Instrumentation for the detailed evaluation of propeller performance

THE NORWEGIAN SHIP MODEL EXPERIMENT TANK.CABLE: SKIPSTANK PHONE: 28020 SKI PS MOD E LLTAN KE N

PREPRINT OF

Instrumentation for the Detailed Evaluation

of Propeller Performance.

by J. W. English, National Physical Laboratory, England.

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DeIft.

SYMPOSIUM

ON TESTING TECHNQUS

IN SHIP CAV[1A110N

RESEARCH

31 MAY2.JUNE 1967

flSTRIThNTATION FOR. TEE DETATTD VfLUATION OP

PROPELLER PERFORMANCE

J. W. Eiglish,

Ship Division, National Physical Laboratory,

E±lánd.

Introduction.

Several theoretical methods exist for designing. non-cavitating

marine screw prppellers, includ.ng the earlier lifting line theories

incorp-orating blade interference factors and. the more recent. lifting surface

methods which require computers for the-fr numerical application. Unlike the

airs crew field where detailed experimentation on screws preceded and. kept pace with the evolution of theoretical methods., in the case of marine screws. very little detailed experimentation appears. to have been conducted. This

is a pity since despite the thoroughness of some of the design methods avail-able today, simplifying assumptions are still necessary.

As par.t of the long term researôh at NFL on marine screw propell-ers and. other devices, it was considered desirable to have

means for

evalua-ting theoretica] methods in more detäi.., and this paper describes some of the work that has been conducted. on this subject, and the preparations being made

for future work.

-The paper is divided, into two parts, the first of which contains a description and. some. results of stationaly-pitot wake survey methods as

applied, to an ordinary marine screw propeller, while the second part describes

with- probes rotating with the screw. This second method will be used for

making blade pressure measurements and.. propeller wake measurements in the - rather special case of fully cavitating and ventilated, screws.

Part I. Marine Propeller Thrust and. ,Torque Grading Experiments Using the

Pitot Thaverse Method.

The method, of measuring propeller thrust and. torque grading was first introduced in work on. airs crews in, 1918 by Stanton and. Marshall, ref..1. At that time only the actuato r disk or momentum theory existed for d.escrib-ing propeller action and. consequently the pitot traverse method was first formulated in terms of this theory. Subsequently with the application of vortex theory to propellers the measurement of propeller thrust grading was

explained more satisfactorily in terms of t.}ii s theory.

In this account of the application of the method to a marine propeller the derivation of the basic expressions underlying the method is

reduced to a minimum, and. for- more detail the reader is referred to -the

original publications. . . , .

i) Torque Grading.

If we consider the flow in a stremtube immediately downstream of a screw operating

in

uniform flow, then the elementary torque at the screw

radius r is given by the áhange in the" angular momentum of the fluid in passing through the screw, or'

dQ= 2p tbWtX dr

(i)

where

t

and wt

are the axial and. tangential vecities respectively in

the streamtube. Ii terms of the usual non-dimensional prppefler

coeffic-ients this may be Written as, '

dKf. . q

!

._=_ja

sin 24r .

-dx 8 '

,thre q=p2 'and.

=.pU2

. Here .0 is the screWadvance

- velocity and V is the resultant of i and. Wt , where 'tan 4c

3

In equation (2) q is the dynaic pressure pf the siipstram

fluid, and. relative to a fixed point in

e

wake of a finite bladed. screw

bot q and. q sin 2 ir vary in a periodic manner with respect tQ time.

The principle of the method of torque grading measurement d.esbribed in ref. 2, hOwever, is to measure the time-mean values of the, quantity

q sin .2 4r with a yawmeter having this form of response and. a recording

system consiting of a conventional liquid filled manometer. The liquid

in such a system has a relatively large inertia and theref Ore a low natural

frequency of oscillation compared with that of the forcing function which

in this case is th& pressure pulses occurring: at blade passage frequency.

As a consequence of this, the manometer does not respond to the aplied.

oscillatory pressures but records a mean value. It is shown in ref. 2, for example, that this. mean value may correspond with the. true time-mean value of the applied pressure signal depending on the. factors governing the

motion of the liquid in the probes, the pressure leads and manometer. These

factors are mainly,the ratio of the frequencies of the applied pressure signal to the natural frequency of. the manometer,. the mii.tude of the fluctuating

pressure signal, and the viscous damping in the system. It i necessary,

therefore, when using this method of recording to ensure that,

1.

The frequency of the applied pressure signas (co) .is thrb

greater than the natural frequency Of the liquid in the

manometers

(w)

or w >> w n

The fluctuations of pressure being recorded are not too

large. . ..

