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POWERING PERFORMANCE OF A VENTILATED PROPELLER

by

Richard Hecker rr ,-

HYDROMECHANICS LABORATORY

RESEARCH AND DEVELOPMENT REPORT

June 1961 Report 1487

nMC--|M»-«U (il-M)

POWERING PEBFORMANGE OF A VENTILATED PROPELLER

by

Richard Hecker

June 1961

Sc 00S 01 01

TABLE OF CONTENTS

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INTRODUCTION......................

DESCRIPTION OF PROPELLER, „ . „ . . „ ,. o „

DYNAMOMETER AND AIR SUPPLY SYSTEM.

TEST PROCEDURES...................

RESULTS AND DISCUSSION...,,,,„..,,

CONCLUSIONS .......................

FUTURE INVESTIGATIONS.............

REFERENCES........................

Page

1

4

b

1

8

10

ii

LIST OF FIGURES

Page

Figure 1 - Drawing of Propeller 3671A. 11

Figure 2 - Propeller 3671A Showing the Holes in the Blades.... 11

Figure 3 - Schematic Diagram of Air Supply system ,... 12

Figure 4 - Effect of Ventilation on Propeller Cavity Formation

at Design J (0.75). 13

Figure 5 - Powering Performance of Propeller 3671A at Design

J (0.75)... 1^

Figure 6 - Comparison of Propeller Cavity Formation at Various

Air Flows 15-17

Figure 7 - Powering Performance of Propeller 3671A (Ventilated)

at Three J Values.................................. 1°

Figure 8 - Powering Performance of Propeller 3671A at Various

Cavitation Indices 19-20

LIST OF TABLES

Table 1 - Physical Characteristics of Propeller 3671-A........ 3

Table 2 - Test Conditions 5

iii

CL :, V% pV'A J

NOTATION

Area, (Chord times unit span)

Lift Coefficients

D Propeller Diameter

r T v ^ e Efficiency, V2TT Q N J

(. V >) J Speed coefficient, V n D y

( 2 > K Torque coefficient, v. n r?!)6 y q

K Thrust coefficient, V p in?D* J

L Lift

n Rotational speed of propeller

P Hydrostatic pressure (submergence)

P Atmospheric pressure (pressure above water surface)

p Vapor pressure

P Cavity pressure c

Q Torque

Q Volume flow rate of gas supplied for ventilation

Re Reynolds number

T Thrust

V Speed of advance

iv

p Mass density P. + P - P

<* Cavitatlon index based on cavity pressure, ^ % p v8 .. C'Vp'vi'')

CT Cavitatlon Index based on vapor pressure r

Pa + P-PA

ABSTRACT

In an effort to achieve supercavitating performance

at relatively low speeds, ventilation of an SC propeller

was Investigated. Tests were run with a two-bladed SC

propeller ventilated through holes in the propeller blades.

The results of the tests show that ventilated propel»

lers operate with a fully developed cavity at speeds too

low for supercavitating operation. Powering performance

was found to be dependent upon the cavltation Index based

on cavity pressure.

INTRODUCTION

Supercavitating propellers, which are desirable for high-speed vessels,

may also be of interest In a speed range too low for supercavitating opera-

tion. In this speed range (20-40 knots), conventional propellers usually

cavltate with a resultant thrust breakdown. Therefore, to have good pro-

peller performance in this speed range, it is desirable to have a fully

cavitatlng SC propeller. Ventilation, i.e., the continuous Introduction

of a gas into the propeller cavity, is one method of producing a fully

developed cavity at relatively low speeds.

In order to keep the cavity ventilated, air which is lost from the

cavity into the stream must be continually replaced. Some basic studies

have been made on the air loss from the cavity behind a flat disc per- i is ia

pendicular to the flow» ** For the disc it has been found that once

trailing vortices appear, it is impossible to supply enough air to make

any further Increase in the cavity pressure. Some studies have also been

References are listed on page 10

made on ventilated foils operating near a free surface»4,6 The foils were

ventilated when their trailing-vortices reached the surface and thus no

air flow measurements were taken. The tests of both the disc and the foils

show that trailing vortices provide a funnel through which large quantities

of air will flow. Since propellers also have trailing vortices it is likely

that a large amount of air will flow through the vortex tubes. Due to re-

strictions in the supply piping the air that can be supplied to the propel-

ler is limited and hence the air should be judiciously distributed on the

blade.

