The Institution o Engineers and
39 Elmbank CresLThis paper will be read ut a meetin day, 4th December, at 7.30 p.m. Y
meeting and to take part in the discussion. Written n utions
should be sent to the Secretar?! before 14th December. roof is subject to correction.
Paper No. 1215
RECENT WORK IN THE LITHGOW WATER TUNNEL AT N.P.L.
By A. SILVERLEAF, B.Sc.* Associate. Member of the Institution.
atìd L. W. BERRY*,
4th December, 1956
SYNOPSIS
Tite first part of the paper describes the Lithgow water tunnel in Ship Division., N.P.L., which has been extensively modified during
the past 3 years. Details are given of the instruments and equipment now available for studies on propellers and on fixed bodies, and of the performance of the tunnel. The second part of the paper
summaiizes some typical examples of work recently carried out. These include comparative propeller tests, research on propellers in non-uniform flows and scale effect studies, tests on rudders, and basic research on water tunnel performance.
PART ILAYOUT. EQUIPMENT AND PERFORMANCE
During the past three years the Lithgow water tunnel at
N.P.L. has been extensively modified in order to improve its
performance, to axtend the range of work carried out in it, and
to provide design data for the much larger tunnel now being built at Feitham as part of the N.P.L. Ship Hydrodynamics
Laboratory. The tunnel circuit, the control and measuring
methods, and the propeller model drive and dynamometer now
differ considerably from those previously described to this * Of Ship 1ivision, Nationa] Physical Laboratory.
hiphui1ders in Sco1aiid
RECENT WORK IN THE LITHGOW VATER TUNNEL AT N.P.L.
SCALE FEET DY N AMO METER r--r-1 L4J IMPELLER MOTOR BO H.P
//7///)/////////////////,
////////7
FLOWWORKI NG] SECTION WORKING LEVEL
IMPELLER CONTRACTION SETTLING RATIO 61
DRAIN TANK
DIRECTION OF FLOW LENGTH FREE SURFACE WATER SPEED //
7-N
TEST SECTION PRESSU REVACUUM PUMP
VACUUM
RECEIVER
Fig. 20.Outline arrangements
of
Lithgow water tunnel.
[Fig. 2ø.
:
7////////////// //,//// /7///////'///./7' 7.' //," ./
DRAIN 4X-4*-MAIN SUPPLY
TACI-IO - GENERATOR
AIR BEARING
LEAD TO PRESSURE CH 0G ES
LEVELLING SCREW
TORQUE BALANCE
WEIGI-ITS_--PICK-OFF
SA SE P L AT E
THRUST ARM THRUST BALANCE WEIGHTS BALANCE WEIGHT HANDLES
7-
THRUST BALANCE WEIGHTSFig. 3(a).Propeller Dyiiamornctcr.
DRIVE FOR
\CK-OFF
HYDRAULIC PICK-OFF/ (MER L PATTE/ GAUGES
--
---MARKER WHEEL LEAD TO PRE SS UP E'N'.
i
TUNNEL FLOW [Fig. 3(a). AIR BEARING CROSS SPRING TO ELECTRONIÇ TUNNEL SHAFT\.\\
30 ftP MOTOR TACHOMETER MAGNETIC PICK-UP THRUST YOKE COUPLINGS WITH SWINGING FRAME CLUTCH AIR BEARING PIVOTS4 RECENT WORK IN THE LITHGOW WATER TUNNEL AT N.P.L.
Institution,' while much additional equipment has been installed. The modifications have significantly improved the behaviour of the tunnel and a wider range of propeller experiments can now
be carried out with greater accuracy and convenience than
previou1y; in addition, a three-component balance is available for experiments on fixed bodies, such as hydrofoils. While there are still undesirable limitations on the tunnel performance
characteristics, some of which are inherent in the tunnel basic design, it is now a compact and versatile laboratory facility.
TUNNEL CIRCUIT
Circuit Componeds. Fig. i is a general view at the working level of the tunnel, while the outline arrangement in Fig. 2 shows the principal circuit details. The tunnel lies in the vertical plane and is of the return-flow or closed-circuit type; the
impeller circulates the water along the lower horizontal limb,
up a vertical limb, into a short settling length, and through a
6 to i contraction into the working section. It then passes through
a short horizontal diffuser and is deflected by a 900 cascade
bend into the vertical conical diffuser which returns it to the
impeller. The 90° swept bend leading into the impeller is fitted yith three splitter plates; the other two corners each conSist of
a 60° bend and a 30° bend containing two sets of three short
splitter plates. At the impeller the tunnel section is circular; along the lower limb downstream of the impeller it changes
to rectangular with rounded corners; in the contraction it merges into square with rounded corners, and remains so through the working section and the horizontal diffuser until just downstream
of the cascade bend it changes back to circular. A drain tank
is situated below the working level. From the impeller bend down-stream to the contraction exit the tunnel circuit is as originally constructed in 1932; the remainder is new. Dimensions of the individual components are given in Fig. 4.
Apart from the impeller bend, which is made from a series of
castings, the tunnel sections are fabricated, mostly from fin. steel plate with welded stiffeiiers. The sections are bolted
together, with Haffite reinforced asbestos gasketsat the joints,
except those at the two ends of the working section, where See bibiiography, p. 51.
100
LOCAL
80
VELOCITY RELATIVE TO TEST
60 SECTION VELOCITY (%) 40 20 O VERTICAL DIFFUSER BEN D L 571.
DIFFUSER CASCADE BEND_
858 / 173 216 25 TEST SECTION CONTRACTION 458 481 DIRECTION OF FLOW SETTLI NG LENGTH BEND DIFFUSER Fig. 4.-.-Componn! /e,sgtII.
(sil,! re! alive' ve!ocizies.
IMPELLER
COMPONENT LENGTH (FEET)
65
756
65
43 R10ENT WORK IN THE LITHGOW WATER TUNNEL AT N.P.L.
synthetic rubber gaskets are used. The internal surface of the tunnel is coated with bitumastic solution from the impeller bend
to the settling length, and with Araldite epoxy resin from the contraction to the vertical diffuser. The inside walls of the working section outer shell
also have a coating of white
chlorinated rubber paint to assist observation and photography of the model. These internal coatings in the high-speed sections were experimental, and have not been entirely successful.Te.st Sectioa. The test section is 30 in. (76 cm.) in length and of constant cross-section, 18 in. square with corners 3 in. radius (equal in cross-section area to a circle 20 in. or 50 cm. in diameter). As shown in Fig. 5, it consists of a cage of Perspex bars forming the boundary of a slotted wall or guided-jet working
section, surrounded by a welded steel shell 40 in. square. to provide a reservoir of water round the jet stream. There are large Perspex windows in the outer shell fitted with
quick-release handles designed to facilitate rapid access to the model. This slotted wall working section was first fitted in November 1953 in order to find out how well it combined the advantages
of the earlier closed-throat section, which had good flow
characteristics, with those of an open jet in which the interference effects of the jet boundaries on the test model are much redieed. Previous experiments at Admiralty Research Lahoratory had
shown that with a slotted wall test section it was possible to
carry out tests with larger fixed models than in a closed-throat section, and it was hoped that this would also be true for rotating propeller models.
A closed-throat test section can be formed by replacing the slotted- wall boundary bars with close-fitting Perspex sheets; the reservoir is then filled with water to maintain airtightness, and to reduce the loading on these boundary sheets. A complete Perspex closed-throat working section has also been designed
and built; it has a Perspex flange, i in, in thickness and 3 in. in depth at each end of the 30-in, length, backed by a loose aluminium collar to take the flange bolts, and a single hatch,
15 in. x 10 in. on the top surface for access to the model. The wall thickness is in. This transparent working section gives an uninterrupted view of the model, but it has not yet been
fitted in place of the large steel shell and internal cage.
