On the nature of scale effect in manoeuvring tests with full-bodied ship models

Evgeny Nikolaev and Marina Lebedeva Krylov Shipbuilding Research Institute Leningrad. U.S.S.R.

ABSTRACT

The paper covers the results of scale effect investigations

on

the basis

or

free-running tanker models. It also deals with the results of captive model tests carried out on the rotating arm in'the KrylOv Institute

basin

presented as

a

fun-ction of hydrodynamic characteristics of drift angle, radius of rotation, speed

and

rudder angle.- The analysis of the

data

ob-tained leads

to

the conclusion that the scale effect occurs as a crisis of the flow around the model's hull. It is caused by the trajectory curvature inherent in the manoeutring ship. To create a stable turbulent boundary layer separation it Is suggested to

tse

Models with sand-rough-ened surface. The paper provides data on free-running and rot mg-arm

sand-rough-cued model tests. NOMENCLATURE length of ship beam of ship draught of ship displacement block Coefficient propeller diameter rudder area speed of ship rudder angle drift angle radius of gyration dimensionless radius of gyration

dimensionless angular velocity lateral force on the rudder dimensionless lateral force on the rudder

On the Nature of Scale Effect in

Manoeuvring Tests with

Full-Bodied Ship Models

Postbus 152 - 8300 AD _EMMELOORD

6 FEB.

₁₉₈₅

IECHESCIIE

,..;ffErf Loboratorlurn vow Soheopshydrontothonloa Archie,

madweg

_{2,2828 CD Delft}

ieL

ots

.785872 -

nut

015 751033

- dimensionless lateral force

on

the hull

- dimensionless yawing moment on the hull

Rn = Reynolds number

V

.

Pn = - Froude number

p mass density

- coefficient of kinematic visco-sity

1. INTRODUCTION

In the last decade scientists have been studying the scale effect

in

full-bodied ship model manoeuvring testa (Refs.

1 through 5). This effect is observed as a considerable discrepancy in turning test data for

Small

and large free-running mo-dels. As a rule, ship steering and manoeu-vring data based on amall model tests are more optimistic than those obtained with large models.

It is -essential to predidt correctly manoeuvrability characteristics of full-bodied ships, since they are conductive to safety of such ships at Sea. it appears im-portant to

study

the causes of the scale effect and Methods of reducing its influ-ence on the ship's manoeuvrability.

The above-mentioned papers review Ma-noeuvrability test data for ship models of various sizes ad well

as for full-scale

ships

and provide conclUSiVe evidence of

the scale effect at different rudder angles. The following assumptions of the causes of

the

scale

effect have been put forward:

(1) differences in the flow around models. Of different sizes and full-scale ships make the forces applied to the hulland

the rudder a function of the model's

(2) relatively greater forces on the

rud-der behind the propeller of smaller models (compared to those normally

ob-tained by the scaling law) resulting from relatively higher velocities in-duced by their propellers account for excessive course stability of small

models.

The published studies deal with some methods of reducing the scale effect,

nam-ely, the use of various fins on models to obtain at the stern a flow similar to that of a full-scale ship and, mainly, the use of large models up to 30 M in length.

Evidence of the scale effect was also obtained during model tests in the Krylov Institute basin when comparison was made between the turning test data for 2 in

and 5-7 in long models. In a number of

cas-es the comparison revealed a substantial discrepancy in values of dimensionless an-gular velocity at small rudder angles.

The point of this paper is to study causes of the scale effect Which is made evident by full-bodied model tests carried out to estimate the steering capability of

full-scale ships. So t,t,p emphasis is made

on the specificity of the turning test da-ta at small rudder angles in the so-called

hysteresis loop.