In order to investigate the problems associated with ventilating a

supercavitating propeller, a research program was initiated at the David

Taylor Model Basin under SR 009 01 01. During the initial investigations

the powering performance of a two-bladed SC propeller with flat-faced sec-

tions was studied. This report presents the results of the initial phase,

specifically the powering performance of the ventilated propeller for con-

ditions of a fully developed cavity. Ventilation is shown to lower the

speed at which a fully developed cavity occurs. Correlation of the per-

formance of SC and ventilating propellers has been found through the

cavitation index.

DESCRIPTION OF PROPELLER

The SC propeller used for this investigation was TMB propeller 3671A

(Figure 1) altered for ventilation by putting holes in the blades. This

propeller was specifically designed to produce a large cavity, therefore

the propeller sections chosen were flat—faced sections operating at high

angles of attack. The method described in reference 6 was used to design

this propeller utilizing lift and drag data from references 7 and 8. The

section at 70 per cent radius was designed to operate at a 7—degree angle

of attack with a lift coefficient of 0.21. The propeller design con-

ditions were: a thrust coefficient of 0.039, a speed coefficient of 0.75,

an efficiency of 41.7 per cent, and a cavitation index tf 0.4. The low de-

sign e6ficiency, which is characteristic of flat—faced sections operating

at high angles of attack, was accepted so that the model propeller would

cavltate fully at the speeds obtainable with the test equipment. Table 1

gives the physical characteristics of the propeller.

TABLE }

Physical Characteristics of Propeller 3671-A

Number of Blades .....,,........,,.. .2

Exp. Area Ratio .,..„..,.........,,..0.154

MWR .................................0.173

BTF ................................,0,055

P/D(At 0 o 7R) .............,,,,...,..,1.500

Diameter ..........................,14,000 in.

Pitch (At 0,7R) ,,.,,,,,,.,.........21.000 in.

Rotation ........................... R.H

Since there was no information available concerning the effect of the

location of the holes on ventilation, the hole positions were chosen arbi-

trarily. Hole 1 was located as near the tip of the propeller as possible

in order to ventilate the tip vortex» Hole 2, which was larger than hole

IjWas placed near the hub and towards the trailing edge of the blade in

order to ventilate the cavity behind the blade (Figure 2).

DYNAMOMETER AND AIR SUPPLY SYSTEM

The TMB 35-horsepower dynamometer, fitted with an air supply system,

was used for the tests. A schematic diagram of the air supply system is

shown in Figure 3. Calibrations of the thrust and torque elements were

made in order to insure that there were no errors in these miasurements

due to the air seal .

The calibrations for determining cavity pressure (P3) were made with

the propeller blades exhausting air into the atmosphere. The atmosphere

was used as an infinite cavity of known pressure. The relation between

the pressure ratio (P3/P1) and rate of air flow (Q ) was thus obtained.

During tests and calibrations the temperature of the air Varied less than

5 degrees Fahrenheit, and so no correction for temperature was made. A

correction for density change and supply pressure was made to all air flow

readings so that all air volumes discussed in this report are at standard

conditions (NTP).

TEST PROCEDURES

The propeller was first tested in the TMB 24-inch variable pressure

water tunnel. These tests were run at 40 fps and the rpm and tunnel pres-

sure were varied to obtain the desired speed coefficient J and cavitation

index a. During the tunnel tests the propeller rpm was varied from 1400 to

3000 rpm and the pressure at the propeller shaft centerline was varied fro«

4 to 38 feet of water.

The ventilated tests were then performed in the TMB high-speed basin.

In the basin the pressure at the shaft centerline was fixed at 38 feet of

water. The forward speed was varied from 10 to 26 fps and the rpm from

900-1800 rpm. During all tests the Reynolds number was greater than

1 X 10».

Table 2 summarizes the tests reported herein.