PER SPEX WINDOW WINDOW FRAME QUICK PELEASE HAND LE - TUNNEL SHELL
40 INS.
RECENT WORK IN THE LITHGOW WATER TUNNEL AT N.P.L.
Lin, thick spacer containing a vertically sliding portion which can be replaced by one wih gauzes and other fittings attached. First Corner. The horizontal diffuser and the cascade bend
form a single unit from the downstream end of the working
sectidn to a horizontal flange at the working level. Full reasons for replacing the original swept bend at this corner are given in item (1) of the bibliography, and the expected marked improve-ments in tunnel performance have been achieved. The horizontal diffuser has a total included angle of 70, and its cross-section
area at the bend is 50 per cent. greater than at the entrance.
A greater expansion before the bend is desirable but could not be achieved with the present overall tunnel circuit dimensions.
The cascade bend is made up of extruded aluminium alloy
vanes mounted on cheek plates in a brass frame which slides into a recess in the fabricated steel corner section; this
arrange-ment simplifies removal of the complete vane unit for
main-tenance or adjustment. The spacing of the vanes was determined
by the methods given in the paper3 by Salter, and the vanes
themselves are similar to those designed and made for another
water tunnel and which were thus readily available. The layout of the bend is shown in Fig. 6.
There are static pressure holes in fitted plugs along the diffuser
and at the exit from the bend, and a number of 3-in. diam.
inspection windows make it possible to examine the flOw round the bend, which appears good at all operating conditions.
Shaft Supports. Propeller models are mounted on a 1-in. cham. stainless steel shaft downstream of the test section,
carried in three plain journal bearings. The bearings are water lubricated and are of ferrobestos. The two bearing housings in the diffuser are at present each supported on three rigid
aerofoil-section struts, as shown in Fig. 6(a), and the position of the
long external bearing, which is mounted on a boss, is adjustable; these three bearings were set up with an alignment telescope.
The distance between the model and the first bearing is
relatively greater than usual in propeller water tunnels. Thishelps to improve observation of the wake and to reduce the
interference effects of the bearing supports. Although the large overhang does not, in fact, adversely affect shaft whirling, the shaft does whirl under certain operating conditions. This was, of course, expected and it is possible to fit another short bearing
WATER SEAL ANO LUBRICATION
VANE SPAN 22
/1 W IN DO WS AT SIDES.
Fig. 6.Voue bend and shaft supports.
PROPOSED BEARING TIES
SCREW POSITION.
DIA.
EXISTING I3EARING STRUTS
IO EECENT WORK IN THE LITHOOW WATER TUNNEL AT NP.L.
at each of the two strut positiQns. However, the resulting slight additional fiexural stiffness would not significantly affect the whirl conditions and would certainly increase the bearing torque,
so the single short bearing at each stut position has been
retained. The support of the model is a critical feature of apropeller water tunnel. Experiments to determine the most
favourable bearing positions to reduce whirling are being made.
and support interference is being reduced by replacing the
bearing struts with adjustable ties, as shown in Fig. 6(b). These ties are much smaller than the struts, so reducing blockage, and can be entirely removed when the tunnel is used for experiments on fixed bodies, such as hydrofoils, and the propeller shaft is not required.
CONTROL AND MEASUREMENT
JVater Speed. The speed of flow is controlled by regulating the impeller speed of rotation, and is measured by the pressure drop in the contraction leading to the working section. The bronze impeller described in the paper' previously referred to is still in
use, since it was found to be in excellent condition when the
tunnel was stripped in 1953. The blade surfaces are in their
original slightly rough condition, but this is not considered to be important. The water-lubricated shaft bearing material was
changed to a synthetic resin (Tufnol), which has been found to be very satisfactory, no wear yet being apparent. The impeller
is direct-driven by the original D.C. motor, developing up to
SO h.p. at 320 r.p.m., fitted with a modified form of
Ward-Leonard speed control. This is operated by a motorized push-button control on the central control panel at the working level. and the impeller r.p.m. are dial-indicated 011 this panel from a tacho-generator driven by the motor shaft. The central control and measurement panel is shown in Fig. 7.
The contraction pressure drop is measured by a differential manomter beside the control panel, an inverted U-tube water gauge being used for test section water speeds up to 15 f.p.s.,
and a mercury U-tube gauge for higher speeds, as shown in
Fig. S. Glass reservoirs are fitted above the limbs of the mercury gauge to collect any air trapped in the water leads to the U-tube. The pressure tapping at the upstream end of the contraction is
RECENT W(flK IN THE LITHGOW WATER TUNNEL AT N.P.L. 11
WATER TRAP
TUNNEL
n
FLOW
jNOPKING LEVEL CAS CONTENT APPARATUS
PRESSURE MEASUREMENT --4-L AIR RESERVOIR GAUGE t. RECEIVER PRESSURE 2 COAMING 3 WATER TEMPERATURE 4
TEST SECTiON PRESSURE
Fig. 8.ConiioL and mi'aslIrenlent s stem l'or pressure and speed.
WATER SPEED MANOMETERS
RECE I VE R PUMP CONTROL PANEL 1 34 GAUGE
Öoc
LL-RECENT WORK IN THE LITHGOW WATER TUNNEL AT N.P.L. 13
a single hole, but four tappings were made at the downstream end, one in each side, connected through individual diaphragm valves to a ring pipe from which the manometer leadwas taken.
This attempt to use a mean downstream
pressure was not successful, and only the bottom pressure hole is now used. linthese and other leads extensive use has been made oftransparent P.V.C. tubing.
Preliminary tests have been made to adapt
automaticdifferential manometers, similar to those used for wind tunnel airspeed measurement, to eater tunnel use. In these manometers the two pressures are applied to bellows connected to a pivoted horizontal weighbeam. A rider weight is moved along a lead-screw by a small motor actuated by a sensing device when the
beam is out-of-balance, and the position of the weight when
balance is restored is a direct indication of the pressure difference. So far the automatic manometer has not worked satisfactorily, partly because of difficulties with the water-filledbellows when both pressures were sub-atmospheric and the water contained appreciable dissolved air, and partly because of difficulties in
achieving high sensitivity without "hunting" of the balance weight. However, neither of these difficulties is insurmountable, and it is hoped to overcome them shortly.
Te.st Section Pre&sure. Until recently the pressure in the test section was controlled as shown in Fig. S by regulating the air
pressure above a water surface in a coaming
on top of the
settling length, with a perforated plate at the upper surface of the tunnel to reduce disturbance to the flow. An air pipe from
a large air tank which can be evacuated by an ejector-type
vacuum pump, passes behind the control panel where isolating valves and pressure gauges are fitted, and thence to the coaming lid. This arrangement worked satisfactorily with a closed-throat test section, but difficulties were experienced after the slotted-wall section was fitted.
When the tunnel is operated at low pressures, air tends to
come out of solution and collect at 'high points " in thecircuit, and with the slotted-wall section the upper part of the reservoir became a main collection region. Air which collected here forced down the water level and consequently raised the water
level in the coaming; for this reason a water trap above the
14 RECENT WORK IN THE LITHOOW WATER TJNNEL ATN.P.L.
valves were also led from the top of the reservoir into the main
air pipe; these valves had to be kept closed while the tunnel
was running, and at intervals the tunnel had to be stopped, the valves opened, and the air at the topof the reservoir forced out. These awkward interruptions were avoided by applying the air pressure control directly to the space above the test section. The perforated plate above the settling length was replaced by a close-fitting airtight one,and the air pipe previously connected to the coaming was led instead to the top f the slotted-wall reservoir. With this arrangement the free surface above the test section is the only one in the tunnel circuit and air which comes out of solution can now readily escape through the air pipe. Air also collects in the top corner of the cascade bend, but this is less important and a manual venting system deals with it adequately.