3. The flow in the fine bore lines is laminar such that.the viscous damping is proportional to the velocity in the lines, also tie damping in all the lines must be

5imil

ar.

i) Thrust Grading.

Using the momentum theory and mean flow principles, the thrust of a propeller may be expressed

in

erms of the rsein total head of the fluid. in passing. thxugh the propefler. A method of deriving a s1 mu ar expression which isLexactlyapplicable to the periodic flbw created by a screw was S.. developed, by. Lock and Yeatman. in ref. _.3, however. Lock showed, for example, that subject to the neglect of profile drag, the. total head. difference

21.

across a propeller section is ven exactly by,

r Zn

p (3)

where i - is the time-mean total head. difference, _{r is the blade}

circu-lation at radius r, _Z is the number of blades ad is the

dT = 2r

or in terms of the thrust coefficient,

dXT

(i-i),

3

(1---dx 4. \. itx2U

If w

is neglected, this expression is exact .when (i -Ib) is the

time-mean difference between the periodically varying downstream total head and. the constant upstream tôta]. head. Due to the non-linear relation between

velocity and. raes sure in Bernoulli 'a equation an error is introduced when

calculating Wt frorn pressure measurements in a periodic flow, but the

effect of this error on the thrust should be ml 1 since in the

-

2nr

()

(6)

FIG.1. VELOCITY DIAGRAM FiG 2 VELOCITY DIAGRAM

AT PROPELLER IMMEDIATELY DOWNSTREAM

OF PROPELLER

rotational speed of the propefler. Referring to fig. 1, the circulation

r may be related to the thrust element by means of the Kutta-Joukowski relation, giving

dT =

Pzr(nr

_{-) dr}

5

above equation () is uaUally sznall óompared. with unity.

The quantity -- in equation () represents. a correction to

Zr

the thrust determined from the total head.

measurements

due to the rotation of the slipstream and may be calculated approximately as foLLows,

From fig. I and. the Kutta-Jkows. relation we. have,

a. Q

=Zpr r

9r sinj9 i dr

₍₇₎

Alo we have,

?ir

r=

K (8)

z

where K is the appropriate value of the Goldstein factor accounting for

the finite number of blades. Substituting

r

from equation (8) into

equation

₍₇₎

and using equation (2) we obtain,

w 11.K'

Q

2r

ii'KJ-sin19i

U

wt w

In this equation -- , K and - sin j9 1 are mutually dependent but an

U

W

approximate value of sin j9 i may be taken as - cos 4r

,

see fig. 2,

U U

and K

may be determined if the hydrod.ynaniic pitch angles are approim.ted.

by,

hr

*

tan

j9 I

tan = ---.--- (io)

ltx 1 -.

smut

j

U

The values of cos r and. sin u may be obtained from the yawmeter

readings but are subject to erPr oh accoUnt of the périod.ic nature of the

flow. Agaifl, however, thjs error should not seriously affect the caJ.cula-tion of

-iii) Section Lift Coefficients,.

The foregoing.a±ialysis can be extended. further to obta-in

approx-imate viues of the section lift coefficients. By resolving the forces -acting on a blade element into the lift and. drag directions the lift coeffic-ients can be. expressed as follows,

.-1 1'

=-

(C)ç)

[t

a.

2

-cos j9 i+-K ''sin _i (ii)

I

I.

-

1+.,j 1+

2

The axial velocity rough the screw is then given by,. 6

tani=

(i6)

The lift' coefficients may be calculated using equations (2),(6),(li) and. (16).

The. hydrodyriamic pitch. angles 9 i ma.y be ób tamed, using a prooedure simil ar to that adopted 'by

Reid.

in ref. Li., except that he neglected the G.oldste4.n

factors whereas they are included here.

2 _{- Ui.ng ej*tions (4) afld (8)}

an4the.ve'locity

_{triangle 'in fig. 1,'}

the thrust element can be written as

dT =2ltrPKwa(U+

2?

(12)

or = i x (1 + a) a K (13)

w

where a = a is the axial inflow

2U

velocity at the . screw. Equation

(13)

is. a quadratic in a and. solving

for a

we obtain, . .

= -I

I +A,J I + ---U J

2L

(15)

7

Description of Instruntation and. Metho& of Experimentation.

)

The, combined yawuieter azid total head probes commonly used in work

on

airscrews in the past coisisted of three open.end.ed. hyp a deririic tubes

attached together side by side.

The two outer tubes 'ormed a Y yawmeter

with an includ.ed angle of 90 deg±ees, and the centtal tube was used for total

head. measurement.

