Propeller cavitation as influenced by the air content of the water

A Station of the

Department of Scientific and Industrial Research

Lab.

y.

Scheepsbouwkunde

See note inside cover

_{Technische Hogeschool}

_{SHIP REP3]L}

Deift

_{August 1962}

ARCHIEF

NATIONAL PHYSICAL

LABORATORY

SHIP DIVISION

PROPELLER CAVITATION AS INFLUENCED BY

THE AIR CONTENT OF THE WATER

by

Crown Copyright Reserved

Extracts from this report may be reproduced

provided the source is acknowledged.

*Paper presented at Symposium on Cavitation and Hydraulic Machinery

organized by International Association for Hydraulic Research at

PROPELLER CAVITATION AS INFLUENCED BY

THE AIR CONTENT CF THE WATER

by

A. SilverleafandL.W. Berry

Ship Division, National Physical Laboratory

SUMMARY

Experiments were made with a propeller model in the new large water tunnel at

Ship Division, 1'PL, to examine the effects of varying the total air content of the

water on the propeller performance under cavitating conditions.

The water tunnel

in-corporates a resorption circuit, or resorber, which enables experiments to be made

over an unusually wide range of air content.

At each set value of air content,

measurements were made of propeller forces, and cavitation patterns were observed for

a range of operating conditions;

the acoustic frequency spectrum was also determined.

For conditions of fully developed cavitation, the results show appreciable effects of

air content on propeller forces, but relatively minor influences on cavitation

patterns, stability of running, and the spectrum of emitted noise.

SOMMAIRE

Cth a fait des essais avec un modle de l'he'lice dans le grand nouveau tunnel

hydrodynamique du Ship Division, NPL, pour examiner les effets des variations de la

teneur totale en air de lteau sur la performance de l'hlice en regine de cavitation.

Le tunnel a un circuit absorbant, ou un rsorbeur, qui permet des essais pour des

variations trs grandes de la teneur en air.

A chaque valeur fixte de la teneur en air

on a xresur

les forces de l'hlice, et on a observé les fornes de cavitation sous des

conditions différentes d'opération;

on a aussi determiné le spectre acoustique.

Sous

des conditions de cavitation bien d&veloppes, les résultats indiquent des effets

significatifs de la teneur en air sur les forces de l'hlice, mais des effets

relative-ment sans ixportance sur les formes de cavitaUon, sur la stabilite d'opration et sur

le spectre du son

nit.

*

Paper presented at Symposium on Cavitation and Hydraulic Machinery

organized by International Association for Hydraulic Research at Sendai,

PROPELLER CAVITATION AS INFLUENCED BY

THE AIR CONTENT OF THE WATER

by

A. SilverleafandL.W. Berry

Ship Division, National Physical Laboratory

INTFODtJCTION

Although ship propellers operate in sea water which contains an appreciable amount of air, either in dissolved form or as entrained bubbles, it has been customary to carry out cavitation experiments with model propellers in water tunnels filled with water which has been at

least partly de-aerated. This test procedure has been necessary

because air contained in the water of a tunnel is released as it passes through the working section, which is generally at low sub-atmospheric pressure in order to induce extensive cavitation at the test body. In

most tunnels this released air does not re-dissolve as it circulates round the tunnel, and consequently the working section soon becomes

obscured by a cloud of travelling bubbles of air. It has been known

for some time that the performance of a cavitating propeller may vary with the air content of the water, and the large water tunnel recently

completed for the Ship Division of the National Physical Laboratory was designed so that experiments could be made with water of different air

contents. This paper gives the results of the first set of experiments

carried out to examine the effect on propeller performance of varying

the total air content of the water.

