MEDDELANDENFRAN

STATENS SKEPPSPROVNINGSANSTALT(PUBLICATIONS OF THE SWEDISH STATE SHIPBUILDING EXPERIMENTAL TANK)

Nr 43 GOTEBORG 1958

THE CAVITATION LABORATORY

OF THESWEDISH STATE SHIPBUILDING

EXPERIMENTAL TANK

BY

HANS LINDGREN

GUMPERTS FORLAGGOTEBORG

GtfTEBORG 1958

ELANDERS BOKTRYCKERI AKTIEBOLAG

1. Introduction

Since 1910, when Sir CHARLES PARSONS built the first cavitationtunnel in order to study the phenomenon of cavitation on marinepropellers, the importance of cavitation laboratories and the interestin their work has grown rapidly.

New laboratories have been built throughout the world and between40 and 50 cavitation tunnels are now in operation. The largest ofthese, the Garfield Thomas Water Tunnel, is situated in the smalluniversity town of State College in Pennsylvania USA. This tunnelis about 30 m long and 9.5 m high, and has a test section 1.2 m indiameter. The water is pumped round the tunnel by means of a2000 HP electric motor. At the present time, large new cavitationlabciratories are under construction in England, Germany, USA andYugoslavia.

The detrimental effects of propeller cavitation include materialerosion, vibration, noise and reduction in efficiency. Due to technicaldevelopments in ship design, not only warship propellers but alsothose of cargo ships of various types now frequently work underconditions conducive to cavitation. In Sweden likewise therefore, ithas become desirable for several reasons to increase the experimentaland testing facilities in the field of propeller cavitation.

The Swedish State Shipbuilding ExperimentalT ank (SSPA) has, for several years, been seeking a grant to builda large cavitation laboratory. Model propellers of up to about 0.5 mdiameter could be tested in the proposed cavitation tunnel. It wouldalso be possible to carry out cavitation tests on propellers in conjunc-tion with normal ship models, i. e. the flow conditions at the pro-peller could be made similar to those prevailing ih the ordinary self-propulsion tests. It was also intended that the tunnel should beused to study flow conditions and cavitation on torpedoes and otherbodies.

Before building such a large cavitation tunnel, which would cost3 to 4 million Swedish crowns, it was considered advisable to builda smaller prototype which could be used both for flow studies to

4

determine the most suitable shape for the large tunnel and for theinvestigation of certain fundamental marine problems. It would alsoenable cavitation research to be undertaken at SSPA without furtherdelay.

A cavitation tunnel based on the above requirements, and suitablefor cavitation tests with model propellers of the size normallyemployed in the ship model tank, is now installed in the mainbuilding of SSPA. A description of this cavitation tunnel and itsequipment is given below, while the final section of this paper dealswith some interesting experimental problems which are at present.being investigated in the cavitation tunnel.

2. Outline of Cavitation TheoryWhen a propeller blade is moving through the water there is-

normally a suction on the forward side of the blade (Fig. 1). If at.some point the pressure falls as low as the vapour pressure ofthe water at the temperature in question, the water begins to-boil and bubbles of water vapour appear, i. e. the propeller begins.

-2.0

Fig. 1.

Region of cavitation

for GrI.8

where

advance coefficient JDn

torque coefficient Ke -e D5

thrust coefficient Ke D4 n2

VE

2

VE VS (1 w)Vs = ship speedw = wake fractionD = propeller diametern = propeller revolutionsQ -= propeller torqueT = propeller thruste = water density

5

to cavitate. When a water vapour bubble is carried by the waterstream across the propeller blade into a region of higher pressure,it suddenly collapses and disappears. This happens very rapidly andthe water around the bubble comes together with such force that itcan cause a damaging mechanical action on the material of the pro-

, Teller. Cavitation can thus lead to erosion of the propeller.This gives a simplified outline of the theory of cavitation. In

.actual practice, however, it is considerably more complicated. Forexample some of the gases -dissolved in the water may come out ofsolution before the water pressure falls to the vapour pressure andcavitation conditions may also be influenced by chemical and electro-lytic processes.