This t'ype of yawmeter produces a pressure difference

proportional to

2

_{sin 2 ifr}

_{over a large yaw range and was admirably}

suitable for work on air screws where, at times, very large

aw angles had to

be measured.

AnOther instrument having a yaw response proportional to

W

sin 2 ir

over a large yaw range is the pitot cylinder.

This type of

ixstrurnent has not been used.

or work on air screws though, because the per-'

formance at the lower Reynolds numbers is dependent upon Reynolds number.

The

Y yawmeter does not suffer from this defect, but due to ihe separation

of the side arms it is di.fficult to make measurements at a point, and in the

case of marine screws where the models are smal comparedwith air screws,

these probes would. tend to be large relative, to the model screw.

In considering the probes to be used in the experiments on a

marine screw, it was decided to use combined totalhead-yawmeter probes of

the "cobr&' type0

With these probes a central hole is used for total head

measurement and the two outer tube, instead of. forming a Y,

are

champ-fered. off at 35 degrees to the probe axis as shown in fig. 3.

ThOse probes

COBRA PROBE V PROBE

FIG.3. COMBINED TOTAL HEAD AND YAWMETERS

have the advantage of compactness aiid their rfora.nce is not seriously

influenced by Reynolds nuniber effects, although their calibratiOn' is

essentially linear or proportional to W2 sin

c

for

mni T, yaw angles

instead of

2sin 2.

.

However, for small yaW angles

sin 2 4'

2 sin 4',

the tangential wake velocity from a fixed pitch screw is always in the

same direction, it was decided, to pre-set the probe heads into the swirl

direction by a small amount, noininafly 10 degrees, in order to ensure that

the instruments were used. within theix linear ranges at all times.

0

6

I

-INCHES

FIG. 4. PRESSURE MEASURING RAKES USED IN WAKE TRAVERSE EXPERIMENTS.

The cobra; probe ra1es were made up and. mounted on their supports

as shown in fig.

.

In all, twelve probe heads at different radii were made

'but these had to be arranged ii-two separate rakes for convenience in

'manu-facturing.

The probes were usedwith the non-nufling technique and.

there-fore they were calibrated bethere-forehand over a range of yaw exceeding that at

No. I water tunnel shaft on a device shown in fig..5, which enabled

0

3

INCHES

9

FIG. 5.

_{METHOD OF HOLDING PROBES FOR CALIBRATION.}

them to be yawed into the tunnel working section flow. _{A. typical set of}

calibrations for a single cobra probe is shown in fig0 6 plotted in a similar inarmer to that adopted by Lock in ref. 5. _{In this figure C, P}

10

and S refer to the pressure rea&ings of the centre, port and starboard

FIG. 6. CALIBRATION CURVES FOR COBRA PROSE

holes of the yawmeter when facing upstream aiid looking towards the shaft axis.

It will be seen from this figure that the quantity

c-4 (p+c)

can be taken

as a measure of the dynamic head. ₍₌ p

ifl

over a considerable range

ofyaw.

The upstream total head, which is nominally constant across the

tunnel working section, was measured by a bank of three open-end.ed. total

head. tubes0 The arrangement of tk rakes and. propeller in the No0 I water tunnel, where the experiments were performed, is shown in fig.

7.

PROBE RADIUS 408 in.

2

-c-(.$) (DYNAMIC HEAD

-FTOR)

\C-P+S)

(YAW FACTOR)

_(TOTALC-Vt (P-B)HEAD PACtoR

RAKE TRAVERSE

TUBE

SCREW SHAFT

SPACE FOR

PRESSURE LEADS

'COBRA' PROBE RAKE

4:47' 2 94" 4Oe'

498'

I $

"'

SIDE ELEVATION 1/4' FLOW DIRECTION -SWEEP OF SCREW N' T.91

TOTAL HEAD PROBES

FIG.7. ARRANGEMENT OF PROBE RAKE

FOR PROPELLER WAKE TRAVERSE

A 10 inch diameter model screw designated T91 was used for the

experiments, this screw already existed and. details of the screw were fairly

well documented in connection with other work. The pertinent section particulars of screw T91 are given in Table I.

END ELEVATION

OF

12

'rARrp I.

Particulars of Screw T91

Diameter 1.0.0 in. NuEiber of Blades 4

Before obtaining results With a probe rake behind the propeller,

care was takefl, to d.etermine an upper timnel water speed lit at which it was

felt safe to operate the rake without probe vibration affecting the results. At a speed of about 20 ft/s there was a faintly d.iscernable vibration and

therefore it was decided to run the experinnts below this speed..

