The cavitation laboratory of the Swedish state shipbuilding experimental tank

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MEDDELANDEN

FRAN

STATENS SKEPPSPROVNINGSANSTALT

(PUBLICATIONS OF THE SWEDISH STATE SHIPBUILDING EXPERIMENTAL TANK)

Nr 43 GOTEBORG 1958

THE CAVITATION LABORATORY

OF THE

SWEDISH STATE SHIPBUILDING

EXPERIMENTAL TANK

BY

HANS LINDGREN

GUMPERTS FORLAG

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GtfTEBORG 1958

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1. Introduction

Since 1910, when Sir CHARLES PARSONS built the first cavitation

tunnel in order to study the phenomenon of cavitation on marine

propellers, the importance of cavitation laboratories and the interest

in their work has grown rapidly.

New laboratories have been built throughout the world and between

40 and 50 cavitation tunnels are now in operation. The largest of these, the Garfield Thomas Water Tunnel, is situated in the small university town of State College in Pennsylvania USA. This tunnel is about 30 m long and 9.5 m high, and has a test section 1.2 m in

diameter. The water is pumped round the tunnel by means of a

2000 HP electric motor. At the present time, large new cavitation

labciratories are under construction in England, Germany, USA and

Yugoslavia.

The detrimental effects of propeller cavitation include material

erosion, vibration, noise and reduction in efficiency. Due to technical

developments in ship design, not only warship propellers but also those of cargo ships of various types now frequently work under

conditions conducive to cavitation. In Sweden likewise therefore, it has become desirable for several reasons to increase the experimental

and testing facilities in the field of propeller cavitation.

The Swedish State Shipbuilding Experimental

T ank (SSPA) has, for several years, been seeking a grant to build

a large cavitation laboratory. Model propellers of up to about 0.5 m

diameter could be tested in the proposed cavitation tunnel. It would

also be possible to carry out cavitation tests on propellers in

conjunc-tion with normal ship models, i. e. the flow condiconjunc-tions 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 be used to study flow conditions and cavitation on torpedoes and other

bodies.

Before building such a large cavitation tunnel, which would cost

3 to 4 million Swedish crowns, it was considered advisable to build

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determine the most suitable shape for the large tunnel and for the

investigation of certain fundamental marine problems. It would also enable cavitation research to be undertaken at SSPA without further

delay.

A cavitation tunnel based on the above requirements, and suitable for cavitation tests with model propellers of the size normally employed in the ship model tank, is now installed in the main

building of SSPA. A description of this cavitation tunnel and its

equipment is given below, while the final section of this paper deals

with some interesting experimental problems which are at present. being investigated in the cavitation tunnel.

2. Outline of Cavitation Theory

When 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 of

the 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 ofcavitation for GrI.8

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where advance coefficient J Dn torque coefficient Ke -eD5 thrust coefficient K e D4 n2 VE 2 VE

VS (1 w)

Vs = ship speed

w = wake fraction

D = propeller diameter n = propeller revolutions Q -= propeller torque

T = propeller thrust

e = water density 5

to cavitate. When a water vapour bubble is carried by the water

stream across the propeller blade into a region of higher pressure, it suddenly collapses and disappears. This happens very rapidly and

the water around the bubble comes together with such force that it can 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. For example some of the gases -dissolved in the water may come out of solution before the water pressure falls to the vapour pressure and

cavitation conditions may also be influenced by chemical and electro-lytic processes.

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

The characteristics of cavitation can be studied in cavitation tunnels

by means of experiments on models of actual propellers and other

bodies.

In the case of the normal so called »open waters tests with model

propellers in a model testing tank, the speed, revolutions, torque and thrust relationships are usually expressed by means of the following

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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 calculated

from 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 comparison between a model propeller and its full-scale counterpart necessitates

a consideration of the so called cavitation number, a. This number,

which for true uniformity must be the same for both model and ship propeller, is defined by the expression:

Po e

a

1/2 p 171

where p, = the static pressure at the axis of the propeller

e = 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, impossible

in 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 SSPA

The building of the cavitation tunnel was begun in May 1956. It was ready for installaticin in Gothenburg in the Summer of 1957 and is now (Spring 1958) in full use after carrying out checking,

calibration and adjustment work during the Autumn of 1957. The tunnel was fabricated and to a large extent erected by a specialist

German 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 to about 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. 2

4), 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

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14,1511kneh-+ow*

-

I-7

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

when changing a propeller or other apparatus, is stored in this tank and can be pumped back again to the cavitation tunnel by means

of a small pump. In the basement (under the ship testing tank) there is also a transformer and Ward Leonard set together with other electrical equipment.