Cavitation can also occur on other bodies in motion in water. Forexample, there may be a risk of cavitation around the forward partof a torpedo travelling at high speed.

The characteristics of cavitation can be studied in cavitation tunnelsby means of experiments on models of actual propellers and otherbodies.

In the case of the normal so called »open waters tests with modelpropellers in a model testing tank, the speed, revolutions, torque andthrust relationships are usually expressed by means of the followingdimensionless coefficients:

6

At any particular value of J, the respective values of KT and KQ.will be the same for the model propeller and the full size propeller,i. e. the thrust and torque of the ship's propeller can be calculatedfrom the characteristics of the model propeller. This, however,presupposes that no account need be taken of propeller cavitation.

If there is any risk of cavitation occurring, a complete comparisonbetween a model propeller and its full-scale counterpart necessitatesa consideration of the so called cavitation number, a. This number,which for true uniformity must be the same for both model andship propeller, is defined by the expression:

Po ea

1/2 p 171

where p, = the static pressure at the axis of the propellere = the vapour pressure of the water at the appropriate

temperature.

Since V E is normally considerably higher for the ship than for'the( model, it is necessary to lower the pressure for the model test-in order to achieve the same value of G. This is, of course, impossiblein an open model tank and for this reason a closed tunnel is used,in which the pressure can be varied as required.

3. The Cavitation Laboratory at SSPAThe building of the cavitation tunnel was begun in May 1956.

It was ready for installaticin in Gothenburg in the Summer of 1957and is now (Spring 1958) in full use after carrying out checking,calibration and adjustment work during the Autumn of 1957. Thetunnel was fabricated and to a large extent erected by a specialistGerman firm, Kempf and Remmers of Hamburg.

The cost of constructing the tunnel, providing the necessary equip,ment, installation work and preparation of the site amounted toabout Svv. Kr. 500,000. The tunnel and equipment were paid for by

Navy while the other costs Were met from a State Fund.The cavitation laboratory is situated at the southern end of the

ship testing tank. In addition to the cavitation tunnel (Figs. 24), its driving motors and measuring equipment, there is a,

concrete tank for storing water: This is'in the basement and holds-about 9 m3 of water, The water, which is emptied from the tunnel

14,1511kneh-

+ow*

-

I-

7

Fig. 2. The cavitation tunnel assembled for testing at the manufacturers' worksin Germany.

when changing a propeller or other apparatus, is stored in this tankand can be pumped back again to the cavitation tunnel by meansof a small pump. In the basement (under the ship testing tank)there is also a transformer and Ward Leonard set together withother electrical equipment.

A specially designed 2-ton lifting beam has been provided on theupper floor. This lifting beam is used for handling apparatus andwhen changing the test section of the tunnel

The tunnel is provided with two readily interchangeable testsections. The smaller one has a section 0.5 m x 0.5 m, and is intend-ed for experiments with propellers and other bodies at very low

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cavitation numbers. In tests in this section, cavitation numbers

(a e ) below 0.15 have been reached without noticeable1/2 e

cavitation occurring in the tunnel itself.The larger test section is generally employed for investigating

cavitation on propellers in non-homogeneous wake fields and alsofor studying flow conditions on streamlined bodies. In order tostimulate the wake field behind, for example, a cargo ship, a modelof the after body complete with rudder can be fitted in the tunnelThe large test section is also intended to be used for preparatoryexperiments for the large cavitation tunnel project.

4. Hydrodynamic ShapeThe optimum shape of a cavitation tunnel, from a hydrodynamic

point of view, depends entirely on the tests for which it is to beused. In the case of the SSPA tunnel, it was specified that it shouldbe suitable for tests on

model propellers of the size normally used at SSPA (about230 mm diameter) in both homogeneous and non-homogeneouswake fields andstreamlined bodies of the largest possible dimensions.

It was also considered desirable that the tunnel should operate asfar as possible without noise in order to facilitate acoustic investig-ations. The tests in connection with the large cavitation tunnelproject had to be borne in mind and a further requirement was thatthe laboratory should be ready for use as soon as possible.

Largely on account of the last requirement, the tunnel has a moreor less conventional shape. In fact an already well-tried design,which could be supplied on quick delivery by the German specialistfirm, was accepted with minor modifications, and a new larger testsection, which has proved to be very useful, was also constructed.

