On a horizontal circulating channel and some examples of experiments with it

(1)

ARCHE

llRoDASKI IIISTITUT

SIIIPBUILDIHG RESEARCH IMSTITUTE

Lab. y.

Scheepsbou'

Technische Hogeschoo

_?APE

_No

₁₅

DeIft

On

a Horizontal Circulatng Channel

and some Examples ot Experiments with

it

by

M. Kinoshia, Dr. Eng. (Kogakuhakushi)

S. Okada, Ba. Eng. (Kogakushi)

The Technical Research Laboratory Hitachi Shipbuilding & Engineering Co., Ltd.

?aper o bepresented at the Symposium on the

Towing Tanic Facilities, Instrumentation and

Measuring Technique

Zagreb

22-25

September

1959.

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*

ON A HORIZONTAL CIRCULATNG CHANNEL

*

e,

_*

AND SOME EXAMPLES 0F

Rfl(2FS WITH IT

e.

,

e

*

-.

*

By M. KinoShita, Dr Eng. (Kogkuhakuahi)'

S. 'Okada, Ba. Eng. (Kcgakushi.)

r

The Technical Research Laboratory

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AND SOME EXAMPLES OF EXPERIMENTS WITH IT

By M. Kinoshita, Dr. Eng. (Kogakuhakush.t) S. Okada, Ba. Eng. (Kogakushi) The Techinica]. Research Laboratory Hitachi Shipbuilding & Engineering Co., Ltd.

1. INTRODUCTION

This report gives a general description of the water circulating channel and the methods and results of a few tests conducted with it, and is not pai'ticularly related to any ordinary towing tank. In a water circu-lating channel, contrariwise to the condition in a towing tank, the model stays in an almost stationary position, and water moves on in circulation. In consequence, although a water circulating channel is found more conveniént than an ordinary towing tank in many points, it is by no means free from disadvantages. By way of examples of convenient points, it may be cited that a water circulating channel allows measuring for an unlimited time. In other words, since water circulates in a circulating channel, there will be no limitation for the measuring time, when a stationary state baa once been established. On the other hand, there is necessarily a limit to the measuring time because of the limitation for the length of a towing tank. The second point of convenience is that the measuring apparatus will not be limited in their size, namely, the model is not needed to move, but

can stay in its position in the measuring section of the circulating channel. Hence,. the measuring apparatus to be normally installed inside the model may be mounted on the circulating channel it8elf or on a measuring table located at any other convenient place. As a result, there will be no limit in their size. The third point of advantage is its high level of the effective Reynolds' number. In a water circulating channel, water is actuated by the impeller to keep circulation, where water holds irtherént turbulence.

Accordingly, it seldom happens that a lmiyir boundary layer ie'geerated

on the surface of the model, even when any artificial device of turbulence stimulation is not applied. So even in case the apparent Reynolds' number is low, the effective Reynolds' number is high, which fact makes it possi-ble to carry out tests with a relatively small model.

Furthermore, as the fourth pQint of advantage, the status of water flow

around the surface of the model can directlybe observed from the ober-.

vation windows. For example, in an instance where such observation is required in connection with the determination of the position of the bilge-keel to be fitted along the direction of stream line, this arrangement will be found convenient.

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ON A HORIZONTAL CIRCULATING CHANNEL

AND SOME EXAMPLES OF FXPERIMENTS WITH IT

As above mentioned, there are many pointa of merite in a water circu-lating rth*me1, but it is by no means free from a number of disadvantages. As water circulates, it is rather d.tfficult to perfectly set the circulat-ing velocity constant and uniform on the basis of time and position, and in this case, the degree of precision for the measurement of the velocity

of flow is some what lower as compared with that based on the running speed by a towing carriage in a towing tank. Besides, it cannot be helped that the

tank water gets in waves to a degree on the free surface, while in consequence of a narrower width of the channel compared 'with a towing tank, it is consi-dered that the side wall effects would be so much the greater.

When the above mentioned merits and. demerits are taken into conside-ration, it will naturally be determined what classes of tests are suitable

to be conducted in a circulating channel. For instance, it will, be unsui-table for the resistance test of model

ships,

but the measurement of propeller race, open test of a propeller and rudder testa, etc., can be expected to be made in a much shorter period of time than by an ordi nry towing tank.