Water tunnel and air content

The experiments described here were carried out in the large No.2

water tunnel at the Ship Hydrodynamics Laboratory at Felthaxn of the Ship

2

been published in ef.1, and a more detailed account is given in Refs.2

and 3. The tunnel has a working section 44 in (112 cm) in diameter and

88 in (224 cm) in length; the maximum velocity of flow through the

working section exceeds 50 f/s (about 15 in/s) and the pressure can be

varied from less than 0.1 atmosphere absolute up to 6 atm. abs. The

circuit outline of the tunnel, fig.1, shows that it incorporates a deep

resorption circuit, or resorber, having a lower horizontal limb of dia-. meter 10 ft (3.05 m) which is 180 ft (55 ni) below the upper horizontal

limb in which the working section is situated. This resorber enables

cavitation experiments to be carried out with water containing a large amount of air while maintaining a clear stream at the entrance to the

working section; the design characteristics of the resorption circuit

are described in Ref.4. There is a de-aeration plant in a secondary

loop linked to the main tunnel circuit to enable the air content of the water to be varied, and the air content range of the tunnel water was

specified to be from a minimum of 4 cc of air at NTP per litre of water (or 5 parts of air per million parts of water by weight) to a normal maximum of 24 cc per litre (or 29 ppm), although it was hoped that a special upper limit of 45 cc per litre (or 55 ppm) might be achieved.

In practice it has been found difficult to reach the lowest specified air content, but higher values than the specified maximum have been achieved, and the propeller experiments described in this report were made with water having total air contents ranging from 7 cc per litre (8 ppm) to

43 cc per litre (52 ppm). At the higher air contents it was evident

that greater amounts of air were entrained as small air bubbles, but the experiments were continued so long as the test body was clearly visible

in the working section.

Throughout this initial set of experiments the air content of the tunnel water was measured by a van Slyke apparatus of NPL pattern fitted so that samples of water could be drawn into it directly from the tunnel. This apparatus measures the total gas content of the water and cannot discriminate between dissolved and entrained air. Preliminary attempts

at measuring these components separately were unsuccessful, but it is

intended to attempt to determine the entrained air by an ultrasonic trans-mission method, such as that described in Ref.5.

Propeller model and test range

The propeller used in these experiments has diameter D 21 in (53.3 cm), pitch ratio (p) 1.333, and three blades with a total blade area ratio

3

Admiralty Experiment Works, Haslar, as one of a geometrically similar series of propeller models of different diameter to be used in worldwide

comparative experiments carried out under the auspices of the Cavitation Committee of the International Towing Tank Conference. This particular model belongs to the ITTC Series i and has the design designation AEW C2;

full particulars are given in Ref.6. The propeller was mounted in the

tunnel working section on a shaft downstream of it having a diameter 2.5 in (6.3 cm) and driven by an independent, external motor.

The experiments were made by the standard NPL procedure to cover a

full range of propeller operating conditions; the propeller rate of

rotation n was kept constant (here 16.67 r/s or 1000 r/min), and the

advance coefficient J (equivalent to the flow coefficient for a pump)

was varied by altering the flow velocity y. At each set value of J

(corresponding to a set value of y) the pressure p in the working section was varied to cover as wide a range as possible of the cavitation

index o-

or o.

For the propeller under test, the design advance

coefficient is J 1.05. Measurements of propeller thrust T and torque ,

visual and photographic observations of cavitation patterns, and acoustic observations and measurements were made at each operating condition thus

defined by its J and o values. This procedure was repeated for each

of several total gas contents. This method of experiment was adopted

because it takes much longer to change the gas content of the water (total volume about 54 000 ft2, 1525 m3) than any of the other variables.

Throughout the experiments the water temperature did not vary by more than

1°C from its mean value 18°C. The full test range is summarized in Table 1.

TABLE 1

uantity Minimum Maximum

Propeller diameter D ft 1.75

Propeller rate of rotation n rev/sec 16.67 constant

Flow velocity y ft/sec 17.5 41.0

Advance coefficient J 0.6 1.4

Pressure at centre line of working section

p lb/ft2 250 7500

Flow cavitation index o- 0.13 25

Propeller cavitation index O 0.04 1.7

Propeller blade section R(0.7) 6.1 x 10 7.0 x 106

Reynolds number

Total gas content of water a.cc/1itre 7 43

V

p-p

Notes: J _-- ; o- = nD 2

VR C07

R(O. 7) V 4

P - PV

o-R - 2

t VR

2 2 2 VR

-

V2 + (O.'7D)

whe re _pv

-

Saturation vapour pressure of water at ambient

temperature toc

p, V

-

Mass density and kinematic viscosity, respectively,

of water at temperature toc

VR - Resultant velocity of blade section at 0.7 radius,

neglecting inflow.