The tunnel consists of two horizontal and two vertical legs (Fig. 4).The test section is approximately in the middle of the upper hori-zontal leg and has two large windows on each side, through whichthe subject of the test can be observed (Fig. 5). After the testsection, in the direction of flow, there is a long diffuser and thesectional area increases as far as the first downstream bend. Thisserves to decrease the velocity of the water and increase the pressure

11

Fig. 5. Test section windows.

so that the risk of cavitation in the critical region at the vanes inthe bend is reduced. After the bend, the water passes into thevertical leg, where the area is maintained constant, and is thenredirected again in the lower bend. Ahead of the impeller whichpumps the water round the tunnel, there is a changeover piece inwhich the section changes from square to round. Behind the impeller,the area increases in a circular diffuser and the section then changesback again from circular to square. The water passes through thelow speed region, i. e. the large lower bend, the other vertical leg,the large upper bend and the honeycomb at the beginning of theupper horizontal leg, and is then accelerated in a nozzle to the higherspeed required in the test section.

This cavitation tunnel differs to some extent from older moreconventional tunnels in that it is comparatively long and also thatthe test propeller is driven by an upstream shaft.

The length was dictated partly by the need for a long test sectionin which long streamlined bodies could be investigated and partlyby a desire for a long diffuser ahead of the upper downstream bend.The long diffuser serves to increase the pressure and reduce thevelocity immediately ahead the bend and thus minimises the risk

12

0-

Tanker20 15 knots

. Cargo Ship25 20 IS knots

00.

Destroyer40 30 20 knots

Motor Tortiedo-Boati II I I -I3I 60 50 40 0 knots

TorpedoII41 I I I I80 70 60 50 40 knOts

0.1 0.2 0.3 0.4 0.6 0.8 1.0 2.0 3.0 4.0 6.0 8.0 10.0

p -e7 , v2

1/2 p

Fig. 6.

of cavitation in the bend itself, which is a critical region. The angleof entrance of the diffuser must be small in order to prevent eddyingand cavitation in the diffuser.

In the case of the small test section, the area is approximatelydoubled between the test section and the first downstream bend,i. e. the cavitation number in the bend is about 16 times that in thetest section. The small test section can be used for tests at cavita-tion numbers down to about 0.15 (Fig. 6).

The area of the large test section is approximately the same asthe area at the bend, so that the cavitation number remains virtuallyconstant. This means that tests at very low cavitation numberscannot be carried out in the large test section without cavitationoccurring in the downstream bend. In practice, a = 1.5 forms areasonable lower limit for tests in this section. An indication of thevalues of a obtained in different tests is given in Fig. 6.

The cross-sectional area of the small test section, 0.25 m2, was

10%13 8a,

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Body ..L/D=14 ..''' ..-., ...--

10.. ..---....- ..--

..-- --...-- --- ---- 6-- __ ------- ---- --

13 I

calculated to be sufficient to allow tests with SSPA's normal pro-pellers without noticeable influence from the walls of the tunnel.At the same time, there is enough contraction in the nozzle withthis test section (area ratio 1:6) to give a homogeneous velocity fieldover the cross section. In order to minimise longitudinal pressureand speed variations in the test sections due to the increasing thick-ness of boundary layer, the walls of the test sections have been madeslightly diverging. Preliminary calibrations have indicated that theflow quality is good in both the small and the large test sections,in spite of the fact that the contraction in the nozzle of the latteris as low as 1:4.

The maximum water velocity is about 11 m/sec. in the small testsection and about 6 m/sec. in the large one.

The degree of influence of the tunnel walls in tests on streamlinedrotationally symmetrical bodies has been deduced theoretically. Thediagram in Fig. 7 shows how the water velocity in such tests isaffected by the limited cross-sectional area of the two test sections(blockage effect). Approximate corrections can, of course, be appliedto compensate for this effect.

