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HYDROMECHANICS == THE HYDROGEN-BUBBLE, FLOW-
oH MISUALIZATION TECHNIQUE

AERODYNAMICS a

George E. Mattingly
STRUCTURAL
MECHANICS

Distribution of this document is unlimited

APPLIED
MATHEMATICS
5 HYDROMECHANICS LABORATORY
RESEARCH AND DEVELOPMENT REPORT

ACOUSTICS AND

Le _» . : February 1966 | Report 2146

} (Rey. 12-64)

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1OHM/18W

TOEO QO

THE HYDROGEN-BUBBLE, FLOW-
VISUALIZATION TECHNIQUE

George E. Mattingly

Distribution of this document is unlimited

February 1966

Report 2146

a vivguicioh ati

TABLE OF CONTENTS

GUMMY 56 9 Fb a DOO FOOD OD OOO MBO OHO
TIMERODUGITION so 5 0 6 Oooo OK HK
USES AND LIMITATIONS OF THE HYDROGEN-BUBBLE TECHNIQUE .. .
TWH IACI 56 5 06 600660
TILIBICIROMNIIG ENGUIUEMOHINP 5 6 6 0 6 Ooo OOOO OO
IGCGEMPINIGS 9 ob 9 © 0 0 D0 oO OOOO oO OOOOH Oe
DROMOGRAPEM oo 6500000000500 00600000000
SKOMIS, OIRVMITOINVAL) IEINO\CINDIUISIS) 6 6 6 Go 6 6500600004506
SOME PRELIMINARY EXPERIMENTAL RESULTS . . . © « » o © © o o
INCISINOWIGIIDIGMIBININS «5 6 6 6°09 O08
RII 5 56 go 0 00 oO OOOOH OOO OOOO OK OOOO

Figure

Figure
Figure

Figure

LIST OF FIGURES

Typical Wake Pattern as Seen behind Foil Shape ... .

_ (Freestream velocity is 1 fps)

Pilegealiaqnm” Wie WOlLerS 5 oo 6 6 ooo Ol lo

Bubble Patterns for Longitudinal and Transverse Velocity
Wi@lel 6 6 oo 060 eo Oo bo ooo HOODOO oO DOO

Wire Configuration for Longitudinal and Transverse

Weloeiltiiy Imoiille DewemmlanclOM sco ..oso00000060
Spider Web Bubble Patterns in the Wake of a Circular
Gyyllatiaslenea) G eoiin’ cHeon epee Omacw chewed LOnIOs ho O0 G, [Ov cy Oona vo) 0° 0
Typical Bubble Patterns behind a Two-Inch Plate
(meestmecm Wellocwthy dis IS os) 6 5 oo 6 7 oo 8
Twelve-Inch ID Plexiglas Closed-Jet Test Section with
IMjO-DimemASiCa@ei Silos Waste 5 o 6560600000000

Enlargement of Bubble Patterns in the Wake of a Flat Plate
with Sharp Trailing Edge (Freestream velocity is 1 fps) .

Leica Camera and Pulse Generation Equipment ...... .

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10-
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Amplifier Unit Built for Hewlett-Packard Pulse Generator
Miolet le PHULEEAN NS AS sohate hceta tela) revs Get oe ike eat yah cra

Two Sylvania "Sun Guns" Mounted in Drained Test Section. .

View Parallel to Stream Velocity (Looking Upstream) of One

Lighting Scheme (a) and an Improved Setup (b) .......
Two General Radio Strobotacs Mounted in Hatch of Test
DECOM, cree rey ties welds), eho. es Eternal Sa- Cou tay ral Moin chmic’d Ue tere ae Coomnee
(Onjalaliaioleal@ely, Iola Se “Grego 6 oc o bo ob) 6 o 6 uo 5 Oo
Streakline Pattern about a Foil Shape, Aspect Ratio is 10:1
(Gesestecin yelOeiuy ds Uos)) oo 0600006 oo oo 6
Streakline Patterns of the Flow about a 1l-Inch Diameter
Cyilialere (Imaseswimecia ellocitey aS IL 98) 6 6 5 5 5 6 5 6 0

Visual Determination of the Position of the Separation
Pomme (HaSeswicacim Voloeiiny a 3 as) 6 o5 oo ob oo oo

Typical Bubble Patterns behind 10:1 Foil Shape Illustrating
Deviation from Two-Dimensionality along Foil Span Length .

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SUMMARY

The hydrogen-bubble visualization technique has been
adapted to the 12-inch variable-pressure water tunnel of the
David Taylor Model Basin. An outline of this adaption and the
operation of the technique; are described.

Photographic techniques and analyses applied to the result-
ing films are discussed. Sources of error are delineated, par-
ticularly with regard to the deceptive streakline patterns that
can be formed and especially the results of exceeding the velocity
limitation imposed by the shedding phenomena taking place behind
the platinum wires. Errors caused by compression and/or stretching
of bubble lines along their length are discussed, and procedures
are given for recognizing this type of error. In addition, cathode-
wire configurations are described by which both longitudinal and
transverse velocity profiles can be obtained in steady or unsteady
water flows.

Various cathode-wire configurations are described through
which qualitative aspects of the flow about bodies as stagnation

and separation point motions are depicted.

INTRODUCTION

The uses of visualization techniques for the determination of the
characteristics of fluid flows have become quite diversified since Reynolds’
transition experiments in the 1880's. The introduction of visible media
into fluid flows has been accomplished in many ways for the acquisition of
either qualitative and/or quantitative flow characteristics. Injection of
dyes into liquids or smoke into gasses through porous bodies or hypodermic
needles, the homogenous mixing of visible “unity oil" - (Sp/Gr. = 1) in

water, the ‘hypodermic: injection of anisol bubbles into boundary layers,

tellurium injection, electrochemiluminesence, etc. are a few examples. The
merits of each visualization technique are based upon the extent of the dis-
turbance to the fluid flow caused either by the injection method or by the
injected medium itself and upon the accuracy with which the desired flow
characteristics can be observed.

In most cases, such visualization techniques as mentioned above are
useful for obtaining only the qualitative characteristics of a specific
flow. Quantitative characteristics are usually achieved by means of such.
techniques as hot-wire anemometry or pitot-tube surveys, etc.

Analysis of flow fields by means of dye-injection techniques to
exhibit streakline: patterns of the flow should be done with care as shown
by ewe This is very important in such unsteady flows as exist in
boundary layer transition and in the oscillating wakes behind bodies. With
a view toward surmounting this ambiguity connected with the streakline
patterns and at the same time achieving quantitative measurements such as
time variant velocity profiles, the following scheme was introduced by
eller,” A swell wise (0.001-in. diameter), positioned in a water flow,
snenedaed waste a negative voltage and a positively energized terminal
positioned in the same flow were so arranged as to construct an electrolysis
of the flowing water. Because of the two-to-one ratio of the resulting
volumes of gas, hydrogen was chosen to exhibit the fluid motion. This hydro-
gen gas is produced in the form of very small photographable bubbles on
which the predominant force is the drag due to local fluid motion.

This hydrogen-bubble visualization technique can be particularly use-
ful in propeller and hydrofoil research as performed in variable-pressure
water tunnels. In addition to such quantitative results as time-variant
velocity profiles in water flows, the bubble technique is qualitatively

useful for observing flows around bodies. Separation phenomena, oscillating

¥
References are listed on page 26 .

flow patterns in the wakes of these bodies, and the time and space relation-
ships for these phenomena are examples of the quantitative value of the
technique -

Unfortunately, the bubble technique is not without disadvantages, e.g.,
certain velocity limitations. Included below is a discussion of the velo-
city limitations and the application of the hydrogen-bubble visualization
technique to two-dimensional unsteady flows. A scheme is put forth through .
which a quantitative analysis of the longitudinal and transverse aspects
of an unsteady water flow is achieved.

The following is a description of the hydrogen-bubble visualization
technique, its diversified capabilities, and its establishment at the
David Taylor Model Basin. The study presented here was carried out under

the General Hydromechanics Research Program, S-ROO9-O101, Task 0103.

USES AND LIMITATIONS OF THE HYDROGEN-BUBBLE TECHNIQUE

Basically, the hydrogen-bubble flow-visualization technique consists
of an electrolysis process created by the excitation of cathode and anode
terminals wetted by flowing water. The resulting gas formed at the
cathode terminal is visible hydrogen gas which may be produced in the form
of very small bubbles. Analysis of the forces on a buoyant sphere in a
steady slow-speed (Stokes flow) water flow shows that the buoyancy to

drag ratio satisfies

B/D = g aa / Asp)

If the bubble size is sufficiently small, say a few thousandths of an

inch, the buoyancy force is very small compared to the drag force. Con-
sequently, the motion of the bubbles is dictated by the local water velo-
city. This predominancy of drag over buoyancy is verified by the negligible
rise rate of the small bubbles. Through this predominance of drag over
buoyancy, water velocity profiles may be accurately obtained in two-dimensional,

low-speed flows.

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To avoid altering the true value of the physical quantity being
measured, the terminals required for the electrolysis process are chosen
to minimize the effect of their presence on the flow. The terminal chosen
for the cathode is a very thin wire supported in the water flow at some
location where the characteristics of the velocity field are desired, and
the anode consists of the metal water tunnel or towing tank wall, or some
suitably installed metallic terminal. Many different materials were used
as cathode terminals and platinum was found to be most suitable for this
purpose because of its corrosion resistance. Other materials used were
stainless steel, copper, brass, bronze, and zinc.