Standard Bourdon gauges on
the control panel indicate
approximate pressures at different parts of the control circuit, but the pressure at the test section is accurately measured directly on a specially designed water/mercury pressure gauge of baro
meter type. A water pressure lead taken from the up-stream
end of the test section is connected to this gauge; the gauge position and its scale marking are arranged so as to indicate
directly the absolute pressure at the test section centre line, and, for further convenience, an additional scale gives the pressure
above the vapour pressure
of waterat a
selected stan-dard temperature. Details of this pressure gauge are given in Appendix I.Air Content. It is not possible to control the dissolved air
content of the tunnel water,
and this imposes undesirable restrictions on tests. If the air content is near saturation level at atmospheric pressure (about 20 ml. air per litre water) then as soon as the pressure is reduced appreciablythe air begins to come out of solution. At low cavitation numbers, clouds of air bubbles form in the reservoir and in the jet issuing fröm thecontraction. A partial solution is to operate the tunnel with
water which has been de-aerated as far as possible. The present
method of reducing the air content
is to fill the tunnel
in-completely so that a free surface is left along the whole upper limb and then to circulate the water slowly while the air pressure
MANOMETER
ECRNT WORK IN THE LITHGOW WATER TUNNEL AT N.P.L. 15
MERCURY SEAL
MOVABLE
MER CURY
RESERVOIR
FROM TUNNEL
Fig. 9.-Gas conten! apparatus.
TO TUNNEL THREE-WAY COCK MEASURING PiPETTE SMALL ORIFICE SPRAY CHAMBER SPRAY TUBE THREE-WAY COCK
16 RECENT WORK IN THE LITHGOW WATER TUNNEL AT N.P.L.
SO as to thrash the water. However, tunnel operation at low air content is not satisfactory since sea water is air-saturated and thus similitude is deliberatelysacrificed. On the other hand.
air bubbles in the stream can be useful in flow visualization
studies.
The gas content of the water is measured on a Van Slyke
apparatus as modified by Williams,4 shown in Fig. 9, so placed
that samples of tunnel water can be taken readily under all
operating conditions and avoiding long leads in which stagnant water can collect. In this apparatus the water saftiple is turned
into a mist of small drops by forcing it through a fine orifice
into a small evacuated chamber; gas dissolved in the water is then rapidly released from solution and by measuring its partial pressure the total gas content of the water can be determined. The method is rapid and simple, and far more accurate than the Winkler chemical method formerlyused. The chemical method. which measures the dissolved oxygen in the water, could not in any event be used because the sodium nitrite now added to the water as a rust inhibitor affects the oxygen content. However, both methods make no distinction between dissolved and
en-trained aira distinction now believed to be significant. Many
attempts are being made to
develop more discriminatingmethods, and it is possible that ultrasonic techniques may
provide a better measurement of entrained air content.
Water Temperature. No attempt is made to cool the tunnel
water, since the energy input
from the impeller and a test
propeller is not sufficient to raise the temperature rapidly,particularly in view of the large radiating surfaces of the tunnel walls. Equally, no external means are provided to raise the
water temperature in order to increase the effective test Reynolds' number and lower the minimum cavitation number which can be reached. The water temperature is measured by a
mercury-in-steel thermometer inserted
in the vertical
diffuse.r just upstream of the impeller bend,with a dial indicator on the
control panel.
MODEL TEST EQUIPMENT
Propeller Dynamometer. Propeller models are directly driven by a D.C. motor, developing 30h.p. at 3,000 r.p.m., whichforms
RECENT WORK IN THE LITHOOW WATER TUNNEL AT N.P.L. 17
an integral part of the propellerdynamometer, as shown in Figs. 3(a) and (b). The motor is of the swinging frame type and the two stator bearings are mounted in gimbals añd supported by compressed air. The torque reaction force is balanced by a hydraulieally'operated pick-off which keeps the stator frame in
its null position; the bearings are air lubricated in order to
reduce initial friction to a minimum and were optically aligued with the bearings supporting the test propeller shaft. Current is supplied to the motor through contact arms dipping in mercury
cups. The motor torque is transmitted to the propeller model shaft through a 4-arm s1iding chftch which does not absorb
thrust; a yoke on 'the shaft transmits the thrust developed by
the model to a cranked beam pivoted on cross-springs. The horizontal end of this beam bears on anpther hydraulic pick-off,
so that the propeller thrust, reduced in value by the
1: 5 crank arm ratio, is balanced by the force exerted bythe pick-offin keeping the beam, and thus the yoke, in its null position. The air bearings and hydraulic pick-offs are based on designs by the Mechanical Engineering Research Laboratory.5
The motor speed is regulated by a Ward-Leonard control,
remote operated by push-buttons on
the control panel, to
which electronic speed-holding control is shortly to be added. A tacho-generator on the motor armature is used forapproximate speed measurement, while an electronic tachometer developed at N.P.L. gives accurate repetitive readings. The block .diagram
in Fig. 10 shows the method of operation of this precision
tachometer.6 A small permanent-magnet detector surrounded
by a coil is fixed close to a marker wheel
mounted on the propeller shaft. Round the periphery of this wheel there are 100 equally-spaced soft iron inserts; as each insert passes the magnet a voltage pulse is generated in the coil. These pulses, suitably modified, are passed to a counter through an electronic gate which is kept open for precisely 1 sec. in every 5 sec. by external timing markers. The counter displays the total number of pulses admitted to it on a series of cold-cathode scale-of-ten valves in directly readable form. The maximum counting rateis 10 kc. per sec.; since the counter receives 100 pulses per shaft revolution, the maximum measurable shaft speed is 100 r.p.s., and the overall accuracy 001 r.p.s. The count remains on display for about 3 sec., the counters are then automatically.
MAGN ET IC PrCK-UP MARKER
o
WHEEL GAT E OPEN FOR I SECOND EVERY 5SECS-lis. IO.- Efrctron
(block diorarn),
(OPEN
CLOSE
ELECTRONIC
CALIBRATION CHECK PULSES
I
GAT E GATE OSCILLATOR (WHEN REQUIRED) PULSES PULSES TIME PULSES GATE DISPLAY TIMER ßJTOMAT IC RESET ELECTRON IC I PER SEC CLOC K OPER ATOR UNIT PULSE MODIFIER DECADE COUNTER0000
RECENT WORK IN THE LITHGOW WATER TUNNEL AT N.P.L. 19
reset to zero, and at the next external timing pulse the gate
opens and the cycle of operation is repeated. Calibration pulses
of known frequencies can be fed to the tachometer to check
its accuracy at selected parts of its operating range.
The hydraulic pick-off is shown in Fig. 11. It consists of a
fixed piston and a close-fitting cylinder which is free to move vertically. The cylinder is rotated slowly by a small motor in
order to reduce initial friction between the lapped internal
contact surfaces. A pump supplies oil under pressure to the space above the piston through the upper ports in its side, thus
lifting the cylinder to its null position, in which the ports are closed. Any load applied to the cylinder then depresses it, so opening the upper ports and admitting more oil until it is raised again to its null position. Removal of load allows the cylinder to rise, thus opening the lower ports and so discharging some of the trapped oil to a sump. The cylinder then falls until it reaches
its null position again. The applied lead is determined by
measuring the oil pressure above the piston on a mercury
mano-meter beside the control panel with its scale marked directly
in terms of propeller thrust or torque. In the propeller dynamo-meter two pick-offs are used to balance part of the thrust and torque: dead weights are used to extend the range, so that the high sensitivity of the pick-offs is available for accurate measure-ment at low thrusts and torques without reducing the maximum forces that can be handled. These dead weights are carried on cam-operated pivot arms, by which they can be lowered onto
scale pans suspended from knife-edge supports on the stator
frame and the thrust beam opposite the pick-offs. Similar sets
of dead weights at the pick-off positions are available as
calibration weights or for the measurement of reverse forces.The maximum thrust that can be measured is ±550 lb., the
maximum torque ±70 ft..lb.; the maximum load applied to the pick.offs is 2() lb., corresponding to an oil pressure of approxi-mately 11 lb. per sq. in.
