Experimental research on the cavitation performance of ducted propeller series

7 MEi 1980

ARCHIEF

EXPER1NENTAL RESEARCH ON TUE CAVITATION PERFORNANCE

OF DUC TED PROPELLER SERIES

By

Y. P.

Yeh d

Y. 4.

Cheng

China ShÏp Scientific Re9enrch Center

Wuih, Kiansu Provìncc

The People's Repub1c of China

1978

Lab.

y. Scheepsbouwkunde

Technische Hogeschool

EXPERIMENTAL RESEARCH ON THE CAViTATION _PERFORMANCE OF DUCTED PROPELLER SERIES

By YP

Yeh and Y. M. Cheng

I _INTRODUCTION

The najority

_{of stern vibratIon, propeller erosion}

endncise symptom

mro or less related with _{cevitational phenomena of}

propellers.

3g-nificant progress has been made recently in the

_{prediction of the type}

and extent of cvitat1on

_{on the blades of conventional}

propellers

throu,h-out the efforts of

_{many institutions such}

_{as SSPA, NSMB, NSRDC,}

eta.

With regard to ducted propellers, since RINA

_{Symposium on Ducted Propellers}

(1973), many valuable

reports have been published,,

_{such as NSMB's}

investi-gation of the interaction between

ducteJ propeller and the hull In depres-surlzed tank I1

¿anese contributions

_{on' the observation of}

cavitatlonal

phenomena of prototype ships

_{and on the prevention of}

duct-erosion by mecns of air injection system _{[,3, ¿,5}

We have learnt that SSPA has done systematic testa of ducted propellers in cavitation _{tunnel CJ}

,

al-though we have not yet seen their report. _{As a first step in the}

research

prograne of ducted propellers, test of 'I-seres" in uniform _velocity

field

_{vere conducted in our}

_{cavitational tunnel, k:}

_{brief sumsary of}

the

test results is given In this

paper.

Il Experiments In Cavitational Tunnel

Altogether twenty variations of ducted propellers,

_{two different}

nom-zles in combinatIon with ten D series propellers were tested in _{the SSIiRJ} cavitational tunnel in the year 1976. The test _{oction' of the tunnel was} 600 x 600 rn- _{The diameter of model propellers vea D}

240 mm. The range of Reynolds number was 0.63 z lO _Rn .

3.72

10 .

The air

con-tent as measured by means of Van Slijke's

_{method was found to b%5= 0.2}

---0.5, and was controlled by experience through variation' of deaeratIon period according to vIsual observation. _{Through observation of} cavlta-tional. phenomena cf varIous tests, it was 3hOWfl that the D-serles prope1-1ers wuich were marked vi th the adoption _{of linear pItch variation} matching the velocity disttlbutlon Inside aecelcrating ducta, effectIvely avoided face tip cavItation in the designed working _{range. SInce}

noniIn-ear distribution of maxImum blade thickness was adopted, following that of Kaeeries of NSMB, bubble cavitatIor _{near the middle portion}

_f

_chord

scarcely occurred. When the cavitation number decreased down to super-cavitating condition of operation, the flow blockage increased, causIng

separation of flow on the outer surface at the leading edge of nozlo. At lover cavitatIon number (e.g. 6= 1.7) and

_ifl

the range of low advance

ratio, the boundary layer separated anJ :atered bubble cavitation

deve-loped, whilst in the open water tests such kind of flow separation occu red oniy In the regIon of high advance ratio.

III Regression Analysis and Wall-Effect Correction

In order

to process the experimental data of ducted

propeller series

tested in cavitation tunrel, a methd for the rgre33ion analysis

'.a3

de-visod ani cmputing programme for

_{CJ-719 type}

computer was composed. Examples of computed results are shown in Figs 1 and 2.

With the open

water

test data as basis, wall effect was corrected in a method somewhat similar to the Kr -Identity method used by Van Lam-meren et. al. in handling the test results of B-series propellers (7).

The Cr -.±.dentity method was employed _{as a new attempt to tackle the}

pro-blem of water speed corrections. _{From th. identity of the propeller}

thruat loading coefficients ('TP

in both ewapheric and open

water tests,

the ratio K=f/ and hence

the

_reiationsp

_{!KJCCrp) nere obtained}

Then the thrust coefficient curvo

_{influenced by cavitation was corrected}

wIth this factor, the cavitation number was

_{corrected accordir.g to}

Bur-nil's suggestion E3 . The open-water thrust coefficient curve of

noz-zle and the torque coeffIcient curve of propeller vere used instead of those of the atrospheric condition in the cavitation tunnel.

