70

(昭和53年11 月 日本造船学会秋季 講演会において講演)

On Some Tank Test Results with a Large

Model Propeller•\ 0.95 m in Diameter

Part I

by Tadaaki Yamasaki, Member*

Summary

The propeller characteristics, surface pressure distributions and oil film streaks

on blades were obtained in the towing tank with a large sized model of which diameter

is 0.95 m. Some new facts on the scale effect of propeller characteristics are revealed

in the test results. Measured values of the propeller characteristics and surface pres-

sure distributions agree well with those which are calculated by means of the lifting

surface theory with the correction for viscous effect. The idea of the so-called equi-

valent profile method is examined by integrating measured sectional load distribu-

tions on blades.

1.•@ Introduction

The propeller open water test is one of the most

basic experiments for accurate estimation of the

shaft horse power of ships and evaluation of the

propulsive performance. Usually it is put into

operation at relatively low Reynolds number•\

in the Kempf's definition•\of the order 105 with

propellers less than 0.3 m in diameter. It seems

that the Reynolds number of this order is very

different and low, in general, in comparison with

that of model hulls in towing tanks or aerofoils

in wind tunnels. Considerable scale effect of

propeller characteristics had been pointed out by

Kempf1) half a century ago.

Recently, the tank test techniques are advanc-

ed in variety of ways. Flow visualization and

pressure measurement on blades are sometimes

added to the test programmes together with the

open water test. Tamura and Sasajima2) demon-

strated by means of the flow visualization that the

flow in the boundary layer on blade surfaces of

ordinary models is almost laminar at the Reynolds

number of the order 105 and the typical turbulent

flow can not be expected until the Reynolds

number becomes more than 2 •~ 106.

On the other hand, it is difficult to realize

high Reynolds numbers of the order 106 in open

water tests with models smaller than 0.3 m in

diameter. Larger models become necessary to

investigate the scale effect of propeller charac-

teristics. Only a few tests have been actualized at such high Reynolds number as above even with larger models. Gawn's series test,3) in which he used models of 0.5 m in diameter and a 24 kW conventional dynamometer, is one of the limited examples. Lap and van Manen4) used a 0.883 m model propeller and a 30 kW special dynamom-eter in their test, but the number of rotation did not come to the prearranged value of 3.5 rps. Their report suggests some difficulties on the practical tank test techniques with such large model propellers.

The author sought to develop a testing system with which larger models may be tested easily and made a new dynamometer driven by a control- lable pitch water turbine in place of an electric motor. The water turbine is lighter than the elec-tric geared motor of the same output power and any additional power supplies are not necessary.

This paper presents the results of such tests as open water test, measurement of surface pres-sure of blade and flow visualization of limiting stream lines with a four bladed propeller of which diameter is 0.95 m. The measured values of the characteristics and surface pressure distributions are compared with calculated ones by means of the lifting surface theory with the correction for viscous effect which was developed by Okamura.5) Finally, some discussions on the scale effect of the characteristics are also included.

2. Model Propeller

As a model propeller, MAU type was selected because it is the most common in Japan. Fig. 1

* Research Institute, Ishikawajima-Harima Heavy Industries Co., Ltd.

On Some Tank Test Results with a Large Model Propeller- 0.95 m in Diameter 71

gives the configuration of the model and the particulars. The diameter of the model was decided to be as large as possible. In practice, 0.95 m is the largest possible one in the IHI Ship Model Basin. Pitch ratio of the model is desirable to be larger when the basic feature of the dynamometer is taken into account. With some preliminary studies it was decided upon. 1.2. Shape of its boss was modified into a cylin-drical form to provide necessary space in the cap where instruments for the pressure measurement were installed.

The model was manufactured within tolerances of ISO—S class. The local pitch on the face and the blade thickness were measured at more than three points along helical chord lines of each 10% radius of every blade. As the outcome, the

pitch is set within the tolerance, but thickness is 3.5% in excess of the designed value on the aver-age. The curvatures of leading and trailing edges were also examined by making replicas with small plastic pieces. The values are shown in Table 1. The surface of blades and boss was

polished like a mirror. Besides the large model, a GEOSIM propeller

of which diameter is 0.25 m was used to get the supplemental data. For small models, in general, it is very difficult to make the curvatures of leading and trailing edges within a certain toler-ance of the designed value, but the GEOSIM was

finished with the greatest possible care.