The propeller is mounted on an upstream shaft and the flowconditions behind the propeller can therefore be determined morerealistically than if a downstream shaft had been adopted. It isthus possible to study vortex formation behind the point of theboss cone (Fig. 8). Also the whole dynamometer assembly, with

14

Fig. 8. Cavitating propeller with boss vortex.

the driving motor, dynamometer and propeller shaft, can be movedaxially, so that it is possible to investigate the effect of altering theposition of the propeller in the aperture.

Certain measures have been taken with a view to reducing thenoise level as far as possible. In the first place, the nozzle and thevanes in the bends have been very carefully designed in order tominimise the hydrodynamic noise. The vanes are shaped like bentstreamlined profiles. In this connection, account has been taken ofthe results of a large number of tests which have been carried outin USA.

In order to reduce external noise and to prevent noise beingtransmitted to the test section, the tunnel has been mounted onrubber blocks and the connections between sections have been rubber-

:

- ......wmalftwalw'meromme°4 wit-*way

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15

Torque, Q 5 mkg

40 50 3*C.I I I

2500 3000 3500 yrnin.

Propeller Revolutions

Fig. 9.

insulated. The impeller shaft is mounted in a rubber gland in thewall of the tunnel while the sliding bearings of the propeller andimpeller shafts are made of poly-amid material with a low coefficientof friction.

5. Measuring ApparatusThe capacity of the torque meter and the propeller driving motor

required for different revolutions and water velocities has beencalculated for tests with propellers of different diameters, pitch ratiosand blade area ratios (Fig. 9). The curves are based on systematic

10 20 301 I I 1

500 1000 1500 2000

16

open water tests and assume an advance coefficient J = 0.75where J. is the J-value corresponding to maximum efficiency.Similar curves have been derived for the required capacity of thethrust meter. The following values have been determined from thecurves:

Propeller motor

Torque meter

Thrust meter

maximum

maximum

maximum

maximum

power

revs.

torque

thrust

21 HP17 HP3500 r/min3000 r/min.5 kgna4 kgm120 kg

(limited periods)(continuous loading)(limited periods)(continuous loading)(limited periods)(continous loading)(continous loading)

The stator of the motor is freely suspended and is connected to abalance on which the propeller torque is weighed by means ofweights. Small variations in the torque can be read on a scale(Fig. 10).

The thrust is transmitted through a thrust bearing to a balance onwhich the force is measured in a similar manner to the torque.

Two pairs of contacts are mounted on the end of the propellershaft. One pair form a circuit to give an impulse at each revolutionto a counter, which thus indicates the propeller revolutions. Theother pair gives an impulse every revolution to the stroboscope andis arranged so that a picture of the propeller cavitation can beobtained with the propeller in any desired position.

The DAWE type stroboscope gives a short flash (30 its) once perrevolution. Due to the high frequency and short duration, the eye isnot normally able to distinguish the separate flashes and the pro-peller appears to be standing still in even light. The stroboscope isalso used to provide single flashes for photographic purposes and inthis case a condenser is used to increase the light output of thestroboscope.

The water velocity in the tunnel is normally measured on theventurimeter principle, i. e. the fall in pressure in the nozzle ismeasured by means of a manometer. A mercury manometer can beused for high speeds and a water manometer for low speeds.

The difference between the pressure on the model under test andthe atmospheric pressure is measured by means of a mercury mano-

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18

meter. The absolute pressure is determined by measuring the atmos-pheric pressure, using an accurate aneroid barometer.

The temperature of the water can be read on a built in thermo-meter near the upper upstream bend:

6. Electrical EquipmentAs mentioned previously, the Ward Leonard set is housed in the

basement and consists of an A. C. motor (88 kW), two D. C. gene-rators for the impeller and propeller driving motors and an exciter.The latter provides the excitation for the driving motors.

The armatures of the D. C. generators are connected to the arma-tures of the respective driving motors. The motors are thus controlledby regulating the generator fields and the revolutions can be controlledeither electronically or, if necessary, by hand. The electronic controlprovides particularly stable conditions and thus facilitates accuratetesting.

The power of the impeller motor is 70 HP and, as mentionedabove, this permits water velocities of 11 m/sec. in the small testsection (area 0.25 m2) and 6 m/sec. in the large test section (area0.49 m2). The revolutions can be controlled up to 1500 r/min. Thepower is transmitted to the impeller shaft by means of V-belts andthe pulley ratio is 1:3.