When the thin cathode wire is energized with a de power source, a
continuous sheet of hydrogen bubbles is produced in the water. The rows
of tiny bubbles which constitute the sheet are distorted according to the
local characteristics of the flow field.

Velocity profiles in two-dimensional flows are obtained by pulsing a
voltage to such a wire. The cyclic generation of hydrogen along sine wire
produces patterns like those of Figure 1. Figure 1 shows an actual size
view of the bubble patterns in the wake of a symmetrical foil shape (chord-
thickness ratio is 10:1) at O-deg angle of attack. The view is parailel
to the trailing edge and perpendicular to the chord of the foil.

Figure 1 illustrates both the qualitative and quantitative aspects of
the hydrogen-bubble technique. In addition to the quantitative data, such
as the longitudinal velocity profile available at the vertical platinum
wire 2 inch downstream of the foil trailing edge, qualitative information
is provided on the reversal of flow at the platinum wire. This reversed
flow which is present at the vertical wire is noted to extend upstream of
the wire, past the trailing edge, and into the boundary layer of the foil
shape. Such a reversed flow exists because of flow separation and continues
as far upstream as the location of the boundary-layer separation point on
the foil shape.

Figure 1 also illustrates the manner in which the platinum wire is
Supported in the wake of the foil shape. The heavy wire or rod frame is

constructed and mounted so as to avoid errors induced by wibrations caused

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by the flow around it. An insulation (a vinyl plastic coating: Chem-

Sol Plastisol material) is applied to the portion of the wire holder which

is submerged. The platinum wire is soldered to the horizontal rods that

are visible at the top and bottom of the figure. These two horizontal rods
are welded to a vertical strut barely visible (and out of the plane of focus )
in the right background of the photograph. This vertical strut is positioned
so that it is not in the plane of the bubbles (the plane of focus) and, con-
sequently, does not interfere with the bubble patterns near the platinum
wire except near the soldered ends. The region behind the platinum wire
affected by the horizontal wire supports is easily observed, and the wire

is always positioned to utilize the center portion of the bubble patterns
for flow analysis. Figure 2 shows four platinum wire holders.

The distance between the bubble rows behind the wire depends on the
velocity at the wire and the period of the pulsed ieee The velocity
at the wire is directly proportional to the bubble-row separation and
inversely proportional to pulse period; the constant of proportionality is
the scale factor encountered in the photograph (see analysis below). For
the 0.00l-in. wire shown in Figure 1, the diametral Reynolds number is
below 40 for velocities in water up to 5 ft/sec. Consequently, there is
no vortex shedding behind the wire itself as shown on page 17 of Reference 5.
The velocity recovery is assumed to occur within a very short distance
downstream of the O.00l-in. diameter wire. By this means, a close approxi-
mation of the local longitudinal velocity profile is achieved. Note that
the determination of such a longitudinal velocity profile is achieved by
these means only when the stretching or compressing of the bubble rows along
their length is minute compared to their horizontal translations (see below).

For a Reynolds number, based upon the cylinder diameter, less than hO,
the two vortices remain attached to the cylinder independent of the time
variable. That is, there is no oscillatory feature in the wake, and
disturbances downstream of the cylinder appear to be rapidly damped out
near the cylinder. In the range of R, petween 40 and 150, the flow

is termed "stable." The flow behind the cylinder is characterized by

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a laminar flow and by vortices being shed from the regions downstream of the
separation points. The vortex streets in the wake are ultimately dissipated
by the viscosity far downstream from the cylinder. In the transition range,
encompassing Re from 150 to 300, laminar-turbulent transition begins to
appear in the free-stream layer that has separated from the cylinder. For
Re from 300 to 10°, the flow is characterized by the shedding of vortices
consisting of turbulent fluid whose source seems to be the separated shear
layer. Therefore, rows of hydrogen bubbles present in the cathode wire
wakes for these latter three Re regimes can portray very deceptive patterns
of the fluid velocity profiles. Accordingly, it is most desirable to
operate the technique in fluid-flow velocities where the diametral Reynolds
number is kept below 40. It is apparent that this can be done by controllin
the fluid velocity, by selecting a suitable wire diameter, or by altering
the kinematic viscosity of the fluid.

In addition to the applicability of the hydrogen-bubble technique to
water flows, it has recently been established that the technique operates
very successfully in water-glycerine mixtures (personal correspondence and
Reference 6). Such mixtures are extremely useful for changing fluid viscosity
by means of temperature control. In this glycerine-water mixture, several
wire configuratioss were used to obtain velocity profiles in which there
existed an increasing vorticity distribution in time throughout the profile.
Of particular significance, however, was the higher degree of photographic
clarity and contrast obtainable in such mixtures. The velocities attained
in this study were on the order of 6 im. /sec.

It is believed that velocities considerably in excess of this value
can be investigated in these mixtures without sacrificing the bubble quality.
The greatest percentage of glycerine used in the cited study was 40 per-
cent. However, it is felt that higher percentages could be used and
acceptable bubble quality retained.

The determination of velocity profiles in two-dimensional flows in
which longitudinal and transverse components of velocity are of comparable
magnitude should be done with great care. Figure 1 is a good illustration

of such a flow. When only longitudinal displacements of successive bubble

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rows are taken as indicative of local velocity, large errors can occur.
This is due to a visually undetectable stretching or compressing of the
bubble rows along their length.

Streamline patterns for steady water flows are also obtainable with
the hydrogen-bubble technique. As described in Clutter et ail,” 1G ws}
possible to uniformly "kink" a 0.002 or 0.004-in. diameter wire by feeding
it through a pair of gears. A 0.001-in. diameter wire was found to be
inadequate in this respect because it would not retain the necessary kinked
configuration. When the wire is kinked and such a wire is excited by a
de voltage, electrolysis occurs along the entire length of the wire, but
the hydrogen is dragged off the wire by the flow only at the downstream
points of the kinks. In a steady flow, the resultant pattern is that of a
series of streamlines. In unsteady flow, it is that of a series of streak-
lines.

Obviously, the larger diameter (0.004 in.) "kinked" wirec causes a
small disturbance to the flow. Therefore, the use of the kinked wire is
limited to a velocity range in which the influence of the wire on the flow
ean be neglected. It is emphasized that care should be used in the employ-
ment of the larger kinked wire for this reason. However, for R,& 40 such
a method does give a rapid means of visualizing the flow field by streak-
line traces.

When a pulsed excitation is imparted to the kinked wire, the streak-
lines are changed into dashed lines. These dashed lines are then illus-
trative of the accelerations and the velocities which exist in the fluid.
It was found that discrete dashes could be achieved with a voltage pulse
having very small rise and drop wainaes c= Otherwise, dashes with blurred
ends result, i.e., the tips and the tails of the white bubble lines are
fuzzy. In the work carried out at DIMB it was found, however, that even
with very short rise and drop times (less than 15 ms), when the velocity of
flow past the kinked wire is sufficiently large, i.e., 10 to 15 ft/sec, the
tips and tails of dashes become blurred. This is felt to be the result of

vortex shedding from such a wire configuration.

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A means of uniquely marking fluid particles so that both components
of velocity are illustrated was accomplished by inserting two cathode wires
(one kinked, the other straight) in the two-dimensional flow. The resultant
bubble pattern is a combination of time and streaklines. (Time lines are
the loci of fluid particles which were located at the platinum wire at a
previous time (Reference }}).) The two wires were installed so that the
resulting hydrogen bubbles were contained in planes which practically coin-
eided. Enough space should be allowed between these plans so that the
presence of one wire does not influence the motion of the bubbles from the
other wire. The direction of observation should be perpendicular to both
planes of bubbles. A sketch is shown in Figure 3a.

The longitudinal displacement of the intersections of bubble lines is
proportional to the longitudinal component of velocity, and the transverse
displacement of these same intersections is proportional to the transverse
component of velocity. The incorporation into this analysis of the longi-
tudinal streaklines from the kinked wire enables determination of the
transverse component of velocity at the downstream extremities of the kinked
wire. Obviously, this particular technique for marking specific fluid
particles will be inadequate when the flow has velocity components of com-
parable magnitudes in all three spatial directions.

The analysis to obtain the longitudinal and transverse velocity profiles
at a Single Location in steady or unsteady flow then proceeds as follows. As
a transverse bubble row is swept off the straight wire, intersections with
each of the longitudinal lines are Visible looking perpendicular to both
planes of bubbles. These intersections are then dragged downstream in
accordance with the velocity profile. When the subsequent rows of bubbles
are dragged off the straight wire, another series of intersections is
observed. Since these intersections are formed after an interval of time
equal to the pulse period, the transverse displacement referenced to some
visible datum point divided by the pulse period is proportional to the
transverse velocity. This velocity is a quasi-steady ome taken over the
pulse period and, accordingly, the pulse period should be much smaller than
the period of any flow oscillations. The initial transverse spacing of the

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horizontal bubble lines is that of the wave length of the kink in the
kinked wire. Increasing or decreasing this spacing is therefore achieved
by suitably adjusting this wave length. A photograph of a spider-web
bubble pattern can be seen in the wake of a circular cylinder in Figure }.
Note that an analysis like that described above can be performed only when
the two bubble-generating wires are positioned as shown in Figure 3a. The
wire configuration shown in Figure 3a and the previously described analysis
were successfully employed, and results can be seen in Reference 6.

