The use of cavitation tunnel experiments to avoid full scale effects

A RCHIEF

THE NORWEGIAN SHIP MODEL EXPERIMENT TANK.CABLE: SKIPSTANK PHONE: 28020 SKI PS MOD E LLTAN KE N

SYM POS UM

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ON TES]1NG TECHNQUES

IN SHIP CAVITA11ON

RESEARCH

PREPRINT OF

The Use of a Cavitatiofl Tunnel in Preventing Full Scale Defects.

THE USE OF CAVITATTQ TUNNEL EXPERIMENTS

TO AVQID FULL SCALE DEFCT$

A. Enierson,

University of Newcastle.

Introduction.

The first cavitation tunnel was iriade so that thrust

breakdown could be studied on the model scale.

These experimerts

showe

the nature of the cavitation problem and, with more

precise

methods of design it becrne possible to avoid unexpected

thrust

breakdown on merchant ship propellers.

The emphasis shifted to

the aVoidance of rapid erosion.

Cavitaton tunnel experiments

at this sg were mislea4ing because they

did not represent the

ship popeller con4itiQp; correctly.

Mpt Of the progress i

practical design depended on advances in theory and inference

from

full scale performance.

During ±his period propelle's could

usually be designed with a margin of safety in the

form of extra

blade area.

More recently the increase in size and fullness of

single sOrew ships has m&de the design problem more acute.

With

the large variatiOn in flow conditions at the stern, excessive

blade area no longer provide

m.rgin ofsafèty;

.voidance of

back erosion easily leads to face erosion.

Tunnel methods have iprov?d and $n uniform flow tlie

conditions can be set and observed with sifficjent accuracy.

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erosion histories can afford the necesary guidance tO the

de-signer.

The use of tunnel experiments to avoid rapid propeller

erosion continues, but a related problem is increasing in

importance.

A' propeller may be designed so that it has

a reasonably

long life, but the noise and vibration at the stern

may be

in-tolerable.

_{A substantial part of this difficulty may be avoided}

by making use of existing towing tank procedures.

_{These allow}

the observation of streaixi.line flow on the hull, wake measurements

and wake analysis.

_{If carried out in time they can allow redesign}

of the after body and can provide guidance on the most suitable

number of propeller blades to avoid vibration.

The main

weak-nesses in this approach lie in the difficulty of obtaining.

sufficient accuracy and in predicting the ship wake from model

wake measurements.

_{The incorrect pressure conditions, which could}

be corrected by transferring ti-le ekperiment to

a cavitation tunnel,

are less important.

_{It would not be difficult to estimate the}

changes in local thrust and torque due to cavitation.

Apart from the vibrations exdited by wake fluctuations,

there are noises and disturbances at the stern which

ocOur at

random intervals of time.

The noise is associated with cavitation.

Itis convenient, to consider, the use of tunnel

experi-ments in two parts., first dealing with prevention of erosion t

the propeller and then with the prediction of noise and damage!

to the hull.

Avoidance of rapid propeller. erosion.

(a)

Uniform flow tests.

Consider first the simplest representation of the working

propeller: the model propeller at the estimated ship propeller

thrust. coefficient in

ünifórm stream and at a cavitation nurner

corresponding to the mean speed. of

dvancê.

The mOdel propeller

then shows an "average" picture of the cavitation, and in

consi:-dering possible cavitation damage this simplification may have

advantages.

For the single screw ship there will be large

dev-iations of short duration corresponding to the increased incidence

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at the top of the. aperture, and smaller deviations with reduced incidence 1atiig for a nuch'-longer part of each revolution. It

is not easy to deduce the relative importance Of the factors

in-volved, - the different type of cavitation on the blade back, the different duration - but the comparison of model piture and fi4l

scale erosion indicates the importance of any trace of face cavi-tation in. the model picture. FOr this reasn the "average" picture, which understates the alarming cavitation on the blad back, i

a useful. guide to possible erosion areas. The ship prOpeller does

not operate at only one coriditiofi even

_'the

repres-entation would show the model propeller with a range o advance

coefficient J and cavitation number . An example is given in

Fig. 1 for a 16TT diameter model of a six bladed propeller for a large tanker. .

JO35O. Oi656.

LOADED

TRIAL

CONDITION.

JO.34I. O= 19.75.

SERVICE

CONDITION.

/

/

J=0368, CII33.

BALLAST

TRIAL

CONDITION.

MEAN CAVITATION NUMBER.

A more complete picture of the successive conditions encountered by each propeller blade in óne revolution can be ob-tained b using the est:ated ship wake velocities to calculate.

the local values of J atid a. These instantaneous steady state cOnditions can theiibe taken from the uniform flow tests as shown

L.

/

/

J=0158.

J=O.290.

J=O.475. J=O.5O1.

J=O.449.

O'6&2.

O2I4.

O'9.O2. O'9O4. OII.76.

LOADED; TRLAL CONDITION.

FIG. 2.. UNIFORM FLOW REPRESENTING LOCAL

CONDITIONS AT d 459d; I35AND I8d

FROM TOP CENTRE POSITION OF BLADE.

It is known that when the incidence of aerofoil sections

is altered rapidly there is a hysteresis effet in the lift

co-efficient. It is necessary to consider whether the bhanges in

incidence as the propeller blade turns through one revolution

cause appreciable changes in lift and in cavitation from the steady

state values. The tanker wake distribution was created in the

tunnel by means of a wake grid - actually a 1/8" steel pate with

an array of 318'! diameter ho]es - placed three feet ahead of the model propeller position. The water speed in the tunnel ws then adjusted so that the propeller operated at the service thrust

coefficient. Th sketches jn Fig. 3 should therefore compare. directly with those in Fig. 2.