The propeller driving motor is rated at 12.5 kW, but it can beoverloaded for short periods. The revolutions can be regulated atall speeds up to 3500 r/min.

Starting the Ward Leonard set and setting the propeller revolu-tions and the water velocity can all be carried out from a controldesk placed beside the dynamometer in the test room. The vacuumpump (0.38 kW at 1450 r/min.) which is used for lowering thepressure in the tunnel to the desired value, is also operated fromthe control desk. In addition, the electronic equipment, which auto-matically controls the revolutions of the driving motors is mountedhere (Fig. 11).

7. Special EquipmentSome items of special equipment for increasing the uses of the

tunnel have already been obtained and others are at present beingdesigned or manufactured. The following are some of those itemswhich are now or soon will be available.

Fig. 11. Controls and instruments on control desk.

For the purpose of measuring local water velocities in the testsection and for calibrating the velocity distribution, there is a 6 mlong strut which can project into the test section from the upperdownstream bend and on the end of which is mounted an armholding 14 Pitot tubes (Fig. 12).. This arm can be rotated in thetest section or it can be moved longitudinally, so that a completecalibration of the velocity at various points throughout the testsection can be rapidly carried out.

A model of the after-body of, for example, a cargo ship can bemounted in the test section forward of the propeller, in order tosimulate the velocity distribution existing at the propeller; specialequipment which will permit an after-body to be installed relativelysimply, is at present under construction.

Two types of test which would be of considerable interest- arecavitation tests with contra-rotating propellers and cavitation testswith propellers in non-axial flow. Equipment is under constructionwhich will enable both these types of test to be carried out.

In the case of tests with contra rotating propellers, they will bemounted on concentric shafts and driven by the existing motor. Itis intended to measure the thrust and torque by means of wirestrain gauges.

After preliminary investigations, the necessary apparatus is nowbeing made to enable acoustic studies of cavitating propellers andnoise level tests of other types to be carried out. For such tests, abox is mounted against the plexiglass on one side of the test section.

19

20

Fig. 12. Pitot tube mounting.

The connection to the window is made watertight by means of rubbersealing and the box is filled with water. A hydrophone is then placedin the box to pick up the cavitation noise from the propeller or otherbody. This method appears to be suitable for determining forexample, the limits at which cavitation begins and ceases, but atthe moment, insufficient is known about the laws of scale to beable to use it for quantitative noise level measurements.

8. Experimental WorkIn addition to ordinary cavitation tests and special investigations

for the Navy and others, part of the work of the new cavitationlaboratory will be devoted to testing and experimental work of moregeneral interest. Some work in this category has already been begunand is described below.

Various cavitation criteria, evolved on the basis of experience,indicate the extent to which the blade area ratio of a propeller shouldbe increased if the cavitation number decreases, or if it is desired to

l) The numbers within brackets refer to the list of references on page 26.

21

increase the margin of safety against cavitation in any particulardesign. In such cases, it is usual to increase the area over the wholeof the blade i. e. both root sections and tip sections are lengthened.Generally, in fact, each section is lengthened in proportion to itslength, according to the method adopted in most systematic seriesexperiments with model propellers.

For the majority of loading conditions, however, the risk of cavita-tion is limited to the sections near the tip, while the root sectionshave a margin of safety against cavitation. This is the case at leastso long as the radial pressure distribution corresponds approximatelyto the BETZ optimum distribution. The length of the root sectionis subject to a lower limit due to strength requirements and at thesame time limited by the maximum thickness length ratio of theprofile to 0.18 or 0.20 from drag Considerations (see also Fig. 14).If therefore, it is necessary for any reason to reduce the risk ofcavitation in a propeller design, it would be logical to increase thelengths of the sections near the blade tips and leave the toot sectionsunchanged.