Another way of achieving these intersecting patterns of hydrogen
bubbles is accomplished without the use of kinked wires. If straight
(0.001-in. diameter) wires. energized with de excitation are installed like
rungs in a ladder (Reference 4) in the flow so that the line of vision is
parallel to the rungs, the steady sheet of bubbles from each wire rung appears
as a line to the observer. Positioning another wire in the usual manner
(shown in Figure 3b), i.€., perpendicular to the direction of vision, enables
creation of the transverse rows of bubbles such that an intersection of
bubble lines is visible. Additional intersections are obtainable with
additional wires. The wires installed as ladder rungs which are oriented
parallel to the direction of vision do not have to produce bubbles along
their entire length to create these intersections. Wires which are
coated with a thin waterproof insulator except for some small interval
at the center of the wire suffice for the production of a bubble line as
seen by the observer. It is important that the insulation be thin to
avoid yorticity shedding from the insulated portions of the rungs. The
wire which receives pulsed excitation (viewed perpendicular to its length)
can then be positioned in or out of these ribbons of bubbles. Such a scheme
has several advantages. One is the lifting of the velocity limitation due
to the shedding phenomena behind the 0.004-in. diameter kinked wire. Another
is the flexibility of chcice of spacing between the longitudinal lines.
Positioning the transverse wire in the sheets of bubbles streaming from
the rung wires also enables visual determination of whether or not vortex

shedding is taking place from the transverse wire, i.e., for R,? ho.

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As the above scheme for measuring two-dimensional steady or unsteady
velocity profiles is accomplished at specific locations in a plane, it is
necessary that the bubble-line intersections be sufficiently numerous to
enable a "continuous" determination of the velocity field. That is, the
separation of the streaklines from the wire rungs and the separation of the
time lines have to be such that the resulting quantitative data enable a
smooth extrapolation for the velocity value points of measured values. In
this way, one is then able to describe velocity profiles by a smooth
curve. However, a photograph with an inordinate number of bubble line
intersections can be tedious and difficult to interpret for quantitative
results.

The velocity field may be calculated in another manner when a motion-
picture film strip has been taken of bubble distortions in a particular
velocity field. When particular fluid particles, as marked by individual
bubbles, are followed from frame to frame, division of the vector distance
between bubbles by the small known time interval between frame exposure
determines the magnitude and direction of velocity components throughout
the field. This procedure and the one previously described for determining
velocity profiles from a single frame are greatly expedited by the use of
film readers such as the Benson-Lehner Corporation Oscar Model F (GS 1026 G)
System. This method of velocity field determination can become extremely
difficult, if not impossible, when there are many similarly sized and
indistinguishable bubbles on successive film frames. One way to eliminate
this difficulty is to use the spider-web bubble patterns to enmesh the
desired velocity field. The bubble translations can then be traced relative
to some datum point observable on the film frame in. a very organized fashion.
As previously mentioned, the adjustable grid size enables one to specify
the number of intersection points in the se lootty a ISlUGl,

The qualitative aspects of higher speed (3 to 5 ft/sec and above this
range) oscillating wakes may be caecmred without a photograph as follows.
Consider the flow about a foil shape or flat plate behind which the
shedding vortices are moving into the wake so rapidly that physical

visualization of the bubble patterns is difficult when a continuous lighting

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scheme provides the illumination. Using stroboscopic lighting, let a
straight platinum wire, oriented perpendicular to the wake of the body (as
shown in Figure 1) be energized with pulsed excitation. If the strobotac
frequency is adjusted to be equal to the frequency of vortex shedding, the
motion of these vortices will cease. Now if the pulsed excitation frequency
is adjusted to freeze the bubble rows and space them about 1/4 in. apart

in the free-stream velocity field, the entire bubble pattern will become
stationary. The pulse frequency required to achieve a completely frozen
bubble pattern must be an integral multiple of the strobe frequency. The
spacing of 1/4 in. between free-stream bubble rows is an approximate value;
the spacing should be such that the wake is not excessively congested by
needless amounts of hydrogen bubbles. (Should the strobotac (or shedding)
frequency be desired, it can easily be numerically determined using an EPUI
(events per unit time) meter to monitor the strobotac output signal.) Figure 5
shows a photograph of such a frozen bubble pattern. The transverse develop-
ment of the wake in the longitudinal direction is apparent from such a photo-
graph. In light of Hama's work, ~ it is stressed that care should be used in
interpreting the photographic results such as those shown in Figure ye,

In order to specify the actual positions of vortices in the wake of
such a foil or flat plate body, the following procedure should be used. Let
a platinum wire holder configuration (as shown in Figure 3b) be positioned
in and perpendicular to the wake of the body. With both wires properly
energized, the resulting bubble patterns should be illuminated using the
improved lighting scheme discussed in the LIGHTING section. Photograph the
spider-web patterns with a motion-picture camera using a film speed chosen
to achieve a sufficient number of frames of the cyclic phenomena taking
place in the time interval of the shedding period. This could be 10, 20,
or 30 frames per second, depending on the continuity desired between film
frames. Guiding values for such a film rate can easily be obtained using
the strobotatic illuminating scheme described above. Having this strip
of film of the cyclic translations of the bubble line intersections, use a
film reader to quantitatively analyze the time variant, two-dimensional

velocity fields. The velocity of the wake vortices is obtained by

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multiplying their wave length by the frequency of the strobotac lighting
which "freezes" their motion. When the velocity of the vortices in the wake
is subtracted from this determined velocity field, one is then able to

specify the actual centers of rotation of the wake vortices to within the
accuracy permitted by the clarity of the photographs. Such a result enables
quantitative analysis of the entire wake flow field, provided of course,

that the transverse dimension of the wake permits the bubble line inter-
sections to be observable on the screen of the film analyzer. Excessive
bubble line congestion can cause this analysis to be tedious, if not impossible.
in addition to wake width dimension, which is dependent on body sizes, water
speeds, etc., the experimenter may be able to eliminate bubble line congestion
by altering the pulsed excitation frequency and/or the spacing between the

the platinum wire rungs of the ladder wire configuration.

TESTE FACILITY

To achieve a two-dimensional water flow, the test section of the 12-
inch variable-pressure water tunnel at the David Taylor Model Basin (Ref-
erence 7) was modified in the following manner. A plexiglas circular tube
(Figure 6) was installed in the existing open=-jet test section to form an
axisymmetrical closed-jet test section. Into this plexiglas tube were
installed the straight, parallel plexiglas liners that are shown in Fig-
ure 6. The perpendicular distance between the liners is 6.66 in. The
flow which precedes this test section was made to change smoothly from the
circular upstream tunnel shape to the straight-sided plexiglas section by
means of two aluminum transition pieces installed in the entrance nozzle.

A pitot survey of the longitudinal velocity distribution revealed that
departures from a total average velocity (averaged over the whole test
section) were less than 3 percent. Transverse velocity components were not
measured. Departures from the average test section velocity occurred pri-
marily in two places. The first occurred mear the wall of the test section
as could be expeeted due to the presence of the boundary. The other occurred

due to a slowly moving "slug" of water centered in the test section. Similar

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departures were found in a previous survey in the open-jet test section,
where the deviations from the average were from 3 to 5 percent (see Ref-
erence 8). This agreement with the previous survey was comforting in that
the two-dimensional section introduced no new departures from the average

and, in fact, reduced those known to exist.

ELECTRONIC RQUIPMENT

Because the load resistance (i.e., the electrical resistance across the
terminals of the pulse generator ) ean vary with each water tunnel or
towing tank, the electrical specifications of one pulse generator which
operates very well in one water tunnel may be insufficient to produce
similar bubbles in another water facility. The proper equipment specifica-
tions can be obtained as follows. First, an estimate should be made to
determine the maximum dimension of the flow fields which are to be visually
studied. For instance, in the case of a hydrofoil study, the width of the
wake (looking perpendicular to the chord and parallel to the span length)
might be the largest dimension. For a visual study of the wake looking
perpendicular to the span length, the span length would be the maximum
linear dimension needed for the wetted platinum wire. Once this length is
determined, a platinum wire of this dimension should be installed in the
center of a test section of the water tunnel. This wire is then energized
with a de power supply using the tunnel (or tow channel or suitably installed
anode) as the other terminal. Readings of voltage and current taken for
different flow velocities allow one to determine the load resistance of
' the water tunnel or tow channel.

From the load resistance, a desirable size for the lengths of the
Yows should be determined. It has been found that a value of 0.040 in. is
an appropriately photographable width for the rows of hydrogen bubble
clusters. The bubble rows in Figure 7 are of this size and therefore the
above value was so chosen. This is not a sacred number, however; it is

dependent upon the lighting and photographic setup in that whatever can

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be made to show the fluid velocity patterns clearly on film will suffice.
At some indicative value of the total pressure, the coulomb transfer across
the surface area of the wetted wire can then be determined. In the time
interval of the pulse width, this coulomb transfer specifies the required

current. “Thus cuisrenu as expressed, by:

al (a Sh) b ear
Ape eer

; RD Wr
where
is the eumcenu,
e is the charge of an electron,
A is Avogodro's number,
p is the pressure at the water surface,
Yy is the specific weight of the water,
h is the depth of the platinum wire,
d is the width of the hydrogen bubble row,
L is the length of the hydrogen bubble row,

R. is the Universal Gas Constant,
W is the pulse width, and

T ais the temperature.

