THE NORWEGIAN SHiP MODEL EXPERIMENT TANK CABLE $KIPSTANK pHONES 28020 SKI PSMOD E LLTA N KE N
PREPRINT OF
The Cavitation Laboratory at Skipsmodelltanken.
General Description.
by J. A. Andersen arid R. Franck-Petersen, Chr. Mchesens Institutt, Norway, L. J. Abeseth, I. Eggestad and J. Vassenden, SkipsmodeHtanken, Norway.
De1
31. MAY2.JUNE 1967
ARCEF
GENERAL DESCRIPTION
L.J. Abelseth, I. Eggestad and J. Vassenden, Skipsmodelltanken, Norway
J.A. Andersen and R. Franck-Petersen, Chr. Michelsens Institutt, Norway
Introduction.
As Skipsmodelltanken resumed working in l96 after the break caused by World War II, it was considered necessary to
supply its laboratories with a cavitation tunnel as soon as possible.
During the war, its facilities had been used for other purposes,
with the result that no progress had been made in the field of
ship research. Several leading ship nations had already had
tunnels working for many years, and these facilities had proved their right for testing ship propellers, and for other
hydrodyna-rnical investigations. As early as in l97 an application was sent
to the Norwegian Technical Research Association (NTNF) for funds
to build a cavitation tunnel of medium size. This application
was not granted, however, and the plans had to be postponed for some years to give way for other urgent instrumentation and modernizing problems of the existing towing tank facilities.
From 1953 on NTNF for many years paid the salary for one or two scientific officers whose assignment was to plan a
cavita-tion laboratory to be built as an extension to the existing tank
laboratory.
This tunnel was originally meant to serve as a model for the larger tunnel, and was also extensively used for studies of flow in the different sections of a cavitation tunnel circuit.
Meanwhile, various aspects of water tunnel testing pro-blems were discussed, for instance at the Seventh International
Conference on Ship Hydrodynamics (the later ITTC) in Scandinavia
in l95'4 (1) and at the Symposium on Cavitation in Hydrodynamics
at National Physical Laboratory, London 1955 (2). New requirements
to such testing facilities were discussed and new tunnel designs were presented at these meetings which led to some reestimation
of our own tunnel plans regarding both the tunnel circuit design
and size. Another period of studying cavitation testing techniques
started, and the first realistic plans were presented in February
1957. During this period the small tunnel was enlarged as much as
possible and supplied with instrumentation for small scale
cavi-tation tests. This enabled our staff to serve our customers to a
certain extent, and it also gave valuable experience in tunnel working technique.
The small tunnel was financed mainly by private donors. A Planning Committee was appointed in 1958, and a pre-liminary project based on the mentioned "1957-layout" was in 1962 handed over to the Building Committee who has directed the erec-tion work on both the large tunnel and the laboratory building
from the summer 1963 to the official opening. The members of
these committees, the consultants and the main contractors are listed at the end of the paper, together with a short financial statement.
The reader will have noticed the long time span from the date of the presentation of the preliminary plans to the date
of the completion. The delay has mainly been due to the particular
financing form used for this laboratory, as one third of the
building sum had "a priori" to be collected from the shipping trade and the shipbuilding industry, unfortunately in years which were not the best ones for these branches.
The contents of the laboratory.
about 7800 cu. m and a floor area of about 1700 sq. m including drafting and administration offices, and is connected to the Tank building through a bridge containing an enlarged propeller workshop and a canteen.
A general view of the No. II Tunnel is presented at the end of the paper (Fig. 36).
Fig. 1. The Cavitation Laboratory, viewed from North.
The Cavitation Laboratory comprises the No. I Tunnel, the
No. II Tunnel and a laboratory for miscellanous tests equipped with apparatus for accelerated cavitation testing, facilities for water analyses etc.
The No. I Tunnel, shown in Fig. 2 is still in the Tank
building. It will, however, be moved to the new laboratory and
will probably then appear somewhat like shown on Fig. 3 equipped
with a "combined resorber and an air separator", which is more or
less a design idea which will not be discussed in this paper. The
No. I Tunnel was presented at the Zagreb Symposium in 1959 (3)
The outer shell of the open jet test section is seen in the center of the picture.
The No. II Tunnel. Size and water velocity requirements.
The description of the new large tunnel plant is the main topic of this paper.
The choice of a tunnel size which should satisfy both scientific and commercial requirements, and at the same time be within a possible economical frame was naturally an object of
yearlong consideration. The plans for the laboratory were based
on a ceiling of .,9 million kroner ( 25.000).
From a hydrodynamic point of view the final requirements can be summed up in the following few statements.
i. Propellers in ahead condition, like the open water tests should if possible be tested at a Reynolds number
.lo6 based on the relative velocity and the chord
length at 0,7 R.
Propeller tests in this condition should presumably cover a range similar to the well known design charts of the Troost series.
0 0 Ahead dyncimometer I5HP. Nr3000RPM 5 r
GENERAL VIEW OF NO.1 TUNNEL (KTID)
WITH RESOR BER/SEPARATOR.
4500
Air bobble release and atOmization system
wI4fl,ArAVAv$fl,gJ.2,jrgJfl Aiifl AYfflL
Plan view of impeller drive 65HP N800RPM a
'ii
--880 30008)0j
L______rry
\///
//":'/Fig. 3. A draught of the planned reconstruction of the No. I
Tunnel in the cavitation laboratory building.
0U, U, ometer 00 RPM ,Sehind'dyna 12 HP, N
Propellers in behind condition should be tested at a Rn of at least 2,5l0, on the same basis as above.
Tests behind both aftership (dummy models), and behind geosim hull models under the influence of a free surface, should be considered.
Hydrofoils should be tested, both two- and threedimen-sionally, without any comprehensive installations or mounting work of tunnel sections.
It should be aimed at Rn of about
1i06
based onchord length.
The resulting model sizes of propellers when the above mentioned requirements are taken into account, should be
used for measuring cavitation noise as a basis for full scale noise estimation.
This point, however, was not intended to be decisive for the choice of the tunnel size, as too little was till then known about correlation between noise from model propellers and full scale cavitation noise.
Regarding propeller tests in ahead condition the scale effect considerations reported at the Seventh International Con-ference on Ship Hydrodynamics (i) recommend the Reynolds number
figure of 4.106, to avoid scale effects on both thrust and torque
measurements.
Thus to be able to predict correct power from tests in
the 'tunnel, this basic Rn 106 was used even though a large
turbulence level in the tunnel flow was likely to reduce this suggested critical number by a certain amount.
To apply trip wires at the propeller model surface to assure turbulent flow over the whole propeller radius can obviously be used with success at open water tank tests, but to apply wires at cavitating conditions was supposed to be greatly misleading. Further investigations on this project should be done in a tunnel
Fig. 'I. Nomogram showing the Reynolds number dependence on
diameter (D), speed of revolution (n), speed of advance coefficient (J) and chord length() at
x 0,7 and x z 0,2. 20 -15 5.0
(*)D2
A4á
Ad! i -- WAiM
- - - -
-__I WA VA g
2.0rariw.
