A high-speed multi-source spark camera

" by J. H. deLeeuw. 1. 1. G!ass

and

L. E. Heuckroth

FT

A HIGH-SPEED MULTI-SOURCE SPARK CAMERA by FEBRUARY. 1962 J. H. deLeeuw. 1. 1. Glass

and

L. E. Heuckroth

.

.

~:

ACKNOWLEDGEMENTS

We wish to thank Dr. G. N. Patterson for his interest and encouragement during the course of developing the camera and applying it to various problem s.

We are indebted to Mr. R. J. North, National Physical Labora-tory, England, for his communications and assistance while constructing the camera along the lines which he first developed at N.

P.

L.

We appreciate the assistance received from Mr. S. A. Gordon and Mr. L. E. Harris in selecting some of the optical and electronic compon-ents and from Mr. W. H. Kubbinga and Mr. B. G. Dawson in the construction of the spark-source unit and lens plate for the camera.

The financial assistance received from the Defence Research Board of Canada and the United States Office of Naval Research is gratefully acknowledged.

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.~ Some details are given of the design. operation. and applica-tions of an eight-channel Cranz-Schardin type high-speed multiple spark source camera suitable for optical studies of nonstationary flow phenomena in gases and transparent liquids and solids. The camera has been success-fully applied to the investigation of cylindrical and spherical explosions and implosions. underwater blasts. stress waves in plastics and impact in glass panels.

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T ABLE OF CONTENTS

1. INTRODUCTION

2. DESIGN AND CONSTRUCTION FEATURES

2. 1 Optical System 2. 2 Mechanical Design

2.3 Spark Circuitry and Multi-Channel Delay Unit

3. OPERATING PROCEDURES

3. 1 Optical Alignm ent 3. 2 Operating Techniques

4. APPLICATIONS

4. 1 Explosions and Implosions in Gases

4. 2 Underwater Explosions

4.3 Stress and Impact Experiments

4.3.1 Photo-Elastic Records of Stress Waves and Stress Concentrations

4.3.2 Shattering Characteristics of Tempered and Plate Glass Panels

4.3.3 Hypervelocity Impact Analogy

5. CONCLUSIONS REFERENCES FIGURES Page 1 2 2 4 6 10 10 13

16

16

17

17

17

18

19 20 21

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1. INTRODUCTION

Optical methods of visualizing steady and nonstationary com-pressible flows have proven to be extremely useful for qualitative and quantitative work (Refs. 1 and 2). Wave-speed schlieren records of the time-distance plane are particularly suitable for quantitative s.tudies of one-dimensional (planar, cylindrical, and spherical) flows containing shock waves, expansion waves and contact surfaces (Ref. 3). As long as such flows are ideally symmetrical and free from viscous and other disturbances they can be analyzed and observed on the basis of two independent variables, namely the radius and time. In actual flows asymmetries and disturbing effects occur so that the flows may depend on two or even three space vari-ables. Under such conditions it is essential to observe the flow deviations in the various space planes in order to assess the validity of the

one-dimensional wave-speed measurements. A high-speed framing camera usually fulfils this requirement.

Even when the deviations from one-dimensional (symmetrical) flows are small, a high-speed framing camera is very desirabie since it provides valuable complementary information that aids significantly in the interpretation of the wave-speed records. In facilities such as shock tubes, hypersonic shock tunnels, or any other short-duration flow apparatus a high-speed framing camera frequently becomes an indispensible diagnostic instrument.

It is beyond the scope of this note to review or assess the various types of framing cameras that can be used for such purposes.

Photographic definition, economy, ease of operation, and particular timing-synchronization needs will usually determine the type of camera for a

specific job. Although some types of useful com mercial cameras are now available, very of ten a special camera must be designed and built to meet some particular requirements.

The camera described in this note is of the so-called Cranz-Schardin type. It is basically of the same design produced by Mr. R. J. North, National Physical Laboratory, England, and is dealt with in Refs. 4 and 5. The UTIA camera is a modification of the NPL type. The modifica-tions, operating procedures, and applications are described in the hope th at they prove to be useful to other laboratories .

