The experimental cavitation tunnel at the Norwegian Ship model Experiment Tank

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RCHE

IIODARSKI IIISTITUT SIIIPBUILDIIIGJESEAR(H INSTITUTE

ZA6REB

The Experimental Cavitation Tunnel at the

Norwegian Ship Model Experiment lank

by

Hj. Christensen, L Eggestad, and J. Vassenden

Norwegian Ship Model Experiment Tank, Trondhelm

Lab.

v. Scheouwkjnde

Teciisc

gIqoJ

Deift

Paper to )3C presented at the Symposium on the

Towing Tank Facilities, Instrumentation and

Measuring Technique

Zagreb 22-2 5 September 1959.

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THE EXPERIMENTAL CAVITATION TUNNEL AT THE NORWEGIAN SHIP

MODEL EXPERIMENT TANK

by

Hj. Christensen, I. Eggestad, and. J. Vassenden

Synopsis

The present paper deals with an experimental cavitation tunnel at the Norwegian Ship Model Experiment Tank.

After a general description of the tunnel, mention is made of the elecrtical drive of impeller and propeller, the electrIcal measuring equipment and a flash unit for photographic purposes.

An account is rendered. of the manometer system for measuring pressure and velocity in the test section and. also of a pressure balancing system for the propeller shaft

A few words are given on the auxiliary equipment such as

vacuum and filter plant and to the inside treatment of the tunnel and. the heat conduction through the tunnel shell.

Special emphasis is given to the design and investigation of important tunnel components such as nozzles, test sections,

dif-fusers, and. the impeller.

Introduction

The Norwegian Ship Model Experiment Tank was inaugurated in

1939

at the outbreak of the second world war and the facilities then comprised a main towing tank for commercial and scientific tests, including the necessary equipment for performing towing and propulsive tests. In addition there was a small towing tank for

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educational purposes.

After the war the two tanks nave been equipped with wave

generators and we cen now unërtäke commercial and scientific

tests in head. on seas. As the next step it has been considered

of primary importance to build a cavitation laboratory and

pre-liminary design studies were started very early after the war.

However, after some tine it was found necessary to build a

model of the projected tunnel, as this would make the staff

familiar with many practical problems concerning the building

of a cavitation tunnel as well as providing.an opportunity of

testing different tunnel, elements.

The following, anticipated test progrpmme has formed the

base for the design' studies:

CommerciaL andsoientific tests on propeller mo&els of 600 mm

diameter in a.homogeneois. velocity field. This is expected to

give a Reynolds number sufficient:Ly great to minimize., the:

scale effects. If a slotted wail test section is found usable

for propeller tests, such a section..should be fitted.. This

would presumably reduce the section/propeller diameter ratió.

and thu's keep the cost of building and. driving the.tunnei at

a minimum.

Propeller cavitation tests in behind condition. 'This should

be done by fitting part of the ship model in tue tunnel, and:

the size of the ship model and propeller should 'be sufficient

to yield reliable test results. With the, before

mentioned-re-servations it seems advantageous to carry out these tests in.

a slotted wall section.

Cavitatiqn tests onbodies and. hydrof oils at high Reynolds

number and at cavitation nuinbeisbelow 0.05. Forsuch tests

a c1ose

throat test section also has tobe considered, as

the slotted wall sections show too high critical cavi'tation

number and, has a 't,endency to create, unacceptable pressure

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THE EXiERIMENTAL CAVITATION TUNNEL AT THE. NORWEGIAN SHIP

- . :MODEL EXPERIMENT TANK

This test programme requires a tunnel of large dimensions and with thaximuin test velocities of abOut 20 rn/sec. The tunnel must be very c±eful1y designed if the stipulated low cavitation num-ber shall be obtained, and. it was considered absolutely necessary to make. very thorough design studies before the final design was

carried out. ,. . .

The model of .the.tunnei proved quite adequate. for testing inte-grate circuit elements. Based on the original tunnel model two alternative tunnel arrangements have been built, and. quite a lar-ge variety of small scale cavitation tests cn be undertaken. In addition the three tunnel arrangement.s will: give aluable contri-butions to the education of naval architec _{students at the}

Technical University in Trondheim, as various aspects of cavita-tion on ship propellers can easily be demonstrated.

1. Description Of. the Bxperiiaental Tunne.1

The space available for the experimental tunnel was rather limited. To sothe extent this fact was determinant for the main dimensions. -,

1.1 Arrangement A.

The rectangular, closed-circuit duct loop, oriented, in the vertical plane as shown in Fig. 1, has a height of m and a width of 3,18 rn. The cross-section is circular along the whole circuit. Ecept for the test region and first bend _{and the} slightly conical upper tube on the right the straight _pipe sec-tions all have an inside dimeter of 500 mm.

Only 90° mitre bends are used in the tunnel, and the elbows

are numbered. from the test section in the. d.ireátion of flow. _The

vanes are of the single bent plate type, cut out .of a steel pipe of suitable dimensions. The..gap-to-chorde _{ratio is. about O,3L4.} /Fig. 40/. . ..

Immediately downstream of the fourth turn a flow straightener is fitted / in order. to adjust the flow before entering the

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-THE EXPERIMENTAL CAVITATION TUNNEL AT -THE NORWEGIAN SHIP MODEL EXPERIMENT TANK

Fig. 1.

Tunnel arrangement A. Height,between centerlines,

= 5

in. Width = 3,18 in. Test section diameter = 2oo mm.

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ThE EXPERIMENTAL CAVITATION TUINEL AT THE NORWEGIAN SHIP.

MODEL EXPERIMENT' TANK

nozzle!, and this is made up of square section ducts,, the length 'being twelve times the hydraulic diameter.

A nozzle having a' contraction coeffic.ient .6,25 : 1 makes the

transition for the 2oo nrin closed test section: of the tunnel, which largely is made of Plexiglass. 'The stream of fluid is bounded by an internal pipe of circular cross-section and. the exterior sur-face consists of plane windws. This working section has a

length-to-diameter ratio of '-i-,6 : 1.

The diffuser is of some unconventIonal design, having a total angle of maximum 40°. and an area ratio 'of about 1 : 5.

The flow of the tunnel is provided for by a 11.bladed, 500 mm impeller, through a. V-belt drive connected with a DC-motor of 12 hp. in a Ward. Leonard system. The' speed of revolution of the impeller, measured by'a tachometer, is. continuously variable within the range ± 800 R.P.M. This' equipment gives a maximum test, section velocity of 8,2. .m'Sec.. jFig p44!.

.1.2 Arrangement B.

When the test section and diffuser mentioned, the first turning elbow and. the conical tube is replaced by a shorter slotted wall section and

_a

conventional

₇

degrees diffusor including a profiled. vaned bend, setup B results. !Fig. 2 and

5!.

ALTERED SECTION.S ?.SLOTTED-WALI. TEST

SECTION

2.FG7LEO CASC4OE BEND

3.7 DIFFUSER

Pig.2. - Tunnel arrazRement B.

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-5-THE EXPERIMENTAL CAVITATION TUNNEL AT -5-THE NORWEGIAN SHIP

Fig.3. - Tunnel arrangement B

With the same impeller input power, the water speed in the test section has been increased to 10,2 m per sec. by this

arran-gement.

Preliminari1, these alterations were made to test a slotted wall section which is discussed later in this paper, and. to have

a small scale tunnel for high speed tests.

Depending on the impeller effect to be installed the speed should be appreciably increased. And a very low cavitation number

c should be attained when fitting a constant axial pressure test section.