2. TEST OBJECTS MID

PROCEDURES

The objects of the investigations were tanker models whose hull lines are

shown in Fig, 1 and schematic models with stern shapes that differ from those shown

in Fig. 1 by 'a greater volume of the

dead-wood

as

shown in Fig. 2. To obtain turning

curves, .e. the relationship between di-mensionless angular velocity at a steady

ship circulation and the rudder angle,

the authors of this paper tested self-pro-pelled models in the Krylov Institute ba-sin. The geometry of these models corres-ponded to the following characteristics of the ship:

length -- - L 170 in

beam - B

25.3

m

draught - T -

9.5

in

displacement - W - 31,000 ton block coefficient - Cu- 0.75

propeller diameter - D-- 6,m

rudder area - A,- 24.5

e

Steady circulation paraMeters were .

estimated by recording the model's trajec-tory in the "Spiral" test and by recording the value of the rudder angle in the "re-versed spiral" test. In the former case throughout the experiment the propeller speed was maintained constant assuring a set speed of the model on the straight course. In the "reversed spiral" tests,

as

well

as in all captive Model tests of

the hull,..propeller-rudder complex, the

speed in circulation Varied with the re,-dius. The value of the speed was defined

by the empirical formula:

° + 1.9

where Vo is the speed on the straight

course.

The above procedure was applied for testing four geosim tanker models 2, 4, 6 and 8 m long (Fig. 1). The test data are

shown in Fig. 3, and it may be seen that

the test points corresponding to 4, 6 and 8 m models lie on the same curve with the

width of the hysteresis loop about 100, while the curve with the hysteresis loop

lees than 5° wide corresponds to the model

2 in long. The data obtained prove that in

this case as the length of the model is in.-creased from 2 in to 4 in there occurs a

con-siderable change in the turning curve, whereas with a further increase of the

length up to 8 in no noticeable changes in

the curve are observed. Such an "abrupt" change in the turning curve with a gradual increase in the length of the models had been described earlier (Ref. 1).

Further studies of the scale effect were made with tanker models having their

stern shapes modified as shown in Fig. 2, with the view to obtain a turning curve with a wider hysteresis loop. It was

assu-med that a larger difference between test data for small and large models would help find the causes of the scale effect. The assumption has been found to be true, as teen from the results (shown in Fig. 4) of tests with 2 and 6 m long models made in accordance with Fig. 2. Here the

hyste-resis loop of the small model is about 10% and that of the large model exceeds 20°.

Searching for causes

of

the scale ef-fect the authors undertook Model tests on the rotating arm in the circular basin

of

the Krylov Institute Which provided data on the forces applied to the rudder and the hull of small and large models towed on circular trajectories.

3.

DYNAMOMETRIC TEST DATA AND HYPOTHESIS

OF THE CAUSE OF THE SCAIE EFFECT

Two geosim models 2 and 6 in long with

schematic hull lines (Fig. 2) were tested in the circular basin. The Models were

equipped with.a propulsion device which admired a smooth change in the propeller

speed, as well as with two dynamometers for measuring forces and moments applied to the hull and the rudder. The test pro-gram included measurements of the full la-teral force and its moment applied to the hull Of the self-propelled model with the propeller operating in the steady

circula-tion mode and recording of the lateral force on the rudder and the lateral force and its Moment on the naked hull of the model. As for the signs of the forces,

the positive sign was given to the lateral: force directed to the center of circula-tion and to the moment turning the model

804

to the center of circulation. The moment was measured with respect to the center of

the model. All the measured values were converted to dimensionless form by being divided by the values of dynamic pressure, length and draught of the model according

to the following formulas:

Y = (2)

0.5pV2IT

(3) 2

0.5p2 L T

The data Obtained from measurements of the lateral force on the rudders of the small and large self-propelled models are shown in Fig. 5 and present the lateral force versus the rudder angle and the re-lative turning-radius. The drift angle and the propeller speed correspond to the val-ues of these parameters at a steady

turn-ing. Pig. 5 shows that the scale effect

develops at rudder angles exceeding,10°. In the range of rudder angles -106401f+10°

embracing the zone of the hysteresis loop the values of the forces on the rudders

of the two models ars;Practically identi-cal (in dimensionless form). In this case it was possible to rule Out the force on the rudders as a cause of the scale effect in the zone of the hysteresis loop.

The restate of measurements on the naked model hulls are shown in Figs. 6 and 7 and present the full lateral force

and

its moment versus the drift angle and the relative radius of trajectory curvature.