Initial operation and calibration of this dynamometer presented some difficulties. The two compressed-air bearings, each 5 in.
in length and 6 in, in diam., support a load weighing about
1,000 lb.; the pressure at the bearings is SOE lb. per sq. in., and about 15 Cu. ft. per min. of free air is required by each bearing.
FLOATING CYLINDER PULLEY TO ROTATE CYU N DER FIXED _-_---PISTON TO SUMP FROM PUMP PRESSURE PICK-OFF .Fg. 11.Iiydraulic pick-off t-T
tL,
-rt-L -i--1-v--±1 IRECENT WORK fl THE LITHGOW WATER TUNNEL AT N.P.L. 21
were oniy prevented from binding by using specially purified compressed air and high-quality filters. Air seeping into the hy-draulic pick-off oil system when at rest caused erratic manometer readings and instability; this was overcome by incorporating an oil return pipe from the manometer to the sump and bleeding the whole system before use. To obtain acceptable speed of response in the manometer columns a special oil of low viscosity had to be used. Difficulties were also experienced with the thrust measuring arm and pick-off; it was found impossible to prevent the thrust arm from pressing slightly eccentrically on the pick-off head. This set up a side force between the pick-pick-off and the thrust 'arm, making the pick-off slow to respond and twisting the cross-springs of the thrust arm pivot. This fault was
over-come by substituting a short push rod for the original button
còntaet between thrust arm and pick-off.
Thrust calibration was carried out in stages. First, known weights were applied directly t the pick-off cylinder head, and the resulting mercury manometer reading was compared with the calculated value. Very close agreement was found. Next, known dead loads were applied to the dynamometer shaft 1)0th
stationary arid rotating; this checked the crank-arm ratio and
gave an overall calibration. The overall accuracy of thrust measurement is within fj per cent. Torque calibration could
,not be carried out so directly; while the manometer scales were
checked by applying direct loads to the pick-off, it was only possible to check the torque arm by indirect measurement.
The stator frame was first balanced in its null position. Next,
equal loads were applied to the two torque arms on opposite sides of the stator; any unbalance was then determined; and
from this, and direct measurement of the distance between the knife edges, the effective lengths of the two torque arms were deduced. It is considered that the overall accuracy of torque measurement is within ± per cent.
The total idling torque of the propeller shaft carrying a 3-lb.
dummy" weight, running in the three water-lubricated
bearings in the tunnel and the air-lubricated bearing on the
dynamometer pedestal, is approximately 07 ft.-lb. at 10 r.p.s. to 0-32 ft.-lb. at 4Q r.p.s.
Hydrofoil Balance. Although the Lithgow water tunnel is
22 RECENT WORK IN THE LITHUOW WATER TUNNEL AT N.P.L.
also desirable to be able to carry out experiments on fixed bodies, such as hydrofoils. For this purpose, a hydrofoil balance, which
can be mounted in the test section outer shell, has been built recently; when it is in use the propeller shaft supports can be removed. This balance is similar in principle to one recently completed at Iowa and described by Appel,7 but differs from it in some important details. It permits a hydrofoil to be mounted
Fig. I 2. H vili olai balance suono mo ii ri ted ori ca/turati on rig.
for test horizontally, so that the pressure, and thus the cavitation
index, over its span is uniform, and is designed to facilitate
accurate three-component measurements from which lift, drag and pitching moment can be readily derived. These
measure-ments can be made even when the hydrofoil forces fluctuate
rapidly, and in a way which avoids the movement of any part passing through an air-tight pressure seal. The N.P.L. balance is also designed to allow the mounting and adjustment of two-part hydrofoils. such as stabilizer fins with tail flaps.
RECENT WORK IN THE LITHUOW WATER TUNNEL AT N.P.L. 23
The balance, shown in Fig. 12, consists of four main units;
these are the hydrofoil assembly, its support structure, the
incidence control, and the measuring system. The hydrofoil Assembly consists of the horizontal hydrofoil and a vertical force, plate to which it is rigidly connected in any desired attitude by a locking clamp. When measurements are being made this unit is
linked only by three parallel horizontal flexure rods to its
support structure, a vertical earth plate rigidly fixed to the test section outer shell. These horizontal rods each contain twodouble-flexure joints. so that while they keep the force plate
vertical they do not restrain it from moving parallel to the earth plate. Thus this parallel motion support system permits the
hydrofoil to move vertically and horizontally and to pitch, but prevents yaw and roll.
The incidence control sets the main and tail hydrofoils at the
required angles of attack and locks them in position. Two
independent control handles are fitted to the two parts of a double shaft, mounted in the earth plate, which can be pushed towards the force plate so that two spur gears engage a similar pair on a double shaft which forms part of the hydrofoil assembly. The
main and tail foils can then be rotated to any desired incidences and held there by closing a friction clamp which locks the double foil shaft. This locking device is controlled through a bevel drive by a shaft carried on the hydrofoil assembly; this shaft is rotated by a control handle on a shaft mounted in the earth plate and
which caii be pushed forward to engage the bevel gears, so
temporarily linking the two short shafts. When the foils are
securely locked at the desired attitudes the locking handle is
withdrawn, so disengaging the bevels, and the ixcidence setting spur gears are also disengaged; the hydrofoil is then connected to the earth plate only through the flexure rods.
The hydrodynamic forces acting on the hydrofoil are determined
by measuring the forces necessary to restrain the hydrofoil
Assembly from moving in a vertical plane. By measuring three
restraining forces applied to the force plate at three known
positions these hydrodynamic forces can be resolved into the lift, drag and pitching moment for the complete hydrofoil; to determine forces on the tail foil as well it is necessary to make additional measurements. In practice it is preferable to restrain the force plate by three springs anchored to the earth plate which
24 RECENT WORK IN THE LITROW WATER TUNNEL AT N.P.L.
do not entirely prevent movement of the force plate but restrict
it to an arbitrary small extent.
It is these flexible spring restraints which permit the hydrofoil assembly to respond tofluctuating hydrodynamic forces: however, care must be taken to distinguish between oscillations due to such forces and those
set up by the hydrofoil assembly vibrating at its natural
frequencies. In the N.P.L. balance the springs used are horizontal steel cantilevers having their roots firmly attached to the earth plate, and their tips connected to the force plate by links with free pivoting ends. N.P.L. resistancestrain gauges are
bonded to the cantilevers near their roots to measure the tip forces.
The balance operates with all working parts, including the
measuring system, completely submerged.
Apart from the
incidence control shafts mounted in the earth plate, which aredisengaged when measurements are being made, only the
electrical leads to the strain gauges pierce the earth plate.It
is mounted for storage in a special frame which contains pro-visions for static calibration of the strain-gauge/cantilevermeasuring system.