The flow quality in the vertical diffuser section ought to be

somewhat improved, owing to the area ratio of 6,25 : 1, which is

too large. A value of 'about 4 : 1 has been found proper for tunnel

design applications [i} . This will be dealt with later on.

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-6-THE EXPERIMENTAL CAVITATION TUNNEL AT -6-THE NORWEGIAN SHIP MODEL EXPERIMENT TANK

1.3 Arrangement C.

This tunnel setup also originates from, the A-tuimel, and. has developed from the desire to have a tunnel capable to yield re-liable, though small scale tests on modeipropeflers up to 25o mm in diameter.

As Seen from Fig. 4 and

_5,

the upstream sections are extensi-vely enlarged to make it possible to apply a test section diameter.

ALTEREO SECTIONS: I.SLOTTEDWALL TEST SECTION 2.CONTRACTION 6:1 3.cAS4 BEND 4 9° DIFFUSER ORIGINAL T(NEL CIRCdiT ELEMENTS

Fig.4. Tunnel arrangement C,.

like 360 mm. The slotted wall type section is so constructe that the plexiglass bars can be removed and. thus anopen test section appears /Fig. 25/. The max. speed is found to be 6,2m per sec. at the slotted wall conliguration.

Summing up, two .tuñnel arränements now alternatively can be run for Small scale cavitation tests, and1cover two difThrent areas of cavitation experimentation. The B-tunnel designed for tests.on bodies, hydrofoils, etc. and theC-tunnel should be proper for propellers up to the commOn size used for tank tests.

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-7-THE EXPERIMENTAL CAVITATION TUNNEL AT -7-THE NORWEGIAN SHIP

Fig.5. - Tunnel arrangement C.

The latter tunnel setup, therefore, is equipped with a torque-and. thrust-measuring dynamometer, which is to be described separately. Especially at this C-tunnel, instruments for use in water tunnel work can be developed, and tried out, in order to gain experience for the larger projected tunnel instrumentation.

1.11. Electrical Drives and Measuring Equipment 1.4.1 Impeller Drive

1.4.1.1. Specifications

The impeller drive of about 10 kw power is based on the Ward Leonard system. The design specifications for the drive were

as follows:

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'-8-THE EXPERIMENTAL CAV1TATLON TUNNEL AT THE NORWEGIAN SHIP MODEL EXPERIMENT TANK

a! Delivered shaft power from motor 10 kw at 2670 RPM,

b/ The impeller continuously variable in RPM from to ± 800 RPM

/ 3

1/3 : 1 reduction from motor to impeller is used! c/ The value of RPM set as constant as possible with respect to

mains voltage- and load variations.

d/ Simple run-up piocedure and maneuvering. e/ High degree of dependability.

1.4.1.2. The System Block Diagram and. the Starting Scheme

In order to satisfy the condition c/ of the specifica-tions the drive is

arranged

as a feed-back control system of Ward. Leonard type as shown in the Fig. 6.

10KW

Fig.6. - The block diagram for the impeller drive

Furthermore the system is built with fully automatic start/stop to comply with the condition d/ of the specifications. The power

switch of the DO control amplifier starts and. stops the whole system. The starting sequence is as follows: heaters of the elec-tronic tubes on /0 sec.!, anode and. grid voltages of the ampli-fier on /40 sec.!,

tiin.e

delay for operating of the Y - A switch

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-9-.,

5693 542 CLS ILl ILl

b

..

eø

Ni

yr

I.

AINS

if

0 rACIL dEN.

THE EXPER [MENTAL CAViTATION TUNNEL AT THE NORWEGIAN SHIP MODEL EXPERIMENT TANK.

/220 sec.! with the

finairunUp

of the asynchronous motor /26o Sec.!. The times are;counted fromthe amplifir switch on..

The "zero contact" oathe speed selecting potentiometer ensures start up of the sysenat 0RPM setting only. However, the control amplifier cannot establish control over the whole .loop before the main alternator starts running. . When this occurs the system balances

itself to the. 0 RPM set. This balancing takes, 2 - 3 seconds, during which time interval a motion of the impeller motor usually at speed

of 20 - 30 RPM occurs. This baiancng motion has not been found objectionable for our application and could if desired have been eliminated by suitable precautions at a cost of a few components.

i..l.3.

The ControlAmplifier ..

The design of the control amplifier has been governed by desire for simplicity and. freedom with respect to choice of the system response time.

-- '

- 'T"

Jff

6,6

-240 V

Fig1?. -

The control ailiplifier circuit diagram

-

10

-cso,\7'

cseN&N.

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AC-motor type and. power: Start and stop.:,

Loop gain:.

Bandwidth:

Damping: -.

Constancy, of. ref.voltae:

RPM vaiiàtion with con-stant load:

RPM droop with increasing

load (from .1/2 to 1/i)- :.'

RPM range.:

Max. Shaft Power:

THE EXPERIMENTAL CAViTATION TUNNEL AT THE NORWEGIAN SHIP

MODEL EXPERDVtENT TANK

The diagram of the amplifier is given in'Pig.7. The used. tachometer-generator is a DC type giving a continuously

reversible system. To obtain sufficiently high loop gain an approximatively integrating network is employed at the input to the amplifier. This network also serves as filter. fr the tachometer-generator ripple. The input tube 5693 is provided with a 'compensating diode .f or the zero point stabilization with respect. to mains voltage varIations. Similarly are the

refe-±ence voltage tubes 2 x 85 A2 prestabilized. by means of 2 x 0A2 voltage regulator tubes

1.4.1.14.. The Main Data for the Impeller Drive

The following observed and desig data give the ge-neral picture of the drive: .

: ' Asynchronous motor,. 1L1 kw Automatic = 500 /50 db/.

If

=0,5c/s..

=- 0,55./at 300 RPM! + 109 mV for ± 10 %

Mains voltage variations:+ 0.06' %

0.1% for 20mm. time interval

at 200RPM. 0.4 % at 600 ?M 0.5 % ..

at725RPMO.9%

..

+ 5.

.to + 800 RPM, continuously

reverSiie

1O'kWat800RTh

The drive has nowbeen iwuse ,.for4. years. The arran-gement described has proved itself to be convenient and

reli-able:. . . .. . .,

..

1.4.2. The Dynamometer Drive . ,., ...

The dynamometér driye. is also arra.p.ged as a. fee back

system, the block diagram of which is given in the Fig. 8.

The difference as compared with the impeller drive is employment of thyratrons /Type VXC 3 - 1500 of the soc. Pran.caise

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THE EXPERIMENTAL CAViTATION TUNNEL AT THE NORWEGIAN SHIP MODEL EXPERIMENt TANK

v WHZ

Fig.8.- The block diagram for the dynanometer drive

Radioelectriq.ue/ for the control of the main generator instead of a magnetizing generator.This gives possibility of higher speed of control which was preferable. The reason for this is that the d.ynamometer drive actually serves a dual purpose: it may either be used with the cavitation tunnel 11 kw dynainometer or with the 15 kw d.ynainometer for open water propeller tests to be used on the towing carriage in the main test tank. For the latter application not too slow a system was preferable owing to the limited time duration of .a test run in the test tank.