Force measurements

A typical set of farce measurement contours for propeller c.236 is

shown in fig.2, where the measured thrust T and torque are given

as thrust and torque coefficients KT and K respectively. The

advance coefficient _T directly derived from the velocity VT in the

working section has been converted into an equivalent advance coefficient

J0 for unbounded flow by an axial momentum correction factor

J

K = as derived by Wood and Harris; for the range of these experiments

this correction was small. The cavitation index _0R' based on the

resultant velocity of the blade element at 0.7 radius, was also corrected to unbounded flow conditions by a factor due to Emerson. These

correc-tion factors and the contour method of presentacorrec-tion of propeller data are

described in Ref.'7.

It should be noted that, in the experiments reported here, the flow

velocity VT was determined by relating the pressure drop measured in the

contraction upstream of the working section to its calculated area ratio. The results of a detailed velocity traverse of the working section, now under way, may alter mean flow velocities so derived by up to 1%.

In fig.2 the portions of the curves shown broken are for conditions

where the thrust and torque forces fluctuated violently due to unstable

running of the propeller; this region of instability is discussed later.

Elsewhere the forces were steady and were measured with an accuracy

estimated to be generally within ±0.5%. The results given in fig.2 are

5

air content is shown in fig.4 for the advance coefficient _T 1.05(J0 1.03)

very close to the design value for the propeller, and in figs.3 and 5 for

values of _T 0.90 and 1.20 respectively. Figs.3-5 show that for higher

values of the cavitation index (crR > 0.20), where the propeller forces

were not affected by cavitation, the total gas content had no effect on

these forces. However, for lower cavitation numbers (crR < 0.20), the

air content significantly affected the propeller forces. This was

par-ticularly so for air contents lower than the value for water saturated with air at atmospheric pressure, and at first sight would indicate that the common practice of running water tunnels with de-aerated water leads

to errors in propeller forces. However, it is possible that the force

variations shown in figs.3, 4 and 5 for conditions of fully-developed

cavitation < 0.10, say) are a reflection of the way in which the

cavitation index 0 or aH is customarIly defined. This conventional

engineering definition assumes that the pressure within the cavity p is equal to the saturation vapour pressure p, of the liquid, and is

in-dependent of other properties of the liquid such as its gas content.

However, if the cavity pressure _PC does vary with the total air content of the water, then the effects shown in figs.3-5 may be significantly influenced by correctly relating the cavitation index _°R to the true pressure within the cavity, instead of to an assumed value p..

Experi-ments to investigate this point are now in progress at NFL.

Cavitation Patterns and Regions of Instability

Visual and photographic observations were made of the extent of

cavitation during the experiments. The results of some of these are

summarized in fig.6, which defines the propeller operating conditions, in

terms of J and

O,

for different forms and extent of cavitation. Here

tip vortex inception was taken as occurring when a thin trailing vortex was seen to attach itself to the blade tip, and face cavitation to start when a continuous thin bead of cavitation could be observed on the leading

edge (L.E. ) of the blade face. Fig.6 shows that tip vortex inception was

affected by the total air content; at a fixed advance coefficient J the

inception cavitation index increased with the air content. This is

in accord with generally held views that tip vortices occur more readily on ship screws operating in air-saturated water than on model propellers

running in de-aerated water tunnels. On the other hand, the present

experiments showed no clear differences with air content variation in the conditions for the start of L.E. face cavitation, nor, as far as could be decided from the observations made here, in the extent to which more

e

developed cavities covered the blade face. The same indifference to air content variation was observed in the shape of extensive cavities on the blade back, but this also may be modified when more precise observations are made in a later stage of this investigation. These coiaments apply

also to the conditions at which cavitation starts on the back of the blade section at its maximum thickness, which is close to the mid-chord

position.

Fig.6 also shows the thrust breakdown line; at each value of J

this is defined by the value of at which the thrust is 1% less than

its non-cavitating value. For advance coefficients below the design

value (J 1.05), for which cavitation occurs on the blade back from the leading edge, thrust breakdown was apparently unaffected by air content. However, for higher j values, corresponding to face cavitation condi-tions, thrust breakdown occurred at higher values of 0 as the air content was decreased.