The following is to generalize the water circulating channel, and also to give descriptions about the teat methods a part of the results of a few experiments in connection with propellers and rudders which were

thought

suitable to be caried out in

this

circulating tank.

2. A GIST 0F T CIRCUlATING CHANNEL

This circulating channel was constructed in the compound of the

Techeical Research Laoratoiy of the Hitachi Shipbuilding & Rng(nering Co.,

Ltd., and completed in the 1956 spring for the purpose of experimenting on ships, propellers and other hydrodynaznic subject matters, being 11.500m in length, 3.400m in width and 1.200m in depth. Ita general arrangement is briefly as illustrated in ig.l.

The body of the tank is of steel plate, and conaructed in 13 blocks,

which are joined into an integral one by bolts, and is placed on an adequa-tely reinforced concrete base. Its weight is about 4 tons, and

to

contain water for a quantity presumed to

be

about 20 tons.

The water in the chnnnpl is actuated by a 4-bladed impeller

with

a diameter of 1m driven by 15 HP 3-phase A.C. comutator motor. The water is thus made to circulate in the horizontal direction at a rate of 0.6-1.8

_iVe.

There is free surface at the measuring section alone All other parts form

a closed tunnel. The measuring section is provided with windows for observa-tion of water on the side in the frontal part and at the bottom.

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AND SOME EXAMPLES OF EXPERIMENTS WiTH IT

The windows are glazed with Acrylite (organic alsa). Fig.2 is a photograph illustrating the conditions of the circulating channel around the measuring section.

To measure the velocity of flow, a pitot-tube is used, which is fixea in a position protruding about 300 mm from the central bottom surface at a point upstréam-from the measuring section. The distribution of the velocity of flow was measured by the pitot traverse method. at the measuring section. Fig. 3 gives one of the examples. In this chart, the velocity of the flows

at respective points in the cross-section at the measuring section is showa

in

the ratios to the velocity of flows at the point of the fixed pitot tube at the time when the mean velocity of flow is 0.92 rn/s at the point. With an

exception of the proximity of the side walls and the bottom, the degree of turbulence is about 3%. The rippling on the free surface is insignificant particularly at the time of the low velocity, and is considered to be eligible

for use in hydrodynaniic experiments of these ld.nds. With a view to minimizing the rippling on the free surface at the time of the high velocity, it is further contemplated to fit up the regulating plates, and to prevent the air bubble drawing at the point where the free surface shifts over to the closed

tunnel, a certain device is planned to be attached. In case these devices are found satisfactory, the circulating thAnnel will work with a greater

degree of perfection.

3.

EXAI(PLES OF PESTS CONDUCTED INTEE CIRCULATING CRANNEL

As discussed in the "Introduction", many d.nds of experiments are considered possible to be carried out in this circulating channel to the best advantage. As one of such experiments, a series of experiments in relation

to ship's rudders, particularly about the rudder performance in the propeller race, were conducted, which comprised the measurement of propeller race.

The test methoda and resulta can

be

summarized as follows:

Regarding a series of experimente carried out in connection with ship' s rudders, investigation was made with the performance of an open rudder, par-ticu].arly about the effect of the angular velocity of steering, and then the rudder performance when it was placed behind the open propeller, principally about the effect of propeller race. Lastly, the experiment was conducted with

the rudder when it was located behind the model ship. For use through ail these experiments, a rudder dynanionieter as shown

in

Fig. 4 was specially

designed and manufactured for the purpose of measuring the forces applied to

the rudder.

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w

ON A HORIZONTAL CIRCULATING CHANNEL

AND SOME EXAMPLES OF EXPERIMENTS WITH IT

This dynamometer was planned to measure the three factors, that is, the moment about the rudder-stâck, the force acting perpendicular to the chord of rudder, that is, normal force, and the force acting in the same direction as the chord, that is, tangential force, in the continuation not only in a stationary condi-tion but even in a non-stacondi-tionary condicondi-tion during the process of steering.

In measuring the abave mentioned three factors, hollow steel tube ith a thin wall thickness, in which an electric wire strain-gauge was bonded, was inserted into a part of the rudder-stock, so as to transduce the mechanical quantity into the electric quantity. The rudder-stock was held in

ver-tical position by a pair of bearings at the point higher than the hollow steel tube, and connected with the rudder at the lower end.