____
I 0.2 0.1-1,V
012 015 070 025 0.30 D(m) I I 01.0 0.50 0.601 03 05()Dn
10 .21 30 D VR= n x V= D REYNOLDS NUMBERFOR MODEL PROPELLERS
IjR
V .io6 0.15 -0.2\m
R\
-\
I
EXAMPLE = PROPELLER RELATIVE = ROTATIONAL CHORD SPEED KINEMATIC 1.01 1O : 0.50m DIAMETER WATER VELOCITY SPEED.. LENGTH AT OF ADVANCE VISCOSITY sq.mVsec AT (m) RPS XR COEFFICIENT 20°C CHART Ifl
m
}
-0.4-j::
TD
--'-U
-1.5 -2.0 --3.0 0.27 n = 26 RPSJ0.6
R4.106
°-4.0
---5.0 -7.0which is large enough to attain Reynolds numbers up to the order of
The nomogram Fig. shows the relation between propeller
model diameter D in m, n in revolutions per sec., speed of advance coefficient J, and Reynolds number for a range of chord/diameter
1 ()-ratio from 0,16 to 0,60.
lv
R x R n 1 X)n 2 + (xrr R nwhere the cinematic viscosity of water v i3Oi.i06 sq. m/sec
for water of temperature t z 20°C. This temperature choice is
discussed on page 33.
For merchant ship propellers an average J-value can be,
1
say, J 0,6. Assuming (_ 0,27 to be a representative lower
limit for those types of propellers, we can easily find the
corre-sponding values of D and n for the required R io6. D 0,5 m,
corresponds to n 26 RPS, D 0,6 m corresponds to n 18 RPS
and so forth.
In a similar way, for typical propellers for the navy,
17
was assumed to have values lower than 0,50 infrequently and the corresponding D and n for those propellers can thus be found
by following the respective dotted chord/diameter-ratio in Fig. '+.
1
When referred to °' , Fig. shows that the J-value has
considerably less influence on the Rn_number than the speed of
rotation. This means that to attain the highest possible Rn over
the J_range, the propeller tests should be carried out mostly with constant n, while the tunnel water velocity should be varied. This was also
adopted as a standard testing procedure at our small tunnel, and is, as far as we know, practiced by the majority of water tunnel operators.
Based on this testing procedure, the desired water velocity was discussed.
As stated above
Ci)
the operating range of the tunnelshould be such as to cover the B_6 diagrams for conventional propeller design.
Fig. 5 chart I shows approximate K - and J . -values
Q max iran
deduced from B 200 and the corresponding 6-values. For
practical reasons, the 6-values are converted to the speed of
advance coefficient J - , where v, n and D are in metric
units. Thus
J 101,3
4
where N RPM, D ft. and V knots.
V Similarly we find KQ
k.B2J5
, where k0,9l6l0.
Hence niin 101,3 max K 0,916.l0_3.(200)2 5 Q max mmAlong the abscissa also the optimal pitch ratio
P
is indicated for the corresponding Br-values (but with no
6 reduction). Jmax_vaes are on an average put equal to
P
1,1 . The approximate Br-ranges for different types of
propellers are also indicated. Thus chart I can be used for a
quick estimation of the testing range for a given propeller design.
As a basis for the determination of the necessary maxi-mum test section water velocity, a
anticipated for merchant ship propellers. A water velocity equal
to v 18 rn/sec. is thus necessary to test such propellers at
n 50 RPS at R
410.
Sirnilarily, chart II and III of Fig. 5show that this velocity is sufficient to test navy propellers up
P
to D 0,27 m with pitch ratio - 1,3 at a constant n of 50 RPS.
Furthermore, it was suggested to be of importance to attain the stated R -value for a certain range of diameters for
n
the different types of propellers. Chart III shows that this is
fulfilled for diameters D 0,36 and D 0,27 for merchant and
navy ship propellers respectively, as the rotational speed range of the large propeller motor goes up to 3000 RPM, see page 42.
D 0,55 rn was chosen as the maximum diameter for model propellers.
This is 50 % bigger than the minimum merchant ship model propeller diameter that fulfills the Rn_requirement.
Summing up, the various evaluated data from the stated R_va1ue for different types of propellers require a test section
diameter above 1,1 m, ref. (i, s), and a water velocity up to
about v 18 m/sec. As a tunnel with a test section diameter
D0 z 1 2 rn was found to be within our economical means this
dia-meter was principally chosen as the basic dimension for the tunnel
loop design. As discussed later, the inlet nozzle diameter is 3,0 m
corresponding to a contraction ratio of 6,25/1.
It was most interesting to learn that the large
NFL-Tunnel test section diameter was determined to nearly an equivalent
6
size based largely on R
210
for the root sections (x 0,2-0,3)of a representative merchant ship propeller series (6). For
com-parison this R -value for x 0,2 and the corresponding J-values
are indicated on Fig. 4, chart III.
Propeller tests in the behind condition should presumably
5
be carried out for Reynolds numbers not less than 2,510 as stated
rF C) 1j PJticD 1 0 CD '-J
(DO
C) pi o H 11 U) < U) pJI-Qpi
CD H' CD U)tt
CD H) 'ii CD CD0O
o pi ()1O
CD ç CD pi 'ii (DC) a c+i
U) C)OHr-f H
:i U) -OQr
CD I pii
pi I .cD H rf H a 0.12a
010_
--
_--
______--_
T--- J.
'
'08 I)?,_ U
I01__
VA
0.31__
V
021_U__
--0.151__U______
Bp iI III
0.12 nD 1.2 11 1.0 0.9 0.8 0.7 0.65 0.6 0.55 0.28 0.26 0.24 0.22 0.?0 fl1 ESTIMATION ON TEST I I tHIGH SPEEDi I I '.15 SECTION SPEED v ecBOATS (NAVY ETC.)
I I
PASEN(ER LINFRSi
APPROXIMATE
Jmax - VALUES CAN
FAST MERCH SHIPS
LAR1
TANJcFUG
a
BE TAKEN FROM CHART I FOR
BOAST
A PROPELLER WITH A GIVEN BpVALUE OR A GIVEN
P0.7,/iJ
- RATIO.
'3,
EXAMPLE Bp=19,AS AN AVERAGE VALUE FOR
MERCH. SHIP PROP
} CHART I
Jmax
1.0
D036m (R4106 AT n=50 RPs)
0.27)
v18m,Sec (PROVIDED TEST PROCEDURE n=CONST.)
LI
Bp = 90 (FOR PROPELLER WITH LOW PITCH RATIOS) (KT/K0 )max
10.5, FROM CHART
i
16'HPi
pi I X CD hjoa
H) C) pi a :j Pi H piH)ti
(DC)ai
0. CD sPl aH)'l
i ;<Hr1
H CD O 0t Pi H < rF HPJ CD CD HiQi
rF a CC) (1J (I) 'd :i P11+i
I a CD Hto be the lowest permissibe Rn_limit to avoid 'scale effects on propeller efficiency', taking to some extent account of the in-fluence of the more turbulent flow in a tunnel than in a tank.
Various types of wake simulating devices were discussed, such as grids, large number of nozzles (7), or devices to produce
radially symmetric wake distribution. All these principles were
more or less found to be unsatisfactory, greatly due to the lack of circumferential flow vector, and the lack of an induced velocity rrrror effect on the hull surface from the propeller in an
aper-ture. Dummy aftership models and geosim hull models were
there-fore preferred to simulate full scale wake, as mentioned above (ii). Wake simulation by use of grids or similar devices can certainly be applied, for instance in conjunction with inclined shaft tests to approach the in-flow condition to the propeller of large tankers and other hulls with large block coefficients.
For testing propellers behind dummy aftership models in the closed throat test section, arrangements like the ones shown
in Fig. 6 were suggested. The respective propeller diameters for
all those ship types yield Reynolds numbers well above 2,5lO. This is obtained even in flow velocities according to Froudes number, but considerably higher velocities should be used provided no separation or other changes in the wake flow character appeared
because of too large model/test section - area ratio. Thus the
dummy model sizes shown in Fig. 6 are considered to be definitely
the largest ones to be applied. The afterships shown are all cut
down to about 0,6 m width and will normally be fitted with meshes or other roughness producing devices to simulate friction drag alongside the hull.