Sub-seq.uently, ±o1ational speed of 20 rps was chosen and. all the tests were

run at this speed thus ensuring, that, in the T range of interest, the

water speed did not exceed 15 ft/s. The probe rakes were made and. mounted

in

the tunnel in such a maimer that they could be rotated slowly about the shaft axis by using cOntrols outside the tunnel. This facility was

incorporated in the desi of the equipment for use i.n later experiments Radius FractiQn x Chord-Diameter Ratio c/ Thickrss-Chord Ratio t/ Camber-Chord Rati0 ."c Face Pitch Angle . 0.2 0.2381 0.1694 0.0216 57°

36'

0.3 0.2690 0,1287 .0.0216 .1° 21' 0.4. 0.2929. 0.1004 0.0205

38°

12' 0.5 0.3079 0.0796 0.0206 32° 10' 0.6 _0.3130 0.0645 0.0221+ 27° 4-7' 0.7 0.3069 ö.0518 0.0218 23° 52' 0.8 0.2817 0.01+19 0.0210 20° 48' 0.9 0.2264 0,0300 0.0150 18° 40' 0.95 0.1764. 0.0238 0.0119 ' 17°

:13

when it was proposed to test screws in non-uniform inflows, in which case

the downstream total head. - as well as the upstream total head - would be

a function of circumferential posi1on in addition to radius. Before táidng a complete..set of ±'eadi.ngs this facility for rotating the rakes was, used, to check the constancy of the yaw and total head. readings with the rakes set at different angular positions in the .plan nQraJ. to the shaft

axis. Slight differences *ere found but on the whole these were no greater than the differences arising when repeating nominally the same conditions. All the readings taken after these initial measurements had been made were obtained with the probes set at the same angular position.

periment Results.

The thrust and torque grading results obtained' from the total head and. yawmeter readings are shown in figs. 8 and 9 respectively. Of the

06

05

04

03

2 O4 0.5 O6 O7 0.8 ' 0.9 1.0

X

CALCULATED EXPERIMENTAL

--1L4.

FIG.9. TORQUE GRADING CURVES - SCREW -T 91

twelve prObes aa-i3.able two wéré found to ,be inoperative early in the

experinents due to blockages *iic1 coul4 not be cleared., and. the yawrneter

at aiother radius w. also zuspcted of beijig faulty, so that only nine of

the twelve yawmeters méasued torque and ten total. head tubes rneasuie,d tirust.

Some scatter appeared in the. results and faired ctirves 'were put throui the

experimental spots.

The probes that were found. to be defective were

situated at the outer radii and therefore the definition of the curvôs in

15

The thrust and torque grading curves were integrated in the

radial &irectio±i to Obtain ovôrafl values of thrust and torque and. these

are compared in fig. 10 with the values obtained from measurements of the

F1G.1O. NPARISON

OF Ky, KQ AND

VALUES - SCREW 191

thrust and. torque using the normal propeller dynamometxy. The comparison

is not too, encouraging at the higher J values since the differences between the integrated, and. overall measurents are rather larger than

16

considered. satisfactory. This is somewhat surprising in itself since the theory underlying the method indicates that a better agreement should be achieved at the higher J values and lower loadings.

It should be noted that in reducing the measured tunnel results

to values of thrust and. torque, it has been necessary to make an allowance for the slipstream contraction. In the basic theory the contraction of

the slipstream is neglected, but in practice it does occur of course and therefore the radius fractions of the probes in the wake are relatively higher than would be determined from the probe radii and the screw radius. This allowance for slipstream contraction has been made by using x values based on the slipstream diameter rather than the propeller diameter when calculating values of thrust and torque from equations (2) and (6). To

obtain the slipstream diameter, visual observations of the position of the cavitating tip trailing-vortex core at the axial position of the probe

heads was made. These results are shown in fig.11.

l00

no.96

0.90 I i I I

0.5 06 07 0.8 O9

.3

FIG. 11. MEASURED SLIPSTREAM CONTRACTION - SCREW T 91.

The section lift coefficients derived from the thrust and. torque gradings arid equations (ii) and (16) are shown in fig. 12.

06 05 O4 03 02 0 0I .. I -1 - -1 --p EXPERIMENTAL p ---- CALCULATED

FIG. 12. SECTION LIFT COEFFICIENT - SCREW T 91

In the case of the thrust, torque and lift coefficient adings, the measured results are compared with values calci4ated. using Hill' s

propeller desii and analysis method of ref.

6.

These comparisons are

shown in figs. 8, 9 and 12 respectively. On the whole, the ra9il dis-tributions of thrust and torque cornpare reasonably well but the integrated

values how some significant differences over the J range. tested as shown

in fig0 10

Conclusions.