3 . Instrumentation and Test Techniques

3. 1 Instrumentation Fig. 2 shows a profile of the measuring system

used in the present tests. The large model, as mentioned before, was driven by the controllable

pitch water turbine with two blades. It was installed about 5 m behind the model and the generated power was transmitted through the multiplying gear box to the propeller shaft.

The diameter of the turbine was desirable to be about twice of the model in this case. However, the lifting range of crane and the space between two instrument rails of the towing carriage restricted severely its diameter and the number of blades within 1.3 m and two respectively. These arose shortage of the output power of the turbine. Thus the tests have been carried out in the very light region of propeller loading, i.e. less than 12% as the slip ratio.

Fig. 3 shows the block diagram of instruments and Table 2 the overall accuracy of them. The devices for measuring thrust and torque of the model do not differ essentially from those of usual dynamometers used in daily tests. A new idea was applied to only their signal telemetering system. Two precision MODEM of double FM type were installed for both of the torque and

pressure transducers. There are two different methods to detect the

surface pressure of blades. The one is direct fitting6),7) of transducers on blade surfaces and the other is the method using drain tubes,8) in which the transducers are equipped in the boss or cap. The former has such a distinguished advantage as the transducers do not receive the effect of centrifugal force. The latter has another advantage such as high precision transducers may be selected because of sufficient fitting space. Although the latter system is influenced by the

Fig . 1 Model propellers

Table 1 Curvature of leading and trailing

edges

72 日本造船学会論文集 第 144 号

Fig . 2 Profile of measuring system

Fig . 3 Block diagram of instruments

Table 2 Accuracy of instruments including signal conditioners and telemeters

On Some Tank Test Results with a Large Model Propeller- 0.95 m in Diameter 73

centrifugal force, the author adopted it. Because

the accuracy of the transducers palys an import-

ant part in the reliability of measured data.

3. 2 Measuring Thrust and Torque

The dynamometer with the model propeller

was mounted, as in usual tests, on the instrument

rails of the towing carriage. When the carriage

started, the turbine of the dynamometer began

to rotate. The rotation came up to steady state

as soon as the acceleration of the carriage was

over.

This dynamometer was apt to work originally

under the condition of constant advance co-

efficient of the model propeller in the tests when

the pitch of the turbine blades was fixed. Then

the rate of rotation changed following after the relative speed as indicated in Fig. 4. This is one of the demerits of the dynamometer in the open water test, but, on the other hand, an extreme merit in the pressure measurement in which several test runs are necessary to obtain a set of data for one advance coefficient. If the pro-

peller was proposed to be operated at another advance coefficient, the pitch of the turbine blades must be changed.

Fig. 5 gives the histograms of deviations of the relative speed and the obtained advance co-efficient from each mean value, which were measured in the pressure measurements. In the tests the current meter was set 0.2 m deep as in usual tests and the intervals of each test run were taken more than 15 minutes for example, 30 minutes when the carriage speed exceeded 3 m/ s.

In the initial test plan, it was expected to

obtain the Reynolds number of 3.5 •~ 106 in early

autumn. However, in actual test runs the

carriage speed was limited within 3.5 m/s owing

to unforeseen vibration of the instrument rails

which might be generated by the turbine blades

passing across the wake of the dynamometer

frameworks, so that only 75% of the expected

Reynolds number was obtained.

The idling thrust and torque were measured

under running conditions for both of the large

and small models.

3. 3 Measuring Pressure Distribution

48 holes were distributed half and half on the

back and the face, mainly at the tip side. The

opening diameter of drain tubes was only 0.8 mm

on the surface and the radius at the position of

each hole was measured from shaft center within

the tolerance of 0.5 mm.

In the tests, the pressure scanner was driven

step by step and the hole was selected in order

of the face side after the back. Before each test

run static pressures were measured for all holes

which were scheduled to be selected in the next

run. These static pressures were quite stable

during the tests. This means that the instruments

worked without any troubles.

The records included the effect of centrifugal

force caused by drain tubes, which should be

rejected. The pressure due to the centrifugal

force was examined in the laboratory and it was

made clear that it is given in the form ƒÏ•çƒÁƒÖ2dƒÁ.

Then the true pressure acting on blade surface

is obtained by the following formula :

where Pm is the recorded value, r the radius at

Fig . 4 Time historical records of measured

data

Fig . 5 Deviations of relative speed and ad-

vance coefficient from their mean

value

74 日本造船学会論文 集 第144 号

the position of holes, co the angular velocity and

Po the effect of eccentricity of the transducer,

which was small and decided by experiments.