Accordingly, it Ça so arranged that the hydrodynamic forces applied to the rudder will be transmitted to the hollow steel tube bonded to the electric wire strain-gauge as they are.

While the upper end was connected with the worm and the rudder angle indica-tor, and the varioha for recording the rudder angle. A 1/32 liP commutator motor was utilized as the driving motor for steering, and two-step reduction

system including the worm and worm-gear was so desigend that the rudder-stock would be turned round at a proper velocity. Fig,5 is a photograph to illust-rate the condition in which the aforementioned dynainometer was installed in

the measuring section. The reading of the rudder angle was available by the indicator moving around the dial with the angle graduations, and also by an electric bridge formed by the variohm directly coupled with the rudder-stock, which converted the changes in the rudder angle into the changes of electric resistance, so that continuous measuring could be made by use of an oscillo-graph.

The unbalanced valtage induced by the electric wire strain gauge were also amplified and rectified respectively and led to the oscillograph. Fig.6 shows the amplifier, oscillator, for the electric wire strain gauge, bridge-box, electro-magnetic oscillograph, as installed by the side of the circulating channel. Besides, before and after experiments, known amounts of torce and moment were applied to the rudder-stock by means of the weight hung through a pulley, and by changing the amount of weight, calibration was made so as to confirm the accuracy of measuring apparatus for conducting

experiments.

One of the examples of the oscillograms thus obtained was given in

Fig. T. This is to show an exämple of test results obtained by measuring the

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AND SOME EXAMPLES OF EXPERIMENL'S WiTH IT

As the result of the analysis is unrelated to the object of this paper, it uil]. not be given here. As to its details, it is requested to refer to the literature under-mentioned. (1) (2)

Henceforth, description will be given in this paper about the resulta of

experiments conducted to find the relations between the scale effect and the Reynolds' number.

In respect to the Reynolds' number of the rudder model experiments,

Dr. Schoenherr (3) publicly announced in 1939 that Reynolds' number was required to be 1.5 x

io6

at the minimum in order to obtain a satisfactory agreement in every case. Later in 1948, Dr. van Lasmeren (4) stated that for a rudder of ordinary shape, a reliable results can be obtained even at a Reynolds' number of around 0.20 x

io6,

basing upon his tank experiments on open rudders for Reynolds' number ranging from 1.66 x iO6 - 0.05 io6. In the present

experi-ments, prior to various experiments on models, tests were conducted with

geometrically similar models in two sizes so as to ascertain the scale effects which might arise in the circulating channel. The shape and size of the model rudders used were as shown in Fig. 8 (Model No.1), and another one which was similar in its shape to Ì4odel No.1 but twice as large in its size (termed Model No.2). Fig, 9 is the photograph showing these two models. As to the

rudder section, N.A.C.A. aerofoil No.0018 was selected in the sense that it is

equal in the thiclesa-chord ratio to those commonly used among ordinary ships, having a symmetrical section, and its characteristics are alrady oun.

The experiments were conducted with Model Nos.l and 2, in which the

rudders were held in a condition that the upper end was submerged under water for oneha].f its height. Besides, with a view to preventing the rudder-stock from disturbing the water surface by

m*king

waves, or causing air-draw, a cutwater with a stream-lined section was prepared, and made the rudder-stock

pass through the inside of the cutwater, so that the force acting onthe outer surface of cutwater would not be transmitted to the rudder-stock.

With the Model No.1, experiments were conducted by changing the velocity of flow in three ways, and with Model No.2, in two ways. The experimental resulte were as shown in Fig. 10 and 11, where the rudder angle o is plotted

along the abscissas and CN, CD, and

4/c

values were laid down.

N

j

CNkrvA

,

Where N: Norea]. force

D: Drag

: density of the water V: speed of the rudder

A: area of the rudder

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ON A HORIZONTAL CIRCULATING CHANNEL

AND SOME EXAMPLES OF EXPERIMENTS wrr

IT

C: chord length of the rudder

.1: distance of the center of pressure from the leading edge of the rudder

The comparison between the two Figures affirms the agreement in the tangent of the coefficient of normal force ), the stall angle, etc., with the exception of a part of i/c, arid there was noticed no substantial difference between the two. Although slight difference was seen with the maximum normal force coefficient, etc., this was due to the effect of rough-ness of the surface of the !nod?l, and is a phenomenon appearing even when Reynolds number is as large as 5.5 x

106.