It is necessary, if improved agreesent is to be obtained, in

future, to account for the differences between the integrated thrust and

torque values and. the values measured directly using the water tunnel dynamometers. Generally speaking, the discrepancies in the results found with a marine propeller, and reported in this paper, are similar to those

reported pre-iously by othei' investigators worhng with ai.rscrèws, as rnar

be seen in ref-s. 3 and.

_7,

for -example.

0.2 O'3 04 OS 0.6 0.7 08 0.9

18

The above differences may be attributed to the te of

instru-ment that has been fused and -its behaviour in measuring periodic flows rather than any other factor. This conclusion is based on the results of some

work publi,shed in ref. 8 which became available after the tests described. in this paper were complete. The work in ref 8was perfOrme. using a

Conrad probe which is similar to the cobra piôbe, cept that the cent±'al total head tube is omitted. _{The Conrad probe is normally used. for flow}

direction measurement by rotating it in the flow until the pressures .n the two tubes are identical when the direction of the instrument corresponds

with the flow direction. _{It was shown in ref. 8, however, that when used.}

in a turbulent flow created

in

_{the mixing region of an underwater jet, this}

instrument behaved as an effective turbulence probe.

.Although it- i _{difficult to copare the operating conditions in}

the propeller wake with those in the jet used. by Jezdinir., it seems clear that they were sufficiently similar to say that the periddic flow in the propeller wake caused the cobra probes to measure values of total head and

W

sin 2 r Which Were _{qt exactly those required in equations (2) ath (6).}

This could also bethe reason why the cobra probes appear to have measired values of the above quantities which were greater than, rather than less

than, the true or effective values. The above reasoning does little more

than confirm the results given in ref.

_7,

when Y yawmeters were used for

measuring air screw torque, and. they were also shown to be subject to error; when used in periodic flows.

In conclusion, it is considered. that this method of

exper-imenta-tion is worthy of.further consideraexper-imenta-tion, particularly in the cases of

conventional screws with widely differing radial circulation di3tributQns and. also due-ted. propellers. Before embarking on such experiments though,

1.9

combined. iotalhead-yawmeter can be developed. which is. less sensitive to

errors arising in periodic flows0

Part U. Instrumentation for Fully Cavita'ting Propeller Research.

A programme of research on fully óavitating proellei is being

conducted at Ship Division with the. objects of improving our understanding

of the action of these screws and clear1r defining their range of applioa-biity, as well as improving and. extending desigi methods.

A paper read before the A.S.M.E. in 1965, ref.

_5,

described

the semi-empirical approach being pursued on the des4.i of fuJ1 caitating

propellers at I'PL, and Part II of the present paper describes the

experi-mental equipment being a embied. for tbre. continuation f 'this work. Th

addition to the work on fully cavitating propellers, it is anticipated. that this equipment wifl also be useful for work on fully cavitating pumps and. inducers, since the interest in these devIces is consideráblê and. expanding.

The Choice of Measuring System.

The observations and measurements of cavity size produced by a fully cavitatingpropelle± and reported

in

ref. 9, stiggested.that a more

detailed study of the flows associated with such screws should be made, and in particular an attempt should be made to measure as many of the following

quantities as prac-ticable.

i) 'Pressures within the cavities - it is possible that the

cavities are not completely vaporous, particularly at the

inner sections of propellers. If they contain bubbly

mixtures of vapour, gas and. water, the cavity pressures may not be constant.

20

iii)

Geometry of the cavities op.

e blades nd. in the wake.

iii)

Fluid velocities between the cavities and. in the wakes.

)

Pressure distributions and. hence the forces acting on the

blades.

-To. cari

out research on these items it was clear that add.itioal

instrumentation was required and. consideration was given to using two methods

for this0

The firt consisted of holding fixed meauring probes. in the

propelle± wake and recording the wake data at high speed as it passed, while

the second employed, probes rotating with the screws so that the wake data

appeared stationary relative to the probes.

The earlier experience obtained

ith prpbes held in fully cavitating propeller wakes and. supported. from the

tunnel wall suggested this method would. not be suitable when attempting to

measure instantaneous pressures.

This is because the instrumentation

would be subject to large periodic forces at blade frequency with the

passage of the vapour-water interfaces.

Such. conditions can easily cause

breakdown in waterproofing and mechanical failure after sufficieht cycles

have 'elapsed.

Iii the experiments on cavity shape measurement reported

eariler, it was only necessary to observe the time intervals at which large

resistanOe changes occurred, and accurate recording of the level of

eJ.ec-trical outputs was not needed.