Two alternative propeller driving systems for the dummy models are shown in Fig. 7.
PROPELLERDIA.=O24M (95")
a) FAST CARGO AFTERSHIP
c) PATROL BOAT AFTERSHIP
Fig. 6. Afterbodies of different types of ships, and the
corresponding model propellers, compared with test section size.
The desired propeller cavitation tests behind geosim hull models and the appurtenant free surface channel was an object of
comprehensive and longlasting considerations. Various channel
designs were discussed and one particular was model tested, but
with no great success. The most intricate problem concerns
ob-viously the down stream sections. These are the same for the
tunnel and for the channel. To avoid air suction into the reentry
proved to be an extremely difficult problem. Though the problems
are by far overcome, this project is not given up yet.
The plan was to replace the closed throat test section by a free surface variable pressure channel of sufficient size to make possible propeller tests influenced by the free surface at
R not less than 2,5lO.
A channel 2 in wide was anticipated as a maximum dimension
with respect to the upstream tunnel tube diameter of 3 m. Although
there is found no definite restriction to the channel width in relation to the breadth of the model (8), it was decided that the
channel width should normally be three times the model breadth. With a chosen moderate channel water speed well below the
theore-tical critheore-tical speed where H channel depth, this suggested
width should prevent the bow wave from being reflected from the
side walls on to the model stern, Fig. 8. For a number of ships
within our tank testing program investigations were conducted to establish the necessary model size which would make the propellers
fill the minimum R requirement of 2,5lO.
From the listed ships and the respective size of models in table 1, it can be seen that a number of actual ships could be
tested in the projected channel. Even large car ferries with a
LIE-ratio equal to 3,7 require a channel of about the size
dis-cussed. Large tanker tests, however, can hardly be carried out
without reducing the model size unduly. These ships would
con-sequently require a channel that would not be possible to combine with the projected tunnel size.
Channel water velocity and the necessary depth was
suggest-ed from table 1. For all the actual model sizes the water velocity
GEAR
D.0 MOTOR 4
CABLE TUBE -.&....r
b.
Fig. 7. Two arrangements of propellers behind dummy models
in the closed throat test section.
a) Propeller driven by the large dynamometer outside the tunnel.
b) Propeller driven by a submerged dynamometer,
remotedly controlled through a water tight cable tube.
- THRUST AND TORQUETRANSDUCERS
Model size and water speed for different types of ships, based
on Froude number similarity,
and Reynolds number Rn
1Q7
VR
- 2,5l0.
V
Model length (L), breadth (B), draught (d) and diameter (D) in meters (m). Model speed v in rn/sec.
Propeller model speed n in RPS.
Wake speed Va in rn/sec.
SHIP Scale A MODEL Type of ship L/B B/d V/'T D N L B d D fl WT Va Large tanker (90000 tdw) 250 6,28 2,74 0,570 6,94 114 31 8,10 1,29 0,470 0,224 10,6 0,50 1,36 2,65
Tankers of med. size (52000 tdw)
216 6,80 2,74 0,687 6,70 110
36
6,00 0,88
0,321 0,184 11,0 0,45 1,25 2,26
Fast cargo ships (6500 tdw)
130 7,23 2,95 1,090 4,70 150 32 4,06 0,562 0,190 0,149 14,1 0,24 1,55 2,03 Passenger liners (16000 tdw) 168 7,00 2,96 0,928 4,80 150 31 5,53 0,788 0,265 0,155 14,0 0,16 1,70 2,03
Large car ferries (6000 tdw)
140 7,00 3,71 1,020 3,80 186 29 4,83 0,690 0,186 0,131 17,0 0,12 1,83 2,10 Trawlers (2700 tdw) 70 5,32 2,51 0,920 3,05 250 26 2,72 0,512 0,204 0,114 21,0 0,36 0,91 1,41 Fishing boats (115') 35 4,50 2,25 1,750 2,20 210 15 2,33 0,518 0,230 0,147 13,6 0,23 1,16 1,52
ref. (9, 10) and (ii), also referred to in (8), shallow water
effects occur when v 0,7/gH. The depth was thus pessimistically
estimated such that the velocity should not exceed 80 % of this critical velocity, which means that the channel depth should be
H 1,8 m (63").
A sketch of the projected channel design is shown in
Fig. 9. A device to increase the velocity above the critical one,
ref. (12) , is also indicated.
The impeller power estimation for the channel is dis-cussed on page 19.
Regarding item (iii), it should be possible to carry out hydrofoil tests both in the closed throat test section and in the
free surface channel. In the first case, two and three dimensional
tests on Rn .l06 should be possible based on a chord length of
0,25 m. In the free surface channel, R
iio
is possible fora hydrofoil chord length of 0,5 rn if the water velocity can be increased to two times the critical velocity referred to the
1,6 m depth, i.e. v 8 rn/sec., as hydrofoils are not necessarily
to be tested at subcritical velocities.
Fig. 8. Bow wave propagation and reflection from the walls
in the projected free surface channel.
Conclusive remarks regarding the chosen tunnel size should thus be the following:
A closed throat test section diameter equal to 1,2 m de-termined from ahead test condition requirements should coincide well with the necessary size for behind test condition with re-spect to both dummy models in the closed throat test section and
geosim model tests on a free surface channel 2 rn wide.
Determination of installed impeller and propeller motor power. The tunnel loop design discussed in a later chapter
(page 23) led to a calculated power loss factor 0,25. The
power loss factor is defined as
E. h 1 2 v E 0 0 2g
where h head loss, m (water column)
v0 water velocity in test section, rn/sec.
E. net input energy at the impeller, kpm.
E0 velocity energy in test section, kpm.
Thus impeller power on output shaft is
E.
pVv02
1 0 Ps = 11mrn 75 1] fl 75pm
m 0.53+ Sp
mwhere hydraulic efficiency of impeller,
= mechanical (friction) efficiency,
p = = density of fresh water,
D test section diameter, m,
0
1 HP 75 kpm,
- 0,53k. 75
Putting ii 0,80 p n 0 95 m D 1,2 m 0 v 18 rn/sec. 0
0,25, the impeller motor power is
PS 1500 HP.
The installed motor power was then chosen to be 1700 HP (1250 1KW). The power loss factor for the free surface channel de-signed as indicated on Fig. 9, is assumed to be of the order of
1,3. With the installed power of 1700 HP, velocities up to
7 - 8 rn/sec. should be obtained at a depth of 1,6 m, or 11 - 12
rn/sec. at a depth of 0,8 in.
WAVE DEMPING DEVICE
6M
HATCH
INLET CHAMBER FILLED WITH WORTEX DEMPING DEVICE
PRESSURE CONTROL
SYSTEM DEVICE AIR OUT
PLAN
Fig. 9. A draught of the projected free surface channel.
So far only the large propeller drive for ahead
con-dition tests has been built. However, smaller dynamometers for
To determine torque and thrust transducer ranges for the dynamometers, the same basic considerations as were used to eva-luate the nomogram Fig. 5, were applied.
Delivered power at the propeller is
Q.2IT.n
a KQflD
(HP)
75 75
which is presented in a nomogram Fig. 10. An average K0 max_value
for 'heavyt' merchant ship propeller is, say KQ 0,06, see also
Fig. 5. To test such a propeller of diameter D 0,55 in, a n-value
6
of 22 is required to reach R l0 . From chart I and II we find
n
Q 150 kpm, and from chart III the corresponding D 275 HP.
