of ship model above the waterline.

As = projected side area of ship model above the waterline.

Loa

length-over-all of ship model.

Hs = height of centre of gravity of As above the waterline.

Distance from tunnel wall

120-100

80

60- 20-0 L

m

Natural boundary layer over open sea, scale 1:300 Boundary layer at

tunnel wall, scale 1:1

U

= local wind velocity

V = wind velocity in the free stream

Figure 2 - Velocity Profiles.

V

0i

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BOUNDARY LAYER

The tests were carried out in the natural boundary layer

existing at the tunnel wall. In Figure 2 the velocity profile of

the tunnel boundary layer is shown together with a typical natural

wind profile scaled down to model size (1:300). With the model

scales used, the tunnel boundary layer forms an average between the natural wind profile and the uniform wind with no boundary layer.

As the real wind field around the full-scale ship in fact is composed of the natural boundary-layer wind and the uniform speed wind, we believe that using the tunnel boundary layer gives us the

best compromise for practical purposes. Therefore we do not remove

the boundary layer with fences or suction.

The whole boundary-layer problem has been investigated at HyA by means of the so-called "slice method", in which a segmented model

of the passenger liner was used. By a combination of model testing

and calculation it was possible to find the wind forces for any composition of speed wind and natural wind, so that the different directions and velocities of the natural wind relative to the speed

wind could be taken into account. The results have supported

the correctness of using the tunnel boundary layer for normal

ship-model tests in the wind tunnel. Further details about the "slice

method" can be found in [i]

SECONDARY WIND EFFECTS

In addition to the primary wind forces on the ship a number of secondary wind effects influence its behaviour.

The side force and the yawing moment make the ship sail with certain drift and rudder angles which increase the resistance of

the ship. HUGHES [2] and WAGNER [3] have found these augmentations.

Especially WAGNER's results show a surprisingly high effective wind

resistance (several hundred per cent larger than CX). If WAGNER's

findings are correct they certainly make the direct use of CX in trial trip analyses somewhat illusory.

Another secondary wind effect is the influence of a drift angle

on the wake pattern. This was first pointed out by JGENSEN and

P1o1iASFC [it] who showed that a drift angle changes the wake appreciably

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with drift angle, so that for a right-handed propeller the wake

frac-tion can be

50 %

larger with starboard drift angle than with port

drift angle. Later tests at HyA seem to indicate that this is typical

for fast cargo ships, whereas the wake patterns of tanker hulls are less influenced by drift.

There might be a clue to better trial trip analyses in these two secondary wind effects, which are not incorporated in any standard trial trip analysis code today.

ACKNOWLEDGEMENTS

The author wishes to acknowledge, with thanks, the co-operation of his colleagues Messrs. Finn Jensen, Verner Jensen, and Holger

Pedersen of the HyA Aero-Department in the preparation of this report.

REFERENCES

Aagé, C.: WIND FORCES ON SHIPS, Report No. A_Li.,

Hydro- og Aerodynamisk Laboratorium, Lyngby, Denmark,

1971.

(Not yet published).

Hughes, G.: THE EF.I'JiCT 0F WIND ON SHIP PERFORMANCE, T.I.N.A.

1933, pp. 79-121.

Wagner, B.: WINDKRI4FIT AN tJBERWASSERSCUIFIi'EN, JaLhrbuch der

Schiffbautechnischen Gesellschaft

61.

Band

_1967,

pp. 226-250.

[Li] Jorgensen, H.D. and Prohaska, C.W.,: WIND RESISTANCE, 11th I.T.T.C.

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-

10

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