CONFIDENTIALNot for Publication
THE DEVELOPMENT OF AN HYDRAULIC
FLUME
FOR
SHIP-MODEL TESTING
(Research Rein H.6)
BY
ORKNEY; B.Sc.,, A.R.T.C.,, PhD., G.I.Mech.E,
(Engineering Laboratory, Cambridge University)
Issued by the Council to the Members of the Association
THE BRITISH SHIPBUILDING RESEARCH ASSOCIATION
5 CHESTERFIELD GARDENS, CURZON STREET LONDON, W.1
Copyrightall rights reserved
REPORT No 165
1955(R.B. 997)
THE BRITISH SHIPBUILDING RESEARCH ASSOCIATION
THE DEVELOPMENT OF AN HYDRAULIC FLUME FOR SHIP-MODEL TESTING
by
J. C. ORKNEY, B.Sc., A.R.T.C., Ph.D., G.I.Mech.E.
SUMMARY
The report states the advantages of an hydraulic flume as compared with the
normal method of tank-testing ship models. An outline design for such a flume is
given.
The suggested dimensions for the full-scale flume were 6-ft wide by 3-ft deep with a maximum velocity of 6 ft per sec, which would accommodate a ship model 4-ft long.
Experimental work has been carried out on a model flume 14-in, wide by 7-in. deep, and the results scaled up to full-size. The design of the various parts of the
flume, namely stilling arrangements, contraction shape, working section, outlet control,
etc., is discussed and figures are quoted to show that for the same channel Froude depth number, uniform flow would be produced through the working section in the
THE DEVELOPMENT OF AN HYDRAULIC FLUME FOR SHIP-MODEL
TESTING
by
J. C. ORKNEY, B.Sc., A.R.T.C., Ph.D., G.I.Mech.E.
Introduction
All resistance, propulsion, and steering experiments on
ship models involve the use of a lengthy tank in which the models are run, and a travelling carriage, which spans the
tank waterway, is required to accommodate measuring instruments and staff. The method of towing models is
simple, satisfactory, and well understood, and has the primary advantage that a uniform velocity is easily obtained
and maintained. The main disadvantages are that all
experimental readings must be obtained in a limited period
of time and that visual observation of fluid flow is rarely
possible. The experiments could, however, equally well be performed with a model at rest, floating in a stream of water moving with uniform velocity.
This report outlines the work which can be carried out in an hydraulic flume and gives an outline design for such
a water channel. The experimental work for the
develop-ment of this design was undertaken at the Engineering
Laboratory, Cambridge, on behalf of the British Shipbuilding Research Association.
Object
The object of the investigation was to consider the
factors affecting the production and maintenance, over a
range of speeds, of a steady and uniform velocity profile of low turbulence and a flat free surface in a circulating-water
channel, and to design such a channel of a size suitable for
the testing of ship models.
Uses of Hydraulic Channels
Items of research which could be done more easily in a
flume than in an experiment tank may be enumerated as
follows :
Development of a technique for the detection of
transition from laminar to turbulent flow in ship models.
Development of apparatus for the measurement of
pressure, velocity, and direction of flow around
ship models.
Application of the pitot-traverse method of measuring drag behind models.
Investigation of the effect of roughness on the
resistance of plane and curved surfaces. Boundary-layer and wake investigation.
Investigation of restriction in tank cross-sectional area due to the presence of a ship model.
Suggested dimensions for a ship-model testing flume were as follows:
6-ft wide by 3-ft deep with maximum velocity of 6 ft
per sec, which would accommodate a ship model
4-ft long.
Description of Model Flume
Development work was commenced at Cambridge using
a 6-in. by 3-in, high-speed channel, and this showed that a uniform velocity profile could be obtained at high speeds,
but that at low speeds, particularly below the critical velocity,
waves formed in the channel. At all speeds there was an objectionable wake thrown off by the top of the trumpet ', or contraction, which fed the water into the open channel.