Therefore, this may be considered

riot to relate to the difference in Reynolds' number of this experiments. Beyond the critical angle which causes a stall, the phenomenon became unsta-ble with differences arising in CN, however this is inevitaunsta-ble since it is a mattér of natural course. Table I shows the -Reynolds' number at the time

of experiments. It must be noticed that they cover a range of 0.085 x iø6 -0.22 x 106.

Table. 1 Reynolds' Number at the time of Experiment )

As above-mentioned, the maximum Reynolds' number at the time of

experi-ment was over 0.2 x 106 as advocated by Dr. van Lauimeren, and. the minimum

niAmber is so small as one, place lower than the aforementioned. Yet these results showed a good agreement. The principal reason why these experimental 'resulte gave reasonable values for important items euch as the slope of normal force,

the coefficiente of maximum normal force, and the rudder angle at which the stall occured as compared with other experimental results, which were carried out higher Reynolds' number at the ordinary towing tank, is that in the present experiment, a circulating thmne1 was used, which condition was quite different

from other cases of experimente.. Namely, an experiment by use of a circulat-ing c)nnl differs from an ordinary towcirculat-ing tank experiment with the modGi advancing through the still water, in that the water itself is activated into circulation by means of the impeller, and the water flow had come to include

its own inherent turbulence.

-6-Rudder

- Dimensions of Rudder (a/rn)

(Height i Chord length z

kz)

Velocity of Water Flow (im/e) Reynolds' Number 0.62 0.085x106 No.1 200 z 120 x 21.6 0.85 0.117 z 106 1.04 0.143 z iø6 No.2 _{400 z 40 z 43.2} _0.64 _{0.153 z 106} 0.93 0.221 z 106

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AND SOME EXAMPLES OF EXPERIMENTS WITH IT

By this reason, the so-called "effective Reynolds' number" grows biggerjust

the same as when a turbulence stiimiltion. grating is inserted, a larger

Reynolds number was brought into action in the present experiments as compared

with ordinary towing tank experiment, and ii1irished the portion of

1pm1rir

flow in the proximity of the leading edge of the rudder, and inspità that

the apparent Reynolds' number was below 0.2 x io6, the experimental res4ts

showed agreement as good as any experiments conducted at a largerReynolds'

number. In this way, it had been affirmed that reasonable values could be

obtained even at apparent Reynolds' number of around 01 z 106 in experiments

by use of circulating channel.

Next, a series of experiments were conducted in order to emmne the

performance of rudder in the propeller race. Prior to the execution of the experiments, the condition of the propeller race was measured by use of a

flow meter specially prepared for the purpose.

For the purpose of driving the propeller, a column having a a1am-lined

section as given in Pi6. 12 was fixed at the center of the measuring section in the c.rculating

r1,inp1.

The propeller was fitted at the rear end of the pro-peller shaft projecting horizontally rearward from this column for about 0.8m. The fore-end of this propeller shaft was engaged with a bevel gear through a thrust block. On the other hand, inside the column along the vertical direction, another shaft was provided, the lover end of which was engaged with the propeller shaft by the medium of a set of bevel gear, while its upper end vas connected with a 1/4 ! driving motor by the medium of pully and belting. The revolutions of the propeller were counted by the revolutions of the another

shaft, which were reduded to 1/20 of the revolutions of the prope]ler by means

of a worm and a worm gear, utilizing the system of mb1g and. breaking electric

circuit which was related to each revolution of the another shaft, so as to give the revolutions of the propeller.

In order to endne the race of a propeller working at definite

revolu-tions (a definit advance coefficient) by a driving apparatus such as

above-mentioned,

the 5-furcated pitot's tube as illustrated Fi6. 13 was used.

It is provided with a static pressure tube in the centre, and 4 pieces of total pressure tubes were amounted at the 4 pointa above and below, and on the right left sides of the central tube, each of which was inclining towards the center at an angle of 45°. Namely, this 5-furcated pitot's tube consisted of 5 pressure measuring tubes combined in one.