The

full

lateral force on the naked hull is the Algebraic sum of hydrodyrAmic and

centrifugal forces affecting the towed model. Here of specific interest is the

run of curvatures obtained for the small model. The lateral force on the hull is

generally assumed to change monotonically

as

increases and to approach asymptotic-ally the meaning corresponding to R = In the figures shown here such pattern of curvature is observed only with the large model. The curvatures obtained from the results of the small model tests are cha-racterized by extrema which suggest occur-rence of an unsteady breakdown of the flow around the model's hull.

The study of the influence of the Reynolds number on hydrodynamic perform-ance of the naked hull was carried out on

heabove-mentioned geosim tanker models. Rn ranged from 10° to 10'. The Froude num-ber was kept constant at 0.217. The test data are given in Figs. 8 and 9. The dia-grams make it clear that the shape of the curvatures obtained corresponds to the

re-lationships typical of the unsteady boun-dary layer separation (Refs. 6, 7). Of the

two groups of curvatures those of the lateral force exhibit the greater influe-nce of the Reynolds number. Turning leads to some differences in these relationships as compared to those obtained in transla-tional movement tests. Because of turning,

crisis phenomena are observed even at zero drift angle, whereas in translational move-ment the crisis appears only at drift ang-les larger than 40°. In addition, as the drift angle increases turning shifts the location of the extrema of the relation-ships Y(Rn) towards lower values of(Rn), while in translational movement the locat-ion of these extrema does not practically depend on the drift angle.

The test data on the lateral force and the moment applied to the hull of self-propelled models with an operating propel-ler are shown in Figs. 10 and 11. The rud-der angle is zero. The variables are the radius of the trajectory curvature and the drift angle. Here again there is evidence of the validity of lateral force relation-ships obtained for the small model. The pattern of these curvatures which resemble the ones noted in the description of the naked hull test data makes it possible to affirm that the unsteady breakdown of the

flow around the hull of a small model takes rise even with the propeller in operation.

A study of the hydrodynamic characte-ristics obtained for self-propelled models and their hull has outlined the region where the breakdown of the flow occurs. The experimental data given in this paper

provide sufficient evidence showing that the region is located somewhere near the midstation section rather than at the stern counter as has been assumed so far. This can be proved by the fact that crisis phe-nomena are revealed mainly in lateral force relationships and have virtually no effect on curvatures of the hydrodynamic moment.

Thus the analysis of the data obtained in. these tests proves the unsteady break-down of the flow around the hull of a model to be the cause of the scale effect.

4. TEST DATA FOR SAND-ROUGHENED SURFACE

NODEIS

The authors of the paper have made an attempt at stabilizing the breakdown of the flow near the hull of

a

model through arti-ficial turbulization of the flow Use was

made of a turbulization methOd convenient for model tests with variable drift angles. This method had been developed by members of the Krylov Institute for

wind

tunnel

model tests (Ref. 7) and consists in apply-ing an artificial roughened layer to the surface of the model.

After the surface of the small model was given a sand-roughened layer with an average size of grain of about 1.5 mm the model was tested again according to the procedure described above. The reediting test data are shown in Figs. 4, 9, 10 to-gether with the data for amotith hull

models.-A comparison of the.two sets of data indi-cates that the characteristics of the amell model with the sand-roughened surface are substantially different from those for the same model before application of the

identical with the characteristics of the large smooth model. This hold true both for the turning curve and the hydrodynamic per-formance of the hull of the self-propelled model and the naked hull. The value of the lateral force on the rudder at small angles practically did not change (Fig. 5).

This flow turbulization method was checked on other models, in particular, on the basic tanker model (Fig. 1). The test

data for 2 in sand-roughened _{surface tanker}

model are shown in Figs. 3, 6, 7 and demon-strate a perfect agreement with those for

large smooth models.