-We summarize system data below /without giving any diagrams which are rather complicated!:

AC-motor type: Synchronous motor Start and stop: Manual

Loop Gain: = 320 /50 db/

Band. Width: f = 1 c/s

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-ThE EXPERIMENTAL CAVITATION TUNNEL AT THE 4ORWEGIAN SHIP MODEL EXPERIMENT TANK

Damping: C =

0.5

RPM variation with

constant load: + 0.05 % for 3 miii. time interval t 1500 RPM

RPM droop for load

increase from 0 to 1/4: 0.05 % at 1500 RPM

Max. shaft power: 11 or

₁₃

kw /dependung on dynamometer/ RPM range:

_{± 30 ±}

300 RPM. Reversal through field of main motor and tachometer-generator switching at zero speed. The speed range 0 to ± 30 is also covered but the speed within this range shows a

certain ainou.nt of "vow".

1.4.3. The Dynamometer Speed Measurement.

The above measurement is done electronically by means of a photoelectric pick-up and. an electronic frequency meter of the counting type. The piok-up is a slotted wheel on the dynamo-meter shaft which interrupts a light bean directed. on to a

photocell. This wheel may be seen in the Fig.9, which also shows the sliprings of the torque measuring system, the stroboscope contact and the fork of the thrust measuring system.

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THE EXPERIMENTAL CAViTATION TUNNEL AT THE NORWEGIAN SHIP MODEL EXPERIMENT TANK

The frequency meter., Cintel Frequency Monitor, permits. counting times of 1 second and 10 seconds. With 100 slots onthe rim of the pick-up wheel the speedmeasurement can be performed with an accuracy of 1 RPS and. 1 RPS using 1. and.

100 1000

10 sec.. timing intervals respectively.

l.L11. Torque .nd Thrust Measurements.

The quantities propeller torque and thrust are meaáured electrically by means of e1ect.ric1. strain gauges. The measurements are at present carried out as static values determination by means of following chain: electrical strain gauges, oscillator/amplifier! detector and recording potentiometer.Apart from the gauges this chain is common to the both uantiti.es; by means of a switch one Is able toselect:eitherthe thrust Or the torque chnne1.

l.Ll.4.l. The Torque Measurement

The arrangement .fbr the above measurernent is -

shoin.

in

the Fig. 10. The hollow part of the propeller shaft e±tend.s to a point just beyond the last.shaft bearing. The shaft wall is here

Fig. 10 - The propeller shaft arrangement for torque measurement

thinned down

to give sufficient sensitivity to the hereon fixed

strain gauges, WhiCh are of torque sensitive type.

and

connected in full bridge. It will be seen that this arrangement indicates

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-14-THE EXPERIMENTAL CAViTATION TUNNEL AT -14-THE NORWEGIAN SHIP MOI)EL EXPERIMENT TANK

propeller torque directly without any bearing torques corrections.

The leads from the bridge --are brought. out through the .,hollow shaft

to the sliprings/brushes connections to the oscillator/amplifier! recorder. The recorder is the Minneapolis Honeywell Extended Range

Re c order cove ring the range 0 -

_25.5

mV, : in five 'ranges.. As each

my gives 2" 50.8 mm we, have. a full effective chart width of neary1 .3 m, -giving high reading accuracy. Pig. 11 shows the ca-libration curves for the torque andthrust measurements. The cur-vesare very close to straight lines.

11 8

I

7 6 5 4 2 I

Pig. 11.- The torqueand thrust calibration cu±ves.

The high sensitivity of the recorder toTnain voltage disturbances made use of primary or secondary batteries to feed-the oscillator/amplifier unit necessary. It was feed-then logicai.to make the unit transistorized. The temperature and tixfle rifts -of

-

15-3 Torue 4 70 15 ThruSt. kg 20

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THE EXPERIMENTAL CAVITATION TUNNEL AT THE NORWEGIAN SHIP MODEL EXPERIMENT TANK

the transistor circuits employed are -sti]i reuiring soe

impro-vement. For this reason we do not' enclose any circiIt diagrams.

220V-60%

L502

Ii

7)!grtmn,f.

1.4.4.2. The Thrust. Measurement .. :' "

Mechanically the 'thrust system consists of a 'thin

bus-hin. totsionaily deflected. ythe difference-of the t-hrust.añd

a couiiterforce rovided by 'weights. he foicé' difference is

-re-corded. electrically by Strain gauges fixed on to the. above

men-tioned bushing. The total thrust is thus indicated, by the sum of electrical recording and. the weight used for approximative balance.

At present-the- thrust -arrangement shows a, certain amount

of torsional vibràtións which we -intend to reduce to more accep-table level by a, suitab).e damping arrangemnt'.

-1.4.5. ' Micro-second Flash-Unit . . . -'

In order to. obtain sharp photographic images of the Ca-vitation fenomena a microsecond electronic flash unit of high

HIGH VOLTAGE RECTIFIER

IA CONDENSER BANK FLASH UNIT Ca..fv S.itch

16

F IN I/p o/p 0-17.5KV

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discharge energy has been built by us. The circuit diagram of the flash unit is given in the Pig. 12, and. the genera]. appea-rance of the apparatus in the Pig. 13. The condensor baiik

Pig. 13. - The electronic flash unit

maximal charging voltage is 15 kv /170 Joule charge energy!. The LSD2 flash tubes are connected in series and are made conductive at the command of trigger impulses initiated. by camera switch and propeller shaft rotary contact. The flash duration is about 3 micro-second.

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-THE EXPERIMENTAL CAVITATION TUNNEL AT -THE NORWEGIAN SHIP

Fig. 11-I-. - The control desk of the cavitation tunnel

1.5 Pressure and Veloóity Measurements

The manometers f or measuring tunnel pressure and velocity and their connections to the tunnel loop are sketched on Fig.15. These symbols are used:

Atm = atmospheric pressure in mm of mercury /Hg/

h5 = pressure below atmosphere in mm Hg

h, h, hL

, and hc in mm of water

hg

in mm Hg

reduced by density of water Hg and hg in. mm of mercury /Hg/

= specific density of water

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THg

A 0

contraction ratio

= speed of water in test section

g = acceleration of gravity

k, k,, axid = constants found. by calibration

Contraction Test Section

Fig. 15. - Velocity and. pressure measuring system

Accordi.ng].y the absolute pressure in test section can be observed

as

p0

=Hg - 8h

= hg mm Hg hg Kg/Ui2

19

-,Valves

1. ily Barometer Tube 4. Hg U-Tube tilled with Water

2. Water Level Tube. 5. Hg Cisterne filled with Water

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or:

THE EXPERIMENTAL CAViTATION TUNNEL AT THE NORWEGIAN SHIP

or:

_V2

P0 =(Atm-h5) + B(h_h0) =(Atm-h5) + [h - _2g mmllg

= -F-(Atm_hs) + [h

_{- ____}

-Similarly the velocity in center of test section is

= \/k

'c =

V0 = k1 _{THg - T} "ic = k2 ''c m per sec.

1.6 Propeller Shaft Balancing System

A pressure balancing system is developed to eliminate the influence on the thrust from the actual tunnel pressure relative to atmosphere.

This is obtained by fixingai annular piston to the propel-ler shaft just outside a pressure chamber. That simultaneously serves as a water trap outside the waterseal ring in the tunnel

wall.

The pressure in the chamber is read on a mercury U-tube manometer, and. is calibrated against the pressure in the test section, i.e. the pressure at the other end. of the propeller shaft. The chamber is fed. by water from a pressure reduction valve connected to the public water supply.

However, the temporarily applied valve appears not to be quite suitable, as it is. not sufficiently sensitive to respond to the pressure fluctuation in the public pipes, which sometimes can be rather large.