At many advance coefficients, generally for J < 0.9, a region of instability occurred as the pressure p, and consequently the cavitation

index o,

was reduced. This instability caused a violent torsional oscillation at the propeller, with unsteady propeller forces and noise output, but when the pressure was reduced still further the unstable

running conditions ceased abruptly and smooth operation was restored.

The conditions for this unstable running are indicated by the broken curves in fig.2 and the region of instability is also shown in fig.6.

The present observations did not indicate any clear differences in this zone of instability with changes in air content, though it is possible that closer examination will show some variation. The phenomenon is difficult to investigate because it is not desirable to continue running in these violently fluctuating conditions. Although the explanation for this in--stability is not yet clear, it appeared to occur when there was a

substan-tial cavity on the blade back, at least part of which collapsed close to

the trailing edge of the blade. In this respect it has analogies with

buffeting of aircraft wings as the speed increases from subsonic through the transonic range to supersonic, and the limits of the instability zone

in fig.6 resemble the buffet boundaries for such a wing.

Acoustic spectrum

Measurements of the noise emitted by the propeller were made for

cavitating and non-cavitating conditions. A self-generating barium

titanate transducer, in the form of a disk 3 in (7.5 cm) in diameter and 0.5 in ¿1.25 cm) thick, was mounted in a brass housing by peripheral

7

O-rings so that it could vibrate freely as a diaphragm. The transducer

was immersed in water in a container which was placed on the top window of the working section, keeping wet contact between the outer surface of the window and the lower surface of the container. The transducer

out-put was fed to a wave analyser to determine the sound pressure frequency spectrum of the propeller and the flow field. The background noise

level was determined by test runs with the propeller not operating.

Fig.? shows the derived acoustic spectra for the propeller in a non-cavitating condition LIT 1.05, o 1.6) at different total air contents.

This suggests that the propeller has a flow-excited natural frequency of vibration about 2 kc/s, which is essentially a "singing" note; as the

gas content was increased, this was picked up much less strongly by the

transducer. This diminution may be due either to increased acoustic

absorption by the water as its gas content increases, or to damping of

the blade in these conditions. It should be noted that the ambient

pressure was so high for the measurements recorded in fig.? (6000 lb/ft2,

or almost 3 atm. abs. ) that, even at the highest gas content value

(52 ppm), the water was not over-saturated and no entrained air bubbles

could be observed.

Fig.8 shows similar spectra at the same advance coefficient

(J 1.05), but at a much lower pressure (BOO lb/ft2) corresponding to a

cavitation index TR 0.15. These show the measured acoustic pressure at

the blade natural frequency to he much reduced at all air contents; this

may well be due to disturbance of the vortex shedding pattern at the blade trailing edge by the cavity on the blade back. In general, these

frequency spectra show that, while cavitation has undoubtedly affected the noise emitted by the propeller, it is difficult to distinguish the effects of total air côntent over the frequency range in which most of the acoustic energy is then concentrated (say between 100 c/s and 10 kc/s). It is recognized that the technique used in the present experiments is liable to many errors, and that the results given in figs.? and B can only be regarded as a tentative indication of possible effects.

8

Conclus ions

The results of this preliminary investigation of the effects of the gas content of the water on propeller cavitation suggest

that:-Propeller performance in non-cavitating conditions is

un-affected by the total air content.

In conditions of extensive back cavitation, the propeller force coefficients increase significantly as the total air content is reduced below the value for water saturated

with air at atmospheric pressure. This may be due to

changes in the effective cavitation index because of variations in the cavity pressure with air content, and may influence the interpretation of results from water

tunnels filled with dc-aerated water.

In conditions in which face cavitation occurs, the cavita-tion index at which propeller thrust breakdown first

occurs increases as the air content decreases.

Conditions for the inception of tip vortex cavitation are

affected by changing the air content; the inception

cavitation index increases with air content.

The extent of visible cavitation on the blade back or face is not significantly dependent on total gas content.

An unstable region of operation, associated with fairly extensive back cavitation, is relatively unaffected by

variations in air content.

The effects of air content variations on the propeller sound pressure field are not clear, except for a possible reduction in a flow-excited blade vibration with increas-ing air content.

Acknowledgements

The work described in this paper forms part of the approved research programme of the National Physical Laboratory, and the paper is published with the approval of the Director of the Laboratory.

The authors wish to thank Mr. S. Grant, who carried out the ex-periments described here, for his valuable help.