During the ventilated tests the air temperature (t), air flow (Q ),

and metering pressure (p.) were measured. The cavity pressure was then

obtained from the calibrations shown in Figure 3. Determination of the

cavity pressure in this way is reliable only if there is no critical flow

(choking) anywhere in the system. Therefore, in order to obtain reliable

values of cavity pressure, care was taken during the tests to avoid criti-

cal flow.

As an aid in running the tests, Polaroid photographs were taken during

each run and developed immediately. These pictures were often used as| a

guide during the later tests. Still photographs were also taken to aid in

analyzing data.

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RESULTS AND DISCUSSION

The effect of ventilation on propeller cavity formation is shown in

Figure 4. From this figure the cavity formation with and without air at

8, 10, and 12 knots can be compared. The effectiveness of ventilation in

producing a full cavity at speeds below 12 knots is quite apparent. At

speeds above 12 knots the propeller has a fully developed cavity without

ventilation.

A composite plot of the powering performance at design J is given in

Figure 5. Included in this figure are the data obtained previously from

tests in the 24-inch watejr tunnel. The scatter of the ventilated test data

is believed to be due to inaccuracies in obtaining cavity pressure. Equip-

ment, newly developed at TMB, will provide direct measurements of the cavity

pressure. This is expected to reduce the scatter of future tests.

The close agreement of the test results shows that the powering per-

formance of supercavitating and ventilated propellers can be correlated

through the cavitation index. For the ventilated condition cavity pres-

sure is used instead of vapor pressure.

Figures 6a, b, c show the cavity formation at several speeds and air

flows while operating at design J(0.75). These figures are keyed to cor-

responding powering performance datai of Figure 5 and show that the propeller

had a fully developed cavity at cavitation indices below 3.7. During this

test,ventilation at all speeds above 8 knots increased the cavity pressure

enough so that the propeller operated at a cavitation index less than 3.7-

and hence all the ventilated data appear below this value in Figure 5.

Figure 7 shows the ventilated performance of the propeller at J = 0.6,

J = 0.75 (design);, and J = 0.9. This range of J was chosen so that all

measured quantities were kept within the range of existing instrumentation.

The tests show that at the lower J, where the section angle of attack is in-

creased, ventilation will occur at higher cavitation indices. The tendency

to supercavitate, and hence ventilate, at high angles of attack has been

shown to occur also on hydrofoils .

Figure 8 shows the performance of propeller 3671A at several slgraas

as a function of the speed coefficient J„ Data for nonventilated opera-

tion (obtained In the 24-inch water tunnel) and ventilated operation are

included in the figure. The "solid" test spots are for ventilated opera-

tion. Most of the ventilated data was with air supplied at atmospheric

pressure (q psig) although some data for air supplied at 10 and 20 psig are

Included.

Figure 8 shows that the performance of a ventilated SC propeller is

dependent upon, the cavitation index based on cavity pressure. As pre-

viously mentioned the scatter cf the ventilated test data is probably due

to inaccuracies in the cavity pressure measurements.

Figure 8 can also be used to illustrate the effectiveness of ventila-

tion in achieving aupercavitating operation ^Ifftder the section on test

procedures it was stated that the tunnel tests were run at 40 fps. For

the a = 0.4 tunnel test the net pressure at the shaft centerline was 9.95

ft of water. The ventilated data for a = 0.34 was obtained in the high- c 0

speed basin at 26 fps and a pressure at the shaft centerline of 38 ft of

water. These conditions would produce a a based on vapor pressure of 3.61.

The ventilation of the cavity raised the cavity pressure and caused ac to

be reduced to 0.34.

Figure 8a is the same as Figure 8 with the test spots removed for

clarity. The designed Kt of 0.39 and efficiency of 41.7 are shown on the

curve. The figure shows that, the propeller performed as predicted. The

predicted values were CAI zulated as show^ by reference 6 with the excep-

tion that, since this propeller has flat-faced sections, the lift-drag data

from reference 7 and 8 were used.