Now the power per pulse can be specified and the electrical equipment output
determined when line losses are incorporated into the above result.

The hydrogen bubbles observed in the work reported here were produced
by several different power supplies. The de power supply which was used to
excite the kinked wire (DIMB unit, Type 140A, Serial 101) transmitted a
maximum of 150 V and 500 ma to the wetted wire. A continuously variable
voltage amplitude was available by means of this de power supply. The
insertion of a suitable switch in the output of this power supply permitted
a polarity reversal which was very helpful in keeping the wetted wire free
from platinum oxides.

Tn addition to the de power supply, a Hewlett-Packard Model 214A
Pulse Generator was used for the pulsed power supply. This unit delivered
O - 170 V into the approximately 300-ohm load resistance and produced

excellent bubble rows for flow velocities up to about 5 ft/sec. The output

Signal of this pulse generator had a reversible polarity and a 10-cps to
l-meps continuously adjustable repetition rate with a pulse width range
from 0.05 to 10.0 ms. The sharply rising and dropping voltage pulse,
attained with this instrument, was found to give distinct edges to the
bubble rows. The characteristics of the output of this unit, such as pulse
width, period, and amplitude, were accurately monitored by a Tektronix
RM-503 Oscilloscope (see Figure 8).

For clear and concise bubble rows at higher flow velocities, the out-
put of this pulse generator was amplified by the circuit shown in Figure 9.
Through this amplifier, the voltage transmitted to the wetted wire could

be increased to more than 4O0OV.

LIGHT ING

Ordinary floodlights were found too bulky to mount properly in the
hatch of the water tunnel, and they produced insufficient light. Accordingly,
the existing tunnel flash tubes (Edgerton, Germeshauser, and Grier FX-22)
were installed in the test section of the tunnel in their 28-inch glass tube
mounts in the hatch of the water tunnel (see Reference 7). They improved
the situation somewhat but could not be focused properly upon the bubble
rows. These lights were used for stroboscopic lighting and gave 5000-w,
4800-V flashes.

In order to attain increased clarity between the white bubble rows
and the black background, two 100-w Sylvania "sun guns" were mounted
beneath the hatch cover and above the water surface. Figure 10 shows two
"sun guns" mounted in the drained test sections. These very compact and
extremely bright lights proved very satisfactory for flow velocities as
high as 5 or 6 ft/sec. Not only can these particular lights withstand water
being splashed on them, but they can also operate continuously when com-
pletely submerged; the submerging advantage obviously eliminates the water-
surface reflection effect. This submerging characteristic is a distinct

convenience because the proper illumination of the bubble rows requires that

the light be directed at the bubbles to form an angle of about 125 deg with
the line of vision. For this type of illumination, the “sketches in
Figure 11 show a means of increasing the amount of light properly
directed at the bubble rows.

Such an increase in the amount of light would undoubtedly allow a
faster shutter speed for photographic purposes. The bubble motions in the
higher speed flows would be satisfactorily "stopped" to permit the use of
this steady "sun gun" lighting for higher flow velocities.

When higher flow velocities were achieved in the water tunnel, the
steady "sun gun" lighting was insufficient. Reference here is to velocities
in the range of 10 to 16 ft/sec. To achieve suitable lighting at these
velocities, strobotacs were used to slow down and "freeze" the motion of the
rapidly moving bubble rows. Figure 12 shows two General Radio Type 1531-A
Strobotae units mounted in the tunnel test section. These units were driven
synchronously by a General Radio Type 1217-B Unit Pulse Generator. The
flash rate was accurately determined by the digital readout of Hewlett-
Packard Model 522B Electric Counter. The flexibility available through the
continuously variable flash repetition rate and the continuously variable
pulse frequency to the wetted wire proved to be extremely helpful in the

subsequent film analysis as described below.

PHOTOGRAPHY

The resulting bubble patterns could easily be observed visually and
photographically. To achieve sufficient photographic contrast between
bubble patterns and the dark background, a unique photographic recipe was
developed. The most satisfactory 35-mm still camera was a Leica Model M-2
with a 50-mm Summicron lens and a dual range finder for close focusing.
Plus-X film rated at ASA 400 was found to give the best -résults. This )
film was developed in Acufine for the recommended time plus 25 percent.
The printed results were made by Polycontract "F" paper using a No. 9

filter and developed in D-72.

Motion-picture photography of the bubble motions was successfully
accomplished with 35- and 7O-mm Mitchell High-Speed FC Chronograph cameras.
The chronograph attachment was not used on either of the cameras. Although
higher film speeds were available, the 7O-mm Mitchell camera was operated
at approximately 25 frames per second. This frame speed was found to be
acceptable in stopping the flow patterns for free-stream velocities up to
about 5 ft/sec. Figure 8 shows the photographic and pulse-generation equip-
ment in the observation booth of the water tunnel. Prints of these motion
films enabled observation of the cyclic motion of the separation point on a
solid surface, the subsequent roll-up of this fluid into discrete vortices,
and the shedding of these vortices into the downstream wake of the body.
For reasons of compactness and ease of film handling, the 35-mm Mitchell
camera replaced the 7O-mm. Film analysis, i.e., the cyclic portrayal of

events behind solid bodies, proceeded in exactly the same manner.

SOME OPERATIONAL PROCEDURES

The electrical operation of the technique proceeds as follows. A
platinum wire of appropriate length is properly oriented in the velocity
ri@lle (Figure 3h). A suitably installed anode terminal is connected to
the ac or de power source. In the case of de power, the amplitude of the
voltage is adjusted to form a white and photographable sheet of bubbles.
The bubble diameter should be such as to minimize bubble rise due to
buoyancy. it is noted that excessively high voltages cause sporadic forma-
tion of large bubbles which rise due to their buoyancy. In the case of ac
excitation, the pulse frequency should be chosen in accord with an analysis
Similar to that. in Reference 4. The analysis is directed toward optimum
measuring conditions and is pertinent when the cross-derivative terms are
negligible in the series expansion of the longitudinal velocity. When the
pulse frequency has been properly chosen, the pulse width is then adjusted
to render the bubble lines photographable without excessive buoyancy

effects.

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The clarity of the bubble lines is very sensitive to the cleanliness
of the cathode wire. Removal of debris which accumulates on the wire can
be achieved in several ways. For example, the wire may be removed from the
water and carefully etched with a 20-percent solution of nitric acid. How-
ever, debris accumulates so frequently that this method becomes cumbersome.
When convenient access to the cathode wire or its wire holder is available,
a delicate striking of the wire holder with an electrically insulated object
suffices to loosen accumulated debris. It is of considerable importance to
incorporate this accessibility feature into the design of any closed test
section in which the hydrogen-bubble technique is to be used. Another is to
strike the metal side of the water tunnel with a small hammer.

The following method, which was discovered quite by accident, is fre-
quently more efficient and incomparably more convenient than any of the
others cited. In order to observe the bubble lines from an anode wire, the
pulse polarity was reversed on a wire which had been just operating for some
time as a cathode hydrogen-generating wire. When the polarity was changed
back again, the resulting hydrogen-bubble lines were amazingly concise and
distinct. The technical details of such a wire-cleaning procedure were not
investigated but it was felt that it was due to an electrostatic repulsion
process. Therefore, it has been found very convenient to arrange the
energizing circuit so that one has such a means available to clean the wire
while it is in the moving flow.

Another method which has been found effective is to energize the wire
before the fluid is in motion. The generated hydrogen rises immediately
and an upward flow is produced all about the vertical wire which sweeps
debris up and off the vertical wire. However, of all these methods, the

polarity reversal methos is the one most frequently used.

SOME PRELIMINARY EXPERIMENTAL RESULTS

The bodies about which the flow was visualized are shown in Figure 13.
The foil shapes are TMB modified NACA 66 profiles. Chord-to-thickness

ratios are 10:1 and 5:1; both chord lengths are 6 in. The circular

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cylinder is 1 in. in diameter. The two remaining bodies in the figure are
both plates. The plate on the right in Figure 13 has dimensions 2 by 1/2 alia c
and all surfaces are flat; the other also has a 2-in. chord but a thickness
of 1/4 in. and a rounded leading edge with a flat trailing edge 1/4 in.
thick. The spanwise dimension of all the cylindrical bodies is 6.66 in.