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,

/

J=0158 J=0290 J=0475 J=0501

J=0449

a482. 02I4.. 0Q2. 0O4. c= 11.76.

LOADED TRIAL CONDITION.

FIG. 3. WAKE SIMULATED BY GRID. CAVITATION AT

O 459O I35AND I8O FROM TOP CENTRE

POSITION OF BLADE.

A1hogh it -is not possible to be peci$e abOUt the tunnel

con-ditions when a propeller is working in non uniform flow, there is little evidence of any difference between c:avitation in a transient c.pndLt±on and n the same steady state condition..

The ship propeller after about one year in service shows some TTbrightening" or "polishing" on the back near the tip and the face erosion near the leading edge shown in the photograph (Fig 4)

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FIG. . SHIP PROPELLSR FACE.

(b) Non uniform flow experiments.

Reference has already been made to the use of a wake grid to produce a particular velocity distribution - to simulate

a ship wake. With this arrangement it is possible to see the variation in cavitation with propeller blade position and this is

a valuable demonstration of the effects of velocity variation.

When a grid is used to give a non-uniform flow it also introduces

its own turbulence - another unknown influence. It is a little

difficult to believe that the velocity distribution produced by an upstream wake grid will be affected by the propeller action in

the same way as the flow round a ship hull. (This comment applies also to the use of variable streams in the tunnel section).

Con-sidei for example the conditions at the top of the aperture, shown

7

il-- ____r2r -.

FIG. 5. PROPELLER BEHIND MODEL HULL.

This naturally leads to the use of a model hull in the tunnel to produce the correct propeller conditions. The

dis-advantages arise mainly from size. If there is adequate clearance

round the model hull so that the tunnel wall does not affect the velocity distribution produced by the model hull, then the model

is small and the experiments are subject to large scale effeOts. For large model propellers at high values of Reynolds? Number, direct representation of the ship propeller conditions is not un-reasonable. As the models become smaller the dangers of scaling

errors increase. When a compromise solution is attempted it is

necessary to distort the true hull shape or to have a variable

tunnel section; and to use devices such as wire gauzes to produce the required velocity distribution. So far as is known it is not

possible to have a simple representation of hull and propeller

interaction on a suitable scale.

There are clearly many ways of attacking this problem

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then thère is some Ios.s of accuracy in correcting local velocities

for tunnel wall interference, or

n other words, in knowing hpw

the propeller action affects the velocity dstributioti.

In sbme

non uniform flow methods the grid turhuience.affects the propeller

action.

None as yet represent the free surface condition and there

is still some doubt about how to obtain a ship wake distribution

from a measured model distributiOti.

To avoid the difficulties on

full scale discussed in the next section it will be necessary to

overOome these objections .to wa.ké simulation in the tunnel.

obtain informatiqn about possible popeler erosion there

are:ad-vantages in using uñ-iform flow tests.

Noie

id damage to the huh.

Trial trip and voyage reports from full single screw

ships describe the disturbance experienced as ran4om bangs, or

peaks in the noise level, and assOciated local vibration in the

structure tiear the propeller.

The noise suggests cavitation as

the cause.

In some ships the disturbances are much worse than

in others.

How can tunnel experiients prevent bad cases occurring?

In testing the 16" diameter six-bladed propeller behind

the wake grid, there was apparent interfëretice be.tween the

cavita-ion produced on adjacent blades near the top of the propellex disc.

In general as a blade makes otie revolution the cavitation at any

p6ition is a result of the local veloc-ity and pressure conditions

at that position.

In this case., near the top, the cavitation on

one blade affects the cavitation on the following

blade.

"following" blade ca±'ries a. larger cavity and so has a bigger

effect on the next blade.

After a succession of about 18 blades

(three revolutions) the mass of cavitation between the blades

-

re.Ohes a maximum, the bubbles are ejected upstream and collapse,;

the cycle starts again.

Figs. 6 and 7 show photographs taken at

-FLG. 6.

The nOise level rises sharply as the bubbles are ejected

and collapse.

bangs d

not Ocir at regi.Ilar i.rtervas bu

average abOu

one per th-tee revolutions.

This average freqency

in ternis of

evQ4utioris is apro

ately the same when the

experi-rnent is repeated at a much lower speed.

Because Of the aflalOgy with ship propeller observations

it seethed worth while making a fiuir and Cound iecord of

ex=

periment.

The SecOid part of the film is taken by contthuoUs

lighting so that the bUbb]

bursts and bangs Oan be noted.

It is

perhaps prSraatu'e ±0 draw conglusions at tiis stage.

The

pheno-menon occurS With ãiternaive

designs of. 16" six-bladed popellers

with appreOible differences in cavitation on each propeller.

t

has riot been visi1e on Sma.11er

propeJers behind a model hull..

It

is co

dered that this build up and collapse haS oOdurred

for-tuitouSly bCcaus? of the hydraulic pump drive tO -the

opefle

shaft and the variation in thrust and torque on

the 16" model

pro-pelle±.

The tentative conclusion is that to study the

possible

propeller excited digturbances behind

ship the usC Qf a dynamic

model to represent the engine-shafting-propeller system IS at

Acknowledgements.

Most of the mate'iaJ. used in this paper has been drived from the joiit research progPamme of Messrs. StoneManganese

Marine and the University.

The ue of.a modei hull the Tuinéi. shown in Fig. 5

is part of an investigation made in collaboration With Mr. D.;I.

Moor, Superintendent of the St. Alan's and Dunbarto Tanks of Messrs. Vickers Limited.

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