When the depression inside the tunnel is denoted by p = Atm - p0, including also the pressure fall in the nozzle h0, the balancing condition is with reference to Fig. 16.

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(h.. YHg + Atm + hg (d.

- d) = Atm .

.21

-Pressure balancing Piston Propeller Shaft.

Mercury U Manometer Pressure Reduction Valve

£ Adjusting Screw. Air Bleeding Valve.

7. Air Vessei.

& Shut off Valve. 9. Calibrated Orifice.

Mair *bter SUDDIW.

Pig. 16. - Pressure balancing system for propeller shaft

Hence

= (d

- d) YHg

ha = P IHg - ha

where

P = d - d = constant,

p can be easily deduced. from Pig. 15.

To eliminate drag forces on propeller boss and shaft another term being proportional to the velocity head can be added and. the equation above takes the form

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ThE EXPERIMENTAL CAVITATION TUNNEL AT THE NORWEGIAN SHIP MODEL EXPERIMENT TANK

p

htl=

YHg

P1af

2g

and. is to be calibrated against pressure p and the velocity

v0.

17 Conduction of Heat through Tunnel Shell

When projecting a large cavitation tunnel the necessary

capasity of the refrigating plant will depend on the natural

con-duction of heat from the tunnel shell to the surroundings. To get

an idea about the actual heat transfer the temperature rise in

the B-tunnel was measured during a long time run of 30 hours at

constant tunnel speed, i.e. constant input power.

When putting H

= input power expressed in thermal units

= heat absorbed by tunnel water

H0 = heat conducted. away by convection and

radiation,

then

H =H+H0

The heat conduction is assumed to follow the law

=

tdh,

where

= heat transmission number,

A = total area of tunnel shell,

= temp. difference to surroundings,.

h = unit of time /hour/.

By integrating the plotted temperature versus time curve, the

heat transmission number can be found .as

H -H_

_i

_W

llkca1/hin2C.

I/A tdh

The tunnel shell is coated with a middle grey semiglossy

painting.

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-ThE EXPERIMENTAL CAVITATION TUNNEL AT THE NORWEGIAN SHIP

1.8 Inside Treatment

The inside of the tunnel tubes cannot be treated too care-fully when the material is mild steel. Our experience is that first of all the steel surface must be tediously sandblown.Then a selfetching primer is supplied, whereafter three or four coats of a chlorcaoutchouc painting or three coats of an epo resin base painting has shown good properties for velocities up to

7

rn/sec.

In really high speed sections and where cavitation can periodically occur it .is reasonable that some anticorosive mate-rial should be applied as far as can be done from an economical point of view.

1.9 Auxiliary Equipment

Undissolved air bubles in the test section flow are highly unfavourable. At all tunnel setups an air collection dome there-fore is placed on top of the fourth bend. The dome is partly filled with water, and connected to a vacuum tank of 0,27 m3. By means of a two-stage wing pump it is possible to attain a

depres-sion of about

₉₅

per cent of atmosphere.

A sand filter plant with a capacity of

7,7

rn/h is connected

to the tunnel through a bypass loop. By means of a two-way valve the filter is always connected when the tunnel is refilled with water from the main supply, and. it can be run continuously if desired also when cavitation tests are carried out.

A storage tank of 0,/40 m3 is at disposal when the water level is lowered for changing models in the test section.

It should also be mentioned that the tunnel is equipped with apparaturs for measuring the contents of gas in the water. The applied method, which terminates in extracting dissolved gases from the sample by evacuation, is due to van Slyke and Neil [/4]

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-ThE EXPERIMENTAL CAVITATION TUNNEL AT THE NORWEGIAN SHIP MODEL EXPERIMENT TANK

2. Tunnel Components

Some of the most important tunnel components

are

fur-ther discussed in the, following chapters.

2.1 Contractions

The purpose of the nozzle in a cavitation tunnel 'is firstly to convert potential energy into velocity energy in the flow, without an unnecessary amount of head. loss. It is also essential that the velocity travers in the test region is as uniform as possible over the majority of the cross-section diameter, and this is attainable only by means of a contraction.

There are various considerations to be taken in designing a proper nozzle. With a certain contraction ratio the boundary layer decreases with the length of the nozzle, but as the curva-ture increases at the same time we thus take the risk of getting an unfavourable boundary pressure gradient. At the entrance of the nozzle a rise in pressure usually occurs, while the x.ozzle discharge at the same time is exposed to a pressure dip.

A study of litterature led to selection of a nozzle like

that of the Garfi.d Thomas Tunnel

_[3].

Here the boundary curve has continuous first and second derivatives, and. each part of the curve is found. from the following equation:

d=D--1

(_-)3e1/'2[l_(x/L)2]

where terms and dimensions in our case appear from Pig. 17.

Pig.17.- Nozzle contour at arrangement A and B

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-2k-THE EXPERIMENTAL. CAVITATION TUNNEL AT -2k-THE NORWEGIAN SHIP MODEL EXPERIMENT TANK.

An approximate precalculation of the contraction flow is

carried out by means of a grafica]. method depending on the. 1'

01-lowing equation for the velocity distribution in a bent canal:

dv_'

v

'db

where b' 'is the crbss-sction widthöf the duct, P the radius

of curvature and v the flow speed.

10 0.9. 06 04 02 o -02. - 0,1 Arrangement a Boundary Curve Cdlculated Pressure Measured Pressure a

Tjjfl!I:

3. 0 41 42 0,3. 04 0,6 0,6. 0J 0,8 49 10 Length of Contraction 25 -Arrangement C

aN

1. Boundary Curve 2. Calculated Pressure 3. Measured Pressure 0 0,1 ? 43 0,4 0,5 06 47 08 '. 09 1.0

Length 'of Contraction

Pig. 18. - Contraction boundry curves;and wal]prffssure Thisestimate.'gave anihiet bbUMaryè'ssu±'e 'ôf -i-'%of the mean test section dynamic pressure, and the pressure dip at the

(27)

The corresponding measurements gave about 2 % and 0 % res-pectively, as shown in Fig. 18.

The above mentioned nozzle was constructed for arrangement A but it is also used in arrangement _{B. In connection with} the slotted'waul section of this setup the velocity traverse caused by the contraction did not appear to be as uniform as desired /Fig. 22/. This was assumed to be due to the compara-tively small radius of curvature in the nozzle exit.

With this fact in mind, the nozzle for arrangement C was given some more gradual boundary curving in the outlet part at the expense of the length of the entry, and a simple sine func-tion /Fig. 19/ was selected to describe the boundary form in-stead of the more complexe third degree equation used for the first contraction.

Fig. 19. - Nozzle contour at arrangement C

In spite of the comparatively short inlet part of this nozzle the measurements did. not indicate _{any marked pressure rise in} this region, and. the pressure dip also here was zero /Fig. 18/. The non-uniformity of the actual wall pressure curve may be a consequence of an inaccuracy arisen during the manufacturing of the nozzle which is made of cast iron and not machined on the inside, except for the outlet part.

(28)

-ThE EXPERIMENTAL CAVITATION TUNNEL AT THE NORWEGLN SHIP MODEL EXPERIMENT TANK

J

2.2 Test Sections

Three test section configu.rations have been tested and.

some dimensional .ata are listed in the table below.

Ve]ocity and pressure measurements were carried:out' ith pi'tot.:

and pitot.static tubes or by conecing a rnuiti!tibe rnanoete to. pressure taps in the section wails.

2,21 ClOsed Throat Test Section /Fig. o/

-This section was made of plexiglass. All presstthe- measure-ments show quite good agreement with reslts

_scuse.