The above factors, together with the

non-avelLabiity of suitable high frequency response miiiiaturised. pressure

transducers, mainly cortributed to the decision to adopt a rotating probe

system, although it was also realised- that it would be difficult to make a

sufficiefitly slender atid. rigid probe Support Systern with a nturJ. frequency

well '.n excess of' 180 cpm, the maximum blade paüage frequency likely to be

met.

a probe system rotating with the 'screw, the wake cavities

appear statiOnary relative to the probe, and the problems of the

21

mechanical impact do not arise0

Their place is taken however by the

problems of transferring pressure data from a rotating shaft to a

stationary system..

Nevertheless, it wasfeit.that t

swas the simpler

problem tQ overcome, and. also tl4s method wouJ4 pernit blade surface

pressure measurements to be made., which the other method would not.

In the general context of this type of experimentation, several

attempts at measuring the pressure distributions on rotating non-cavitating

model propeller blades have been made with varying degrees of success,

and. ref s. 10 and 11 are tb psrIies in which

1ightlr different methods

have been 'used.

In the former report, a single pressure transducer in

the boss was connected in turn .iia channels in the blades and a scanning

valve in the boss to static pressure taps in the blade surfaces, while in

the second report several pressure transducers in the boss were usôd.

Elec-trical slip rings were then used to transfer the information to recorders

outside the tunnels.

Both methods used water in the blade channels as the

medium for transmitting the pressure to the transducer(s).

Using water for

this purpose, meant that great care had. to be taken to prevent the centrifugal

pressure drop occurring in the radial channels causing the water in the

channels to. vaporise and make the centrifugal pressure oQrrectipns that iust

be applied uncertain.

Certainly at the low, ambient presures at which fully

ca.vitating propellers ire kested this problem would be insurmountable if an

attempt were made to use water a

the pressure conducting fluid.

To overcome

this difficulty it was decided at the outset to use the air-blowing pressure

measuring teôhnique in the measurement of the rotating probe pressures and

the wetted blade surface pressures, and the equipment described in the next

section wà.s designed. with that in mind, although its use with water as, tile

pressure conducting fluid was not excluded when no cavitation, was present

22

ôavity pressures on the blades and in the wake, It will not be necessary

to d.isharge air into the cavities and. a simple direct connection between the pressure ta.s and th 'essUre recordér is all tht. will be' required0

If, however, the cavities contain a bubbly mixture of vapour and water, as they may do at the inner p±op iler radii, it may then be necessary to inject some air in order to obtain a measurement of the mixture pressure.

The air-blowing techiique fQr water pressure measurement Was developed by Dr. Gadd of Ship Division and a detailed account of its use is contained in ref. 12, while a brief sumary i given n the appendix to

this paper0

Desii of the Rotating Pressure Measuring Device.

Fig. 13 is a schematic arrangement of the pressure rneasui'ing equipment th worcig seption and diffuser of No0 I water tunnel.

FIG 13 ARRANGEMENT OF ROTATING PRESSURE MEASURING

EQUIPMENT IN N° 1. WATER TUNNEL.

propeller drive. shaft (i) passes from the drive motor through . one to one

gear box and. into the tunnel to support and drive the screw. A second

shaft or tube (2) surrounding the drive shaft and. rotating with it gives support to the drive shaft. and. also acts as the probe traverse tube. _This

23

probe traverse tube -is surrounded. ty another tube

(3)

which carries

the channels conveying the air or water pressure from the rotating

equipment to the manometer. The upstream end. of this tube is driven by the probe traverse tube (2) and. carries the probe which rotates with the sOrew when wake measurements are to.'be made..

Vhen blade surface pressures are required, the probe end. is

detached from tube

₍₃₎

and. the pressure leads are connected directly to

the static pressure channels in the screw.

The function of the one to one gear box in the transmission is to enable the probe traverse tube (2) to be rotated slowly relative to

the drive shaft (1) while the system is rotating at high speed.. This

feature will enable circumferential traverses to be made with the screw

operating and thus speed up the proOess of data acqU3tion. It will

also be possible to move the tube (3), and hence the probe relative to the drive shaft and screw, in the axial direction. Finally, to obtaizr pressure

data at ffarent radii it will be necessary to use probes with lengths appropriate to the radii required.

A 10 channel hydro-pneumatic slipring assembly surrou.nds the tube (3) and is designed. so that the assembly can be moved axially with the tubes (2) and (3) while the channel signals are taken out via, the stationary

support tube and passed through a hollow strut to the manometer. The principles On which the sliprings operate are shown fig. lu-, where it

FROM PRESSURE MEASURING POSITION

SPACERS

FIG.14. PRINCIPLE OF HYDRO PNEUMATIC SLIPRING

use with air or water as the pressure conducting fluid. They have proved

satisfactory in the trials conducted so far.