AUxILIxY EQUIPMENT
Illumination and Photography. Cavitation due to a model propeller under test is observed by stroboscopic lighting from a gas discharge tube. The tube is triggered from a shaft contact
which can be rotated at will to alter the phasing of the flash
relative to the model position. The tube, housed in a reflector on the top Window of the tunnel section, has an energy of i joule per flash and an approximate flash duration of 20 micro-sec. This repetitive flash is also useful for visual observation of cavitation on stationary test bodies. A separate single flash tube operated with a greatly increased energy of 150 joules and a flash duration of approximately 200 micro-sec. is used for photography. This straight discharge tube gives a light which spreads sufficiently to avoid hard shadows; it is mounted together with the camera in a frame fitted to a side window. Tests are being made with
alternative equipment giving sufficient energy but a shorter
flash duration now that larger diameter, faster-running propeller models 'can be tested in the tunnel. Experiments on high-speed
RECENT WORK IN THE LITHGOW WATER TUNNEL AT N.P.L. 25
ciné-photography of cavitation have been suspended until a
better high-speed camera is available.
Noise Measurement. There is a growing interest in acoustic
measurements of cavitation, both as a criterion of cavitation inception and as a meaxs of studying its nature and causes.
Hydrophones incorporating barium titanate crystals have been
made and used for these purposes; these are placed either in
an aperture in a boundary bar of the slotted wall test section, or
B
Fig. 13.Pressure tubes and combs.
APigot-sta.tic tube.
BComb on vertical strut.
CComb on horizontal tube.
in a water-filled anechoic ch amber pressed SC against tiì c outside of a side window. The output from tltese hydrophones is fed to a wide-band amplifier and then either through a series of band-pass filters to a meter or display tube, or to a panoramic wave analyzer for frequency analysis of the acoustic spectrum. When used simply as an on-off" switch to indicate cavitation inception or suppression the amplifier output can be fed directly
to an oscilloscope:
the trace width increases suddenly as
cavitation begins. Used with care, this technique provides the best criteria yet devised for cavitation inception; equally,26 RECENT WORK IN THE LITHGOW WATER TUNNEL AT N.P.L. frequency analysis of cavitation noise is a valuable tool in
studying cavitation scale effects as well as its mechanism. Pitot Tv bes. A number of pressure tubes of different type are available for examining the velocity and pressure distributions in the test section: sone of these are shown in Fig. 13. A Pitot-static tube, with an aerofoil section strut carrying a i-in. (ham.
head, has been used for calibrating the contraction pressure
drop in terms of test section velocity. Combs or rakes of tubes have been used to measure the velocity distribution across the
test seetion, and a long multi-hole static pressure tube was
loaned from A.R.L. to examine the axial pressure distribution along the section.
A multi-tube manometer board is permanently fitted next to the central control panel for connection to these pressure tubes or other pressure measuring devices. The water in the leads
from pressure point to manometer tends to (lamp out fluctuations and reduce the frequency response. Pressure gauges giving an electrical output and having a higher frequency response have been considered but have not yet been used in the tunnel, although they have been employed for other purposes in Ship Division. Tnrhuience Measurement. The measurement of low and high -frequency velocity fluctuations in water has always presented difficulties. Hot-wire anemometers have been developed for use
in the Lithgow tunnel by Leathard,8 but were not entirely
satisfactory. A turbulence sphere 25 in. in (ham. has been used
to obtain a comparative turbulence level measurement by
determining the mean velocity corresponding to an arbitrary
pressure drop between the front and the rear of the sphere, but this method is not sufficiently precise or sensitive. Recently a new type of hot-film anemometer has been developed at Iowa by Ling : one has been used in the Lithgow tunnel and, after initial difficulties had been overcome, gave promising results.
It
is hoped that this instrument will be suitable for detail studies of free stream turbulence and its effects on cavitation and model performance.TUNNEL PERFORMANCE
Power Factor. The relationship between mean speed of flow through thé test section and the power input from the impeller
RECENT WORK IN THE LITHGOW WATER TUNNEL AT N.P.L. 27 is given in Fig. 14, in which the effects, of successive changes to the tunnel circuit are shown. The marked improvement due to replacing the original radius bend at the first corner by a vane cascade bend, and the higher resistance of the slotted-wall test section, can be seen. Fitting the vane cascade bend reduced the power required to reach a given speed of flow to half that needed with the original swept bend. The maximum speed was thus considerably increased, hut a new impeller is required to
SWEPT BEND CLOSED THROAT TEST SECTION
VANE .. SLOTTED WALL 80 60 MOTOR OUTPUT H. P. 40 20 O s 0 0 20 30 40
TEST SECTION SPEED FT/SEC.
Fig. 14.Relationship between tesi section speed and impeller power.
obtain the highest speed possible with the existing impeller
motor.
The quality of a tunnel circuit is commonly assessed in terms of its power factor, defined as the ratio of the power input to the impeller to the rate of flow of energy through the test section. This is equivalent to
power factor=P/( p/2)AV3,
where P is the power delivered by the impeller motor, A is the cross-sectional area of the test section, and V is the mean speed of flow of water of mass density p. Power factors for the Lithgow
r
f
S RECENT WORK IN THE LITHGOW WATER TUNNEL AT N.P.L.
tunnel are summarized in Table I. For a tunnel of good modern design, with a closed-throat test section and a long diffuser before the first bend, a power factor as low as OO can be achieved.10
TABLE I
POWER FACTORS Max.
Tunnel condition flow speed. Impeller Power
ft. per séc. h.p. factor
O-72
O-31
03S Oross-section area of test section 21 8 sq. ft.
Limiting Condition-s. The range of experiments possible under cavitating conditions is restricted by the minimum test section cavitation index 0T which can be maintained with the particular
body under test. This clearly depends on the tunnel circuit
design, the body being tested, and the model support system;
it also depends on what cavitation effects can be tolerated.
Generally, cavitation first occurs in some part of the circuit at
a higher test section eT value than that at which it so affects the flow that steady conditions cannot be maintained.
In the
Lithgow tunnel, without a mocll in place, cavitation now first occurs in the diffuser entrance, particularly on or close to the shaft bracketsthis can be seen and heard, but it does not
appreciably influence the general flow. However, at lower eT values the flow is affected because the tunnel 'chokes" at some point, ad the free surface water level rises. The minimum test section eT value that can be steadily maintained in the" empty" Lithgow tunnel has been arbitrarily taken as that for which the free surface water level rises 4 in., and Fig. 15 shows the tunnel performance at present on this basis. Two features may be noted; first, that the minimum eT value attainable decreases as the speed of flow is increased, and second, that slightly lower aT valuescan be reached with a closed-throat test section than
with a
slotted-wall section. A screw model under test tends to lower
\. Original (with radius bend) 31 80
B. Original closed-throat test
section; new cascado bend,
diffusers, etc. 41 80
C. Slotted-wall test section;
new cascade bend, diffuser.
20 05 ¶0 2 ¶4 16 ¶8 20 22 24
TEST SECTION SPEED
IN FT/SEC.
Fig. 15.Test section liniiting conditionsminimum auairniMe cavitalim,
,,i,rniav.
TUNNEL
30 RECENT WORK IN THE LITHOOW WATER TUNNELAT N.P.L.
the minimum attainable csT value, while a hydrofoil or other fixed body raises it.
This "tunnel" cavitation index
is defined by al=(pTe)[
(p/2)VT2, where PT and VT are respectively the static pressure,
and flow velocity at the test section centre line, and p and e
are respectively the mass density and vapour pressure of the water.
The maximum value of Ta can be reached is also important, since it is clearly desirable tobe able to carry out tests under conditions in which cavitation does not occur. For experiments with propeller models, this generally demands that the maximum
tunnel" cavitation index should not be less than iO.