The photographic results for the various cathode-wire configurations
and experimental setups are shown in Figure 1 and in Figures 14 through 17.
in Figure 1 the flow characteristics downstream of the trailing edge of the
10:1 aspéct ratio foil shape are portrayed by the bubble patterns. MInstan-
taneous velocity profiles at the location of the O0.00l-in. diameter platinum
cathode wire are obtained in the manner previously described. For such an
undertaking, an enlargement of the photograph of the bubble patterns in
the wake region behind the wire facilitates the velocity determination.
Enlargements of different photographs are shown in Figure 7. The bright
area in the left side of the photographs is a light reflection off the sharp
trailing edge of the plate. The specification of position in such a compu-
tation is best done with an observable datum point wiich appears in the photo-
graph and is located in the plane of the bubbles. For unsteady flows,
similar to the one shown in Figure 1, the time-variant velocity profile at
one location in the wake is obtainable with a series of photographs encom-
passing the period of oscillation. An entire wake survey can be obtained
with successive locations of the cathode wire downstream of the trailing
edge. Although this can be a laborious process, a result of considerable
Significance is achieved. Such a survey was performed, not in a wake
flow, but in a very thick boundary layer, by Hama and Nutant (Reference Be
They were among the first to use the hydrogen-bubble technique and
excellent photographic results were obtained in this low-speed investigation.

Figure 14 illustrates a typical bubble pattern about the 10:1 aspect
ratio foil shape for a situation in which a kinked wire was positioned
upstream of the leading edge and energized with de voltage. Note the dis-
tortions of the mainstream flow due to the presence of the body. In
addition, the streaklines demonstrate the involvement of the free-stream

flow in the wake region behind the body.

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To visualize flow characteristics very close to the body, i.e., in the
boundary layer of the body, it is necessary to insert the streaklines into
the boundary layer itself. This is done by positioning the kinked wire
very close to the leading edge of the body so that the streaklines begin
in the stagnation region and are dragged around the body in accordance with
the boundary-layer characteristics. Caution should be used, however, in
interpreting these streakline patterns. Hama has shown that very deceptive
streakline patterns can be obtained in shear flows. That is, streakline
rollup patterns can be wrongly interpreted as illustrating concentration
of vorticity in shear flows in which there is no wave amplification what-
soever. Streakline patterns about the l-in. diameter cylinder are shown
in Figure 15. In this series of photographs the streaklines portray the
eyelic behavior of and the mainstream participation in the flow developments
behind the cylinder.

The hydrogen-bubble technique can also be conveniently utilized to
determine the position of the separation point of the flow about a body.

To use the technique in this manner, however, two wires operating in the
flow simultaneously are usually needed. Because of the separation from the
body of the streaklines due to boundary-layer characteristics, it becomes
necessary to put bubbles into the flow downstream of the separation point.
Thus, the streaklines upstream of the position of separation exhibit the

expected velocity gradient:

tei) es

where vu is the longitudinal velocity of the flow in the boundary layer and
y is the transverse coordinate perpendicular to the surface of the body.

The position of separation is defined as the position on the body where

au | m
es Oban

Downstream of the separation point, the flow is reversed and

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a certain position on the foil surface, and a similar specification of a
reversed flow condition proceeding in the opposite direction up to a certain
position, indicates that separation must occur between these respective
locations .” When the interval between such points is very small, the
prediction of the location of the separation point is accurate. Figure 16
shows such an interval between positive and negative velocity gradients.
The two wires in the flow are straight and excited with a d-c voltage. One
is positioned at the front stagnation point of the foil shape and illustrates
a positive velocity gradient. The other is smoothly attached perpendicular
to the surface of the foil and supplies bubbles into a reversed flow region.
In the thin interval between these groups of bubbles lies the separation
point on the surface of the body. The separation streamline is contained
between these two groups of hydrogen bubbles. Motion-picture photography
of such a separation region obviously enables determination of any cyclic
motion of this point and the correlation of such motion with wake phenomena.
One of the more severe limitations of the hydrogen-bubble technique,
which has also been found by other investigators, is the low-velocity
flows to which the technique has been confined. This is primarily due to
the delicate hardware used to install bubbles in a water flow. To install
a 0.00l-in. diameter platinum wire in a water flow so that the wire remained
straight, the following was done. A heavy rod or steel wire frame, which
was adequately insulated from the water, was made into the configuration
visible in Figures 1 and 2. The thin cathode wire was soldered across the
two cantilevered portions of the wire holder. Before the two ends of the
wire were soldered, however, the cantilevered portions of the wire holder
were bent together very slightly. When the remaining end of the wire was
soldered, these portions of the holder were released and stretched the
thin wire taut. As one can easily imagine, the amount of wire-holder

bending required extensive practice before competent wire installations

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were achieved. The result of such a process was a wire installed as in |
Figure 1. However, such a configuration required very little lateral drag
force to cause breakage of the wire - usually at the solder joint. Suf-
ficient lateral drag force for breakage was produced at water velocities as
low as 1 to 3 ft/sec. Obviously, the susceptibility to breakage also depends
on the initial wire tension.

To surmount this difficulty, a new wire-support mechanism was devised
by Mr. John Coon of the Model Basin. This new manner of positioning wires
utilized two rods of elliptical cross section (major axis 1/4 in., minor
axis 1/16 in.). One such rod was mounted through the top and the other
through the bottom of the closed-jet test section. The plexiglas test section
was modified to enable positioning the platinum wire at any location in the
streamline direction. Because of the elliptic section, rod vibration due to
vortex shedding was radically reduced and the increased rigidity of the
supports was apparent. A disadvantage of such a support system was that the
wire could not be easily stretched into a straight configuration. When
this was attempted, breakage usually occurred. The non-taut wire was then
used and, although the wire assumed the form of a catenary, an extremely
high water velocity could be obtained before breakage occurred. In fact,
tunnel velocities as high as 16 ft/sec were achieved. Figure 5 shows the
results of such a high-velocity flow situation.

For the above-mentioned velocity profile determination, it is not of
critical importance that the cathode wire be straight. The significant
features are bubble-line separation in the transverse direction or loca-
tions of particular bubble-line intersections in successive film frames,
pulse frequency and motion-film rate, and the scale factor encountered in
the photography. For the streamwise specification of the location of the
computed velocity profile, it is convenient to adjust the catenary-shaped
wire so that the vertically sloped section occupies the center of the
wake. This provides for initially straight and transverse bubble lines
in the wake region at the streamwise location of the wire.

Additional difficulties in using the hydrogen-bubble technique are
caused by vortex shedding behind the thin cathode wire. This vortex

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shedding causes rapid diffusion of the otherwise distinct edges of the
bubble rows. Further problems, which time limitations in the present
investigation did not permit studying, related to the possibility of
excessive, but undetectable, wire oscillation and the deceptive velocity
fields behind these thin wires. It is obvious that such a body in a flow
has its own wake and this may obscure the wake profile that is being
investigated behind a foil or other shape. However, as seen in the photo-
graph, the delicate hardware can be arranged to install hydrogen bubbles in
flows having velocities far beyond speeds of 1 ft/sec. It is logical to
expect, therefore, that small, more delicate wires may be similarly
installed in flows where velocities reach 10 to 15 ft/sec without intro-
ducing the shedding errors that undoubtedly exist behind an O.OOl-in.
diameter wire in flow velocities of 10 to 15 ft/sec (R, > ko).

The following cathode-wire configuration has been found convenient in
determining the general characteristics of the three dimensionality of a
supposedly "two-dimensional" flow. The wire should be installed in the
- flow in such a way that it is parallel to the generatrix of the body
about which the flow is being studied. For instance, if the wire is
positioned parallel to the trailing edge of the foil shape and perpendi-
cular to the flow direction, the "plan view" of the velocity field can
be observed. In fact, this was done by Hama and Nutant and the wire was
energized by a pulsed excitation. Figure 1/7 illustrates such a wire con-
figuration. The excitation of the wire near the trailing edge is with de
yoltage. The upstream kinked wire is energized similarly. In this
case, the trailing-edge wire is crudely attached to the straight walls of
the test section by means of tape. It should be mentioned that photography
parallel to the resulting wake bubble patterns produces blurred and
apparently poorly focused photographs. This is especially true for the
short depth of focus encountered with the necessary shutter openings. As
was done in Reference 3, photography should be performed perpendicular to

such bubble patterns; unfortunately, time did not permit the alterations

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necessary for such photography. However, with the naked eye (or proper
photography) one can gain considerable insight into the three dimensionality
encountered in the wake of these "two-dimensional" bodies. The literature
verifies that such flows can be extremely predominant (Reference 10) in
reality.

When experimental investigations are considered in water, it is
apparent that with discrete sizing of characteristic body lengths of models,
it is important that the water velocities needed to meet specific parameter
values are adaptable to the necessary bubble quality. Although it has been
definitely verified that velocities can exceed certain values without wire
breakage, there is a decrease in bubble quality with increased water velo-
city, not to mention the increased likelihood of vortex shedding error which
undoubtedly occurs at higher flow velocities.