Pressure Recording Equipment and Probes.

It is convenient to experiment with model fully cavitating propellers of about 10 in. diameter in No. I water tnêl at speeds of

rotation around L1-0 rps and at tunnel pressures at the shaft axis of ahout 250 psfa. Under these conditions large extremes of pressure will have to

be measured. For example, when blade surface pressures are being recorded on fully cavitating propellers, they can be as low as vapour pressure, on the other hand when, say, a five hole measuring probe is being used in t wake, dynamic pressures a high as 100 psia will be encountered on the

probe head0 With the air-blowing pressure measuring system, the very

large pressure reductions in the rotating radial channels do not occur due

to the lower density of air and. therefore pressures close to the above values have to be measured. For this purpose a five channel air-blowing

large .pressure range recorder has been designed and built, and the TO MANOMETER

WATER FOR

001-02 SOURCE FOR PRESSURE MEASUREMENT RELIEF VALVE PRESSURE AUSE

(Z)

25

components comprising a single channel are shown in Fig.15. To

'-Ow PRESSURE SIDE AERODYNAMIC RESISTANCE 'C' AERODYNAMIC RESISTANCE 'B' VA LV E

J-PRESSURE SUPPLY

FIO.15.ARRANGEMENT OF AIR- BLOWING MANOMETER (ONE OF FIVE CHANNELS)

accommodate the large pressure range that has to be measured, high quality dial recording pressure gauges have been adopted with change over valves

to split the ranges. Additionally, electrical pressure transducers have

been incbrporated. to cover the range 0 to 2 psia and 0 to 125 psia, and

these will also enable graphic recording of the pressure outputs. The

aerodynamic resistances in the system that are required to damp out pressure fluctuations have been made easily interchangeable and two air flow meters have been incorporated to split the air flow rate into convenient ranges.

0 HIGH PRESSURE SIDE FLOW VALVE VALVE ELECTRICAL PRESSURE TRANSDUC ER S FLOWMETE RS 01.- 2O ft! hr PRESSURE REGULATOR

ALTERNATIVE PROBE

LL

PROPELLER POSITION 26

Aspreseritly conceived, the wake velocity measuring

instru-ments will, take the form of a 5 hole spherically-ended probe similar to that shown in fig. 16, which also shows the connection of a probe to the

BALANCE WEI AS5CHBLY

FIG. 16. ARRANGEMENT OF PROPELLER WAKE VELOCITY

MEASURING PROBE.

rotating tube assembly. The top of the measuring probe will be cranked tboint -into the peripheral flow direction since this velocity is several

times the ad.vanOe velocity at the outer rad.ii. In use the p±'obe will be pitched into the oncoming flow in order to reduce the flow dii'ection angles to the central total head hole. The non-nulling technique, coonly usd.

with such probes, will be used.

in

recording and the prObes will be calibra-ted. at their correct settings on the rotating assembly using the rotatona1

speed and tunnel water speed as the means of covering suitable yaw and. pitch ranges. The calibrations will, of course, be conducted, at reduced tunnel pressures to simtlate the actual operating conditions as closely as, possible.

As mentioned. previously when it is required to measure blade static .pressures the probe end. assembly w4l. be removed from tube

₍₃₎

5 HOLE PROBE

PROSE AXIS

MEANS OP TURNING PROBE ABOUT AXIS

27

in

fig.13 and the channels in the screw will be coupled directly to those in tttbe (3).

In order to detexmine the wake cavity surface shapes, a probe

consisting of, a radial arm carrying nuiiiber of points or pins spaced in

the radial direction will be traversed circumferentialiy until, as observed with stroboscopic lighting, the pins first pierce the cavity surface. Then from a knowledge of the radius of the pins and. the relative circumferential positions of 'the pins and. the screw the geometry of the cavity surfaces

will be known.

Acknowledgements.

Mr. K! Poulton designed the equipment described

in

Pait I of the

paper and., together with Miss S. Wallace, 'performed the experiments.

At the outset of the work on the rotating pressure measuring rig Mr. A. Emerson of Newcastle University kindly supplied details of a rotating probe system he has used in the University water tunnel. After. considera-tion it was felt that for' the fully cavitating propeller applicaconsidera-tion rather di.fferent features were required and therefore the design described in this report was produced.

Throughout the design of the rotating rig, emphasis has been placed on keeping the size and weight of the rotating assembly to a minimum, particu-larly in the vicinity of the screw. Largely due to the efforts of Mr. H. B. BOyle and Mr. D. Webb both .f Ship Division, a

design

has been evolved which,

it is believed, it would be difficult to make smaller without compromising the

functional requirements. Credit is also gladly &.ven for the design concept of using a one to one gear box to obtain the facility of traversing the probe

during the screw rotation.