This wake could be reduced by boundary-layer removal. The model flume now reported was 14-in, wide by 7-in.
deep which was the maximum size that the Laboratory
pumps could supply. Fig. 1* is a perspective view of this
apparatus illustrating the circuit. Water from the Laboratory main was supplied to the stilling tank through a 6-in, control
valve and a pipe line ending in a submerged ' colander '.
Stilling was provided by a perforated wooden baffle and two phosphor-bronze wire-gauze screens inside the 4-ft wide and
16-ft long tank. (The pressed steel sections forming the tank were 4-ft square.) The wooden duct leading to the
contraction contained two more wire-gauze screens. The contraction, which was made in four pieces with the top
easily removable, was originally designed as part of a wind tunnel)* It had an area ratio, i.e. ratio of cross-sectional area at entrance to cross-sectional area at exit, of 7.25, contracting
from just over 3 ft by 1 ft 6 in. to exactly 14 in. by 7 in.,
in a length of 3 ft W, in. The duct and the contraction were of resin-bonded mahogany plywood, manufactured to
B.S. 1088:1951 (Plywood for Marine Craft). This material proved completely satisfactory. The channel, which was 8-ft long and 11.1.-in. high, composed of three pieces of plate glass, was supported on a steel frame by means of which the channel slope could be adjusted. At the end of the channel
there was a sharp-edged weir of variable height over which
the water flowed freely into a large measuring tank
under-neath, or was returned directly to the Laboratory sump.
To enable pitot traverses and surface-wave plots to be
made at any point in the channel, a carriage was developed, consisting of a bridge spanning the channel and free to move along it, to which was attached a saddle capable of traversing
the bridge and carrying a depth probe and a pitct tube with
double wedge fairing, each having a 12-in, vertical movement.
The manometer to which the pitot tube was connected was
mounted at one end of the bridge, and was of the short
sloping-tube type. The meniscus was set between two cross
wires by raising or lowering the tube mounting, and the position of the mounting was read from a Mercer clock
gauge which was given a 12-in, range by the use of a series of extension rods. If care was taken to remove air bubbles
by siphoning through the line, and a small quantity of
*Figs. 1 and 2 will be found on pp. 8 and 9, and the
bibliography on p. 7.
'
wetting agent was added to the water in the sloping tube,
this manometer would measure changes of 0-001 in. in total head although its zero setting was within only ±0-01 in.
Experimental Results
From the first it was clear that the major problem was the elimination of gravity waves rather than the production
of a uniform stream.
The main criterion involved is the channel Froude depth number defined
V gd where d= depth of water,
V - velocity,
g- acceleration due to gravity.
Froude number also serves to define the flow regime
relative to the critical velocity, for which 1. Above this critical speed the flow resembles a partly encased jet of water,
and only below this speed can trains of gravity waves be
formed. The change from below the critical speed to above will take place smoothly, as over a spillway, while thereverse
change can take place only through an 'hydraulic jump.'
In the experiments described, the channel Froude depth num-ber was always below unity. Flow velocities above the critical would be of limited value in normal model testing since the transverse wave train
behind the ship could not then
exist.
It was found that the contraction with its top in place
would produce trains of gravity waves at all channel Froude depth numbers below unity. It was found that these waves could be reduced very nearly to zero amplitude by extremely
careful adjustment of inflow quantity, channel slope, and outlet weir height, but that the interference due to even the
pitot tube in its fairing would upset these adjustments. In
addition, there remained the problems of the wake from the contraction top, its associated surface roughness, and diagonal waves springing from each corner of the contraction lip and
forming a small plume where they crossed in the centre of
the channel.
With the contraction top removed, a uniform velocity
profile and a flat surface were obtained for channel Froude depth numbers from zero up to values between 0-4 and 0-6. Above these values gravity waves were formed, their
ampli-tude increasing with
increasingchannel Froude depth
number. These waves were eventually found to be a function
of the
contraction shape. In accelerating through any open-topped contraction the surface of the water drops by an amount appropriate to the final velocity head.If the
water surface drops the whole of this amount within the contraction, and thus enters the channel with a horizontal
surface, then a flat surface results in the channel.