Further flexibility in the bubble technique is attained by allowing
a particular portion of a metallic model to act as a cathode terminal (see
Reference 2). When this was attempted, the bubbles were unevenly dragged
from the body, apparently because of excessive boundary-layer thickness.
Therefore, when: the model itself is used as a cathode terminal, the local
fluid velocity has to be sufficient to drag the bubbles into the flow. A
dielectric covering (paint or plastic tape) can easily be applied to the
entire surface of the body in the fdlow with the exception of the region
where the bubbles are to be produced. Another way to achieve the same
effect is to imbed a platinum wire or other conductor into the surface of
a plastic or nonconducting body. One has then only to energize either the
entire body or the imbedded conductor with a de voltage to obtain a steady
stream of bubbles from any point on the surface of the body. Pulsed
excitation was found rather unsuccessful due to the uneven shedding of
bubbles between electric pulses. An attempt was made to utilize the trail-
ing edge of the foil shapes in such a manner, but the velocities in this
region for very small angles of attack were apparently insufficient to
drag bubbles into the flow evenly all along the entire span of the body.
Although the bubbles were not dragged evenly (as a sheet of bubbles) into

the flow along the span length, one could observe whether or not the

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downstream stagnation streamline intersected the trailing edge of the body.
Such an observation resulted from the visible reversed flows on the upper
and lower foil surfaces or the absence thereof in the region of the trailing
edge of the body. Therefore, determination of the satisfaction or the vio-
lation of the Kutta-Joukowski condition is possible; the degree of accuracy
depends on the accuracy with which the conductors were positioned on the
surface of the foil. Such a determination would undoubtedly employ the
definition of separation point and reversed flow discussed earlier and the
lighting scheme as sketched in Figure 11. An experiment using essentially
this same method for specifying the position of separation is described

in Reference 9.

For instance, in the surface of a plastic foil shape, wire conductors
which can be energized separately are placed so that they are parallel to
the trailing edge and spaced, e.g., 0.010 in. apart; then the point of
separation could be determined on this foil to an accuracy of not less than
0.010 in. Such accuracy is achieved by observing between which two wires

the velocity gradient is normal to the surface and changes sign.

ACKNOWLEDGMENTS

The author wishes to express his gratitude to Dr. F. R. Hama for
stimulating his initial Aeeeeet in the hydrogen-bubble technique.

Specific thanks also go to Mr. John Coon, who greatly assisted in the
establishment of the technique at the Model Basin.

Last, but certainly not least, many thanks for the helpful suggestions,
endless patience, and expert photographic experience which were made avail-

able to the author by Mr. Berkley Ball of the David Taylor Model Basin.

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REFERENCES

1. Hama, F. R., "“Streaklines in a Perturbed Shear Flow," Physics of
Fluids, Vol. 5, No. 6 (Jan 1962).

2. Clutter, D. W., et al, "Techniques of Flow Visualization Using Water
as the Working Medium," Douglas Aircraft Company Report ES-29075 (Apr 1959).

3. Hama, F. R. and Nutant, J. A., "Detailed Flow Field Observation in the
Transition Process in a Thick Boundary Layer," Proc. of the 1963 Heat

Transfer and Fluid Mechanics Institute, Stanford University Press: ~ (1963).

4. Schraub, F. A., et al, "Use of Hydrogen Bubbles for Quantitative Deter-
mination of Time Dependent Velocity Fields in Low Water Flows," Report
MD-10, Thermo Sciences Division, Department of Mechanical Engineers,

Stanford University (Feb 196).

5. Schlichting, H., "Boundary Layer Theory," Fourth Edition, McGraw Hill
Company (1960).

6. Nowell, R. W., "An Investigation of Fluid Motion with Variable Vis-
cosity,'' M. S. Thesis, Department of Aerospace and Mechanical Sciences,

Princeton University (June 1965).

7. Bowers, W. H., “The 12-Inch Variable Pressure Water Tunnel Propeller

Testing Procedures," David Taylor Model Basin Report 505 (Nov 1943).

8. Venning, E. and Shields, C. E., "Wake Studies in the 12-Inch Propeller
Tunnel," David Taylor Model Basin Report 1536 (Jul 1961).

9. Schubauer, G. B., "Air Flow in a Separating Boundary Layer," NACA
Report 527 (21 Annual Report (1935)).

10. Mattingly, G. E., “An Experimental Investigation of the Three-
Dimensionality of the Flow Around Circular Cylinders," Institute for

Fluid Dynamics and Applied Mathematics Report BN-295 (Jun 1962).

11. Morkovin, M. V., "Flow Around a Circular Cylinder - A Kaleidoscope
of Challenging Fluid Phenomena (Including Flow Instabilities and Transi-
tion to Turbulence)," Proc. American Society of Mechanical Engineers

Symposium on Fully Separated Flows (May 1963).

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Bubble lines from straight wire
positioned behind plane of
Streaklines. A separate pulsed
power source is used.

——

Straight Wire
Streaklines from kinked wire energized with

dic. power source. These lines are referred
to in later discussion as horizontal lines.

Kinked Wire

v(At)

a u(At)

Figure 3a - Bubble Pattern for Longitudinal and Transverse Velocity
Field

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8

Rows of Bubbles
from Vertical Wire

Ribbons of
Bubbles from
Rungs

i 1 di

Anode Terminal
(‘ On (Copper Sheet)
Anode
Terminal
(Copper Uy Pulsed Excitation
Sheet) lh ~

Figure 3b - Wire Configuration for Longitudinal and Transverse Velocity
Profile Determination

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Figure 5 - Typical Bubble Patterns Behind a Two-Inch Plate
(Freestream Velocity is 16 fps)

Figure 6 - Twelve-Inch ID Plexiglas Closed-Jet Test Section with
Two-Dimensional Sides Installed

34,

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Figure 7 - Enlargement of Bubble Patterns in the Wake of a Flat Plate with
Sharp Trailing Edge (Freestream Velocity is 1 fps)

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ee

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Figure 8 - Leica Camera and Pulse Generation Equipment

wy San

Pulse Out

Pulse In 4 4/600 V

— : 22 K/|W
INIT20

= A

ZA S|

400 ut/500 V

O- 120 VAC Input

Figure 9 - Amplifier Unit Built for Hewlett-Packard Pulse Generator
Model 214-A

Figure 10 - Two Sylvania "Sun Guns" Mounted in Drained Test Section

Sun Gun Mounted
above Water Level

Water Level

Side Wall Painted Black for
Contrast with the Rows of Bubbles

Line of
Observation

Plane in which Bubbles Appear

Closed Jet Test Section

Air Water Level
Water

Sun Gun
(Submerged)

Plane in which Bubbles Appear

Line of
Observation

Black Side Wall

Sun Gun
(Submerged)

(b)

Figure 11 - View Parallel to Stream Velocity (Looking Upstream) of One
Lighting Scheme (a) and an Improved Setup (b)

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Figure 12 - Two General Radio Strobotacs Mounted in Hatch of Test Section

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Figure 14 - Streakline Pattern About a Foil Shape, Aspect Ratio is 10:1
(Freestream Velocity is 1 fps)

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Unclassified
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DOCUMENT CONTROL DATA - R&D

(Security classification of title, body of abstract and indexing annotation must be entered when the overall report is classified)

1. ORIGINATIN G ACTIVITY (Corporate author) 2a. REPORT SECURITY LASSIFICATION }
26. GROUP ‘
David Taylor Model Basin Lieit ee ee

3. REPORT TITLE

The Hydrogen-Bubble, Flow-Visualization Technique

4. DESCRIPTIVE NOTES (Type of report and inclusive dates) K
Final

5. AUTHOR(S) (Last name, first name, initial)

Mattingly, George E.

6. REPORT DATE J 7a. TOTAL NO. OF PAGES 7b. NO. OF REFS
Februa 1966 O JLaL

8a. CONTRACT OR GRANT NO. 9a. ORIGINATOR'S REPORT NUMBER(S)

b. PROoJECTNO. S=ROOQ-O1LOL

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Qualified requesters may obtain copies of this report from DDC.

f 11. SUPPLEMENTARY NOTES 12. SPONSORING MILITARY ACTIVITY

None Bureau of Ships

| is. speak SUMMARY

The hydrogen-bubble visualization technique has been adapted to the 12-inch
variable-pressure water tunnel of the David Taylor Model Basin. An outline of
this adaption and the operation of the technique are described.

Photographic techniques and analyses applied to the resulting films are dis-
eussed. Sources of error are delineated, particularly with regard to the decep-
tive streakline patterns that can be formed and especially the results of exceed-
ing the velocity limitation imposed by the shedding phenomena taking place behind
the platinum wires. Errors caused by compression and/or stretching of bubble
lines along their length are discussed, and procedures are given for recognizing
this type of error. In addition, cathode-wire configurations are described by:
which both longitudinal and transverse velocity profiles can be obtained in
steady or unsteady water flows.

Various cathode-wire configurations are described through which qualitative
aspects of the flow about bodies as stagnation and separation point motions are
depicted.