4 . Open Water Characteristics of the Large

Model

4. 1 Thrust and Torque Coefficients, and Effici-

ency

The open water characteristic curves were

obtained at three different Reynolds numbers;

i.e. Rn(K)= 1.6, 2.0, 2.4 •~ 106, where Rn(K) means

the Reynolds number (Co .7R/ v) •ã Va2 + (0. 7 ƒÎnD) 2.

In Fig. 6, two of them are presented. The thrust

and torque of the large model are quite high in

comparison with those of the small model which

was tested at Reynolds number of the order 10.

Moreover, the thrust changes even at high Rey-

nolds number above 1.6 •~ 106. The maximum

efficiency of the large model is more than 80%.

In addition to the measured data, the theoreti-

cal values calculated by Okamura's method')

are also shown in Fig. 6. In this case, the so-

called METHOD-I in his paper is adopted, which

is an application of Preston's idea9 to propellers.

The calculated characteristics agree remarkably

with the measured values in case of Rn(K)=

2.4 •~ 106. However, for the lower Reynolds

numbers discrepancy appears on both the co-

efficients and the calculated values are higher

than the measured ones.

4. 2 Pressure Distribution on Blade Surfaces

It took hours to measure pressure distribution

on the blade surfaces, so that the conditions

tested were limited. Full measurements for the

prearranged 48 holes on one blade were carried

out under only two conditions of Rn(K) =2.0 •~

106, J= 1.054 and 1.163, where J means the

advance coefficient V / nD . In addition to the

above, the measurements at 70% radius were

done at Reynolds numbers of 1.0, 1.3, 1.6, 2.0 •~ 106

and J= 1.108. Figs. 7 and 8 show a part of the

results, where the theoretical distribution calculat-

ed by Okamura's method (METHOD-I) is also

presented. In the case of Rn(K) =2.0 •~ 106,

J=1.054, the pressure distribution at 70% radius

was measured twice each in different day, and the

repeatability of the measurements was confirmed.

Double circles of the marks in Fig. 7 mean that

there are two measured points of the same value.

In general, the measured distributions agree

well with the theoretical ones except in the neigh-

bourhood of the leading edge. At the thicker

sections, agreement is not so good on both the

surfaces. These suggest that further improve-

Fig . 6 Open water characteristics

Fig . 7 Pressure distributions on blade sur-face (1/2)

On Some Tank Test Results with a Large Model Propeller- 0.95 m in Diameter 75

merits may be expected in theoretical works.

Although the experimental and theoretical

pressure distributions differ partially a little

from each other, the sectional lift coefficients

which are obtained by integrating both of the

measured and theoretical load distributions at

70% radius take almost the same value; i.e. at

R.(K)== 2.0 •~ 106 and J = 1.054, 0.134 as measured

value and 0.136 as theoretical one, and at J=

1.108, 0.112 and 0.114 respectively. The measured

characteristics, as mentioned before, also agree

well with the theoretical ones. Thus there are no

contradiction between the results of the open

water tests and those of pressure measurements.

4 . 3 Limiting Stream Lines on Blade Surfaces

The flow on blade surfaces of both of the large

and small models were visualized by means of

the oil film method. Fig. 9 shows a part of the

results. Meynen) concluded in his paper that

the radial streaks are originated by the centri-

fugal force when the boundary layer on blade

surfaces is laminar. Following after lVfeyne, the

angle made by tangents of a streak and a circle

is limited within 40•K even in case of the rotating

disc of which boundary layer is laminar. However,

the angle in the author's results is too large as

the laminar declination except at Rn(K) =2.5 •~

106. Then, this may suggest the presence of separ-

ation bulobles on both of the back and face. These

separation bubbles on small models are also

surmised from the works of Tamura-Sasajima2)

and Okamura." Even in case of the large model,

the oil streaks also indicate more clearly the

separation bubbles on the loack at Rn(K)=- 1.1 •~

106. ,However, when Reynolds number comes up

to 2.5 •~ 106, the flow on the bach and face looks

two d imensi onal .