The sand-roughened small model was also used in and additional experiment aimed at substantiating the assumption that the zone of the boundary layer breakdown affecting the manoeuvrability performance

lies in the region of the _{bilge along the} parallel middlebody. With this end in view the zone along the port side of the model was cleaned of sand and made smooth _again. The turning curve obtained for such a medal is shown in Fig. 12. The dotted lines are the turning curves of smooth models. It is obvious that the curves for the starboard and port side turnings differ _{considerably.} In the port side turning test _{when the zone}

external to the Center of circulation was smooth the turning curve for the sand-roughened surface model practically coin-cided with that for the small _{smooth mode/.} During the starboard turning _{tests when the} bilge of the external side was roughened the curve for the small model _{with the} sand-roughened surface was identical _{to that for} the large smooth model. The data obtained in the additional experiment make it poss-ible to assert that the zone of breakdown of the unsteady flow lies in the region of the bilge of the side external to the cen-ter of circulation.

5. CONCLUSIONS

The cause of the scale _{effect in} -him-ing model tests of full-bodied ships is a change in the hydrodynamic force on the hull of the model which takes rise as the size of the model is varied. The change has a crisis nat-ure characteristic of unsteady break-down of the flow.

Of decieive importance

for

_the

occur-rence of the unsteady breakdown of the flow during manoeuvrability tests is the circulation of the mode/. The zone of the flow breakdown lies in the region of the bilge of the side external to the center of

circu-lation.

Use of models with _{sand-roughened} sur-face is an effective method _of stabi-lizing the flow breakdown around the model. It helps prevent the scale ef-fect when small models are used to estimate the manoeuvrability of

full-bodied ships.

-806-REFERENCES

Okamoto, H., Tamai, H., _{and Oniki} H., "Correlation Studies of Manoeuvrabili, -ITTC, 1972, pp.227- .

-ty of Full Ships", Proceedi

s of the 1 th Okamoto, H., Tamai, H., _{and Oniki,} H., "Some Experimental _{Studies of} Manoeuv-rability of Ships", Proceedings _{of the}

14th ITTC, 1975, _PP.591-605.

gomoto, K., and Fujii, _{H., "Studies} of Model-Ship Correlation _in Manoeuvrabili-ty by Use of Large Size

Free-Running Mo-dels", Proceedings of the _{14th ITTC, 1975.}

1313.3T)Riano, H., and Asai, Sh., _{"On the}

thusuel Phenomena in Manoeuvring

Motions of a Full Ship Model", Mitsubishi

Technical

Bulletin, No. 116, 1976.

Sato, S., and Nakamura _I.

"Some

Conapect for ULCC with Small

Selected papers from the Journal _of

_Sodi

of Naval Architects of Japan, _{Vo .1 , 4,}

Treshchevsky, V. N. Volkov _{L. D.,} and Korotkin A. I., _"Aerodynamic Elperi-ment in Shipbuilding",

Sudostroenie, Lenin-grad, 1916, pp.89-102 (in Russian).

Treshchevsky, V. N., and _Korotkin,

A. I. _{"Some Characteristics of the Flow}

around Ships at Different Drift _{Angles in} Shallow Water", Proceedings of the 11th Symposium on Naval Hydrodynamics, _1976,

pp.693-703.

Nikolaev, E. P., and Lebedeva, M.P., "On the Scale Effect in Ship Model Manoeuvring Tests", 16th ITTC _Newsletter, No.1, Oct. 1979.

1

NOWA

IL

tate.) 111

Fig. 1 - Hull Lines of Tanker

Pig.

2

Hull Lines of Model With.

an

Increased Width of Deadwood

-15-o-.6- Large Models

s

Small

Smooth

Model

Stall Sand-Roughened Model t'

Fig. 3 - Comparison of Turning Curves of Geosim Tanker Models

Large Model Small Smooth Model

Small

Sand-Roughened Model 0

Pig. 4 - Comparison of Turning Curves of Models with an Increased Width of

Deadwood

k;

WL

uu

0,.03 0.02 0.01 -0.01 Large Model

Small

Smodth Modei . Small Sand-Roughened Model

krT

--70)

\i

5D P w

0,'LL ER= De

-5

II

6R- 5°

0

SB --= 10°

Fig. 5 Comparison of lateral Forces on Models' Rudders (Starboard Circulation)