It should be noted that this type of camera is essentially com-posed of a compact number of individual schlieren systems. The camera makes use of a pair of common main mirrors and a group of distinct but

identical light sources with corresponding identical lenses and knife edges. Each separate schlieren system is triggered at a predetermined time from some initial - _ puls~ originating from the flow phenomenon to be

In the present design a given flow is viewed frorn eight slightly different angles and recorded as eight distinct photographic images of 1 in. dia. on a single sheet of 4 in. x 5 in. standard cut film. The number of views can be increased by increasing the number of sources and lenses. However, design considerations, size of image, and film size will usually dictate the actual number of views that are practicabIe.

The time between exposures is continuously variabIe from an

interval of 2 jJ. sec (equivalent to 500, 000 pictures per second) to a total

observation time of 100 milliseconds and more, if required. This is achieved electronically by means of short-duration pulse techniques and time-delay units. Consequently, there are no moving parts in the camera.

The camera has an apparent disadvantage in that only 8 frames are available, but in many applications this is easily compensated for by the simplicity of design, economy, excellent resolution, and the real advantage of a preset variabIe or constant framing rate to suit a given experiment. However, there are certain alignment and operating difficulties and these are

discussed in some detail in ~he text. The camera has been used successfully

in the study of spherical and cylindrical explosions, implosions, and wave interactions; underwater blasts; stress waves in plastics; impact experiments in glass plates; hypervelocity irn pact analogy.

2.. DESIGN AND CONSTRUCTION FEATURES

2. 1 Optical System .

The camera has been adopted to fit an existing wave-speed schlieren system, which is described in detail in Ref. 6. The common

schlieren system mirrors make it possible to take wave-speed or (x, t)-plane records and rnulti-spark source photographs of blast waves (Ref. 7). How-ever, these are not taken simultaneously but during different experimental runs. A schematic drawing of the schlieren systern is shown in Fig. 1, for the central steady source S (solid line) and for one of the eight off-set spark sources Sl (dashed line).

- - - + - -

---_=:....::::=- ___

w-++- __

±_++w ±_++w ±_++w

-36 72" roz .. r02'1 r'] 2." _{( 6Y+"} o tb Y2,"

Fig. 1 Schematic Drawing of Schlieren Systern

,

.

S

=

steady central source, 2 watt concentrated zirconium -arc lamp Sl

=

typical spark source, eight sources on a 2-7/8 in. dia. pitch circle,

equally Spaced.

M 1 = 12 in. dia. parabolic mirror, 72 in. focallength W

=

test section window

M 2

=

matching parabolic mirror, as for M 1

K

=

typical knife edge, total of eight, set at varying distances from the lens. L

=

typical camera lens, 0.87 in. dia. x 6.50 in. focal length; eight

lenses on a 2-7/8 in. dia. pitch circle eqllally spaced to match the spark sources.

p = photographic plate

E

=

spark source and lens offset angle (1. 150 )

For the sake of resolution and definition, it was decided to use an 8 channel multiple schlieren system giving a maximum image size of ab out 1. 0 in. dia. on a 4 in. x 5 in. standard sheet of cut film. It is seen th at the arrangement shown in Fig. 2 provides eight 1. 0 in. dia. spark images on a

2-7/8 in. dia. pitch circle, that will give a clearance betweeIt each image of approximately 1/8 in. and an edge clearance of 1/16 in.

Fig. 2

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Image Spacing on a Standard 4 in x 5 in. Sheet of Film Without Vignetting Effects

Since the schlierèn beam is 12 in. dia .• it was felt that the

maximum size image of approximately 1. 0 in._ dia. (magnification factor of

1/12) would be desirable for subsequeht photographic enlargement purposes. '

The central image is usually not used during a run, since its corresponding

source is a steady, 2-watt, concentrated. zirconium -arc lamp (Western~Uri.iQ:à)

which is used for initial alignment purposes only. (A multiple stray central image is also formed by tne centrallens from the superposition of reflected and transmitted light from all the spark sources and consequently it is not useful for flow observation purposes).