4 . 4 Analysis of Open Water Characteristics by

means of the Equivalent Profile Method

In on alysing propeller characteristics, the equi-

valent profile method12),13),14) has been used. Here

the lift and drag coefficients for each propeller

characteristics .were calculated by Lerbs' method,

as- shown in Fig. 10, where the representative

section of blade, was set at 70% radius. Lift

curves relating to the theoretical characteristics

and ,the measured sectional lift coefficiLnts are

also put in the same figure in which the flag of

the marks indicates the sectional, lift coefficients

at corresponding Reynolds number.

From Fig. 10, I ollowing facts are revealed:

i.e. The lift coefficient grows larger even at

Reynolds n umbers above 2.0•~106; That of the

large and small models differs from each ,other

surprisingly enough; The zero lift angle aug-

ments , with ReYnolds number, while the lift

curve slope increases in case of the theoretical

culculation; The minimum drag coefficient is

extremely low, to the frirtional drag coefficient

of turbulent flow, for example, Prandtl-Schlicht-

ing's friction line, at even such a high Reynolds

Fig. 8 Pressure distributions on blade sur-face (2/2)

Fig . 9 Limiting stream lines on blade sur-

face

76 日本造船学会論文集 第144 号

number as 2.4 •~ 106.

As also shown in Fig. 10, the sectional lift

coefficients obtained from the pressure measure-

ments are very close to the lift curve by means

of the equivalent profile method. This is an ex-

perimental verification for availability of this

method which was justified theoretically by

Yamazaki15) on the lifting line theory.

4 .•@5 Scale Effect of Propeller Characteristics

As mentioned in the previous section, the

circulation around the blades changes with

Reynolds number, which is one of the most

fruitful results obtained in the present experi-

ments. This is firmly concluded from the facts

that both of the thrust and torque curves move

in the same direction and the lift grows consider-

ably with increase of Reynolds number. Further-

more, the sectional lift coefficients of the large

model also grow with Reynolds number. Kempf's

prediction comes in actuality.

Many studies have been continued on the scale

effect of aerofoil characteristics and the theoretical

works were achieved by Preston,9) Spence,16)

Beasley17) and others. According to their studies,

the thickness of the boundarylayer causes origi-

nally the scale effect. The displacement thickness

on the blade surface attains twice or more at the

downstream of the separation bubble.18) This

may be one of the fundamental reasons why the

discrepancies appear in the lift coefficients of

both of the small and large models. This test results deny the concept of the so-

called critical Reynolds number. The oil streaks on blades clearly recorded the laminar flow on wide area in the neighbourhood of the leading edge even at so high Reynolds number as 2.5 x 106. Moreover, the laminar separation bubbles at lower Reynolds number and the transition of the flow in the boundary layer just behind the leading edge owing to the so-called leading edge separation bubble make the behaviour of the

propeller characteristics complicated, so that the meaning of the critical Reynolds number becomes more vague.

5 . Concluding Remarks

The open water characteristics, pressure dis-tributions and oil streaks on the blade surfaces were obtained at high Reynolds number of the order 106 with the large size model, although only one was tested under the limited conditions.

The measured characteristics and pressure distributions are very close to the calculated results by means of the lifting surface theory. The circulation around blades plays an important role on the scale effect of the characteristics. The equivalent profile method gives the approxi- mate values of the sectional lift coefficients.

This test technique opens a route to fine im-

provements of propeller studies. The larger models may be used in the large towing tank with more powerful towing carriage. On the other hand the daily open water tests are usually carried out with smaller models. Many studies, in addition to the laborious experiments, should be mapped out with small models, on which the

present test results show vexatious phenomenon, that is, separation bubble. The improvement of the flow on the blade surfaces is supposed to be one of burning questions.

Acknowledgements

On achieving this study, Prof. Ryusuke Yama-zaki of Kyushu University has given the author many valuable suggestions. The author expresses his heartfelt thanks to Prof. Yamazaki. Dr. Tatsuo Jinnaka, Dr. Ryo Tasaki and Mr. Masa-ald Namimatsu have guided and supported the author's works as managers of the IHI Ship Model Basin. Mr. Naoaki Okamura gave the author great helps on numerical calculation of the theoretical characteristics and pressure distributions for the tested models using his

programmes. Mr. Kei Tanabe, Mr. Yasushi Shirose and many other experimenters have operated laborious experiments with their best efforts in the towing tank of the IHI Ship Model Basin. The author appreciates all of them.

Fig . 10 Lift and drag coefficients by means

of equivalent profile method

On Some Tank Test Results with a Large Model Propeller•\0. 95 m in Diameter 77

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