Within the same torque and power ranges even propellers up to
D 0,60 in, should be tested, although at a lower KQ. Such a case
is shown with another dotted line in the chart I and II, D 0,60 m,
n 25 RPS and KQ
max = 0,03. The power D 300 HP is read from
chart III.
From these considerations a d.c. driving unit for the
largest dynamometer was chosen to give Q 150 kpm at N 1500 RPM,
Q = 70 kpm at N = 3000 RPM. The three torque transducer ranges
for this unit are indicated in chart II on Fig. 10.
The maximum thrust to be measured, corresponding to
torque Q 150 kpm, was derived from the relationship
T =
max Q max K
The (vi) -ratio is also to a certain degree depending
Qmax
-upon the B -value or the pitch ratio. This relationship is shown
1<
in Fig. 5, chart IV. The largest () " 10,5 occurs for propellers
Q
with low 1< -value. For the largest propeller models we thus
Qmax
-get
T 10,5 . 150 2630 kp.
max 0,6
The load cells applied as thrust transducers for the maximum range
4
3
2
1
0
Fig. 10. Nomograrn for torque Q(kpm) on propeller shaft and
horsepower
D depending on KQ n(RPS) and propeller
diameter D (m).
-I__
.4111
IPWA
Ill
__
IIUVA
N
II
ifl
IVANSD
CERRANGE
IrRI_cOO
-ii_
P PR1:
ii
'I
, ,,,D(m),,
,,I,
SQ(kprnS D = n = Q = = = = D = n= Kç Q= PROPELLER SHAFT HP 10 HP HP 100 200 SHAFT10iu
II
PROPELLER DIAMETER ROTATIONAL SPEED TORQUE ON PROPELLER KQn2D5 kpm DELIVERED HORSEPOWERQ2fln
= 2iTQ KQn3D5(m)
, RPS , WATERI
II
ill
II
EXAMPLE 75 75 DENSITY OF FRESHf102 kpsec2/m
: O55 m } CHART 22 RPS 0O6}
., 150 kpm 275 HPFurther description of the instrumentation for torque and thrust measurement for this large dynamometer is given on
page 5.
A medium size dynamometer for testing high speed
pro-pellers in behind condition, is now being designed. This
dyna-mometer will be placed inside the stilling length section (section
upstream of the nozzle). The torque and thrust transducer ranges
are assumed to be 15 kpm and 600 kp respectively. The motor rating
is 120 HP at 6000 RPM. A hydraulic motor will be applied, due to
its minimum space requirement, Fig. 11.
The laboratory instrumentation will also soon be supplied
with smaller dynamometers like the one shown in Fig. 7 b), for use
in dummy models, for inclined shafts and other similar conditions. These dynamometers will be designed for use also in tank tests for
comparative tank/tunnel propeller tests to eliminate errors due
to different measuring devices.
BEHIND DYNAMOMETER 120 HP HYDR. MOTOR 3 STRUTS WITH OIL SUPPLY AND CABLES
I
IOTOR UNIT GEAR UNIT 2000/6000 RPM T AND 0 TRANS. UNITII
Fig. 11. The large behind dynamometer (120 HP/6000 RPM) to
be placed in the stilling length section upstream of the nozzle.
Strength of model propellers.
The tensile strength of the applied material in the model propellers may introduce restrictions regarding test range, and thus the obtainable R.
The stress formula used to determine full scale pro-peller blade thickness for both fixed pitch (FF) propro-pellers and
controllable pitch (CP) propellers from ref. (13) can be rewritten
as t
xJ
C nfl -D Q max x)2AK
where t blade-thickness at xR xA, B and C are constants depending on material quality, Fx/D-ratio and chord/diameter-ratio etc.
To indicate stress limits for model propellers, nfl-values
are shown in Fig. 5, chart III for one FP propeller, and one CF
propeller both of conventional design, made of high quality bronze
material (NI-CU-AL). White bronze with s 7,0 kp/sq. mm is normally
too weak for model propellers to be tested at high Reynolds numbers.
As stated in ref. (6) and (ilI.) the hydroelastic effects
should also be accounted for. Keeping in mind that the condition
for hydroelastic similarity is that the product nfl must be constant for the same material used for model and prototype, this condition is also satisfied when the model is tested at the nD-line valid for the material concerned.
Tunnel pressure.
The pressure range for the tunnel was determined from
a cavitation number consideration. The lower pressure limit was
chosen as low as possible, to facilitate the testing of
pro-pellers at low cavitation numbers and at moderate water velocities. This means that the lowest obtainable pressure is about 95 %
The upper limit was decided to be 6 kp/sq. cm although
a pressure of kg/sq. cm abs. was assumed to be sufficient in
most cases. The price of the tunnel still was, however, found
to be only little influenced by this pressure increase.
General remarks on the tunnel ioop design.
The nozzle contraction ratio 6,25:1 was determined from theoretical calculations, and was found satisfactory also by tests
in the No. I tunnel. The measured velocity field in No. II tunnel
test section is constant within % of the central core velocity
across 95 % of the radius, Fig. 12.
The longitudinal pressure distribution from the nozzle
outlet into the first part of the diffuser is shown in the same
figure. A general view of the tunnel loop is shown in Fig. 36.
The test section diameter was chosen constant as a tapered
test section to obtain constant longitudinal pressure distribution
had less interest for the tunnel test program outlined previously. A transition section leads to a 7° diffuser which in-creases the diameter from 1,2 m to 1,7 m ahead of the first bend. Thus the mean velocity is decreased to 50 % of test section
velo-city before entering the bend. A stilling length 1,5 times the
diameter was necessary to reduce the core velocity at the diffuser
outlet for reasons of cavitation at the vanes. Based on a suggested
minimum cavitation index a 0,05 in the test section, the
cavi-tation number at the bend entrance is about 0,7, which is found
to be satisfactory for a 5° bend. The vanes are profiled (NACA
016-006) and are mounted at increasing spacing according to ref. (15).
The 145° bend also brings the length of the propeller
shaft down to a minimum and shortens the building length of the whole tunnel plant.
The diffuser is continued after the first bend and
in-creases the tunnel diameter to 2,1 m ahead of the second bend.
0.7 0.6 7j0.5 0.4 03 0.2 01 -0.1
LONGITUDINAL WALL PRESSURE DISTRIBUTION
y00.9l0
TRANSVERSAL VELOCITY TRANSITION
DISTRIBUTION 83.a03o6 .
82
Fig. 12. Nozzle contour, longitudinal pressure distribution
and transversal velocity distribution.
pressure (5 m beneath the test section center line) justified the
choice of a 9Q0 bend with unprofiled curved plates as cascade
vanes. The vanes are rounded at the leading edge, and tapered at
the trailing edge.
The cross section of the tunnel is still increased to
2,1+ m in diameter before the impeller bend. A conventional 45°
circular bend was chosen on the contractor's recommendation. 3.5.
DIFFUSER
After a short 8° diffuser the diameter is kept constant
ahead of the contraction will to some extent serve as stilling
length. Disturbances from the cascade vaned bends and large
scale turbulence is reduced sufficiently in the flow straightener
ahead of the nozzle. The straightener consists of square tubes
50 by 50 inni and 0,5 m long, built up of 0,2 mm thick brass plates.