'The work described

in

this paper forms part of the research programme of the National Physical Laboratory.

References.

Stanton, T. E. and

Marshall, D.

a.

Kronauer, R. E.. and.

Grant, H. P.

Lock, C. N. He and.

Yeatrnan, D.M.

4- Reid, E0 C. Wake stud.ies of eight model propellers0

N.&CA, Tèchnica]. Note No. 104-0, 194-6.

50 Lock, C. N. H., Bateina.n, Measurement of thrust and torque grading H and Nixon, H. L0 on high-pitch model airscrews.

Aeronautical Research Couxipil, Reports and. Memoraida, No.

_2477,

1950.

The d.esiga of propellers.

Trans .The Soc of Naval Architects and. Marine Engineers, Vol.

63,

1955, pg.365.

Douglas, C. P. an4 The measurement of t.rque grading along-an

Coombes, L. P1.

6.

Hill, J. c-. Jezd.insky, V. English, 3. W. 10. Auslander, J. 28

On a method of estinating, from observations on the slipstream-of an airscrew, the

performance of the elements of. the blades, and. the total thrust of the sOrew.

Advisory Cornui.itee for Aeronautics, Reports and. Memoranda, No.

4-60, 1918.

Pressure probe. response in fluctuating flow..

- Proc.2nd.U.S.Nat.Congress Applied Mechanics,

1954-. .

Periodic flow behind an airscrew.

Aeronautical Research Committee, Reports and

Memoranda, No. 14-83, 1933.

airscrew blade0

Aeroiiautical Reearch Committee, Reports and. Memoranda, No. 992, 1925.

Measurement of turbulence by pressure probes. American Institute of Aeronautics and

Astro-nautics. Vol.4-, No.11,

pg.2072,

November 1966. An approach to the d.esii of fully ca.vitating

propeUers

Symposium on cavitation in fluid machinery

The .Amercan Society of Mechanical Engineers,

1965.

Meatrë ment of pressure distrthution on blades

of marine propeflers.

David TaylOr Model B8sin Report, Prepared fQr the Amerlc'1an Towing T Conference, Berkeley,

Mavlud.off, M. A.

G.ad.d, C-. Es

29

Measurement of pressure on the blade surface

of non-cavitating propeller model.

Written Contribution, 11th International Towing

Tank Ccnference, 1966.

An air blowing technique for measuring pressures

in water.

- Nomenclature

2

30

áxiáJ. inflow velocity factor

at

propeller disk = w a 2U

p n2

D4 Q

pi?D5

1-

i2

2 P "a

C chord length of propeller section

C centre hole readig on cobra probe

D propeller diameter

aximum camber of propeller section

total head of fluid far upstream.

total head of fluid iirediateiy dOwnstream of screw

TI

J propeller advance ratio -- =

nD

T KT propeller thrust coefficient

KQ propeller torque coefficient

propeller speed of rotation rps

port hole reading on cobra probe

dynamic head of fluid relative to probe head

Q propeller torque

d.inamic head. of fluid far upstream = p

r local radius

a starboard hole read.ing on cobra, probe

t

a4um

propeller sec.on thicimess

T propeller thrust

total 1 velocity through screw ax.alvelocity far upstream.

z

K

31

Wt

-- tangential induced. velocity at propeller

2

tota1 velocity relativeto probes immediately downstream of propeller

2r

propfler section radius fraction =

D

nuiber of propeller blades

hydropamic pitch angle

a pprol-mte hydrodynamic pitch angle (equation. ic) Goldstein factor

mass density

frequency of pressure fluctuations

natural frequency of flu& in manometers

speed of rotation of propeller rad./5 = 2 n

flow circulation at propeller sectio

_iwt

yaw angle tan

Appendix

The air-blowing techiLique for water pressure measurement

depends on the .simpe principle of slowly discharging air from a small hole in the surface of the body at which the water prssure is required

and. inasuring the air pressure.. It is necessary to incorporate a certain

amount of damping in the air line to prevent surges oourring in the air being discharged from the hole under nominally steady conditions. Due

to the surface tension effects on the bubbles detaching from, the air

outlet hole, and. the small pressure d.rop occurring in the air leads,

the actual air pressure recorded is slightly greater than the true pressure.

However, provided, the air flow rates used are not too high axid the air

outlet hole diameter is not too smaU, so that the surface tension effects

are small, these errors are not large and. can be tolerated. or approximately corrected for. It has been found. desirable from experience to use air

outlet holes of not less than about 0.03" diaraeter and. to keep the air

discharge rates low.