Littge Model

Small Smooth 'Model

Small Sand-,

Roughened

Model

Fig. 6

= Full lateral Force on Model's Naked

Hull

Large Model Small Smooth Model .Small Sand-Roughened Model

0.2

0.1

Fig. 8 - Influence of Rn on Hydro-dynamic Force on Model's Naked

Hull

-809-R 5

- - 0 - -

3 . 5

R 2

Fig. 9 , Influence

_of..Rn

_On Hydro-dynamic MoMent

_on

_{MOdel'a Naked .Hull}

large yodel Small Smooth.

Model

Small Sand-Roughened Model

Fig. 10 Full Lateral Force on Self-Propelled Model's Hull (Starboard

Circulation) -810-Large Model SP411: Smooth Model Small Sand-Roughened

gool

Fig. 11 - Moment on Self-Propelled :Model's Hull (Starboard Circulation)

---- Large

Smooth Model Small Smooth Model Stall Partially Saad,Roughened Model

P

SB

5_

200 15°1O°

502:34r0 150

1:.

t

eq'

e'

Fig, 12 Turning dii±te of Partially

Saad=RoUghened Model

-Discussion

M.Fujii10(Univ. of Tokyo)

The authors should be congratulated for presenting the very interesting results of experiments the purpose of which is to enable one to predict the

manoeuvring

chat-acteristics of

'a

full-scale ship from the

results of testwith a small model. Out of many illustrations, I. am interested in figures 6,7,10, and 11 which show the

hydro-dynamic force and moment on the naked and

the self-propelled models. The hydrodynamic

lateral force, for example, on the

self-propelled

model is considered as the

alge-braic sum of three dominant components; the hydrodynamic lateral force on the naked hull, the lateral force on the rudder be-hind the main hull and the propeller, and the hydrodynamic lateral force on the main

hull

induced by the presence of rudder. Needless to say, most part of the

hydro-dynamic lateral force on the self-propelled model is the hydrodynamic force on the naked

hull as clearly lihderstood by Comparing the lateral force of the large model shown in Fig.6 with that shown in Fig.10.

On the contrary, comparing the lateral force on the small smooth model shown in Fig.6 with that shown in Fig.10, it is found that the hydrodynamic lateral force on the small smooth model remarkably dif-fers from each other qualitatively as well as quantitatively according to whether the model is naked or self-propelled. What is the cause of this great discrepancy ? Moreover, I wish to know how the presence of the rudder and the propeller affects the flow pattern, especially the breakdown

phenomenon of the flow, around the-main

hull because the model of 2 m in length is one of the standard model sizes of our sea-keeping and manoeuvring basin of University of Tokyo.

K. Nomoto(Osaka Univ.)

I would like to congratulate the au-thors for their most interesting finding on manoeuvrability scale effect.

At the sametime there are many data indicating that the different rudder effec-tiveness between models and actual ships

is

a substantial source of manoeuvrability scale effect. For single-screw ships, model rudder is normally more effective than

ship's because of stronger propeller slip stream. This effect is sometimes cancelled by heavy wake of full-bodied models, however.

In this connexion I would like to ask a question. Do the authors consider their roughened model technique useful also for

fine hull forms, for example, destroyers and. container ships ?

-812-H. Tamai(Akashi Ship Model Basin)

By Using sand-roughened surface models, the authors have shown the favorable results as for the scale effect. I

would

like to congratulate the- authors for their interest-ing work.

In the present case, the average grain Size for surface-roughening was described to be abt, 1.5mm, I would like to know how to determine the grain size.

The sand-roughened surface would in-crease the propeller slip ratio in a free running model and result in the relatively

larger rudder force in comparing with that of large smooth models. That may cause some unfavorable effect on the stability index (T) derived from the dynamic model test like the zig-zag test- From this view point,'I suppose that the area for

sand-roughening should be limited only to the necessary zone.