If the
test section pressure cannot be raised above atmospheric, then for c= iO, the corresponding flow speed is less than 15 ft. per sec. Clearly, higher flow speeds cannot be used for non-cavitating experiments unless test section pressures above atmospheric can be maintained. Since the Lithgow tunnel was not designed for above-atmospheric pressures, this imposes a further limiting operating condition. Thislimitation affects propeller experiments unfavourably. If the propeller cavitation index is defined byaR= (pe)/(
p/2)V2, where VR is the resultant velocity of a blade section at OE7 radius, then in general for non-cavitatitig conditions, R must exceed 1. An equivalent form for thecavitation index is cR=(p c)/(p/2)(nD)2, where n and D are
respectively the propeller speed of rotation and diameter and ,
which varies with the advance coefficient J, usually ranges only from about 5 to 6. From this it can readily be seen that, for aj 1.0, if the test section pressure cannot exceed atmospheric, then n cannot exceed 17. r.p.s.
for D=12 in. or 20 r.p.s. for
D= iO in. Thus attempts to minimize scale effects by increasing the experiment Reynolds'number through increasing model sizeand rotation speed are
severely restricted. Further, higher pressures aredesirable so that the tunnel water can be kept more nearly air-saturated.It
is clearthat, perhaps contrary to
common belief,it
is at least as necessary to he able to
operate water tunnels at high pressures as at sub-atmospheric
pressures.
Test Section Flow Pattern. Measurements have been made of the transverse distribution of velocity across the test section at the screw disc position; the results are shown in Fig. 16. After
RECENT WORK TN THE LITHGOW WATER TUNNEL AT N.P.L. 31 the new cascade bend and diffusers had beenfitted. but with the original closed-throat test section still in place, a velocity traverse was made at the screw disc position with the Pitot-static tube shown in Fig. 13. This tube was inserted through a side plate carrying a seal in a ball-and-socket mounting so that the sensing head could traverse a sector of a circle having the ball as centre. This method could not be used once the new test section outer
shell was fitted. A comb of pressure tubes was then mounted horizontally on a vertical strut through which all the tube heads were brought out: this was inserted through the bottom plate
of the outer shell, and
was traversed vertically through the slotted-wall test section. The corn!) was made as small as possible in order to minimize its interference effect in the test section. in consequence, the tubes were too fine for simultaneous measure-nient of total head and static pressure,so these were determined separately, first using the comb with open-ended impact tubes,then after each tube bad been fitted with a cap having side
holes fcr static pressure response. By relating all the individual nicasurcmcnts to the corresponding values of the contraction p1curc drcp, and,hy using the results of towing tankcalibrations of tile ccml'. the velocity distributions were deduced. Later a
similar cernb was fixed to the end of a tube which could be
substituted for the propeller shaft. This could be rotated to
traverse the screw clise position in both
slotted-wall and built-up " closed-throat test sections in a more satisfactory manner. All these measurements with pressure tubes were necessarily confined to non-cavitating conditions, but it is hoped to extend them to some cavitating conditions by usingthe hot-film anemometer.
It can be seen from Fig. 16 that, for both types of test section, the velocity distributions are reasonably uniform; variations from the mean velocity over the useful central region do not exceed about per cent. The mean velocities over representative central disks have been determined by volume integrations of the measured local velocities within each disk; these can he ex-pressed in terms of the constant c in the relationship VT= \/(h/c), where VT is the mean speed and h is the measured contraction pressure drop. For disks 8, 10 and 12 in. in diam., the constant c was fòund to he O-185 for the closed-throat section and 0-191 for the slotted-wall section, with VT in ft. per sec. and h in iii.
32 RECEN! \VORK IN THE LITHGOW WATER TUNNEL AT N.P.L.
of water. The slightly lower value of the constant for the closed-throat section suggests that there is some cross flow intoand out of the slotted- wall reservoir.
The axial pressure distributions from contraction exit to
diffuser entrance were also measured for the slotted-wall and built-up" closed-throat sections, and, are shown in Fig. 17.
The values shown were obtained from static pressure holes in a test section boundary bar and in thediffuser wall, and also from the long A.R.L. static pressure tube mounted in the test section. As expected, the closed-throat static pressure fallsslightly in the
direction of flow, due to boundary layer growth. However, the slotted-wall measurements show an initial rise above the
static pressure at the test section entrance and then a sharp
drop at the diffuser entrance. This pattern suggests an unwanted secondary flow; water first flóws through the boundary bars
into the reservoir and then flows out at the diffuser entrance. so increasing the overall velocitythere; these results follow those found at A.R.L. with a much larger test section. There is also a further sharp local pressure drop in way of the shaft bearing struts, hut this will be largely eliminated hen the struts are replaced by small section ties.
Flow Fluctuation-s. It is essential that the flow through the test section of a water tunnel should be free from low- or high. frequency fluctuations. The difficulties due to the unsteady flow conditions which. originally occurred in the Lithgow tunnel were fully discussed by Emerson and Berry,1 and one of the main purposes of the circuit changes was to remedy them. The
test section flow is now extremely steady, and, except when 'operating at very low pressures, the low-frequency axial velocity
fluctuations as indicated on the water-speed manometer do
-not exceed ±OE2 per cent. of the meanvelocity. High-frequency flow fluctuations can be described in terms of turbulence levels. but these are difficult to determine in water. However, using the constant temperature hot-film anemometer, a first quantit-ative measurement of axial high-frequency velocity' variation has recently been made. A measurement at a water speed of 6 f.p.s. gave a turbulence valueof +- per cent, of the mean free stream velocity, and suggests a turbulence wavelength of about OE5 in. This level
is, of course, higher than that of a
low-thrbulence wind tunnel, hut it is not unduly so.IO INS DIAM DISK OuTER "-s LIMIT OF \ VELOCITY \TRAVERSE f p CONTFkOL SIDE (b) It f
OUTER LIMIT OF VELOCITY TRAVERSE
O
Fig. 18.Velocity distributors across test section
(a) Closed-ti, lout
(b) Slotted- wall
IO INS cRAM
-. DISK
\
/
08
-o-2
AREAS
CALC. PRESSURES FOR NO LOSSES. MEAS. PRESSURES WITH CLOSED THROAT. MEAS. PRESSURES WITH SLOTTED
WALL. DIFFUSER J_. TEST SECTION LENGTH 328 INS. LENGTH 301HS. DIRECTION OF FLOW
Fig. I .Axiai pressure disirihuijon in les! section and di/n'....
PRESSURE TAPPING
500 400 300
CROSS SECTION AREA SQ. FT.
200 loo
O 6. 0-4
9' V2
34 RECENT WORK IN THE LITHGOW WATER TUNNEL AT N.P.L.
PT ITTyPICAL CURRENT WORK
STANDARD PROCEDURE FOR PROPELLER EXPERIMENTS
The procedure for carrying out experiments with propeller
models in the Lithgow tunnel has recently been revised. A complete set of experiments includes one or more groups of
measurements with the test section pressure near atmospheric.
and further groups at reduced pressures.
Each group of
atmospheric" experiments covers a range of advance co-efficients J and defines a curve similar to that obtained from
open-water towing tank measurements. The screw speed of
rotation n is kept constant and the speed of flow VT varied, while
the- test section pressure is set at the maximum that can be
reached in order to obtain non.cavitating conditions whenever possible. Each group of ' reduced pressure " experiments, on the other hand, covers a range of cavitation index a at a selected
advance coefficient J; the speeds of rotation and of flow arc both kept constant and the test section pressure varied from
atmospheric to the lowest value attainable.