9

REFERENCES

ALLAN, J.F. - The New NFL Ship Hydrodynamics Laboratory - Trans.

I.N.A., 1957, 99.

SILVERLEAF, A. - Basic Design of the NFL No.2 Water Turmel

Ship Division, NFL, Report No.15, May 1960.

SILVERLEAF, A. - The Determination of the Operating Range of a Water Tunnel - Ship Division, NFL, Report No.2,

September 1958.

SIL VERLEAF, A. The Design of a Resorber for a Water Tunnel -Ship Division, NFL, Report No.1, September 1958.

RIPKEN, J.F. and KILLEN, J.1. - A Study of the Influence of Gas

Nuclei on Scale Effects and Acoustic Noise for

Incipient Cavitation in a Water Tunnel - Tech.

Faper27, Series B, St. Anthony Falls Hyd. Lab.,

University of Minnesota, September 1959.

e. GAWN, R.W.L. - Results to Date of Comparative Cavitation Tests

With Fropellers - Trans. S.N.A.M.E., Vol.59, p.168, 1951.

7. SILVERLEAF, A. and BERRY, L.W. - Recent Work in the Lithgow Water

Tunnel at NFL - Trans. I.E.S.S., Vol.100, p.211,

DIAMETER

I39

WORKING SECTION

LENG 881N5 MAX. VELOCITY 5OPS DIAM 44 INS MAX, PRES.(AS) 6 AT.

1400E!. SCkEW OIAM. '2 TO 24 INS

4TTI0tI SETTLING R&T&Ó 4.1 LENGTh DIAMETER IO T FIG. I. DIAMETER 13' 9 l'o, CENtRES

PROPELLER

PERFORMANCE

CURVES

SCREW C 236 DESIGN AEW C 2 (ITTC SEPIES 1)

D 21 INS (s3.4cMs) Z 3 _fi 0797 1333

TOtAL AIR CONTENT 8 4 (7 CCS/ LITRE)

0OG 005 004 KQ ₀₀₃ 0.Oz OZB 020

'

Id= 0301

0.0 O. IS O IO 075 I I I 00G 005 004 KQ 003 002 020 OIS

00

0.05 IO 20 30 40 AIR

CONTENT, PARTS PER MILUON

VARIATION OF PROPELLER FORCES WITH AIR CONTENT VARIATION OF PROPELLER FORCES WITH AIR CONTENT

J,.o.9O

T= IO5

FIG. 3.

FIG. 4

IO 0 30 40 50

0O4

003 -

KQ

002 -

_{001 H}

010

-O

J0I1S _(IT=i'20)

010 s. s 0l5 O IO 07S 0.Os ö 010 O, 0

84

AIR C.4TtNT, PARIS PER MILLION

VARIATION OF PROPELLER FORCES WITH AIR CONTENT i1. = I2O

flG. 5.

1.4

Vr

i0

04 AIR CONTENT - 84 PPM Z4 PPM

- - 42 PPM

ICREA$NG

/CK

CAVITATION

¡

FROM LE 11P VORTEX INCEPTION REGION OF INSTABILITY

/ / y/Ji/Ji

/

Z /

'

FIG. 6.

CAVITATION DIAGRAM REGION OF NO C-A'ITATION

\

!ACE CAVIAT

rosi LE

I/ TNST

ß0 N

ALL AIR CONTEN

BACK CAVITA1ON FROM MAYJMLIM THICKP.JSS

-t I I

I.

I I I i L 0G 07 08 O '0 I I 12 AVANCt COEFFICIENT J0

tLTIV PRE5Su5 O-Io (ANAL'5R \0tjTPuT IN TTtV) oc "os

4-'

5 PM

4 '!OAL 1 C0NTtT i

II'

¡ \

\\

/

I

r\\

_f

il

!QtQur-,v C/5

ACOUSTIC SPECTRUM FOR d, - o is (CAVITATING)

FIG.8.

¡t'

fl

too 000 IQ000

EOUNCV c/s

ACOUSTIC SPECTRUM FOR _dR l-6 (NON CAVITATING)

FIG. 7.

RLATNt SOUNb PRESSURES I00 (WALYSE 0UTPUT N IW 00I 0o 00 000 ic000