For a given cavitation number _{and advace coefficIent J}

, let

Krp / KTDt , be the uncorrected

cavitatio7i-Influeflced

moa-cured values of propeller thrust coefficient, nozzle thrust coefficient, and propeller torons coefficient

_repectiveiy

At the point J=i _{, the}

corresponding values not influenced by cavitation _{are K,KÇt and} res-pectively. _{After correction, the cavitation number and advance}

coeffi-cient vili bG-0 and J

. _{Coefficients influenced by cavitation will}

be

,Aand

K

. (subscript t

_{indicates the}

_{uncorrected measured}

valuo from tunnel tests,

o

indicates the corrected vaue. Symbol

indicates value not influenced by

cavitatIon). _{The correction to the} cavItation Influenced propeller thrust

coeffIcient

_{rp- Is done by} uti-lizing the relatlonahip be'tween the water speed correction coefficieflt

K

and the propeller thrust coefficient

_F from to find

J0 and to calculate 6 From the relationshIp _{to obtain}

K6)The correction to the

_{cavitation influenced nozzle thrust ooeffI}

oient _{in done by assuming that the ratio of thrust ratio}

remain con3tant before and after

correct1on, i.e.

p

(' J_J_

t'(S)

The correction to the cavItatIon influenced _{torque coefficient k.} is done by assuming that the ratio uf efficiencIes remain constant be-fore and aster correction, i.e.

'

(J-)

(Jo)

By means of the nbov& mentioned method, calculations _{have been} made for four of the seriase _{in addition, two of the above four}

_series

have been calculated by p - identity method as well. lt was found that the

Crp -

Identity method seems to be more neceptable for _{us than the Krp}

-

identity method.

A comparison of the results as calculated by both

methods is shown in Table I. _{Comparing these two methods, it is concluded} that the inceptio _{of the tip vortex and the limit for onset of thrust} breaFdown are nearly identical (as shown In Fig. 3), but for propeller with large dIsc area ratio, there in a dIstinct difference In performance characteristic curves

IV. Inception of Tip Cavitation

From the analysis of the results of systematic tests, the following conclusions may be drawn:

1. For a given cavitation number defined by

the

speed of advence

( ) , there in an optimum pItch ratio for ducted propeller

cor-respodig to the greatest value of C1 the total

thrust coôfficiet

at the onset of tip vortex, as shown in Fig.

_3.

_{This characteristic} is somewhat different from that of conventional propeller. For a given

cavitation number, the ml1er the pitcI- ratio of a conventional propel-1er, the greater the marginal thrust CT

at the onset of tip

vortex. _{The inception curve of back cav'ation oÍ NSÎ'IB B-er±es} propel-1er at r/R 1.0 is reproduced in Fige

If the induced velocity of propal1e _{d duct be neg1ectd, and the} resultant velocity at 0.7 _{fl of propeller is used to define the}

cnvita-tion nixner G ther all of the P/B curves _{are in reilar order. For}

propellers hav.ing the saiie /JJ,, (5 io proporticnal _{o (-Tr} ,

3-cn-the value of slope _{incroses with the irl3roase}

i?ì i/.

_{i'O' iOuic2i}

,.

the

¿rator the P/B value, the greater 6 _{physically the earlier}

will be the onset of tip vertex. _{or identical , the greater the P/B} value, the smaller _{he marginal CT , as shown in Fig. 5.}

- is proporionai to (P/B - where

0, as shown in

Pig. 6.. With it is possible oy follo1ug the methods of C9) [lo]

to predict the shio speed at which the tip vortex of ducted propeller begins to shed,

V Thrust Breakdown

Since it seems there is no unIversally

_{agreed sharp definton for}

the inception point of the second stage _{cav1tat1o, it may be artificIally} prescribed. _{With regard to the ducted propellers owing to the fact that} the influences of cavitation _{upon thrust breakdom of propeller, of nozzle} and upon torque breakdowa are nearly but not exactly equal, the onset

points of thrust breakdown and torque breakdown will not, in general, occur

at the sane instant. It is tentatvoiy _{suggested thet when the total thrust}

of duct and propeller just commence to fall, the corresponding condition vili be defined es the onset of the second stage cavitation. _{The test} results of propellers with different P/D and i- were faired by reans of regression

analysis.

_{Then by plotting a series cf different poInts of} thrust breakdown corresponding to

different

_{cavitation numbers}

_(say,

_h 85, 90, 95, 98 and

_99.5%,

h - the ratio

of cavitation-thfluencad thrust

to non-Influenced thrust), the onset point of total thrust _breakdown cor-responding to h 100% may be obtained by extrapolation,

The following remarks may be drawn from calculation:

The onset of second stage cavitatIon _as

_determined

_{by the regression} analysis, when considered together with the visual observation through cavitation tests, corresponds to _{a cavitation extent of about 1/7 of the}

back area of the blade, whereas for open propellers, this extent would be

about 1/6 normally at

the onset of thrust breakdown

_{This fact}

Indicates

that the thrust breakdown will

_{occur earlier}

for ducted propeller than

for open propeller.