The calibratïon of the 5-furcated tube was carried out at the towing tank

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ON A HORIZONTAL CIRCULATING CHANNEL

AND SOME EXAMPLES OF EXPERIMENTS WITH IT

The 5-furbated tube vas mounted at an inclination of given angle against the advancing direction of the towing carriage, and the indication of respective

pressure tubes were read as the carriage advanced at a given speed. _{In this} case, the angle of inclination was set at evezy

_5°

up and down wards and

towards the right and left for the range from 00_300 including the angle at which they were combined to the both directions. If the differences in pressure between the upper, lover, right and left tota]. pressure tubes, and the static pressure tube are written as y+, y-, z+ and z-, and the sum total of the four as M, then _{is considered to be greatly affected by angular deviations} in vertical direction, and _{by angular deviations in horizontal direction.} Henàe, the calibration curves obtained were as given in Pig. 14, as these data vere plotted along the axis of ordinates and abscissa, by which it was

possi-ble to learn the angular deviations in both directions. _{Regarding the ve1octy} of incidence, since the value of

M (Mo)

at the time when the two angular

devia-tions are both 0°, is proportional to the square of the velocity, so the ratios of M at respective deviations to M0 were put together into the calibration

curves as shown

in

Fig. 15, from which the velocity of incidence was to be calculated.

In the actual survey of the propeller race, the 5-furcated pitot's tube was mounted on a base which could be moved up and down, and. also to the

right and left at a low pitch

80

as to facilitate the 5-furcated tube to shift

to respective measuring position as might be required, and every consideration was given so that the velocity of flow within any required section which was

at a right angle to the mean flow could be measured speedily and handily.

The statua of actual measuring operation of the propeller race was as shown

in the photograph of Pig. 16.

Prior to the measuring of the propeller race, with a view to checld.ng

whether the effect of wake generated by the pillar-shaped displacing body mounted in front of the propeller so as to drive it, was extending to the propeller position, the distribution of the flow velocity was determined with

the section at a distance of 100 mm behind the propellér, by conducting the measurement with the propeller removed. From the results of these measurements it was affirmed that there would be no objection to regard the flow distribu-tion as uniform, except that at localized points the flow was observed to be inclined to the center line of the channel at an angle of about 1°-20.

Therefore necessary correction was made for such inclination with the results of measurement of the propeller race enforced thereafter.

The propeller race vas measured by use of a propeller of Troost B. 4-40 Type, diameter D=200mn, with a series of advance coefficients, and. at varying pitch ratios.

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-8-AND SOME EXAMPLES OF EXPERIMENTS WITH IT

As a typical example, the test results were given below as obtained with a

propeller working at the pitch ratio of 0.800, end advance coefficient J=0.60. The measuring was conducted with 4 section at 0.25 R, 050 R,

1.00 R, aM 1.50 R. (R: the radius of propeller) bbind the propeller. However, inasmuch as the Troost type propeller used in this case had

a rake angle, the propeller surface, adopted was the èectiori that was at a

right angle to the propeller shaft and which alsoincluded' the intersecting

point of the TT1imnm thickness line of the aerofoil section at 0.7 R with the pitch surface.

The measured points included in a section were 9? in number within a

square of 250 usi x 250 mm as shown in Fig 17. Fig. 17 -' 19 Indicate examples of the results of measuring. These figures were prepared in the form of resultant vector of vertical and. transverse components, and V. respectly,

of the velocity at respective measuring points at the time when the propeller

was viewed from its rear side. Fig. 20 is intended to show the status of distribution of the ratio of the resultaztt velocity composed not only 'of the

and V, but also of' the axial cbmponent V to the general velocity of

incidence. By the figure, it is clearly known tha:t the velocity around 0.5 - 0.6 R is most accelerated.

FIg. 17 - 19 are indicative of the measuring results when à propeller

with pitch ratio of 0.800 was working at the advance coefficient of J=0.60. over the sections at 0.25 R, 0.50 R, and 1.00 R behind the propeller

respec-tively. According to these figure, the circuinferentially accelerated condi-tion of water can well be observed. The direction of flow at every point

out-side the projected area of the propeller s actuàtor disc, is pointing more towards the center of the propeller than the direction o(' the tángent of the

concentric circle which passes the point. This fact gives exact idea that 'the

propeller race isa flow of contraction. At the section at 0.25 R directly behind the propeller, the centipedal velocity is at the largest, and it

decreases in intensity as it gets further backward. This may be deemed to show

that the intensity of the flow of contraction is at its maximum at the point

directly behind the propeller.