CONCLUSION'S

The data presented in this report clearly indicate the effectiveness

of ventilating supercavitating propellers at lev speeds to Induce a fully

developed cavity.

Correlation of the power performance through cavltation index

(based on cavity pressures) has been obtained. In general, the air

supplied to the cavity raises the cavity pressure and thus reduces the

cavltation index. Hence a fully developed cavity can occur at speeds

too low for supercavitating operation.

It was found that raising the supply air pressure will reduce the

cavltation index as long as the mass flow of air is increased. If the

supply pressure is raised too high, say above 20 psig for these tests,,

choking occurs in the supply system and further increasing the supply

pressure does not affect the propellers. The location of the holes was

not thoroughly investigated; however, it was found that unless the natural

cavity enclosed the air supply hole, full ventilation did not occur.

FUTURE INVESTIGATIONS

From the information contained in this report, it has been concluded

that ventilation of SC propellers is a feasible means of inducing a fully

developed cavity at relatively low speeds. However, in order to gainfully

apply ventilation, several aspects of ventilation must be further investi-

gated experimentally and theoretically»

The tests reported herein show considerable scatter; this has been

attributed to indirect measurement of cavity pressure. Newly developed

instrumentation will provide direct measurement of the cavity pressure

and hence, reduce scatter. As soon as manufacturing of this equipment

is completed tests will be performed in which the cavity pressure will be

measured directly.

In the present Investigation it was found that the powering perforra—

ance can be correlated through the cavltation index. This allows the

prediction of the thrust and torque when the cavltation index is known.

At this time, however, it is not possible to predict the cavltation index

at which a ventilated propeller will operate since the relationship between

propeller Iqading, air entrainment, and cavity pressure are not known.

8

Hence they must be obtained. Future investigations will thus consist of

a theoretical study of the mechanism of air entralnment and an associated

experimental study to investigate the air entralnment of simple shapes and

foils*. The information thus obtained will be used to develop a design

method for ventilated propellers.

* These, experiments are currently being performed by the Research and Propeller Branch of the David Taylor Model Basin.

REFERENCES

1. Swanson, W. M. and O'Neil, J. P., "The Stability of an Air-

Maintained Cavity Behind a Stationary Object in Flowing Water," California

Institute of Technology Hydrodynamics Laboratory Memorandum Report No.

M-24.3 (Sep 1951)

2. Cox, R. N. and Clayden, W. A., "Air Entrainment at the Rear of

a Steady Cavity," Symposium on Cavitation in Hydrodynamics, National

Physical Laboratory (Sept 1955)

3. Campbell I. J. and Hilbourne, D. V., "Air Entrainment Behind

Artificially Inflated Cavities," 2nd Symposium on Naval Hydrodynamics,

Office of Naval Research (Aug 1958)

4» Johnson V. E., Jr., "Theoretical and Experimental Investigation

of Arbitrary Aspect Ratio, Supercavltating Hydrofoils Operating Near the

Free Water Surface," NASA Report RM L57116 (Dec 1958)

5. Wadlin, K. L., "Ventilated Flows with Hydrofoilsj" Presented at

the Twelfth General Meeting of the AITC, University of California, at

Berkely, California (Sept 1959)

6. Tactukindji, A. J., Morgan, W. B., Miller, M. L., and Hecker, R.,

"The Design and Performance of Supercavltating Propellers," David Taylor

Model Basin Report C-807 (Feb 1957)

7. Parkin, Blaine R„, "Experiments on Circular Arc and Flat Plate

Hydrofoils in Noncavitating and Full Cavity Flow5" California Institute

of Technology, Hydrodynamics Laboratory Report No. 47-7 (Feb 1957)

8. Wu, T., "A Free Streamline Theory for Two-Dimensional Fully

Cavitated Hydrofoils," California Institute of Technology, Hydrodynamics

Laboratory, Report No. 21-17 (Jul 1955)

10

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I 3

Circled Number» Correspond To Photograph» in Figure 6

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20

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