DD i, 1473 Unclassified

Security Classification

Unclassified
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KEY WORDS

Flow visualization
Hydrogen bubbles
Water tunnel
Streaklines

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pu ‘pessnosip aiv yjdu9] Jey) Suoje sour ayqqngq jo sutyojaI4s

10/puv uotsseidwioo Aq pasnvo siougq -soitm wnujeld ayy

pulyeq aouyd Suiye) eueswouayd Suippoys ay) Aq pesodur uone}

-Iwt] AjID0[9A ayy Julpaeoxe jo sy[nsai ay) Ajjetoedse pur pauoy

oq ed 4BY SUIe}}ed eUT[YBeI4S eAtdao—ap oy} 07 piedai YIM

Ajae[noyied ‘paywaurjap aie 10110 Jo saoanog *PpeSSNosip av Su[T
Surnses 84} 0} parjdde sasdyvue puew senbruyoa, o1ydeisojoyg

*peqtiosap aiv enbruyoa, ay}

Jo uo1yeiedo ay) pu uondepe sty) jo aut;jno uy -urseg [2poW

JoTABT, prawq oy} Jo Jeuuny 19}¥mM oinsseid-ayquiieA YOur-ZT ey) o7
peidepy useq sey enbruyoo, uorezipensia 214q4NqQ-uadoipAy ayy,

GaldISSWIONN “sje
“siderp “snqyt dog ‘tr -996T qaq -Ajsumep “q-y Aq “ANOIN
“HOUL NOILVZITVASIA-MOT4 “ATGENA-NADOUGAH AHL

‘OVIZ 440day 0 -ulspg [2PoW 4ojAdy pang

pue “pessnosip aiv yydue] tay) Suoje sour, eyqqnq jo sutyojeas

10/pue uorsseiduioo Aq pasneo sioug -seltM unuNeyd ay)

putyeq eouyd durye} eueuoueyd Suippeys ayy hq pesodur wore}

-twr] AjIO0[9A ayy Sulpaaoxe jo sqjnse1 ay) Aqjeroedso pue pewioj

aq uvd 48Y4} SUIe}}8d OUITyBeI7S eATdadap ay) 07 prede1 YIM

Ajavjnoyied ‘peyeauljep oie ole jo seoinog *peSsnosip o1B SUITIy
Sunqjnsea ey} 0} perjdde sescyeue pu senbruyoa, o1ydeasoj0yg

*peqtiosep oie enbruyoa, ay}

Jo uonjziedo 24) puw uondepe sty} Jo autyjno uy -urseg jepoy

JopABT, plAv Oyj Jo Jeuuny 10yBM oansseid-ajqetseA Yout-ZT oy) 07
pajdups ueaq sey onbruyoe; uorjezipensta 9[qqnq-uedoipky ayy,

GaIaIsSSVIONA “sje
“saderp “snqt “dog ‘tt “9961 qaq “AisurnyeW “q-D Aq ‘ADIN
“HOGL NOILVZIIVASIA-MOT4 “AIAANa-NADOUGAH AHL

“OVLZ Hoday = -ulsog japow s0jADy plang

‘aD ‘AsuneN *]

aNd

--"$"f)--Sjeuuny Jayeq -¢
senbiuyoay,
--"G°(]--SJauun) 10984 “Ff
SOUITYBO14S

--Aydeidojoy g--seiqqng -¢
uorneZITeNsiA

~MO[j JO SOUTT--Soiqqng -G
senbiuyoay,

--UOlBZI[BNSIA--MO[ “TL
‘

‘aD ‘Apsunqey “]

anid

--"S"(]--S]auun) 107eQ “¢
senbtuyooy,
--"$"(]--SJouun) 10384q “PF
SaUT[yeAI19S

--Aydeisdojoy J--seiqqng -¢
UONBZI[eNsIA

ANY BS 260) FEM Mg ASSYLEE 26
sonbruyooy,
--UOIJBZI[BNSIA--MO[Y “TL

pue ‘pessnosip aiv yydue] s1ey2 Suoje saury atqqngq jo Sutyoqans

Jo/pue uorsseidwos Aq pasneo siougq -selim wnuyed ay)

puryeq eovld durye) euswoueyd Surppays ay) Aq pasodut u0i7e]

-IWI] Aq190[8A ay} Sutpaaoxe Jo syjnsai ayy Ajjetoadsa pus pawioy

oq Ue 4BY47 SUIa}78d auI[yBE23S aATydadap ay} 0} prvdas YIM

Ajae[nonied ‘paysoutyep ie 10118 Jo Sedinog -passnosip aie sw
durj[nsei 94} 0) pardde sesXjeue pue sanbruyoay o1ydeidoj oy g

“paqtiosep oie enbruyoe} ayy

Jo uoMviedo 04) pue uotjdepe siyy jo autjjno uy -ulseg japow

JO[ABL, PIAB| OY} Jo [ouUN) 19y"M onsseid-ayquIIVA YouI-ZT ey) 4
peydepe useq sey onbiuyoay uorezipensta atqqnq-uesoipdy oy,

GaI4ISS VTONN : *sjal
“sidurp “snqyt “dog “11 “9961 qed “AZuneW “aD Aq “AQOIN
“HOUL NOILVZITVASIA-MOTA ‘ATAANE-NADOUGAH FHL

‘QVIZ Hoday = -ulsng japow 40j;A0y plang

pue ‘passnosip oie y{sue] itey) Suoye seury ayqqnq jo Suryoqeays

Jo/pue uotsseidwios Aq pasneo sioug “seiMm wnutjeld ay)

puryeq eovyd Surye] euewoueyd Surppays ay) Aq pesodui uore)

-{ult] AJID0[0A ey) Durpaaoxe jo sj]nsai ayy Ajjetoedse pur pewi0;

8q uvd 4BY) SUIE}Qed OUI[YBAI]S BATydadap ay) 0} predoed YIM

Ajavjnoqied ‘payveutyep aie 1018 Jo seonog -passnosip aie swt
Sunnses 94) 0} parjdde sesAjeue pue sonbruijoaq otydeidoj0y g

*peqiiosep aie enbruyse} ayy

Jo uoijeviado oy] pus uondepe sty] jo ouryjno uy -ulseg japow

JojABL PlABq eyy Jo JeuuNy 103eM oinssosd-a]quIaVA Youl-gI ayy
peydepy ueeq sey enbruyoe; uoneztyensia ofqqnq-uedoipAy ayy,

GaldISSVIONOA *sjol
“saserp “snqpt “dog ‘it -996T qaq “A[sunqeW “gD Aq “ANOIN
-HOGL NOILVZITVASIA-MO14 ‘AIGANE-NADOUGAH AHL

‘OVLZ today = -uIsDg japow 4ojAd} plang

*payoidap aie suotow yu1od uoneaedes pue

UoleUude}S SB SeIpog yNoqe MOT] ayy Jo Sjoadse oaeyITeNb yorym
ysnoiy) peqtosap o18 suoieinsIjuoo o11M-apoyywo Snowe,

*SMO]J 1032M Apvoysun 310 Apeeys

UI peulego aq UBd SajIjoid AyID0]eA aSIOASUBI] pus |eUIPNzIsUOT

yjog yoy Aq paqiiosep oie SuOIjeINsIjUOD o1IM-apoyjeo ‘uoTy

«[ppe uy “10118 jo edAq sty} durziudooel 10} uaAId o1e Soinpadoid

*pajoidep oie suoijow ju1od uoneiedes pue

UuOIeUde}S SB S8Ipog yNoge MOT] ay} JO Syoedse aATyeII;ENb yoryM
ysNo1y} peqiosep o1% SUOTIVINSIZUOD 1IM-opOyjVd SNOLIeA

“SMO]j Joye@M Apvaysun 10 Apees

UI pauteqyqo oq uvod Saptyoid AjID0T0A eSJOASUBIY puv [VUTPNyIsuUOT

yjoq yorym Aq paqtiosap ele SuoNeindju0d aI1M-apoyjyeo ‘uory

eIpp¥ UT “sola jo odAj sty) durztusooeI 10) ueAts 018 Soinpedoid

*peqoidep aie suoow quod uoneiedes pus

uonvudeys S¥ SeIpog jnoqge MOTj ayy JO Sjoodse oanezITenb yorym
ysnoiyy pequosep oie suoteindyuoo e11M-apoyyeo Snowe,

*SMO]j JoyeM Apvaqsun io Apeays

UI pauleqqgo oq uBo Seztjoid AyIOoJaA asiaASUBIy puB [BUIpNIsUO]

yioq yorym Aq paqiiosep oie SuONeIndIyUuOd a11M-apoyyeo ‘UOI}

cIpp¥ uy “10a Jo adAy Siy} Sulztudo9a1 10j udAId ov Seinpaooid

*payoidep aie suotow yu1od uonvaedes pue

uoleude}s Se SeIpog ynoqe MOTJ ayy Jo syoadse aaneytyenb yorym
Ysnoiyy pequosep e18 suOIqVINSIjUOD o1IM-apoyywo SNoWBA

*SMO]j 108M Apvajzsun io Apeays

UI peuleqqo oq UBd SeTtjoid AqIOO][@A aSIaASUBI) PUB [BUIPNyTsuoT

yjoq yorym Aq paquosap aie SuoMeinstyuoo a11M-apoyyeo ‘uOTy

cIpp® uy ‘soize jo adAy siyy durztuso0e1 Joy UaAIS ov Soinpodold

puw ‘pessnosip aie yydue] atey) duoje saul, ayqqnq jo Sutyoqe1s

Jo/pus uorsseidwios Aq pasn¥o siouq ‘seiim wnuNeld oy)

putyeq eovjd durye) euewousyd Sutppeys oy) Aq pesodu uo17e4

-Tut] Aj100]9A ay) DuIpaeoxe Jo sy[nsei ay} AT[Bloedse pue paulo}

aq UBd 4BY) SuIE}}¥d aUITYBEeI)S BAIydadap oY) 07 predal YIM

Ajae[nonied ‘peyseutjep e18 Jole Jo seoinog “passnosip ase sully
duyjnsei 94} 03 parjdde seskjeue pue sonbiuyoe; o1ydeasojoyg

*peqtiosap aie anbruyoe) ay}

Jo uolyiedo ayy pus uodepe styj Jo aur;jno uy -ulseg jepo;