4. CONCLUSION

The above is to generalize the water circulating thRnnel, giving its advantages and disadvantages in comparison with an ordinary towing tank.

Further more description vere made in connectioù with the test methods and results of a few experiments on rudders and propellers which were thought

suitable to be conducted in the circulating channel.

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.

ON A HORIZONTAL CIRCULATING CHANNEL

AND SOME EXAMPLES OF EXPERIMENTS WITH IT

When its advantageous points are utilized, a watr circulating channel is believed to be useful for conducting hydrodynamic experinnts.

References:

-(i). S. Okada "On the results of open test of model ridder&'

Jour, of S. N.A.I'l.E. of Japan Vol 103. _1958.

S. Okada "On the results of experiments on rnod1 rudders

in the propeller race"

Jour, of S. LA.LS. of Japan Vol 104. _1959.

K.E. Schoenherr "Steering'1 Principles of Naval

Archtecture II

W.P.A. van Lammeren, L. Troost & J.G. Koìiing "Resistance, _Propu].sioi and Steering of Ships" 1948.

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-AND SOME EXAMPLES OF EXPERIMENTS WITH IT

PLAN

ff500

Pig.1. Gen.èral arrangement of the water

circulating channel.

Fig.2. Conditions of the circulating ehne1

arouxul the measuring section.

u

-ELEVA TION

fr

I

'I

j I I I I I II I I II I I' I Ij I

II

LIw

L

I

i

Ii

ill

I I I I

lii Ill

) I

i t

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ON A HORIZONTAL CIRCULATING CHANNEL

AND SOME EXAMPLES OF EXPERIMENTS WITH IT

FREE SURFACE

Fig.3. An example of the

distribution of the

velocity of flow.

12

-POSIT/ON 0F THE BONDED STRAIN GAUGE

(15)

AND SOME EXAMPLES OF EXPERIMENTS WITH. IT

13 -Fig.5. General view of the

rudder dynamozaeter

as installed in its

position.

(16)

ON A HORIZONTAL CIRCULATING CHANNEL

AND SOME EXAMPLES OF EXPERIMENTS WITH IT

Fig.7.

An example of oscillogram measured..

NUMERA LS; IN M. M.

Pig.8.

Rudder No. i

14

(17)

AND SOME EXAMPLES OF EXPERIMENTS WITH IT

Pig. 9.

The similar model of rudder.

RUDDER N.!

i

T 0.62m1$

V.0.85'fl/s

T .7.04m13 70 20

30 RUDDER ANGLE

Fig.lO.

15-.

RUDDER N2 2

V:o.64m/s

V= o.93m/s

10 20 30

RUDDER ANGLE

Pig. 1].

(18)

ON A HORIZONTAL CIRCULATING CHANNEL

AND SOME EXAMPLES OF EXPERIMENTS WITH IT

Fig.12.

The apparatus for propeller driving.

1ff

I

Fig.13.

The 5-furcated.

pitot's tube.

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-AND SOME EXAMPLES OF EXPERIMENTS WITH IT

Fig.lk.

Ca1bration. diagrams of the

-furcated. pitot's tube.

Fig.15.

Calibration diagrams of

the 5-furcated pitot's

tube.

.11.-1w'

VAT

(20)

ON A HORIZONTAL CIRCULATING CHANNEL

AND SOME EXAMPLES OF EXPERIMENTS WITH IT

Fig.16.

The model propeller and its race

measuring apparatus.

TROOST B4-40 TYPE 0200mm,P/0 i 0.600

GAp:25mm, ¡:040

z

SCALE THE RESULTANT

VECTOR

Fig. ].7.

NOTE:F7GLJPE SWS THE RESULTANT

* VELOCITY VEC7DRS 0F ¡ANO Z DIRECTIGV VE WED FWA ADS FROH THE AFT AND THE SCALE 0F VECTORS AT 7)/E MEASURED POINTS ARE SHOWN IN PER-CENTAGE 0F THE RESULTANT