If the
surface is still dropping, i.e. has a downward slope,
as it
leaves the contraction and enters the channel, then a train of waves results.Experiments with various shapes of contraction showed
that the water surface
will drop horizontally' a fixedmaximum amount through a given contraction irrespective of the final channel depth. Thus for a given contraction shape there is a maximum fixed speed, rather than a fixed channel Froude depth number, for which the surface in the
channel is flat.
It became clear also that the longer the
contraction the better, but that even more important is the
' gentleness '
of the last part of the shape although the
upstream region of the contraction may be as abrupt as is
consistent with the avoidance of boundary-layer separation. The problem of the contraction shape has been dealt with at length because the contraction in the proposed 6-ft by 3-ft
channel has had to be designed by eye.
However, theproposed contraction is known to be a better shape than the one originally employed in the 14-in. by 7-in, experimental
flume, which produced flat flow up to Fn=0.52. It is also
probably slightly better than another experimental
con-traction which produced flat flow up to Fn=0.65.
It remains to show that these results can be scaled up
to the 6-ft by 3-ft channel:
V
but V2- -2gh where h= velocity head,
217
therefore F.=
The model produced flat flow up to F 0.52on a 7-in.
depth by giving
a'horizontal drop' of 0.94
in., andhld= 0.135. Hence for a depth of 36 in, a channel Froude depth
number of 0.52 would again be obtained provided that the
dimensions of the contraction were increased in the ratio of 7 to 36, the ratio of the depths.
In the experiments, the degree of flatness obtained was of the order of ±0-02 in., which was the order of amplitude
of the pattern of small surface-tension ripples which sprang from the plate-glass sides of the flume. It was found that
the flatness of the walls in terms of both roughness and
waviness was of great importance when looking for quantities of this order.
The uniformity of the
velocity profile outside theboundary-layers formed on the sides and bottomwas found
to be better than 0-06 per cent. With a velocity head of
about 0-8 in., the variations in head on vertical or horizontal
traverses of the pitot tube could not be measured on the
manometer reading to 0.001 in. The steadiness of the flow rate in terms of head was also better than 0-001 in., and the manometer showed fluctuations only as the pitot tube entered a boundary-layer. There was, of course, no wake on the surface.
The lack of turbulence and the uniformity of the profile was confirmed visually by dropping a few small potassium-permanganate crystals into the flow, for on sinking these left coloured traces that travelled the length of the channel
or .F 2=
d Fn= gd-=
-without visible change of shape. Such crystals lying on the contraction floor showed that at the speeds considered (approx. 2 ft per sec) the boundary-layer was laminar for about 2 to 3 ft into the channel.
With a turbulent
boundary layer the 0.06 per cent profile came to within about
1 in. of the walls and floor. The mechanism of the stilling
arrangements which produce these results is discussed later as one of the design features.
With the pumps supplying a given quantity of water,
the speed and depth of flow were governed by a weir or other control unit at the downstream end of the channel from which
the water fell away freely so that disturbances in the sump
could not affect the flow in the channel. The weir, venturi
throat, and ' hump ' were all
examined and the hump
was found to be the simplest and the most
satisfactory.Discharge characteristics were obtained.
At an early stage in the experiments a Fielden Electronic Proximity Meter was used in an attempt to measure first the
water depth by using the surface as the earthed side of a capacitance, and second, the pitot head using a diaphragm, the deflexions of which were detected with the instrument. The water-depth measurements were successful and the
instrument itself found to be stable, but the diaphragms were not stable. Changes of 0-001 in. in pitot head were, however, easily detectable. With this method it proved possible to
show at once whether a boundary layer was laminar or
turbulent, and to get some impression of the degree of
turbulence.