The power loss factor was calculated to be about c 0,25
without propelle- shaft and struts (empty tunnel). Tests have
shown that this calculation was a little too pessimistic. The installation of a reabsorber was discussed at an
early stage. A reabsorber of the Cal. Tech, type would require
excavation of about 000 cu. m rock at an estimated cost of about
250 - 300.000 kroner ( 12.5 - 15.000). In addition to this, added
steel weight and increased mounting costs would bring the total
expenditure up to a much higher amount. Due to these economical
considerations this project was dropped.
The small tunnel, on the other hand, will be equipped
with a reabsorber as mentioned in the introduction. It is
assumed that, by using the same water in both tunnels, it should be possible to introduce corrections to allow for differences in measurements caused by differences in air content.
The outlet part of the nozzle, the test section and the
first diffuser is made of stainless steel, and so are also the
cascade vanes in the first bend. The rest of the tunnel shell is
made of mild steel and coated with a bitumen-epoxy resin. One
area around the impeller is coated with metallic zinc to protect the mild steel against corrosive attack because of the bronze in the impeller blades.
Supporting structure.
The load condition on the upper horizontal limb and the
load transfer to the supporting structure and the tunnel shell
are shown schematically in Fig. 13. The supporting beams are
also dimensioned to carry the heavier weight of the projected free surface channel, indicated by dotted lines.
PROPELLER
MOTOR WEIGHT 2 TONS
LOAD DISTRIBUTION ON UPPER LIMB
INDICATED WEIGHT OF CHANNEL
ilhiin.n...ugl
-/
j
7 TONS PER M. FIXED POINTFig. 13. Load condition on supporting structure and tunnel
shell shown shematically.
The lower limb is partly cast in concrete and is thus
completely founded on the bedrock. The impeller bend is cast in
a separate concrete fundament also carefully bolted to the bedrock. The upper part of the tunnel shell is stiffened transversally
without touching the building construction, Fig. 14.
Fig. Supporting steel structure viewed from beneath
The impeller system.
The function of the impeller is to circulate the water
in the tunnel at velocities which can be chosen at will. The
velocity must remain constant at each chosen value and show no
variations. As shown earlier the maximum velocity is chosen to
be 18 rn/sec.
The impeller head is flow resistance only and is com-posed of head losses caused by friction, and losses in bends and
transitions between the different tunnel cross sections. A
cal-culation gave a resultant head of ,l rn water column at 18 rn/sec.,
or a power loss factor 0,25 for "empty" tunnel. This
calcu-lation was partly based on loss coefficients found from tests in the small cavitation tunnel.
The impeller design was based on a power loss factor of 0,305 and for optimal conditions at 17 rn/sec. due to the variety of tests both in ahead, behind and free surface test conditions.
This gave a head of '-1,5 m. With the chosen test section diameter
1,2 rn the capacity of the impeller becomes 19,2 cu. rn/sec.
For calculation of the impellers Net Positive Suction Head a tunnel height of 10 m between center lines and a minimum
cavitation number of 0,05 in the test section was anticipated.
Based on a forced vortex circulation distribution the following data for the impeller were found:
Speed 200 RPM
Specific speed 280
Outer diameter D 2200 rnni
Boss diameter 770 rp.rn
Number of blades 3
Blade area ratio
Pitch P1 0/D 1,08
Thrust 17000 kp
Flywheel moment 3120 kp . sq. m.
For reasons mentioned above and from economical
powered by a synchronous motor. To gain experience with this kind of impeller system, model testE in the scale 1:12 were
con-ducted in the small cavitation tunnel. During these tests flow
conditions, cavitation conditions and pump characteristics were investigated at different values of the pitch corresponding to definite values of the head and water velocities from zero to
maximum. The results showed that the choice of system was
justi-fiable.
A drawback with this kind of system is the considerable
power requirement at zero pitch. At this position the impeller
also generates some noise.
Satisfactory conditions at low velocities were achieved by employing an auxiliary motor with a speed of about 1/3 of the speed of the synchronous motor.
The pump housing forms part of the lower horizontal part
of the tunnel (Fig. 15). The inlet to the impeller consists of
the third bend of the tunnel followed by a contraction immediately
in front of the impeller. The bend is circular with a middle
radius of 1,6 times the tube diameter.
The impeller is supported by struts in the conical
part which is cast in a free concrete fundament together with the bend.
The impeller housing proper, which is machined inside,
is a straight tube. This is also the case for the adjoining
guide vane section. At its outlet, the guide vane section joins
a diffuser by means of a stuffing box which facilitates the re-moval of the section.
The impeller blades are made of AC-CU-NI bronze and are machined at the tip in such a way that a slot of 3 mm is formed
along the entire profile length at zero pitch. At positive or
negative pitch this slot will increase to both sides of the
turning axis. This is a drawback which could have been avoided
by employing a sphere-shaped impeller section. However, due to
The impeller boss is of the same design as a conventional
propeller boss for a 3 blade propeller. It is made of stainless
steel except for a few minor parts which are made of bronze. A
paraboloid-shaped dome made of reinforced polyester resin serves as an even termination of the impeller boss in the direction of the flow.
All sliding surfaces in the boss are Teflon-lined, and the inside of the boss is in direct contact with the tunnel water. The stainless steel pushrod for the pitch adjustment mechanism passes through a stuffing box in the shaft flange to prevent the water from entering the inside of the shaft. (Fig. 16)
Fig. 16. Sliding surfaces of Teflon in the impeller boss.
The impeller shaft has an outer diameter of 250 mm and
an inside bore of 80 mm diameter. The shaft is made of carbon
steel, and bronze sleeves for stuffing box and bearings are
shrunk on. A coat of reinforced polyester resin, which is placed
between the sleeves acts as protection agains corrosion.
lubricated rubber bearing mounted in a sturdy boss. The impeller
boss is supplied with radial pump vanes. These run in the space
between the impeller boss and the boss for the bearing and supply
the necessary cooling and lubrication. The outside bearing is a
conventional oil-lubricated sleeve bearing, mounted on a stand which is welded to the tunnel wall.
The stuffing box of the impeller shaft contains a carbon
seal which is dimensioned to withstand a water pressure of 6 kp/sq. cm. The leakage water is collected in a container and pumped back to
the tunnel by means of a level-operated dosing-pump.
The thrust bearing of the impeller is built into the reduction gear which is located between the synchronous motor and
the impeller shaft, see page 39. This bearing is dimensioned for
a positive thrust of 20000 kp and a negative thrust of 2000 kp. The hydraulic servo motor is mounted on the opposite side of the gear (Fig. 17), and the pushrod runs through the gear
shaft. The servo motor, which pneumatically controls the impeller
blade pitch and thereby the water velocity in the tunnel, is re-motely operated from the control room via en electropneumatic converter.
Fig. 17. Hydraulic equipment for control of impeller pitch
The propeller shaft system.
The 11,93 m long propeller shaft is surrounded by a
sleeve from the first bend of the tunnel and to a point 3O nun
from the propeller. The sleeve is supported in three places by
profiled struts. The supports are made such that centering is
possible. Where the shaft passes through the tunnel wall at the
first bend the sleeve is part of a welded structure together with the guide vanes of the bend.
The shaft and the sleeve are divided into several sections
to facilitate mounting. The sections are connected together by
means of hydraulic shrink couplings. The part of the shaft which
has the built-in transducer for torque measurements is flanged to the rest of the shaft.