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

The tunnel is provided with two readily interchangeable test sections. 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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To vocuum PE,o0

Doornometel tobIF

(mo.eable m rhe shof r

chrecho 61111. P, 21 HP =3500 r/rn -'POL4M..1:.MITAVN Test iecr,on (changeable) 2400 x700.700 rnm 2200 o500.500 mrn P=70 HP n=1 00/500 r/Hr Gonne, d to moron Fig. 4.

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cavitation numbers. In tests in this section, cavitation numbers

(a e ) below 0.15 have been reached without noticeable

1/2 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 also

for studying flow conditions on streamlined bodies.

In order to

stimulate the wake field behind, for example, a cargo ship, a model of the after body complete with rudder can be fitted in the tunnel The large test section is also intended to be used for preparatory experiments for the large cavitation tunnel project.

4. Hydrodynamic Shape

The optimum shape of a cavitation tunnel, from a hydrodynamic

point of view, depends entirely on the tests for which it is to be

used. In the case of the SSPA tunnel, it was specified that it should be suitable for tests on

model propellers of the size normally used at SSPA (about

230 mm diameter) in both homogeneous and non-homogeneous wake fields and

streamlined bodies of the largest possible dimensions.

It was also considered desirable that the tunnel should operate as far as possible without noise in order to facilitate acoustic

investig-ations. The tests in connection with the large cavitation tunnel

project had to be borne in mind and a further requirement was that the laboratory should be ready for use as soon as possible.

Largely on account of the last requirement, the tunnel has a more

or less conventional shape. In fact an already well-tried design,

which could be supplied on quick delivery by the German specialist

firm, was accepted with minor modifications, and a new larger test section, 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 which

the subject of the test can be observed (Fig. 5). After the test

section, in the direction of flow, there is a long diffuser and the

sectional area increases as far as the first downstream bend. This

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11

Fig. 5. Test section windows.

so that the risk of cavitation in the critical region at the vanes in

the bend is reduced. After the bend, the water passes into the vertical leg, where the area is maintained constant, and is then

redirected again in the lower bend. Ahead of the impeller which pumps the water round the tunnel, there is a changeover piece in

which the section changes from square to round. Behind the impeller, the area increases in a circular diffuser and the section then changes

back again from circular to square. The water passes through the low speed region, i. e. the large lower bend, the other vertical leg, the large upper bend and the honeycomb at the beginning of the

upper horizontal leg, and is then accelerated in a nozzle to the higher speed required in the test section.

This cavitation tunnel differs to some extent from older more

conventional tunnels in that it is comparatively long and also that the test propeller is driven by an upstream shaft.

The length was dictated partly by the need fora long test section

in which long streamlined bodies could be investigated and partly

by a desire for a long diffuser ahead of the upper downstream bend. The long diffuser serves to increase the pressure and reduce the

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12 0-Tanker 20 15 knots . Cargo Ship 25 20 IS knots 00. Destroyer 40 30 20 knots Motor Tortiedo-Boati

II

I I -I 3 I 60 50 40 0 knots TorpedoII41 I I I I 80 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 -e 7 , v2 1/2 p Fig. 6.

of cavitation in the bend itself, which is a critical region. The angle

of entrance of the diffuser must be small in order to prevent eddying

and cavitation in the diffuser.

In the case of the small test section, the area is approximately

doubled between the test section and the first downstream bend, i. e. the cavitation number in the bend is about 16 times that in the

test 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 as the area at the bend, so that the cavitation number remains virtually

constant. This means that tests at very low cavitation numbers cannot be carried out in the large test section without cavitation occurring in the downstream bend. In practice, a = 1.5 forms a

reasonable lower limit for tests in this section. An indication of the values of a obtained in different tests is given in Fig. 6.

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10

% 13 8 a, Z:.; 6 a, .c'_ 4 2 Z.; 0 0.05 Body - Test section 0.5 x 0.5 m L/D=14 0.10 0.15

Diameter, D, of body of revolution

Fig. 7. 0.20 m --Test section 0.7x 0.7m

.

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 with

this test section (area ratio 1:6) to give a homogeneous velocity field

over the cross section. In order to minimise longitudinal pressure

and speed variations in the test sections due to the increasing thick-ness of boundary layer, the walls of the test sections have been made

slightly diverging. Preliminary calibrations have indicated that the flow 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 latter is as low as 1:4.

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

The degree of influence of the tunnel walls in tests on streamlined rotationally symmetrical bodies has been deduced theoretically. The diagram in Fig. 7 shows how the water velocity in such tests is

affected by the limited cross-sectional area of the two test sections

(blockage effect). Approximate corrections can, of course, be applied to compensate for this effect.

The propeller is mounted on an upstream shaft and the flow

conditions behind the propeller can therefore be determined more

realistically than if a downstream shaft had been adopted.

It is

thus possible to study vortex formation behind the point of the

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14

Fig. 8. Cavitating propeller with boss vortex.

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

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

in USA.