Proposed Design of 6-ft by 3-ft Flume
Fig. 2 shows the proposed arrangement of a flume 6-ft
wide by 3-ft deep with a working section 24-ft long, to be run
at speeds up to 6 ft per sec (17-0.61).
Water enters thestilling bay through a distributor and a baffle and is stilled by a honeycomb and four wire-gauze screens.
It then
accelerates through a 24-ft long contraction into the working section. The outlet of the working section is controlled by a hinged hump, over which the waterflows into the sump, whence it
is pumped to the stilling bay through a 4-ft
diameter pipe line.
The stilling process is one of successively reducing the incoming jet diameter from 4 ft to 0-030 in., at which diameter both the turbulence produced and the variations in speed are
so small that they die out within a few inches of travel. A jet of water entering still water will ' mix ' in about five
diameters from entry, giving rise to turbulence, the scale of
which varies with the jet diameter. Although mixed at
five diameters, the water velocity will still be greatest along
the jet axis. The overall velocity profile following a baffle
or screen will be as uniform as the screen itself.
In the
design, the 4-ft jet is split up, and put through a baffle of1-in.diameter holes at 2!,-in. centres, and then 1 ft further
down-stream (well beyond the five diameters of mixing), is put
through a block of
-in. mesh, Furan-treated Dufaylitepaper honeycomb 3-in. thick. This is followed by four
phosphor-bronze wire-cloth screens of mesh 24 apertures
per in. and 31-gauge wire, giving a 52 per cent hole area with apertures 0.030-in. square. At the maximum flow speed
of 0-67 ft per sec, this gives a head drop of approximately
0-19 in. This screen also gives a head-loss coefficient (head loss over velocity head) of 2.4 compared with the suggested, but not critical, value of 2.8 given by Taylor and Batchelor.3
The 24/31 gauze, which is a standard product, is strong
enough to be cleaned by scrubbing and to be tensioned in a
frame.
The original wind-tunnel type of contraction scaled up according to the method already developed, would be 19-ft
6-in. long. The contraction in the design is 24-ft long and is of improved shape, thus there is little doubt that a channel
Froude depth number of 0.61 can be attained with this
shape.
The working section is designed to give a 24-ft length of channel with a flat water surface, while a further 6 ft is
allowed for the drawdown which occurs as the water approaches the outlet. Rails along both sides of the section allow it to
be spanned by suitable
travelling instruments, some ofwhich would be developments of the traversing gear and
manometer used on the model. It appears certain that the
uniformity of profile and steadiness of flow of the Cambridge 14-in. by 7-in, channel can be repeated. The working section
would be sloped slightly downwards to compensate for the
thickening of the boundary layers along its length.
Since the measurement of the flow velocity in a
circu-lating flume depends on a pitot tube and a manometer whose
zero is set from the moving water surface, it is likely that
surface-tension ripples springing from the sides would pro re objectionable.
It was found that these ripples could be
eliminated by allowing a pair of floating rubber tubes,
arranged one near each wall, to move downstream with the
flow. There can be no ripples on the surface between such a pair of tubes or curtains, if they move with the stream to within i0.75 ft per sec, the minimum wave velocity. Each rubber tube, or nylon curtain, can be constructed in the form of an endless belt running the length of the working section and for some way into the contraction. Ripples cannot form in the stilling bay, since the maximum velocity there is 0.67 ft per sec.
In a circulating flume, the control unit at the downstream
end of the working section must serve also as an isolator,
past which pressure fluctuations from the pump cannottravel upstream.
This could be accomplished by allowing the
water to drop away freely into the sump after passing the
control, but the resulting air entrainment would be most
objectionable in a large-scale flume. Pressure fluctuations can travel upstream only in a stream flowing at less than the critical velocity. Thus the hinged hump shown in Fig. 2 allows the stream to accelerate through the critical velocity over the crest, while the back of the unit serves as ashort chute down which the water flows to the sump.