The propeller shaft, which has a diameter of 55 mm, runs
in water lubricated sleeve bearings with Teflon coating. The
bearing sleeves are forced into the shaft sleeve and divided along the length such that the distance between bearings does not exceed
1,36 m. Where it leaves the tunnel the shaft is centered by means
of a roller bearing before it enters the thrust-measuring unit. The lubricating water is taken from the tunnel and is
pumped into the shaft sleeve at a pressure of 5,0 kp/sq. m, and
such that it passes through a series connection of all the bearing
clearances and leaves the shaft system in the test section. The
lubricating water also prevents the suction of air when the pressure
inside the tunnel is lower than the atmospheric pressure.
The shaft has a central bore for the cable which runs
from the slipring unit outside the tunnel to the torque transducer
in the test section.
A numerical analysis of the natural frequency of the
shaft/tube system has been performed. In this connection it has
been considered very important that the end of the shaft with the
mounted propeller is not allowed to whirl. The calculations are
Auxiliary eguipment.
When the tunnel is filled from the city piping system the water passes through a conventional sand filter (Fig. 18).
The capacity is 30 Cu. rn/h, which gives a filling-up time of
about 9 hours from empty tunnel.
It is also possible to change over to continuous filtering
of the tunnel water. In this case the water is circulated by means
of a centrifugal pump supervised from the control room.
The filter mass is rinsed by water flowing through the
filter from the under side of the bottom screen. The waste water
discharge at the top of the sand stuff is led to an open sink in the floor.
The filter plant comprises equipment for chemical water
tr e a trite n t.
33
Fig. 18. Sand filter and equipment for chemical water treatment.
The water supplied to the model propeller and to the
impeller causes the tunnel water temperature to increase.
The working temperature was chosen to be within the
limits of 20°C and 23°C. These values were decided from the
The water viscosity is greatly influenced by the
tempera-ture, as i,ai.io_6 (sq. rn/sec.) for 20°C against V l,l5.l0_6
at 15°C. This means that the Reynolds number is increased by about
114 % by this temperature rise.
The influence of the same temperature increment is also
favourable for the cavitation number, as the vapour pressure increases about 60 %.
One of the reasons, however, why it is not convenient to
raise the temperature to a higher level than 23°C is that the
chemical aggressivity of the water increases rapidly with tempera-ture.
The city water temperature in this part of the country is on an average about 6°C throughout the year, and very seldom
exceeds 10 - 12°C. This fact, together with the chosen
tempera-ture level in the tunnel, makes it possible to use a conventional water cooled plate heat exchanger for cooling (Fig. 19).
The cooling_water supply valve is controlled by a
thermo-stat in the tunnel wall. It can also be remotely supervised from
the control room.
The tunnel water circulation pump has a capacity of
0 cu. rn/h and the pipe water consumption is limited to 30 cu. rn/h.
The air content of the tunnel water is regulated by means of a deaerator tank (Fig. 20) which contains about 900000 small
ceramic rings with a total surface area of 900 sq. m. The water
to be treated is sprinkled over the rings and exposed to vacuum. The capacity of the deaerator is 100 cu.m/h, and the cir-culation pump, which is controlled from the control room, is placed at a level about 10 m lower than the deacrator tank.
The air content of the water is measured by means of Van Slvke's method.
The tunnel is also fitted with an expanding tank (Fig. 20) which covers the volume increase of the tunnel shell when the
pressure is raised from full vacuum to 5 atm. overpressure, as well as the net volume increase of the water when the temperature rises from 20°C to 23°C.
A concrete tank of 80 cu. m is used when the test section
is emptied for mounting of propeller models. A centrifugal pump
with a capacity of 200 cu. m/h is able to refill the tunnel in about 15 mm.
The pressure in the tunnel is regulated by the air pressure
in the expanding tank. A pressure tank and a vacuum tank (Fig. 21)
are connected to the expanding tank through two automatic contro' valves which make it possible to keep constant pressure in the test section or constant cavitation number for the propeller model. Both the compressor and the vacuum pump is fitted with pressure-controlled start/stop switch.
Fig. 21. Pressure and vacuum tank. The automatic control
Filter, deaerator, heat exchanger, expanding tank and storage tank are all connected to the tunnel by a system of
stainless steel pipes (Fig. 22). The tunnel has one main discharge
at the top and two at the bottom.
All the valves are of a diaphragm type. Some of them are
pneumatically operated and remotely supervised from the control room.
The pipe system is arranged so that the auxiliary equipment also can be used for our small tunnel when this is moved to the
new laboratory.
Fig. 22. Pipes and valves. The main pipe at the top of the
rack leads to the tunnel top. The two which go down
into the concrete floor lead to the tunnel bottom. Impeller and dynamometer drives.
General:
Prior to the selection of the drive systems for the im-peller and the proim-peller dynamometer thorough investigations and analyses were made in order to arrive at system configurations
which would give optimum performance at minimum cost.
As far as the impeller drive was concerned, two
alter-natives were investigated. These were:
Alternative 1) Fixed pitch impeller driven by variable speed drive.
Alternative 2) Controllable pitch impeller driven by constant speed drive.
Alternative 2) was found to be the one which in the best way met the requirements.
The propeller dynamometer drive was chosen as a Ward Leonard system supplied with a control loop for speed regulation.
The two drive systems represented an economical solution as it was possible to combine the equipment for the impeller and
the dynamometer drives in a way which offered substantial savings.
The achievement of this optimum design was made possible by taking
into account the power requirements at different test conditions.
The drive systems will be described below.
Impeller drive.
A simplified block diagram for the impeller drive system
is shown on Fig. 23. Fig. 2 shows a photograph of the
in-stallation.
The impeller drive system consists of the following: 3-phase asynchronous motor.
220 Volts, 50 Hz. 375 RPM. 65 kW. Reduction gear. Ratio 5:1. Pneumatic coupling.
To Thyristor PS..
Or
Prop. Dyn. Drive
A
CoupLing
Gear
Asynchr.
motor
Fig. 23. Impeller drive system. Simplified block diagram.
Fig. 2. Impeller drive equipment.
Synch r.
motor
Leonard gen.
1+) 3-phase synchronous motor. Self ventilated. Water cooled. 6.6 K Volts, 50 Hz. 1000 RPM. 1250 kW.
Motor-generator set for exitation of synchronous motor.
The set consists of:
One d.c.-generator, 7 Volts, 170 Amp.
One 3-phase asynchronous motor, 220 Volts, 50 Hz. d.c.-generator.
0-250 kW. 0-220 Volts.
110 Amp.
1000 RPM.
Thyristor power supply. 55 kW at 220 Volts d.c. Output voltage 0-260 Volts.
The above item numbers will be referred later in the description.
As mentioned in a previous chapter the water velocity is
continuously variable between 0 and 18 m/sec. To cover this
range in the best way two different impeller speeds
namely 75 RPM for the water velocity range 0-7,5 rn/sec. and 200 RPM for the velocity range 7,5 - 18 rn/sec.
At 75 RPM the impeller is driven by the asynchronous
motor 1) while the synchronous motor
f)
provides the drive at200 RPM. It should be noted that the impeller is always driven
over the 5:1 reduction gear. The actual speeds are therefore
375 RPM for motor 1) og 1000 RPM for motor 4)
Motor 1) is directly connected to the reduction gear while motor '+) is connected to the gear over pneumatic coupling 3).
thereby disconnecting motor 11.) from the gear.
d.c.-generator 6), which is mechanically connected to the synchronous motor, serves as a starting motor and runs the
synchronous motor up to synchronous speed. In this case 6) is
driven by thyristor power-supply 7). Otherwise 6) serves as
Leonard generator for the dynamometer drive. This is described
in the next paragraph.