{2] and.

f5J

.

Pitot traverse, measurement -at position A 18 shows a uniform velocity distribution within. 1% of the, veloç.ity a:t center across 80 % oZ. the diameter.,, whichis, suppose& to be sufficient1y

accurat!.fQcomop.. tests. On the whple a. -sipl,e closed .throat test section is likely to cause only small design

problems.-- 27problems.--

27-No. SectiOn fDialneter ion

Len h n

diameters Form nominal Outer hydraulic /reserv./ 1 200 mm 4,6 -2

s1ted

2Q0 mm 2,5 Hexago 4,00 1,90

s1ted

360 m

1,0. -' Square corners' .. "4,811. 2,14 4 360 mm 1,0 Square ded corners . 4,84

(29)

-ThE EXPERIMENTAL CAVITATION TUNNEL AT THE NORWEGIAN SHIP MODEL EXPERIMENT TANK

Section A-A

Pig. 2o. - Closed throat test section for arrangement A. 1. Plizometer taps, 2. In.àértion hole for pitot /stationA 18/,,

_3.

Plexiglass window,. LI..

Plexi-glass pipe.

2,22 Slotted Wall and Open Jet Sections,

Special attention, however, was consentrated on the confi-guration of slOtted wall sections. This type Of section is ori-ginally developed for 'wind tunnel application and h*s proved to give valuable improvements to the' flow cha±acteristics of a wind. tunnel test section. Not very much is known about the applicabi-

-lity of similar sections for water tunnel use but the possibi-lity to increase the effective Section area by intrbducing slotted walls is very interesting.. - .

-- ' As little iItfoation about such se'ctionth was availail'e on

-the date of'designing,. -the construction of -the number 2 section" is fairly' sImple /Fig. 21/. Though a test section hardly ca.n 'be viewed separately independeiit-of.the inflOw cond.itións 'caused.

for instance by the-ontract1On, some effêctsseemevident.

(30)

-ThE EXPERIMENTAL CAVITAI ION TUNNELAT THE NORWEGFAN SHIP

MODEL Ek?ERThfl TANK

'1.1

,45.

_-0

41

Section A-A

Fig.21. - Slotted wall test section for arrangement B

- 1. Insertion hole for pitot,

2. P1exg1ass

window,

3. Pleglass bar..

The velocit7 traverse measurements /Pig.22/ show an incre-ased velocity towards the boundary. A possible explanation may

472

42 .43 04 48 46

o,r

48 49 Distance along Diameter:

-fr--

29

-0.28

-0,50

-0,72 0.94

Fig.22. -. Arrangement B. Tes.t section velocity profiles at various distance from entrance. /Re0 ₌ 5 . 105/.

(31)

THE EXPERII4IENTALCAVITATION TUNNEL AT THE NORWEGIAN SHIP MODEL EXPERIMENT TANK-.

be that the contraction used, probably has too small a radius of curvatut'e at the inlet to the test sectiOn /seé page 12/, and thus accelerates the flow in the boundary region more than the core velocity.

Still another explanation must be considered that the flow in the tank accelerates the boundary flow at the inlet end, but retard.s itfurther downstream. Fig. 23 seemsto confirm this assumption. --004 002 000 -002 0.04

p006

Q08 -0.12

DIAMETERS. FROM ENTRANCE

{

-20 -10 0 10 30 40 50 60 70 80 90 100

LENS Til OF TEST SECTION

Fig. 23. -- Arrangement B. ?ressure and. -velocity measure-ments for. the, slotted wall test section.

= '..io5 - io6/

Anyway, the flow resistanc ishigh and accordingly the pressure drop is apreciab1e a seen from Fig. 23 The latter curve trend, however, agrees well with a similar curve [5] _.

s slotted. wan sections haye shown he tendency to create unstabilities in the.jet flow {ioj -some elemen-taryineasurements were made, to get an- idea about. the ed..sting pressuè pulsation

in the surrounding reservoir.

-1,04 1,03 Z02 1,01 hA -J 1,u,I 499 0,98 0,9.7 1,0 2

(32)

A pressure gage

[7]

was attached to a brass plate repla-cing one of the perspex windows and the pressure sencitive dja-phraa brought in contact with the water in the reservoir through a hole in the brass plate. Pressure pulse records

Pig. 24 show that the fluctuations do not exceed ± 1,8 % of the velocity head in. the test section for speeds up to about 10 rn/s 133 fps/.

if

m/sec, -4500 mm of wate, _{- ±1.8%}

2g

Fig. 211.. -

Pressure pulsation in the slotted wall test section of arrangement B.

No resonance frequency was reached within this velocity range, probably due to the rather low speed of the water flow and. a much shorter section relative to the jet diameter than is re-ferred to in [10]

The No. 2 section is, however, much too small to test propellers to any degree of reliability, so no interference considerations, as to propeller performance, were made.

-

31

-sec 00 00 00 00

0----c-0 1 2 3 ₄ ₅ 0 L 400 200 E O ₂₀₀ lip - ±80 2 mm of water 4 5sec. 4 p ± 31.2 mm of water

- 7.7 rn/sec. ie.* -X00rnmof water

-

05 %

(33)

ThE EXPERIMENTAL CAVITATION TUNNEL AT THE NORWEGIAN SHIP MODEL EXPERIMENT TANK

To meet this requirement and to be able to investigate, for instance, the influence of the ratio of _{open to closed} circumference, section No. ₃ was designed. Due to other tunnel alterations this section could be made

₃₆₀

mm in d.iarneter /Fig. 25/.

+

±

Pig.

25.-

_{Slotted wall test section for arrangement C.}

1. Insertion hole for pitot, 2. Plexiglass window,

_3.

Plexiglass bar, L4. _{Propeller shaft.}

Provided

that

the interference

_can

be reduced by

_half

_{the order}

of a closed type section, It was anticipated that common model propellers of diameter ₌ _{200 - 250 mm could be tested, and} thus valuable experience could be gained by doing such

prelimi-nary

cavitation tests.

(34)

As to this slotted wall test section configuration, the slots are tapered to zero at the upstream end [8] . The number of slots br bars! = 12 and. the ratio of open boundary to to-tal boundary 1

t

a/b = 0,1 was found by extrapolating values

from

_[9]

.

The bars are wedged at the downstream end, and pro-pagate a part into the transition section where the nominal

diameter is increased by nearly ₉ % with respect to the inlet diameter. This is done largely to avoid the appreciable pres-sure drop at the downstream end meapres-sured at the No.

2

section.

The violent vortical flow observed in the surrounding tank at

the No.

2

Section is not, by far, avoided by the bell mouth

shown, although the flow is clearly improved. This bell mouth entry is also disniountable and can be replaced, if needed, by some other configuration. This point will be given much atten-tion in further tests to be carried out with this secatten-tion. It should also be referred to {LIJ that recommends vane like modi-fications to be inserted to improve the entrance conditions to the diffuser.

Furthermore, this section is so constructed that the plexi-glass bars can easily be replaced by bars of other dimensions or completely removed so that the open test section No. k

appe-ars.

Pressure and. water speed measurements along these sections are shown in Fig.

26.

Obviously the wall pressure curves are distinctly different from Pig. 23. The small difference bet-ween slotted wall and. open jet axial pressure curves is also

somewhat remarkable. Unfortunately, due to the complexibility of measuring the pressure throughout this region, the pressure is not observed further downstream until now.