At a channel speed of 1 ft per sec the depth of water in the stilling bay will be 8 ft 61 in., while at 6 ft per sec it will have increased to 9 ft 01 in. The difference of 6.1, in. spread
over the surface area of the stilling bay and part of the
contraction represents the volume of water which must be
drawn from the sump. If then a very large sump is to be avoided, some form of control of the level in the sump is
desirable in order to prevent the head of water over the
pump suction intake becoming too low, and also to give control of the point at which the water flowing over the hump meets that in the sump. This is of some importance
in minimising entrainment of air at the top speeds, and in preventing drowning of the outlet control at low speeds.
The simplest method of sump-level control appears to be to make use of air-ballast tanks. In an emergency shut-down, the surges which caused trouble at the David Taylor Model Basin could be eliminated by releasing air from the tanks.
To compare with the uniformity and the freedom from
turbulence and surges given by the stilling arrangements,
the pump, motor, and control set must give an exceptionally steady rate of flow over a wide range of quantity. A decision
on the relative merits of the possible schemes must depend
on economics, on the specifications to which manufacturers
would be prepared to work, and on whether electronic or
mechanical servo control is deemed preferable. Following
the choice made both by the Americans for their flume at the David Taylor Model Basin and the Germans for theirs
at Heidenheim, a variable-pitch axial-flow pump driven by
a synchronous motor is specified now. The design of the sump and the pump-intake arrangements shown in Fig. 2
are speculative and should be checked. At 6 ft per sec and a depth of 3 ft, this channel would require between 31 and 55 h.p., depending on whether the sump level is 1 ft or 3 ft below that in the channel.
To run at a channel Froude
depth number of unity (9.83 ft per sec) would require
approximately 90 h.p.
Conclusions
It is concluded that an effective flume can be built as a result of this investigation.
BIBLIOGRAPHY
1. Contracting Ducts of Finite Length. WHITEHEAD, L. G.,
Wu, L. Y., and WATERS, M. H. L. Aeronaut. Quart., 2 Part 4 (1951), p. 254.
?. An Investigation of Flow through Screens. BAINES,
W. D. and PETERSEN, E. G. Trans. Amer. Soc. Mech. Engrs., 73 (1951), p. 467.
The Effect of Wire Gauze on Small Disturbances in a
Uniform Stream. TAYLOR, G. I. and BATCHELOR,
G. K. Quart. J. Mech., 2 Part 1 (1949), p. 1.
Measurements of the Aerodynamic Forces acting on
Porous Screens. SIMMONS, L. F. G. and COWDREY,
C. F.
Aeronautical Research Council, Reports andMemoranda No. 2276. 1945.
- --.
8'
STILLING TANK
CONTROL VALVE
Fig. 1-Perspective Sketch of Equipment.
CONTRACTION I 20' CHANNEL Be INSTRUMENTS I
/
/
3,7 24.-o* _ = -DIST1:ULTORTILL761G3 RAY CONTRACTION HONEY CO t3AEELL /. [S_EcTIO_NAL _ El-EN/ATKIN.
CONTROL pAmo, PLAN
Fig. 2Sketch Design for 6-ft by $..ft Hydraulic
-24-0
WORKING SECTION
LENGTH OA
26-0
CONTROL/ ISOLATOR PUMP INTAKE BAY
0'0,0 0 0 0 0 Q.0 0 0 0
-OUTLET CONTROL
VAIR BALLAST TANKS VARIABLE PITCH AXIAL FLOW PUMP 41. SYNCHRONOUS-MOTOR
RETURN PlrE.0. I D CONTRACTION
GUN 'DY/AY
_WORKING
SECTION
_T-RA ERS1 G &
E IG11. GE AR_ ET_C_
AIR RELEASE VALVES
PU_P NTAK 6 EBB OARS -WORK I NGDEPTH RETURN PIPE Flume.