The thyristor power supply 7) serves also a dual purpose as it in addition to the function already mentioned will be used as power-supply for the dynamometer of the small cavitation tunnel which will be moved to the new laboratory.
The system operates as follows:
When the start button is pressed motor 1) starts and runs
up to 375 RPM. The impeller will be driven at a speed of 75 RPM
over the reduction gear. The pneumatic coupling will now be
en-gaged and the synchronous motor ) and the d.c.-generator 6) will
thereby be connected to the reduction gear and will run up to a
speed of 375 RPM. When this speed is reached the pneumatic coupling
opens and disconnects the synchronous motor from the gear. At the
same time the thyristor power-supply 7) is connected to the d.c.-generator and the latter runs as motor and pulls motor t) up to
synchronous speed which is 1000 RPM. When the start of the
synchronous motor is completed, the thyristor power-supply is
dis-connected. The pneumatic coupling is now slowly engaged thereby
pulling the impeller up from 75 to 200 RPM. The asynchronous
motor is now deenergized and start is completed. It is possible
to alternate between high and low impeller speed during operation
by engaging and disengaging the coupling and energizing and
de-energizing the asynchronous motor.
The start and stop of the impeller drive equipment is fully automatic and may be performed from two different locations, namely from the control console in the control room and from
cabinets located in the equipment room.
As already pointed out, the water velocity is varied by
regulation is part of the testing procedure, the velocity control potensiolneter is located on the control desk for the data handling
system. The control desk is described on page 57.
Propeller dynainometer drive.
As mentioned earlier, this is a Ward Leonard drive with
constant speed regulation. A simplified block diagram is shown on
Fig. 25. Fig. 26 shows a photograph of the dynamometer motor.
The motor is completely enclosed for noise reduction. The
com-panion generator is d.c.-generator 6), located in the motor room.
The speed of the dynamometer motor is continuously variable
between 300 and 3000 RPM. A tachometer on the motor shaft indicates
the RPM and is incorporated in the loop for constant speed control.
This tachometer has an accuracy of 0,1 %. Its output is also
used in the data instrumentation for indication of the speed of the
propeller under test. The accuracy of the speed control is
speci-fied to be within 1 % at 300 RPM and 0,25 % at 3000 RPM. Actual
measurements show that the obtained accuracy meets these
require-ments by a wide margin.
Start and stop of the dynamometer drive is performed from the control console which is located adjacent to the control desk
for the data handling instrumentation in the control room. As
pro-peller speed regulation is part of the testing procedure, the RPM
is controlled by a potentiometer on the control desk.
The dynamometer motor is a 200 kW, 220 Volts d.c. motor
with a normal speed of 11450 RPM. This speed has been chosen as the
one giving the most optimal system and is determined from hydro-dynamical considerations seen in relation to the characteristics
of d.c. shunt motors. Large diameter propellers are tested in the
lower speed ranges and small diameter propellers are tested in the
higher speed ranges, see Fig. 10. By employing variation of the
armature voltage for speed variation in the range 300-11450 RPM and
variation of the field voltage in the range 11450-3000 RPM the
electrical as well as the hydrodynamical requirements have been met in the whole speed range and the motor size has been brought down
d.c
tra nsf.
Fig. 25. Propeller dynamometer drive system.
Simplified block diagram.
Fig. 26. Propeller dynamometer motor.
Set -
Control
Control.
Co ntrot
point
Unit
Unit
U nit
No:1
No:3
N o:2Leonard
Tacho.
Leonard
Fig. 27 shows a photograph of the control console. From this console the impeller and dynamometer drives as well as pneumatic and hydraulic systems are supervised.
Fig. 27. Control console.
As will appear from Fig. 27 the console may be regarded
as divided into vertical sections. These sections will in the
following be referred to as sections 1 through counted from the
left. The upper part of each section is sufficient to accommodate
3 indicating instruments horizontally and 2 instruments
verti-cally. The lower part of each section contains control
potentio-meters, pushbuttons, indicating lamps and alarm lamps.
Section 1 serves to supervise tanks, pumps and valves. The temperature at different points in the piping system is also checked from this station.
Section 2 contains current and voltage indicating meters
for the synchronous motor L+). Furthermore it contains an instrument
for indication of the current drawn by the asynchronous motor 1). In addition to the start and stop buttons for the above machines,
the lower part of section 2 contains control buttons for compressor
and oil pumps. Alarm lamps for pneumatic coupling, impeller zero
pitch position, pressurized air, oil pressure and temperature are also located in this section.
Section 3 contains the supervisory instruments and the
controls for the dynamorneter motor. Temperature alarm lamps for
dynamometer motor and for propeller shaft lubricating water are also located here.
The upper half of section '4 contains only one instrument at present, namely an instrument to indicate the RPM of the
syn-chronous motor 4). The rest of the upper half is available for
extensions.
The lower part contains lamps which indicate if the speed of the synchronous motor is in the correct range for phasing.
This range is 1000 20 RPM.
The measuring, signal conditioning and data reduction system. General.
Instrumentation of an experimental installation where a large number of measurements of the same kind and of a predeter-mined set of variables are involved is a typical data handling
system. The development of a new class of commercially available
operational and signal conditioning amplifiers, data loggers and small digital computers opens new possibilities in systems design of such experimental installations.
When dealing with a large system characterized by a great
number of variables or complicated computations, a small and fast
digital computer can be a most powerful tool both in data collection
and in on-line data reduction. The use of an on-line digital
computer can however be justified only when a large number of complicated, time consuming and repetitive experimental investi-gations are carried out.
Dataloggers on the other hand can be advantageous when dealing with single experimental investigations of systems
The conventional experimental instrumentation based on strain gauge transducers and carrier frequency signal conditioning amplifiers followed by strip chart recorders will however, with some modifications, prove to be advantageous in many installations.
The present experimental installations at the No. II Cavitation Tunnel is characterized by a restricted number of pri-mary variables - a total of seven - and fairly simple arithmetic
operations to be performed on a consistent set of variables. Even
the simplest on-line digital computers would in the present case be too expensive, and a datalogging system would prevent a good
communication between the investigator and the experiment. In the
present case a sophisticated conventional system including a special analogue data reduction unit therefore would give the most suitable
solution.
System solution.
The primary variables entering all propeller cavitation experiments are:
- propeller torque Q, (kpm)
- propeller thrust T, (kp)
- propeller revolution n, (RPs)
- water velocity in test section v, (m/sec
- difference between pressure in the test section
and water vapour saturation pressure (P - Pd)(kp/sq. m)
- difference between the test section pressure and
the atmospheric pressure (P - Pa), (kp/sq. m)
- water temperature t (°C)
Consistent values of these primary variables are used as the basic variables in the computation of the characteristic
propeller parameters. These derived variables are:
Speed of advance coefficient J
nD
Torque coefficient K Q
q
25
T cor Thrust coefficient KT -pn D K Efficiency coefficient r
-Kq 2rThe variables are determined at constant temperature and pressure
or at a constant cavitation coefficient. The cavitation number
is given by the expression:
-PVrel
When D is the propeller diameter, p specific gravity of water and
v the relative velocity of the water
re 1
2
v p(v + (O,7ir Dn)2)
re 1
A simplified system solution is shown in Fig. 28. The
forces and pressures are measured by means of strain gauge trans-ducers, and the temperature by a resistance thermometer.
A d.c. signal conditioning system has been used throughout. An eight channel oscillograph is used for recording purposes, in parallel with a digital voltmeter where the variables can be read
in calibrated units.