SImilar to the inception of tip

vortez of the

first stage, for a given

cavitation number defined by the speed Of _{advance, there Is an optimum} pitch ratio _{at which the marginal breakdown thrust coïfIcient is the} largest, as shown in Fig. 7 _{Generally speaking, the optimum pitch ratio}

for inception of the first and the

second stage cavitation

are not the

same

If the induced velocities ara neglected _and

_{the cavitatioa number}

_6c corresponding to the onset of

_{thrust breakdown is}

_{defined by the}

resul-tant velocity at o.7 R of propeller, then the P/B

curves

o the CTT

diagram are in regular order, similar to the

G-CTr

curves.

For

propel-1ers of the same P/B values, _{is nearly in proportIon with C}

, i.e.

çç< 3-C'1 The valueS of parameter _{increase as P/B Increases.} For the same _cTT

_,

the greater the P/B

_{value, the greater 6}

, the earlier

onset of thrust breakdoww.

_{For the sane 6.&, the greater}

the P/B vo]-ues,

the smaller the marginal Gr , as :awn in FIg. o.

In thQ pepioçl of development o

-avitation on the ducted propeller,.

the mutuai

_reiatonShiP

propeller torquo are quite noticeable in the following respects:'

With the development of tip vortex,, duct thruzt will break down

earlier than that of propeller normally. Except for small pItch ratio

ducted propeller with low cavitation number, in which case

_propeller

thrust will break down first.

The

_{dlfferenca between the speed of advance at which}

torque breakdown and that corresponding to thrust breakdown Is smaller than the differenee between the speed of advance at which nozzle thrust _{breakdown and that} corresponding to propeller thrust breakdown. _{In' other words, wIth the}

development in

_{cavitation, the breakdown of propeller torque and thrust}

take place nearly simultaneously.

_{But strictly speakIng,}

_{at high}

cavi-tation

number, with propeller of small pitch rctio, torque breakdown will take place after _{propeller thrust breakdom.}

When cavitation

num-ber decreases, the differences 15 the speed of advance _{between both} breakdowns become

smaller. At low cavitation

number, torque breakdown

vili take _{1ace.cavlier than propeller}

thrust breakdown'.

e. WIth further development

In cavltation, the nozzle thrust will drop

off more rapidly than

that of

propeller.

When cavitation number decreases, there is a tendency for the thrust ratio _to

_{increase rapidly,}

as shown in Figs. 9and 10.

VI Trade Off DesIgn Charts

In order to facilitate the design of ducted propeller

_{system, the}

curves of onset ei fIrst and second _{stage cavtntion are}

given on the or-dinary - or _---I)

_desI

_{chart. The left hand side of}

the

curves i _{the safety region and the right hand side ig the}

cavitation

o _{thrust breakdown i'eg'.on,}

see Fig. 11 _[11J

_.

VII

_References

I) M.W.O. Oostervcld,

_{W Van den Berg: "Research}

_ifl

a Deoressurized Tow-ing Tank on Ducted Propeller

- Hull InteractIon" ISP,Vol. 23,

No.263, 1976. 2] Okamoto, H., Okada, _{K., Salto, Y. and Masai,}

K.: Fuil Scale Cavita-tion ObservaCavita-tion on Tankers Fitted With Dacted Propellers" _{NSI4B Symposium}

on High Powered Propulsion of

_{Largo Ships, Dec. 1974.}

51 _{Okamoto, H., O}ada K., Salto,}

Y. and Takahel, T: _{"Cavitation Study} of' Ducted Propellers on Large _{Ships" SNAME, Nov.. 1975.,}

4) Narita, H. Kunitake, _{Y. and Yagi, H:'Correla'in}

aesults of Model and Full Scale Punted Propeller

Cavitation Observations" NSNB Symposium on High Powered Propulsion

_{of Large Ships,}

Deco 1974.

[Si Narita, H. Kunitake, Y, mgi, H :"ADplication' and _{Development of'} Large Dueted Propeller for the _{280,000 dt Tanker M.S.}

Thorsaga". SNAKE, Nov. 1974.

1 _{Hans Edatrand: "The Cavitation Laboratory of the Swedish State}

Ship-building Experimental Tank" Circu1ir f rn Statena _{Skoppsprovningsanatalt} Gbrterborg juni,, 1974, Nr. 32.

[7] W.P.A. Van Laminaran, J.D. _{Van Manen and M.W.C. Oosterved:}

"The Wageningen B-Screw Series" SNAHE, 1969.

181L.C.

_{Burrill: "Seventh International Conference on}

Ship Hydrodynamics"

Stockholm, 1954,

J N. Chandrashekkara: "Analysis

_{of Tip Vortex}

Cavitation Inception at Hydrofoils and Propellers"

_{Schiffstechnik, Band 23,}

(10)

i.Noordzij: "

Noto on th So1in

of

Tir Vortx Cavitation

Totion" IS. V,,l.24, 1\ro.227, S,t,. 197.

(1 î). L.. iiurril,

-... ELrson:"Prcplier Catiori. Furt

r Tsts

on lo

nch..s o"

lIer

ocels in

- '

Jole

Oit'..4-on

Tunn1".

Vol.79,

7art

6, 1963.

*TììjS

article is an airidd vrsicr c'

a roport 'ith th-

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titis.

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