In order to reduce external noise and to prevent noise being transmitted to the test section, the tunnel has been mounted on rubber blocks and the connections between sections have been

rubber-:

- _{...wmalftwalw'meromme°4 wit} -*way

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/

Al. -,1:0 N.-P --- ADIA 0 = 0.65

₀₎

1.. 20 AD/A0 0.20 cs

/

VE = 5 m/sec.

ei

Q

,k/

\

15 t.

0\/

ct

I

f /

._ _{4) \}

_00"

3. 1... ' ''' " -::3 \ ; 27 cu

. \ 0,

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2(3° 0 \ Do_ 4-- /0 _\,..d..0_N 0 -c I-P) 216, = 1.10 f61" D°2 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 the wall of the tunnel while the sliding bearings of the propeller and

impeller shafts are made of poly-amid material with a low coefficient of friction.

5. Measuring Apparatus

The capacity of the torque meter and the propeller driving motor

required for different revolutions and water velocities has been calculated for tests with propellers of different diameters, pitch ratios

and blade area ratios (Fig. 9). The curves are based on systematic

10 20 30

1 I I 1

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open water tests and assume an advance coefficient J = 0.75

where J. is the J-value corresponding to maximum efficiency.

Similar curves have been derived for the required capacity of the thrust meter. The following values have been determined from the

curves: Propeller motor Torque meter Thrust meter maximum maximum maximum maximum power revs. torque thrust 21 HP 17 HP 3500 r/min 3000 r/min. 5 kgna 4 kgm 120 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 a

balance on which the propeller torque is weighed by means of

weights. Small variations in the torque can be read on a scale (Fig. 10).

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

Two pairs of contacts are mounted on the end of the propeller shaft. One pair form a circuit to give an impulse at each revolution to a counter, which thus indicates the propeller revolutions. The other pair gives an impulse every revolution to the stroboscope and

is arranged so that a picture of the propeller cavitation can be

obtained with the propeller in any desired position.

The DAWE type stroboscope gives a short flash (30 its) once per

revolution. Due to the high frequency and short duration, the eye is not normally able to distinguish the separate flashes and the pro-peller appears to be standing still in even light. The stroboscope is

also used to provide single flashes for photographic purposes and in this case a condenser is used to increase the light output of the

stroboscope.

The water velocity in the tunnel is normally measured on the venturimeter principle, i. e. the fall in pressure in the nozzle is

measured by means of a manometer. A mercury manometer can be used for high speeds and a water manometer for low speeds.

The difference between the pressure on the model under test and

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mano-Eletric

motor with fre ly suspended

stator

.Dynomometer table

Contacti for reeolation. counfe

Contacts for stroboscaPl'

. ,wocihti Fig. 10. L_J ,Thrust balance

r- 011 plain; for shaft bearing lubrication

Propellei

shaft

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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 Equipment

As 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 controlled by regulating the generator fields and the revolutions can be controlled either electronically or, if necessary, by hand. The electronic control

provides particularly stable conditions and thus facilitates accurate

testing.

The power of the impeller motor is 70 HP and, as mentioned

above, this permits water velocities of 11 m/sec. in the small test section (area 0.25 m2) and 6 m/sec. in the large test section (area 0.49 m2). The revolutions can be controlled up to 1500 r/min. The power is transmitted to the impeller shaft by means of V-belts and the pulley ratio is 1:3.

The propeller driving motor is rated at 12.5 kW, but it can be

overloaded for short periods. The revolutions can be regulated at all 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 control desk placed beside the dynamometer in the test room. The vacuum

pump (0.38 kW at 1450 r/min.) which is used for lowering the

pressure in the tunnel to the desired value, is also operated from the control desk. In addition, the electronic equipment, which auto-matically controls the revolutions of the driving motors is mounted

here (Fig. 11).

7. Special Equipment

Some items of special equipment for increasing the uses of the tunnel have already been obtained and others are at present being designed or manufactured. The following are some of those items

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Fig. 11. Controls and instruments on control desk.

For the purpose of measuring local water velocities in the test

section and for calibrating the velocity distribution, there is a 6 m

long strut which can project into the test section from the upper downstream bend and on the end of which is mounted an arm holding 14 Pitot tubes (Fig. 12).. This arm can be rotated in the

test section or it can be moved longitudinally, so that a complete

calibration of the velocity at various points throughout the test

section can be rapidly carried out.

A model of the after-body of, for example, a cargo ship can be

mounted in the test section forward of the propeller, in order to

simulate the velocity distribution existing at the propeller; special

equipment which will permit an after-body to be installed relatively

simply, is at present under construction.

Two types of test which would be of considerable interest- are

cavitation tests with contra-rotating propellers and cavitation tests with propellers in non-axial flow. Equipment is under construction which will enable both these types of test to be carried out.