For each individual experiment the tunnel flow speed VT, test section pressure pi' and water temperature O are measured, as well as the model thrust T,. torque Q and speed of rotation n. The effect of any difference between atmospheric pressure and the pressure PT On the measured thrust is also determined as a
pull " T, on the propeller shaft, and the shaft transmission
losses assessed as a bearing torque Qb from an independent cali-bration. These basic data are then converted to non-dimensional form as thrust kT and torque kQ coefficients and efficiency i at corresponding values of the tunnel advance coefficient JT and cavitation index aRT. The effect of the test section boundaries is regarded at present as equivalent to an alteration in the speed of flow through the screw disk, and the direct tunnel results are corrected to equivalent unrestricted open-water values by intro-ducing a tunnel speed correction factor x, defined by = XJT where J0 is the equivalent open-water advance coefficient. This factor x depends on the type of test section, the ratio of screw-disk area to test-section area, and on the screw loading coefficient k,0; it is discussed more fully later. This factor is also used to
RECENT WORK IN THE UTHUOW WAThR TUNNEL AT N.P.L. 35
unrestricted free-stream pressure p0
by the
relationshipP=P opT= j PVT2( lx2). The open-wateì cavitatiuri index R, based on this corrected pressure and resultant
velocity YR of a blade section at fractional radius x, is given by
R=(p0e)/4 PVR2 which can be reduced to either rR=(p,e)/
f pV02' or R(P&f)/fP(flD)2, in which VR2=V02+(xrîrn)2
and where y=1+(xr/J0)2 and
=J+(x7r)2 are convenient
conversion functions. For experiments at fixed speed of rotation n the final form given above for ax is the most convenieat for analysis, particularly as is comparatively insensitive to clìnges in J.Although each group of "reduced pressure" experiments is intended to he at a fixed advance coefficient JT, slight variations from the pre-set fixed values of speed of flow and rotation do sometimes occur, resulting in slight differences from the nominal JT. Each experiment value of kT and kQ is adjusted eithcr to the mean JT for the group, or to a nominal JT, by adding thrust and torque coefficient corrections derived either from preliminary analysis of the results or from previous similar results. A similar, though generally negligibly small, correction is also applicl to ax. The processing and reduction of the experiment data into its final corrected form is now done on high-speed calculating
machines in the Mathematics Division of N.P.L.;
the baie
information is punched directly onto cards and the whole
analysis process, including tabulation of the final coefficients, is then carried out by machine. This system has many obvious advantages, and is capable of considerable extension. It ii also hoped to make use shortly of the newly-developed N.P.L. automatic graph plotting device. The coefficients obtai id inthis way are presented graphically in the form of c.ntour
plottings, based on the carpet " plotting devised for airfoil data by R. F. Sargent of N.P.L. In these a set of curves of kT,
say, on J at different fixed values of aR, as in Fig. lS(a), is
combined with another set of curves of kT on at different fixed values of J. as in Fig. 18(b), to produce what is effectively the three-dimensional diagram Fig. 18(c). In this final contour plotting all the curves have a common ordinate scale for kT, and the scale intervals for J and ax are the same as those in Figs. lS(a) and 18(b). However, the horizontal scales of the componcut
pro-020 T 010 -(Fo ji CONTOUR) c; I I C-4
J
LL (J3)(C) COMBINED CONTOUR PLOTTING
Fig. .18.Contour plotting for propeller experiment results,
(a.) .&- ON J AT FIXED
020
(b)
J2
010 J3
RECENT WORK IN THE LITHGOW WATER TUNNEL AT N.P.L. 37
portional to the intervals between the contour values for each curve. This is achieved very simply. A horizontal scale of J
having the same scale interval as in Fig. 18(a) is laid off in an arbitrary position; each separate curve of Fig. 18(b) is then set
off on a r scale having its original scale interval, hut with its
origin (or any other preferred value) at that point on the arbitrary J scale corresponding to its own fixed value of J. In practice. Fig. 18(c), with equivalent contour plottings of kQ and ,, is set.
off directly from experiment results without any need for
preliminary plotting of Figs. 18(a) and (h), and the separate horizontal scales are not indicated, since they are impliáit in
the contour markings. Two years' experience has shown this form of contour plotting to haste several advantages; it gives a complete and immediate impression of the performance of a screw under a wide range of operating conditions; it emphasizes errors in individual measurements and makes possible logical and justifiable cross fairing; it enables curves for any inter-mediate condition not directly covered by experiment to h
quickly and accurately derived; it facilitates comparison betwee:i
screws.
The cavitation index rp, based on a blade section resultant
velocity, is used as a standard rather than OEA, which is based on
the screw speed of advance alone and takes no account of
propeller diameter or r.p.m. It represents far more precisely the
overall screw bperating conditions and directly specifies th cavitation index of the most heavily loaded blade sections.
Fse of OER involves no loss in generality, since values of J must also be specified and this permits ready cojiversion from one system to the other by the relationship OEA= y OER. The mechanical data reduction process now in use is arranged not only to give the standard values of J0 and OER hut also values of J, OER and
OEA for two values of x, including the uncorrected x= 1.
Colvn'ARIsoN OF PROPELLER DESIGNS
Tunnel tests to assess the relative performance qualities of
alternative proposed propeller designs for particular ships are regularly carried out, often in conjunction with hull resistance
and propulsion experiments in the towing tank. Whenever possible the propeller models made for the propulsion experimeilt
38 RECENT WORK IN THE LITHOOW WATER TUNNEL AT N.P.L.
are used,' but if these are less than 8 in. in diam. new models are made for the tunnel tests. These new models are generally 10 in. (25 cm.) in diam., and are at present made in a tin-based low melting-point casting alloy (E=6-8x 106 lb. per sq. in.). as described in item (11) of the bibliography.
Fig. 1Q(a).Comparison of propeller designs. Standard" design, 4 b1ads.
P/I) l'O, B.A.R. 0'65, 064.
The test conditions are derived from the propulsion and open-water experiment results; these determine the model screw
advance coefficient in open water (J ) and the cavitation number
(az) for each specified ship condition. Force measurements and photographic observations are then macle in the tunnel at each of these values of J0 for values of ranging from above the ship R value to below that at which thrust breakdown occurs.
RECENT WORK IN THE LITHGOW WATER TUNNEL AT N.P.L. 39
The results fòr the alternative proposed designs are then com-pared at equivalent operating conditions.
As at present practised at N.P.L., and, indeed, in almost all water tunnels elsewhere, this apparently straightforward pro-cedure has many shortcomings. The model is tested in a uniform
Fig. 19(b).Compari.on of propeller designs. Special " design, 4 blades,
P/D 095, B.A.R. 050 0645.
stream instead of in the irregular wake stream in which the
ship screw will operate; the tunnel boundaries affect the model performance and, more important, the extent and nature of any cavitation to a largely unknown extent; there are unknown and possibly major scale effects on cavitation inception conditions
and patterns; the relationship between cavitation form and
40 RECENT WORK IN THE LITHGOW WATER TUNNEL AT N.P.L.
ship designs there is also the difficulty that iii the majority of
instances in which cavitation does occur it is only slightly
developed, often showing merely as tip vortices and at blade leading edges at outer radii, and does not affect performance: this makes the selection of a best " design even less conclusive. A typical comparison of this kind is shown in Fig. 19. Continuous
efforts are being made to reduce the gap between model test
conditions and ship operating conditions, but it is still necessary,
for practical purposes, to use correlation factors which aie largely empirical to link model results, particularly propeller
cavitation observations, to full-scale conditions. Two such empirical factors are in current use at N.P.L. The first is intended to allow for the observed differences between propeller r.p.m. as predicted from model propulsion experiments (N) and those measured on ship trials (N). It is defined by N?fl=k1NR, where the factor k1 is largely dependent on wake scale effect present N.P.L. practice, based on recent investigations,12 is to take k1 as 09S. The second factor is intended to allow for some of the
shortcomings of tunnel tests of propellers under cavitating
conditions by carrying out the model experiments at a cavitation number (arn) which is lower than that derived directly from the ship screw operating conditions (a.). Defined by arn=lcia, where the values of a may be of the form a1 or aA, present. N.P.L. practice is to take a tentative value for k2 of 85. This
value was given by Lewis13 as that recommended by the Taylor Model Basbl. while similar but lower values have been used at the Admiralty Experiment Works.14 Before this factor can be given accurately and with precise meaning, it will be necessary to know more about the testing of models in water tunnels and to bave far more ship propeller cavitation data available.