The derived variables are computed in the on-line analogue computing unit shown in Fig. 29, and the results plotted on a x-y
plotter. A controller station in the analogue unit makes it possible
to run the experiments at either constant tunnel pressure F, or
constant cavitation number . The control is achieved by
manipulation of the pressure/vacuum valves, (split action system). Referring to Fig. 28 and 29 a short description of the measuring and data reduction system will be given.
TORQUE,
Q
O-1,5kpm O-l5kpm O-15O.kpm
THRUST, T -50-+lSOkp O-600kp 0-3000kp TACOBENERATOR, n 4,5 /revf WATER VELOCITY.
0_9m15 I5psi) O_17m/s (2Upsi)
PRESSURE DIFFERENCE, ' Pd O-l5psi, O-9Opsi PRESSURE DIFFERENCE, P-Pa 0 - 80 psi WATER TEMPERATURE, e 0 - 30 °C
6
Pressure and friction data Thrust
correctin unit 8 channel voltage - current conner ler Fig. 28.
Simplified block diagram of the experimental instrumentation of the No. II cavitation tunnel.
D.V.M. Channel Sele
ii
Ii
ctor > To Analouge KEY:Indicating meter Full bridge resistance transducer
-a---Power supply Automatic
KEY: Summing Amplifier
-'- High
gain Amplifier -O-Coefficient p0t. meter Sservo set pot.m.
Thrust correctinq unit n Servopotentiometer
4
Icor T-k1PP0)K'+k, V2 P Pd V 0111- d2 ,2Print motor electron
Error signal Manual orint
9<
+ o .):!i-.
..
p-p or 0 reference Fig. 29.Block diagram of on-line analogue data reduction system of the No. II cavitation tunnel and pressure/cl control system.
Set scale - nd El. neumatic valve 'ositioners E -0 icor I uencin i motor PID Controller Memory Penn contrail
Pre ssure Tunnel Vacuum
x-y plotter Er
.
' Tdn)7o
0
The torque measuring units.
Two separate torque measuring units, one covering
0 - 15 kpm and one covering - 0 - 150 kpm have been specially
developed. The units are designed as an end part of the main
shaft and include standard fixing cones for the test propellers.
WATER SEALU, PROPELLER CD MODELL 0 PROPELLER MODELL 1273 TORQUE TUBE 0-15 KPM
--NEEDLE BEARING BEARING SURFACEBEARING SURFACE HOLDING COIL a
°
TORQUE TUBE0-15 KPM
BEARING SURFACE TORQUE TUBE
BEARING SURFACE O-15OKPM
Fig. 30. Torque measuring units, a) 0 - 15 kpm b) 0 - 150 kpm.
The lower range torque unit has two ranges, 0 - 1,5 and
0 - 15 kpm. A simplified drawing of the units is shown in Fig. 30.
The choice of these ranges is evident from Fig. 10. The change
from the lower to the higher range is performed automatically. To prevent hunting a holding coil is included, so that once full scale on the lower range is reached, the coil will be energized and stay energized until the torque is reduced by 10 % (from 1,5
kpm to 1,35 kprn). (17) The torque units are connected to a d.c.
signal conditioning amplifier system, which includes a
The accuracy of the torque unit and the electronic system
is estimated to be better than 1 %. The amplifiers, power supplies
and signal conditioning networks are within an accuracy of 0,5 %.
The torque measurement is not influenced by bearing friction. The
dynamic response of the electronic units is limited to
approxi-mately 500 Hz. A bandwidth of 20 Hz has been chosen by inserting
passive filters.
The thrust measurin system.
The measurement of the axial forces, thrust, can be done
in several ways. A conventional weight balancing system seems too
elaborate in dealing with forces up to 3000 kp. A precision spring
system, on the other hand, will require an axial free movement of propeller shaft, complicating the sealing problems of the shaft. A force balance principle (18) would give the highest accuracy
con-sidering the large measuring range from - 150 - 0 - + 3000 kp. In
addition it would be possible to measure without an axial shaft
movement. However, weight, spring or force balance systems based
on pneumatic, hydraulic or electromagnetic devices will be limited
in bandwidth, and may exhibit unwanted resonances.
The design of the dynamometer was therefore based on
commercially available high accuracy load cells. Total inaccuracy
from linearity and hysteresis of modern load cells are within 1 %
f.s.d. with a mechanical resonance frequency well above the
fre-quency spectrum of the propeller forces. However, to maintain
a high accuracy within the complete measuring range, it was necessary to use three different cell pairs:
The construction principle of the dynamometer is shown in Fig. 31, and a photograph of the thrust measuring unit is shown in Fig. 32.
lower range - 150 - 0 - + 150 kp (3 x 50 kp )
medium " + 150 - + 600 kp (3 x 200 kp)
Propeller shaft Holdin' coil l.oad celLs Spherical roLLer bean ng_ BaLl joints
Fig. 31. Thrust measuring unit.
Roller bearing
Motor shaft2::'
Basically the main propeller shaft (hollow) is suspended on a platform including a spherical roller bearing on which the
thrust force is acting. This platform is connected to the
funda-ment by three sets of load cells (a total of nine), very accurately
positioned around the periphery of the suspending platforms. To
prevent bending and twisting, all the cells are linked to the
platforms by precision ball-joints (zero-play). The total forces
acting on the shaft is found by summing the forces acting on each
load cell. When the thrust is below 150 kp, only the three cells
on the lower range are operating. Between 150 and 600 kp the
medium range cells are also activated.
The range changing is done automatically in the following
way: The set of cells in the lower range is suspended on springs
which give the shaft a displacement of approximately 0,3 mm for
+ 150 kp force, zero travel for negative forces. The set of cells
of the medium range is suspended similarily, but without springs. As soon as the maximum of the lower range is reached, the flange
on the cell shaft will touch the platform and the cells on the
medium range are activated. To prevent hunting, a holding coil is
energized as soon as these cells are activated, (see Fig. 31). A
simplified block diagram of the electronic system is shown in Fig. 33.
RANGE 1. lSOKp I I I ol e2
I---I
l.a
Fig. 33. Thrust measuring unit, simplified block diagram.
II ii Ii ii II ii 3 4K II
K®
II ii ii 1 o3 R II II II II I'Supply Ce(L.1 Measuring Summing Amplifier.
Supjy Cell .3
-Supply CeIL.2
RANGE 2. 600kp
Each load cell has a special stabilized power supply. A check zero
and a check calibration network are included in the d.c. signal
conditioning system. The signal conditioning amplifiers are class
0,1 and only precision components are used in the balancing and
summing networks. Zero check and calibration are controlled by the
digital voltmeter with three digits (highest accuracy 0,2 %).
Once the load cells of the lower range reach maximum out-put, the range shift is done automatically by switching the input
networks on the operational summing amplifier (Fig. 33). The sums
are weighted in the ratio 1:14 corresponding to the range of the cells.
Change to the upper measuring range is done manually by a simple fixing arrangement since measurement on the high range will also require change of the torque unit.
The total accuracy of the dynamometer depends on the accuracy of the various parts, i.e.:
- load cells 0,25 % f.s.d.
- measuring amplifiers 0,1 % f.s.d.
- summing and signal
con-ditioning networks 0,1 % f.s.d.
- measuring instrument 0,2 % f.s.d.
The maximum uncertainty should therefore be within 0,5 %.
The accuracy of the thrust measurements depends further on the shaftmotor coupling.
Propeller revolution.
The propeller revolution is one of the primary variables and is measured by means of a tachometer on the main motor shaft.
The motor speed control system is described on page 142. The