More details concerning these objects are assumed to be published later on, in connection with a more comprehensive analysis of slotted wall section 'characteristics.

The peculiarity of a slotted wall test section to increase significantly the effective capasity of a water tunnel seems very fascinating. However, this test section configuration can

-have certain limitations. As found by more researchers

_t2],151

(35)

1.1

_31_

FLOW

I

.1

a0

Distance along Radius:

f

--

AXIAL VELOCITY. OPEN JET.

- AXIAL VELOCITY.&OTTEO WALL.

BLW0ARY PRESSURE.

AKL4L P5SURE EN E7

----AXIAL MESSURE.SLOTTED WLL.

Slotted Wall Open Jet

0.70 7.02 1.O 098

All__________

Os____________________

4

004

__________

Fig. 26. - Pressure and. velocity measurements for the slotted wall test section of arrangement C.

/Re0 = 1,2 io6i.

Fig. 27. - Arrangement C. Test section velocity profile for slotted wall and open jet. Distance from entrance

D - 0,12. /Re0 = 1,2 . io6/

-50 0 50 100 7. LENGTH OF TEST SECTION

(36)

and which is also verified. b observations at the present

Nc

2

section, the critical cavitation number is

;0,5 - 0

so a slotted wall section cannot in any case be expected to work too satisfactorily where low ø-values and high veloci-ties are desired. The last statement refers to the increased tendency to cause unacceptable pressure fluctuations at high speeds [10].

2.3

Diffusers

As mentioned above, only a limited space was available for our experimental tunnel. Therefore we had to build. it spe cially short. This, in addition to the general desire to make the tbroughgoing propeller shaft not too long, led us to try a diffuser of some unconventional design / Pig. 28/.

Pig. 28. - Diffuser contour at arrangement A.

The area ratio between test section and first bend was about 1

5,

and with a length of only

₅₂₅

mm it was obvious that the diffuser would not show sufficient good bydrodynami-cal properties.

The boundary curve has a linear central part, and. the transition to the test section and. the first bend is drawn on the basis of the sane equation as used in the nozzle design for arrangement A.

(37)

-THE EXPERIMENTAL CAVITATION TUNNEL AT -THE NORWEGIAN SHIP MODEL EXPERIMENT ,TAKS'

The theoretical présauxe' rise ina d.iffusor, 'can be

written as

2g

:Tr'

'

provided, that the resistance to flow is zero. The pressure

efficiencl is then

36 --.

V0

2g'

tTi

where

h is the actual pressure increment, and

_{-f-. the area}

ratio.

. . .. " -

-The, first meaSuremnts gave a mean efficiency of 0.115.

This. value did not seem to ,be quite imrobable as we. had allowed

for separation 'even in the entrancepart of the diffuser, an

assumption which was coxifrmed by pitot tube measurements.

In order to minimise the uñfavourable effect of 'the

extre-mely high area increase, of the 'diffuser, boundary' layer suction

was carried out at a distance of 200 mm from the inlet. This

experiment gave some rise in efficiency, and a ma±imum value

of 0.60 was noted at the higheSt rate of suction discharge.

Pitot tube measurements aláo pointed out to what extent the

boundary layer' suction really influenced the separation.

Another possibility to improve- the. flow conditions in a

diffuser consists in imparting rotation to the flow. A body of

revolution, fitted with, fixed .ide- vanes

,- was

'inserted, as

in-dicated in Fig. 29,a.A 'constant ,entrance ..and 'discharge

angie-were,'Se11ected',for', the. vanes 'which' thus were 'assumed to.. produce.'

a stream condition.something between free vortex and,forced

vortex.

'

- '

In this cise the pressure efficiency was found to be 0.5o,

with a gradüàl increase tôwárds higher values' of' 'Reynold.

number.

(38)

Fig. 29. - The diffuser of arrangement A: a/ with fixed guide vanes, b/ with rotating guide vanes.

due to the fact that the rotation was too intense. Pitot tube measurements showed that a considerable separation took place on the boundary of the body of revolution.

The highest efficiency value, = 0.70, was attained at a simultaneous suction discharge like 18 % of total flowage capacity of the tunnel.

It appeared from these tests that the boundary layer suc-tion method was not effective unless the discharge ratio was unproportionably high. Therefore this course was left, and. work

on the oiher alternative was continued.

In order to vary the tptencity of the rotation a movable blade rim was mounted on a shaft, just ahead of the body of

revolution, after the fixed entrance vanes had.been removed -

37

(39)

/Fig. 29.b and. 30/. By means of a suitable DC-motor this blade rise, within wide ranges, could be given any desired speed of rotation.

Fig. 3°. - Rotating blade rim for diffuser tests at arrangement A.

The design of the vanes was based on a flow conditiob

where the angular velocity had a constant value along the radius, i.e. a forced vortex.

(40)

The blade rim distinctly showed an optimal velocity of revolution, which at constant impeller speed gave an increase in test section velocity of about

_{31,5 %}

/Pig. 31/.

lAO

1.30

1.10

0.40 aw Q LW 120 1i 1

Angular Velocity of Blade Rim

--WI.,

Fig. 31. - Velocity measurements showing the influence of a rotating blade rim in the diffuser of arrangement A.

= 2,2 . 10

- 9,5 . 1O/

Since the tunnel-loop itself was quite unaltered this ve-locity rise must be owing to an improved diffuser efficiency, which was found to be 0.72.

Later these achievements will be followed up for the pur-pose oVdesigning a new fixed blade rim with corresponding pro-perties. If we succeed in this, the loss coefficient s /see

page

₄₅

/ for the tunnel will be reduced from about 0.88 to

_0.35.

As a blade rim like this, placed in the diffuser entry, probably will determine the critical cavitation number of the tunnel, it was important to investigate the cavitation proper-ties of the vanes.

-

39-0

(41)

However, incipient cavitation was d.ifficult to observe within the diffuser. The blade tip therefore was cut off and the shaft was lengthened in order to bring the blade rim into the test section. In this position a mean cavitation number was found to be 2.8.

By arrangement B the diffuser was given quite a conven.-tiona]. design, with a horizontal part upstream of, and a ver-tical part downstream of the first bend. With the exception of the entrance region, which is constructed on the basis of the third degree equation mentioned before, the total ang)e overall is

70

For each part the rise in area is about 1 : _2.5.

Pig. 32 shows the boundary curve and the actual boundary pressure for the horizontal diffuser. The efficiency is calcu-lated to be 0.87. The energy- ration of the flow, , raised from 1.05 to 1.56 in this section.

Length of Diffuser

Fig. 32. - Boundary curve and pressure measurement for the horizontal diffuser of arrangement B.

/Re0

=.io5_io6,

L0-"U 48 0,6 0,4 Measured Pressure Boundary Curve 0 0! 42 43 04 0,5 06 0.7 08 09 '0

(42)

THE EXPERIMENTAL :CAVITATION TUNNEL AT THE NORWEGIAN SHIP

B reason of the non-uniorm velocity traverse caused by the bend /Fig.33/ and the large area. ratiö,separation ocOured.

in vei,ticai diffuser. This, resulted in

a.very.,low'effici-ency which did. not exceed 0.40.

A vertical diffuser upstream of the fourth bend in

arran-geent

.C has an area ratio of about 1:3 and. a. total angle like 8.9.. For this sectionthe pressure measurement gave an

effi-ciency 'of

0.71.