JO[AB], PIAB| ey} Jo [auUN] 1038M oInssaid-a[qulIBA YouI-Z] ey) OF
peydeps useq sey anbruyoe, uorezipensia a[qqnq-uedoipAy ayy,

“OD ‘ApsueW “I

ania

--"S$"f]--SJouuny 10784 -G
senbiuyooy,
--"§"f]--SJeuuny 10384 “Ff
SOUuI[xB019S

--Aydeidojoy J--seyqqng -¢
woNnezIpensi,

=-MOfj JO SourTy--serqqng <<
senbruyooy,
--UONBZITBNSIA--MO]q “T

GaIaISSVIONN *sjol
“saderp “snit “dog ‘tr “9961 qaq “A[sunyeW “aD Aq ‘ANDIN
-HOUL NOILVZITVASIA-MOTA “ATA NG-NADOUGAH AHL

‘WIZ Hoday = -ulsog japow 40jADy prang

pus ‘pessnosip olv yydue] atayy duoje seul, afqqnq jo Sutyoj034S

10/pus uoisseiduios Aq pasned siougq “Selim wnuned oy)

pulyeq eovyd duiye} euewouseyd dutppeys ayy Aq pasodut uo1je4

-IWt] AyID0[8A By} Julpaaoxe jo sj[nsoei ay) A][eloedso pue pewioy

oq uvd 4BY] SUIEe}}ed oUITY{BeI)S AAIdedep 04] 07 predol YIM

Ajaenoyied ‘payeeutjep o1¥ 10110 Jo Seoinog -pessnosip ais sult]
Sunjnses eyy 07 parjdde seskjeue pu sonbruyoe, o1ydeasojoyg

*peqtiosep aie anbruyoay oy)

Jo uo1jeiedo ey) pus uodepe srtyq jo aulyjno uy -ulseg [epoy,

JO[ABT, PIAB 944 JO [ouUN) 1a38M aInsSaid-a[quIIVA YOUI-ZT ey} 0}
poydepe ueeq sey anbiuyoe; uoljezipensta afqqnq-uesoipAy oy J,

“OD ‘ApsuEW “I

anid

--"§')--S]euun 1eyeMq “G
senbiuyooy,
--"§"f}--Sjouuny 1038Mq “F
SeUI]48019S

--Aydvasojoy g--sejqqng -¢
UONBZITensiA

=, MO]J/ JO SOUlT--Selqqnd iG
senbruyoay,
--UONBZI[BNSIA--MO[y “T

dalaIsSVIONOA *sjo4
“sadeip ‘snqjt “dog ‘tr “9961 qeq “ATsuineW “aD Aq ‘ANDIN

-HOUL NOILVZIIVASIA-MOT4 ‘A TAANG-NADOUGAH FHL
“OPLZ odey «-ulspg {2POW soyAdy plang

‘ap ‘ApsueW

anLa

--"S"(\--SJeuun) 1078A °G
senbiuyoay,
--"G"(]--SJouun) 10384q “F
SOUTTYB2I9S

--Aydvadojoy g--seqqng -¢
UOIeZITeNsIA

--MO[j JO SoUTT--Serqqng «G
senbiuyooy,
CALCONGS EZ A FMS NAGY |G TLE

“aD ‘AysuBN “T

anid

--"$"(]--Souun) 1038q °G
senbiuyoay,
--"Q°(]--SJouun) 10384 “FP
SOUI[Y2OI9S

--Aydvidojoy g--Sseiqqng °¢
UOMBZITENsiA

--MO[J JO SOUIT--SeTqqng ~G
senbruyoay,
--UOIJBZI[ENSIA--MOTY “T

pue ‘passnosip e18 yydue] Itayy SuoTe Sout, etqqnq jo Suyojens

10/pue uotsseidwos Aq pasned siouq ‘salIM wnuyeld a4)

puryeq eoujd duiye) euewousyd Sutppays ay) Aq pasodur u0i4e4

-tuit] Aqioo[aa ayy Suipaaoxe jo sqjnsei ey) AT[Btoadsa pus pawioy

aq Ud BY} SUIE}98d aUT[YBeS BATyda0~ap ayy 0} predol YRIM

Ajae[noyied ‘paysauljap 018 10110 JO SaDINOG -pessnosip o1B SUITIy
Su1j[NSei ey) 07 petjdde seskjeue pue senbruyoe} o1ydeadojoyg

*paqtiosap aie anbriuyoa4 ayy

jo uorjeiedo ayy pus uondepe siyj jo aut;jno uy -ulseg [epoy

Jo[AB], praAvq ay) jo [euun) 1038M oinssaid-a[qBlsVA YOUI-ZI ey} O}
pojdeps ueeq sey anbiuyoe, uoezipensta afqqnq-uesoipAy oy,

Ga1dISSVIONA *sJol
“sideip “snq[t “dog ‘tr -g96T qaq “AysunjeW “aD Aq ‘ANDIN
-HOGL NOLLVZITVASIA-MOTA ‘A TAANE-NADOUGAH AHL

“OplZ Hoday -ulsog japow sojADy plang

pue ‘passnosip 018 yjdue] Itayy SuoTe Sout, efqqnq jo dutyoyeIs
1o/pue uotsseidwos Aq pasn¥o siolq “Selim unuyeld ayy
.putyaq eovyid Surye) euowousyd Sutppays oy) Aq pasodut uo1je}
wi] AjId0[0A ey} Sutpasoxe jo s}jNsei ay} AT[etoedsa puv pawoj
aq uvd yeYy SuI0})}Ud aUI[YBeIS BATJdaDap ay) 0} predaI YIM
Ajavjnoyied ‘paywoutjep av JoJo Jo SeoIn0g “passnosip oie SWI[TJ
surj[nsei ayy 07 parjdde seshjeue pue sanbiugqoe, o1rydeidojoyg
*peqtiosap av anbiuyoe; ayy
jo uotjeiedo 043 pus uoldepe siyj Jo oul;jno uy “uIseg apo;
JofAVy, PAV oy} JO JouUN] 1938M OaNSSaid-afqRiiBA YOUI-ZI oy} 0}
peydepe ueaq sey onbiuyoe; uorjezipensta efqqnq-uedoipAy oy,

Gul4ISSVTONA ‘nee
“saderp “snqyt “dog ‘mr “9961 qed “A1sunqeW “aD Aq “ANDIN
-HOUL NOILVZITVASIA-MOTA “ATAANE-NADOUGAH AHL

“OplzZ soday -ulsog japow 4ojAD] plang

*pazoidap aie suoijow yurod uoneaedes pug

UOIVVUde]S SB SaTpog ynoqe MOTj ayy Jo Sjoedse oAr;EyITENb yorym
Ysnoiyy pequosap a1v suoTzeINFIJUOD o1IM-9poyyed Snowe,

*SMO]J JoyeM Apvaysun Jo Apeays

UI pauleqqo aq uvd SaTIjoid AjID0[9A eSIaASUBI] PUL [eUIpNyIsUoT

yjoqg yorym Aq paqiosep aie SUOT}eINSIJUOD oIIM-apoyjed ‘uOTy

«Ippe uy “10118 jo addy Sty} duIziudodeI 10} UsAId 018 SeInpadoid

*payoidep oie suotjow yuiod uoneiedes pus

uoNeuse}S SB SeIpog ynoge MOT 94} Jo Syoodse aANeyITenb yorym
ySno1y) peqiuosep o1¥ SUOTZBINGIJUOD o1IM-BpoOyjed SNOLIeA

*SmO]j Joyem Apvoajzsun Jo Apways

UI peuleqqo eq uv Sazjoid AYIOOJeA BSIOASUBI) PUL [BUIpNyIduoT

yjog yorym Aq paqiiosep oie SUOTeiINdIyUOD o11M-apoyjeo “UOT}

<Ippe® uy “Josle Jo edAj sty) durztusodeI 10j ueAtsd aie Seanpaooid

*pazoidep ai8 suoow yuiod uoyeiedes pue

uoleudes S¥ SaIpog ynoqe MOTJ BY) JO Sjoodse aANezITENb yoy
Ysnoiyy pequosep aiv suotzeinsjuod a11M-apoyjyeo SNOT,

*SMO]J 1032M Apvojsun 1o Apeajs

UI peule{qo aq ued Sajtjoid AyIO0]9aA aSieASUBI) PUR [eUIPNyIsuO|

yjog yoy Aq paqiiosep aie SuOIyeINSIyuOo aI1M-apoyjeo ‘uoty

eIpp@ uy “1o11e jo adAy siyj dulziusodeI 10j UaAId aie Seinpedoid

*pejoidep o18 suoijow ju1od uoneiedes pue

uoleuse}s SB SaIpog ynoqe MOT} ayy Jo Sjoadse sAtjezITeNb yorym
ysnoiyy peqiuosep 018 SUOT|eINZIJUOD a1IM-9poyjzwd SNOIeA

“SMO]J 103784 Apvaysun Jo Apees

UI pauleqqo aq uo Sattjoid AjIO0;9A aSiaASUBI] pue [BUIPNyIsuo]

yjog yorym Aq paqiosep ov SuOTZBINSIjUOS aI1M-apoyyeo ‘uUOIy

eIppe uy “s0I120 jo odAj Sty} duIzIUdODeI 10} UAAIS a1e Sainpeooid

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