PROPELLERS IN NON-UNIFORM FLOWS
Experiments with propeller models in non-uniform or irregular flows were begun in the Lithgow tunnel about three years ago.
in order to facilitate comparison of the results with theory it was
proposed to carry out these eperiments in flows in which the
radial and circumferential non-uniformities occurred in a regular pattern, rather than In the irregular form common in ship wakes. For this reason controlled non-uniform flows were first produced
O6
07
08
09
10
0030
1Q0020
- 0010
1'30_ J20 FLOW EFFKJENCY FACTOR10 I I I
j
0-80
I I I I I I 0.6 Q7 O'B O9 1.0 1.1 12070
060
050
Fig. 20.Noii- z:iujou:ii flow experiments.
[Fig. 20.
F
I
UMFORM FLOW.
0040
V0LUMETRC MEAN SPEED. THRUST IDENTTY
-SPEED. TORQUE DENI1TY SPEED.
020
010
RECENT WORK IN THE LITHOOW WATER TUNNEL AT N.P.L. 41
by wire gauze screens inserted in the spacer ring immediately upstream of the test section. Narrow vertical wire gauzes were used for the preliminary experiments; these. set up a central
vertical strip or band of relative low axial velocity at the screw
disk, its width being only slightly greater than that of the
gauze and its edges quite sharply defined. When the width of the gauze was half the diameter of the screw model, inner sections of the screw operated entirely within the lower velocity wake"of the gauze, while outer sections operated in this wake when vertical and in the higher velocity free stream when horiiontal. Such a flow field can be considered as an idealized simulation of the operating conditions for the propeller of a single-screw ship. Three of the models tested behind such vertical strip gauzes were those subsequently designated BS i, 2 and 3 in item (11) of the bibliography. They were 10 in. ii diam. and had pitch ratios of approximately unity; a gauze 5 in. in width, of S mesh and 24 S.W.G., placed about 16 in. upstream produced a central
vertical strip about 55 in. in width having a velocity about 23 per cent, lower than the two outer parts of the flow field. Force measurements made in non-cavitating and cavitatin.g
conditions were compared with those previously made in uniform flow conditions. A typical comparison for non-cavitating con-(litions is shown in Fig. 20, in which the measurements made in the non-uniform flow have been analyzed in three different ways. First, the values of J were derived from a calculated volumetric mean speed of flow through the screw disk based on a velocity traverse made with the gauze in place. Next, thrust identity
was taken as a basis by assuming the same k in uniform and non-uniform flows to oecir at the same J: finally, torque identity, in which equal lcQ.vRlues occur at the same value of J, was taken as the basis of comparison.
For all three methods of analysis the screw efficiency was appreciably higher in non-uniform than in uniform flow; the
ratio of the efficiency in non-uniform flow to tht in a uniform field, here termed flow efficiency factor, is similar in nature to relative rotative efficiency. The extent to which this factor
exceeded unity seemed to conflict with previous similar results obtained by Lewis.'3 It was suggested that the gauze affected turbulence levels as well as axial velocities, and the distribution of free stream turbulence across the screw disk was measured
42 RECENT WORK IN THE LITHGOW WATER TUNNEL AT N.P.L.
with a turbulence sphere. The results were unsatisfactory, sd further experiments were deferred
until a better method of
turbulence measurement was available; itis hoped that the
hot-film anemometer will fill this need.The measurements and observations made under cavitating
Fig. 21.Propeller in noii - uniform flow.
conditions suggested effects similar to those found by Lewis:
in particular, the cavitation of a propeller blade element in a
circumferentially non-uniform flow does not correspond to is
mean operating condition but varies with its instantaneous
condition. This is seen in Fig. 21, which shows the different
forms and extent of cavitation at the the blade tips in the
horizontâl and vertical positions. . This effect has several
corn-RECENT WORK IN THE LITHGOW WATER TtJNNEL AT N.P.L. 43
parative tests with screws intended to operate in circumferen-tially varying wakes have to be macle in uniform flows, then it is not the mean operating condition which should be simulated but one in which the advance coefficient J and cavitation index correspond more closely to the most severe operating con-dition. The cavitation correlation factor k2 in the relationship , previously discussed does not properly take account of this point.
Much further work is needed to study the effects of
non-uniform flows on propeller models; the preliminary experiments
outlined here have perhaps helped to emphasize some of the
difficulties in this work. There are many ways in which it can be clone: the recently completed tunnel at the Netherlands Ship Model Basin fittd with a flow regulator provides one valuable method.
PROPELLER SCALE EFFECT STUDTES
The comparison of results from models of different sizes is essential in developing a reliable method of full-scale prediction.
Experiments with a series of geometrically similar propeller
models can provide useful data for this purpose, and two such sets of experiments are being made in the Lithgow tunnel. The
first set is part of the programme of comparative tests being
carried out in propeller water tunnels throughout the world for the I.T.T.C. ;16 the second set is complementary to these and is being made with an N.P.L. geosim series to investigate the extent
to which tunnel test conditions affect the results of routine
experiments such as those previously described. The effects on screw performance, both cavitating and non-cavitating, of model size and of the way in which propeller operating conditions are reproduced are awkward tè study in water tunnel experiments since it is difficult to separate these effects from those due to wall interference.
The first N.P.L. geosim series is a group of 4 screws having diameters of 12, 10. and 6 iii. (approximately 300 mm. to
150 mm.) which are of the type termed BS2 in item (11) of the bibiliography.
In the preliminary experiments the range of
operating conditions, defined by loading and speed coefficients. were simulated at three different absolute values of speeds and44 RECENT WORK TN THE LITHGOW WATER TUNNEL AT N.P.L.
pressures for each model, chosen so that the three sets of tests on each screw covered a range of Reynolds' number R, which overlapped that for the other screws. The force measurements and observations of cavitation inception and patterns,'7 showed significant differences with changes in test conditions and with model size, but there were several rather unsatisfactory features of the results. Some of these were due to instrument troubles which have now been overcome, but other inconsistencies were clearly due to more fundamental reasons; for instance, open-water tests in the towing tank also gave different performance curves for the 4 models and some scatter in the measurements with each screw The results of a concurrent investigation into boundary layer flow on propeller models'8 suggested that
turbulence stimulators should be fitted to these geosim models; this was done, and subsequent open-water experiments gave far more consistent results. In consequence, trip-wire stimulators are now being fitted to many tunnel models which are also tested. without them, and the geosim series will shortly be re-tested in this way.
EXAMINATION OF PROPELLER DESIGN FEATURES
A specific propeller design feature can be examined by carrying out experiments with a group of models in which this feature alone is systematically varied. In spite of the difficulties in re-lating model results from tunnel tests to full-scale behaviour, such comparative experiments can yield useful information on the general effects of changes in design characteristics. Two such investigations have recently been made in the Lithgow tunnel; the first was a study of the effects of blade-section shape, and
the results so far published" included tentative practical
con-clusions with which several leading' designers were in general agreement, and which have subsequently been used in N.P.L. propeller design work. This investigation is now being extended to include further comparable propellers designed by recently developed methods incorporating other flow curvature and lifting surface correction factors. An important feature of this work is the comparison between measured and calculated performance values under non-cavitating conditions, in which agreement is essential before the more difficult problem of calculating