. . . .. 1,2 1,0 os 08 C 0 0.5

C04

2. 1 Upstream of Elbow 2. Downstream àf Elbow

Diameter Perpendicular lo Vanes

I I I F 1. I I L I

of

0304

0,5 .0,5. 7 0,8

Distance along Diameter X10

Fig. 33. Arrangement B. Velocity profileà

at first. bend.

/Re0= 4,5

. lo/.

2.4 Bends :

The van.ed elbows are not thoroughly investigated as compre-hensive test data are available fro ljtterature [i.- cii].. Simple vanes like bent plates of cOnstant thicess have been weighed against, the more elaborate profiled vanes for economi-. cal reasons. It is assumed that the former, say, just a 1/4 senent of a steel tube 'for a 90°-bend can be app].ied with,

success, where pressure and water speed most likely do no cause cavitation, or vibration, provided that the mechanical sfrength'

(43)

also is sufficient. Normally,. .this means that only the first bend downstream of the test' section must be fitted with

prO-filed. vanes.

--. Under no circumstances must thè bends be viewed

separa-tely independent of the 'oncoming flOw. ith an example the fl.ow at the oütlét of the diffuser will influence appreciably both' loss and. cavitation characteristics in the- elbow.

At tunnel setup B /chapter 1.2/ the water speed and. pres-sure rating for high speed tests were expected to promote high losees, perhaps vibrations and probably also cavitation at a

simple bent plate

cascad.e. It

was decided therefore to insert, at this elbow, a profiled vane cascade, designed on data found

from (1] . Some tests have, been carried out with this bend.

con-figuration/Fig. 34/.

VanOd Elbow Data

Pig. 34.-

900 elbow for arrangement B. The- vane

cas-cade dimensions are, shown to the left.

42

-Twmel Elbow NuffIb áf _e° _e/c

- I 14' 450- _Q.

A

.23 4' ' 16 500 Q327

1. 11 - 314 9Z$ Q347. ..

(44)

According to the equation

h =k

+

2g

t _2g

this bend appears to have loss coefficient k = 0.168, when the velocity distribution upstream is as shown in/rig.

33/.This

k-value is supposed to be quite reasonable and agrees fairly well with indicated values in [i)

A pressure loss coefficient for bent plate vanes / a quarter circuinference/ has been found [11] to amount to 0.2o, but under some dessimular conditions, for instance, a nearly uniform inflow velocity. As described the setup B is not yet equipped for velocities or cavitation numbers at which cavita-tion in this bend can be feared. At a test seccavita-tion

= 052

evidently no cavitation occured. in the elbow, neither could it be expected, as the a0-va].ue corresponds to a

= 7.5

for the bend, whereas the critical = number for the bend has been found to be

_Ca.

2.25 [1)

2.5 Impeller Design

The impeller was designed originally to cover a wide head range, as circuit element models of extreme flow resistance were meant to be tested. This was achieved by constructing the impeller 11th adjustable blades /Fig.

35/,

and by applying a variable speed motor drive. This arrangement baa also proved to work satisfactorily at both final tunnel setups described earlier.

Tunnel resistance calculations determined the maximum estimated value of the head loss factor

/1/

V0 2g

wbee

= total head loss in meters of water, i.e. the

pressure

increment

at the impeller,

water speed in test section, meters per second (mis) g = acceleration of gravity (rn/a2)

(45)

Pig. 35. - The impeller. ID = 500 mm!.

The head. loss factor is evidently similar to the power factor, defined as:

= _{1/2 p}

where

= impeller thrust /kg!

u

= water speed throu impeller (m/s)

p = specific mass ( )

A0 = test section are:

/m/

(46)

THE EXPERIMENTAL CAVITATION TUNNEL AT THE NORWEGIAN. SHIP

-As will be well Imown, this factor _{is frequently used. for} qualitative comparation of tunnel circuits, and has for. eff-ic-.

ent tnel constructions the maitude c1

_{0.20 -, -0.25. :.}

Introducing the impeller é±fióiency i the drive power factor including the impeller characteristics, resuls:'

p '= =

.1/2

P;A0

₌

1/2 PvA0

where

M = torque at impeller shaft /kgm/,

= angular velocity of impeiler.:shaft / 1/s

_1,

P = impeller power/kgm/s /

For the original tunnel stup a max. head loss iactor _C _0.68 was assumed, ta.king intoaccoun.t the sections wibh the highest

estimated flow resistance. From /1/ and. Fig. _{39 follows, as} the selected desigu point v0 = 6 m/s, that the corresponding total head loss h =

_1.25

meters of water. An adequate speed

of the available DC-motor was found to-be _{n = 600 RPM.}

Except for a slight mod.ificatio aSs.

othewaof applying

the method of caiculàton., the impeller was desigued according

o.[6] . . . .

The driving conditions previously mentioned shouM make it preferable to fit the impeller with nearly constant blade biedth. To take precautions against óavitation,the lift coef-fiient CL should be kept at a relatively low value-, e.specially that at the blade tip. However, these .requirements will _surely tend to decrease the. attainable ipel1er efficiency. . .

A preliminary dimensional ,'desi, regarding also the blade

root stress, lead to the .fb]lowing- choice:

- Outer dianëte± D = 50Q mm, .

Boss diameter

_Db=O;35

D =

₁₇₅

Number of blades N

4

Cord leigths at the root, at .--- = 0.6 and at

the tip are c _135,

₁₄₇

and

₁₁₅

mm respctively. The blade/area ratio is thus

_O.5o7

(47)

THE EXPERIMENTAL CAVITATION TUNNEL At tHE. NORWEGIAN SHIP MODEL EXPERIMENT TANK

The blade sections were selected from well experienced profile forms, and theliftcOefflcientat the ithpeller.tip was assunied. not to exceed CL = 0.5mákingsureto avoidcávitation. also in the most unfavourable conditions.

These data were applied to the fOrmula 4erive4'from:a.sim-plified propeller theory by A.R. Collar [6 , Fig. 36.

Pi. 36. - Symbols used in. eq. /k!,

/5/,

and /6/ shown schematicly.

1/2 p .N. a u2

. cotg, or

dT

.12

=21=.NCuG.cotg

= 1/2 P N C u2 Hr cotg q'

where G and. H depend on the angle p , the CL and CD-values, and the rotational. inflow factor a2.

Providing the pressure increment p to be independent of radius, and as the other values are already fixed the equation

/5/

s]iows that the aimed properties of the impeller will be achieved if the product G cotg ' takes the value: derived from

/5/

by

putting. p = = 10

125 0.13

kg/cm2. Applying the method

of appro.rnation jointed out in [6] wbich is not very elaborate,

leads to tb.e result shown, Fig.

_37.

-46-j4j

/5/

/6/

(48)

'a 70 30 calc. necessary final values 20 8 10 12 14 16 18

- L.7

20 22 24 26 r (cm)

Pig.

37. -

Theoretical G cotg -distribution along impeller radius, and final values found from the method of approximation 50 .45

-'

_E

_U

ii"

II

:411

-

____

30

41lI1il

P

M.J'dr.

T .J(r.

Tu 552 kgan 2'

UR

I.__NUR

_{227 kg}

U!

15

-8 i

10 12 14 16 78 20 22 24 r (cml

Pig. 38.. - Calculated torque /M/ and thrust /T! distribution along impeller radius

24 22 20

l.18

16 14 12 10

(49)

1.

60 'ZO

Test Section Velocity (m'per see)

Fig..

39. -

Impeller head-capacity diagram

This should indicate hat the design method

[61

is applicable with sufficient accuracy also for, axial flow impellers for use

in water 'tunnel.