In the case of tests with contra rotating propellers, they will be

mounted on concentric shafts and driven by the existing motor. It

is intended to measure the thrust and torque by means of wire strain gauges.

After preliminary investigations, the necessary apparatus is now

being made to enable acoustic studies of cavitating propellers and

noise level tests of other types to be carried out. For such tests, a

box is mounted against the plexiglass on one side of the test section.

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Fig. 12. Pitot tube mounting.

The connection to the window is made watertight by means of rubber sealing and the box is filled with water. A hydrophone is then placed in the box to pick up the cavitation noise from the propeller or other

body. This method appears to be suitable for determining for

example, the limits at which cavitation begins and ceases, but at

the moment, insufficient is known about the laws of scale to be

able to use it for quantitative noise level measurements.

8. Experimental Work

In addition to ordinary cavitation tests and special investigations

for the Navy and others, part of the work of the new cavitation laboratory will be devoted to testing and experimental work of more general interest. Some work in this category has already been begun

and is described below.

Various cavitation criteria, evolved on the basis of experience,

indicate the extent to which the blade area ratio of a propeller should be increased if the cavitation number decreases, or if it is desired to

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l) The numbers within brackets refer to the list of references on page 26. 21

increase the margin of safety against cavitation in any particular

design. In such cases, it is usual to increase the area over the whole

of the blade i. e. both root sections and tip sections are lengthened. Generally, in fact, each section is lengthened in proportion to its length, according to the method adopted in most systematic series

experiments with model propellers.

For the majority of loading conditions, however, the risk of

cavita-tion is limited to the seccavita-tions near the tip, while the root seccavita-tions have a margin of safety against cavitation. This is the case at least

so long as the radial pressure distribution corresponds approximately

to the BETZ optimum distribution. The length of the root section is subject to a lower limit due to strength requirements and at the same time limited by the maximum thickness length ratio of the profile 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 of

cavitation in a propeller design, it would be logical to increase the

lengths of the sections near the blade tips and leave the toot sections

unchanged.

In order to test the results of this method and to obtain further knowledge on the reliability of the theoretical methods of design, a family of three model propellers has been designed on the basis of the vortex theory [1].1) The propellers have all been designed for a

loading. condition corresponding to

VE

J=

64 Dn KT D4 2 0.19 e n

The propellers are thus suitable for a ship model which was tested in 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 axial flow, the theoretical design calculations were based on cavitation

numbers of 3.2, 4.8 an.

respectively. The blade outlines are

shown in Fig. 14.

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22 12 10 8 2 0 12 14 16 18 20 22

Ship Speed, V. in knots

' Fig; 13.

Model No. 720, in open water tests in the towing tank and in cavitation experiments in homogeneous flow. Tests in the cavitation

tunnel behind a model of the after body have not yet been carried out. The results obtained so far both confirm the theoretical region

of 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 by

an increase in blade area of 53 % in order to maintain immunity

from cavitation. In this case however, an increase of 48 °/,, appears

sufficient.

A complete analysis of the experimental results is being made and a full report will be issued as soon as the tests have been completed: The experiments described above were based on the assumption that

the radial pressure distribution on the propeller blades corresponds

approximately with the BETZ optimum conditions. With this

distribu-24 26

_

kIA,

Ship' wake 'fraction, w= 30

NISIm

. .... ,

=Propeller centreline ubelow

N

,,,

,,% , .

.'`I

.

,1\

.

gt.."..._\-__

_

_..__

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Fig. 14.

tion, there is a considerably greater risk of cavitation at the outer sections than there is at the root sections; by adopting a different

distribution, however, and transferring part of the load from the outer sections to the inner sections, it is possible to reduce the lengths

Of the outer sections and still maintain the same theoretical margin

of safety against cavitation. This, of course, implies abandoning the conditions corresponding to the maximum »ideal» efficiency, but by

reducing the blade area ratio, the friction losses are reduced at the

same time.

It is planned to carry out experiments with a further series of

propellers in order to study these conditions. The three propellers in the series are each designed for the same loading and the same cavitation margin in the critical blade region. The radial loading distribution, on the other hand, is different in each case.

An investigation of scale effects in model propeller tests has been described in reference [3]. This investigation was concerned with a

family of four geometrically similar model propellers with diameters

00

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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 to determine the wall effect in propeller tests in the cavitation tunnel

and this family of propellers will be rested in both the small and the

large test section of the SSPA tunnel A similar series of tests will

also be carried out using a family of four propellers each having

different 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 with

6pp = 0.675, Part I*, SSPA Publication No. 39, 1957.

NORDSTRoM, H. F., EDSTRAND, HANS, LINDGREN, HAN'S: *On Propeller Scale

Effects*, SSPA Publication No. 28, 1954.

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25 10. Acknowledgement

The 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 the

Tank for all their assistance.

Thanks are also due to Mr. P. D. FRASER-SMITH, who translated

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