In order to test the results of this method and to obtain furtherknowledge on the reliability of the theoretical methods of design, afamily of three model propellers has been designed on the basis ofthe vortex theory [1].1) The propellers have all been designed for aloading. condition corresponding to

VEJ= 64Dn

KTD4 2

0.19e n

The propellers are thus suitable for a ship model which was testedin the course of another investigation (Model No. 720, see ref. [2]).They are designed for cavitation numbers of 4, 6 and 8 respectively(see Fig. 13), a value of about 8 being suitable for Model No. 720,but in order to allow a 20 % margin for scale effects and non axialflow, the theoretical design calculations were based on cavitationnumbers of 3.2, 4.8 an. respectively. The blade outlines areshown in Fig. 14.

The propellers have now been tested in self-propulsion tests with

22

12

10

8

2

012 14 16 18 20 22

Ship Speed, V. in knots

' Fig; 13.

Model No. 720, in open water tests in the towing tank and incavitation experiments in homogeneous flow. Tests in the cavitationtunnel behind a model of the after body have not yet been carriedout. The results obtained so far both confirm the theoretical regionof freedom from cavitation and show that as a (or J) is reduced,cavitation occurs in the critical region, Fig. 15.

According to the cavitation criterion of, for example, Prof. BuRRILL,a decrease from 8 to 4 in the value of a should be accompanied byan increase in blade area of 53 % in order to maintain immunityfrom cavitation. In this case however, an increase of 48 °/,, appearssufficient.

A complete analysis of the experimental results is being made anda full report will be issued as soon as the tests have been completed:

The experiments described above were based on the assumption thatthe radial pressure distribution on the propeller blades correspondsapproximately with the BETZ optimum conditions. With this distribu-

24 26

_

kIA,Ship' wake 'fraction, w= 30

NISIm. .... , = Propeller centreline ubelow

N ,,,,,% , ..'`I .,1\.

gt.."..._\-__ _

_..__

. _ _. .

Fig. 14.

tion, there is a considerably greater risk of cavitation at the outersections than there is at the root sections; by adopting a differentdistribution, however, and transferring part of the load from theouter sections to the inner sections, it is possible to reduce the lengthsOf the outer sections and still maintain the same theoretical marginof safety against cavitation. This, of course, implies abandoning theconditions corresponding to the maximum »ideal» efficiency, but byreducing the blade area ratio, the friction losses are reduced at thesame time.

It is planned to carry out experiments with a further series ofpropellers in order to study these conditions. The three propellersin the series are each designed for the same loading and the samecavitation margin in the critical blade region. The radial loadingdistribution, on the other hand, is different in each case.

An investigation of scale effects in model propeller tests has beendescribed in reference [3]. This investigation was concerned with afamily of four geometrically similar model propellers with diameters

00

23

24

Fig. 15. Propeller P755 cavitating at= 3.0 (design 4.8)

J = 0.64 (design 0.64).

of 150, 200, 250 and 300 mm. Experiments are now in progress todetermine the wall effect in propeller tests in the cavitation tunneland this family of propellers will be rested in both the small and thelarge test section of the SSPA tunnel A similar series of tests willalso be carried out using a family of four propellers each havingdifferent pitch ratios, but the same diameter and blade area ratio.

9. References

LINDGREN, HANS, JonNssoN, C. A.: *Propellerberakning enligt virvelteorien.Riikneexempel och hjalpdiagram», SSPA Allman Rapport Nr. 2, 1956.

FREIMANIS, E., LrNDGREN, HANs *Systematic Tests with Ship Models with6pp = 0.675, Part I*, SSPA Publication No. 39, 1957.

NORDSTRoM, H. F., EDSTRAND, HANS, LINDGREN, HAN'S: *On Propeller ScaleEffects*, SSPA Publication No. 28, 1954.

et,

25

10. AcknowledgementThe author wishes to express his gratitude to Dr. HANS EDSTRAND,

Director of the Swedish State Shipbuilding Experi-mental T a n k, for his valuable advice and to the staff of theTank for all their assistance.

Thanks are also due to Mr. P. D. FRASER-SMITH, who translatedthe paper from the Swedish.

ContentsPage

Introduction 3

Outline of Cavitation Theory 4

The Cavitation Laboratory at SSPA 6

Hydrodynamic Shape 10

Measuring Apparatus 15

Electrical Equipment 18

Special Equipment 18

Experimental Work 20

References 24

Acknowledgement 25