In Manoeuvrability tests by models of blunt hull form, abnormally stable

charac-teristics are sometimes obtained. Previous-ly, Prof. Nomoto confirmed, by flow survey,

the =usual flow like separation of

boun-dary layer at the stern of the outside of turning circle and explained that this un-. usual flow pattern is the cause of the ab-normally stable characteristics (11th ITTC). The- unsteady flow, described as the cause of the scale effect, in_ the present paper and the flow survey result by Prof. Nomotd seem to have a common feature except for their occurrence zone. I feel deer, interest

in using the- sand-roughened surface for blunt hull form model which Clearly has the abnormally stable Characteristics in case of smooth surface.

Author's Reply

E. Ni)colaev(KSFIl)

1. I thank very much Professor M. Fujino for his very interesting and valuable discussions. He has raised two questions.

The hydrodynamic lateral forces on the self propelled small smooth model differ indeed quantitatively and qualitatively. It might be explained by the fact that un-steady breakdown of the flow occurs in both cases. Presence of a working propeller and rudder changes the

flow

pattern, which is reflected in Figs.6 and 10, but does not change the principal feature Of the flow, namely unsteady character of the breakdown. Taking this in View we made an attempt to stabilize the breakdown of the flow of self-propelled small Model by means of

sand-roughness.

The result was rather

suffi-ient as it might be seen from

Figs.6 and 10.

There is satisfactory

agreement between the

characteristics of large smooth model and

smA11 sand-rOughened model in

both cases: for bare and self-propelled hulls.

2. Now I have the pleasure to express

my gratitude to Mr. H. Tamai for his very

interesting dicussions concerning the im-portant questions of principal character that he has raised.

Sand-roughened surface really might

influence an exaggerated rudder force. To check this we carried out force measurements on the models' rudders. We find that this

influence is insufficient in the rudder helm range of + 10-°, i.d.

in

the range

em-bracing the hysteresis

loop.

Some exagger-ating Of the rudder force on the-sand-rough-ened model was found at larger rudder angles. This might lead to Some error in estimation of the tactical diameter and may cause an unfavourable effect on the zig-zag test results too, if the rudder angles are too

large. It seems to me that such a test is

hardly appropriate- for the purposes of evaluating of stability characteristics due to strong non-linearities Inherent to a.

full-bodied

-The proposal to test partially sand-roughened models seems to be attractive if one takes into account results s/lOWn in Fig.11- It's a pity we can't define the necessary and sufficient zone_

I value very highly the results ob-tained by Japanese Colleagues in the field of manoeuvrability scale effect. Especially

interesting results were reported by Prof. K. Nomoto. Phenomenon observed by Prof. K. Nomoto and the one described in the pre, sent paper are not similar in one more

respect besides the place of breakdown zone.

From my viewpoint in the case described by Prof:. IC Nomoto, there was a stable

break-down of the boundary layer that generated a strong flow behind the model which im-proved her course stability. Here we deal with unstable breakdown of the flow on the

hull of a small model. Stabilization of the breakdown by means of sand-rougheness makes small blunt models more unstable. This is similar to large models with res-pect to dynamic stability Characteristics.

If

I was right

in

suggestion that, in case of abnormally stable Characteristics, the breakdown Of the flow on the blunt model was stable, then I would hardly expect any

positive result due to sandroughening such a model.

And one more question concerning the

grain size measurement. We used for this purpose a micrometer.

I at Very pleased by the fact that

Professor K. Nomoto, one of the first

in-vestigators of the manoeuvrability scale

effect, has paid attention to our paper. He has raised the important question if the sand-roughened model method is applicable for slim-bodied models. In my opinion this method might be effective in cases of unsteady breakdown of the boundary layer

Which behaviour strongly depends on the

size of the model in a manner characteris-tic to crossflow breakdown around a blunt body. That occurs when we deal with full-bodied models. I at not very optimistic

regarding the extrapolating of the method to fine hull forms.

Finally I would like to thank once

again Prof. M. Fujin°, Mr. H. Taipei and Prof. K. Nomoto for their discussions and to express my gratitude to the Japanese

Colleagues who have carried out so many

original and important investigations in the field of manoeUvrabilitY scale effect and who provoked in a way the presented paper.