-Ii.'

-500 UC cp 520 600 580 500 R.PM .

-ThE EXPERIMENTAL CAVITATION TUNNEL AT '-ThE NORW GI N -SHIP MODEL EXPERIMENT TANK

Now, both andH Fig. 58 can be 'aflca11y integrated, which consequently leads to the theoretical:effiCieflCy

Tu

= 0.71

As mentioned. 'previously, this:figure may, probably be

appreci-ably increased when a tunnel power factor varies within but narrow limits..

The bron.se impeller was fabriôàted'in our own workshop, and the following performanc,e investigations

show

very good agreement with calculations. Actually, the measured head. at

n =609 RPM .ad nominal seting

angle of, incidence is

(50)

ThE EXPERIMENTAL CAVITATION TUNNEL AT ThE NORWEGIAN SHIP

2,6 Impeller investigations

The pump is placed just downstream of the second bend, as

shown in. Fig. 14-o, and the impeller shaft is led. through the

cas-cade vanes without any f airing. An axial ball bearing is fixed

to the tunnel Iundaiaent, and. the body with the inside water

lub-ricated rubber bearing, is supported by six struts that serve simultaneously as guiding vanes at the inflow.

Fig. 40. - Impeller section and 2. bend

Fig. l. - Schematic sketch of traverse stations for impeller section

Similar vanes at the outlet are not yet fitted, as the flow pat-tern at the respective cross section had first to be explored. As stated below proper vanes will surely improve the working

characteristic of the pump considerably and will soon be con-structed.

(51)

To check the method of impeller calculation a number of tests were made, specially those concerning the head-capacity relation and the efficiency.

As no thrust measuring device could be attached to the im-peller shaft, the thrust had to be integrated from pressure increment measurements with pitot-static tubes across the im-peller disk. The diDection and. maitude of velocity ahead and behind the impeller were measured with angle sensitive pitot

tubes. The results are plotted in Pig. £1.2 and 43.

10 8' 6' 4, U. 20 -2 0

po

c44 0 4 2

-6 -44 1.0 0.8 .5 04 Q2 0 Q2 04 L6

Distance along Radius

f

Pig. 42. - Flow angle and velocity upstream and downstream of impeller in a horizontal plane through impeller shaft.

/See Fig. £1.1/. 50

-U

_____ Total Velocity

pv

r 4 4 AxIal Velocity 6 0,8 1,0

(52)

I0 8 4 Velocity 0 -Q4 LO 08 0.6 04 2 0 Q2 04 48 (*6 1.0

Thstanc. oiong Radius

f

Pig. 11.3. - Plow angle and velocity upstream and downstream of impeller in a vertical plane th±ough impeller shaft.

/See Fig. 41/.

The torque on the impeIr shaft was calibrated against the arma-ture and. field current at. the DC-motor by applying a friction brake. Thus the net torque could be read by direct current mea-siireznents, and together with derived values from Fig. 42, 43, the efficiency

= could be d.ed.uced.

The total circuit loss, found by integrating the pressure curves, has been plotted versus test section speed, i.e. capacity,

(53)

-ThE EXPERIMENTAL CAVITATION TUNNEL AT THE NORWEGIAN SH1P

or volume per unit time, at setup A, both for ttempty" tunnel and. for artificially.increase4.test.section resistance, Fig.

_39.

It can easily be deduced frm this chart., that the measured head, and. capacity'per unit time are respectively 4.6 and 1.7 per cent

lower than the calculated va].ués, at nominal pitch sett.in.g

/ a. = 00/ _{d n = 600 RPM.}

The meásür.edhead 1os coefficient for empty tunnel appears. to be. =

0.55,

while' the anticipated value was C .= 0.68. This

deviation is..supposed tobe responsibie fOr the fact that the optimum efficiency was found when the blades were set at

a. = + 2

degrees re1ati-ê to the nominal p,itch.. This measured efficiency amounts to

_'i=

-0.71 when total output power was conside-red, which is Just the figure calculated at nominal pitch

A fractio 6f the output é'nerr from the impeller /about 11 %'/ is, however, rotational energy, which is not regained by

any considerable. 'amount, as proper guarding vanes are missing.

'When this fraction'is neglected the resulting.'energy is reduced. to = 0.61. The power factor at setup A including impeller

lOSS is then" c

,

=0.9, which is surely very high, and also previOusly presumed. . '

12

Arrangement B

41-. -Mean test section velocity versus'iipel1er input horsepower for various tunnel

arran-gInent5. .

At the present only

12

'1g. available, which gives the test -.

section velocities.at. the various arrangements as shown' in Fig. 1-, where-also the respective Evalues are put in., assuming an impeller efficiency = 065 for B and. C.

- _'52 -ax -a44 12 2 4 6 8 10 Input, Horsepower

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THE EXPERIMENTAL CAVITATION TUNNEL AT THE NORWEGIAN Sill

References:

[i] Ripken,John P. : Design Studies for a Closed-Jet Water

Tunnel. - St. Anthony Falls Hydraulic Laboratory,

Minnesota. Technical Paper No.9, Series B,August

1951.

Straub,L.G., Ripken,J.F. and Olson,R.M.: The Six-inch Water Tunnel at the St. Anthony Falls Hydraulic Labora-tory and. its Experimental Use in Cavitation Design Studies. - Cavitation in Hydrodynamics, a symposium held at the National Physical Laboratory in September

1955. Paper No. 10.

Ross,D., Robertson,J.M. and. Power,R.B. Hydrodynainic

Design of the 48-Inch Water Tunnel at the Pennsylvania State College. - The Society of Naval Architects and Marine Engineers, Transactions, Volume 56, 1948.

Ed.strand, Hans: Cavitation Tests with Model Propellers in natural Sea Water with Regard to tne Gas Content of the Water and. its Effect upon Cavitation Point and.

Pro-pe.11er Characteristics. . - Publications of the Swedish

State Shipbuilding Experimental Tank, No. 15, Göteborg, .1950.

[s] S±lverleaf ,A. and. Berry, L.W.: Recent Work in the

Lith-gow Water Tumiel at N.P.L. - Transactions of the Ins-titution of Engineei's & Shipbuilders in Scotland, Volu-me 100, Part 4, Paper No. 1216.

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ThE EXPERIMENTAL CAVITATION TUNNEL AT THE NORWEGIAN SHIP

Collar, A.R. : The Desii of Wind. Tunnel Fans.

-Technical Report of the Aeronautical Research Committee 1940, Reports and. Memoranda No. 1889.

Cliristensen,H. and Fund.er,J.E. : A Gauge for the

elec-trical Measurement of small dynamic Pressure-Variations on Ship-Model Hulls. - Publication of the Norwegian

Ship Model Experiment Tank, No. .29,. March

_1954.

Laurmann,J.A. and Lukasiewicz,J. : Development of a transonic Slotted Wall Section in the N.AE 30-inch x

16-inch Wind. Tunnel. - National Aeronautical Establishment,

Canada. Laboratory report LR-l78.

Wright,R.H. and Ward,V.G. : NACA Transonic Wind-anne1

Test Sections. - _Report1231,

1955.

King,J.L., Boyle,P. and Ogle,J.B.: Instabilit in a Slotted Wall Tunnel. - Ad.miralty Research Laboratory, Tedd.ington. Unclassified report, May 1956.

[ii] Klein,G.J., Tupper,K.F., and Green,J.J. : The Design of

Corners in Fluid Channels. - Canadian Journal of Re-search, Volume 3, 1930.