The nine lenses are positioned to match identically the cox-res-ponding eight spark'sources and the central steady source. Since the mirrors

MI and\M2 have the same diameter and focal length, a magnification factor of

unity is obtained at the knife edge positions .

It should be noted from Figs. 1 and 2 that all the spark sources

are offset from the optical axis by 1-7/16 in. The collimated 12 in. dia.

schlieren beams will therefore be displaced by an angle

€

(1. 150 ). as shown

by the dashed lines in Fig. 1. so that a portion of the beam will not reach the

second mirror. Consequently. all of the images (except the central image)

suffer from vignetting effects and appear elliptical in shape. The minor diameter of the illuminated image for the geometry shown in Fig. 1, is

(12 - 204 x 1-7/16) or 8 in. The loss in viewing area could be overcome. if

72

required, by using ~arger test section windows and a large mirror M2'

Nevertheless. the offset angle

E:

should be kept as small as possible to avoid

large viewing losses through vignetting.

From Fig. 1 it is seen that for a magnification factor of 1/12

or 0. 083, the ratio of the focal lengths of the focusing lens L to mirror M2 is

given approximately by fL/fM2~1/12 or'fLl:::'6 in. The lenses were chosen

on this basis and are very inexpensive war-surplus achromatic lenses of 0.87

in. dia. x 6. 50 in. focallength.

It

should be noted that the lens diameter

must not be too sm.all otherwise it will act as a (knife-edge) cut-off when sU,

b-jected to astrong optical flow disturbance. With the geometry as shown in

Fig. 1, a more accurate calculation using the simple lens ,equation gives an

image distance LP of 6.25 in. and an actual magnification factor of 0.087.

The unvignetted 12 in. dia. schlieren beam therefore appears as a 1. 045 in.

dia. image. Small changes in the distance of the test section to mirror M2

will not significantly change the image distancé, or the magnification factor,

thus providing ample lattitude for focussing for different experiments.

2.2 Mechanical Design a) Spark-Source Unit

The spark source unit is very similar to the one described in Ref. 5, and 1s shown in Figs. 3 and 4. The overall dimensions of the unit are

4 upright plates of 1/4 in. thick lucite, 3/16 in. aluminum, 1/8 in. lucite, and 1/4 in. lucite, respectively. The four plates are stiffened by 2 x 3/8 in. dia. rods at the upper corners. The first plate is used to mount the 8 pulse transformers (Atkins, Robertson and Whyteford Ltd., Glasgow, U558, 300 volt primary). The pulse to the primary winding comes from the time delay unit through the 884 thyratron, and the resulting high-voltage pulse (about

10 KV with about 0.4

fA

sec rise time) at the secondary winding provides the

final breakdown voltage to the series trigger gap shown in Fig. 5. The first lucite plate also has a 3-1/2 in. dia. cut-out to permit the light from the sparks and the steady 2-watt source to pass unobstructed.

The second plate (aluminum) has a 1/16 in. thick mild-steel electrode plate with 8 holes, 1/16 in. dia. (equally spaced on a 2-7/8 in. dia. pitch circle) that contain the spark discharges (the duration of the spark is about 1/3 ;-;. sec. to half amplitude, see Ref. 5). This plate also has a cut-out which permits the mounting of the central 2-watt zirconium-arc steady source on the central optical axis.

At the rear of the aluminum plate are mounted 8 condensers

of O. 01 ~ f (Telegraph Condenser Company, London, Cathodray, Visconal,

6 KV type CP. 56 QO). Although these condensers are rated at 6 KV, they are usually run at 10 KV or higher to produce a high intensity spark. This overdrive is not too serious, although replacements are necessary from time to time. The aluminum plate also houses the eight fixed central electrodes of the series spark gaps. The condensers also have a brass fitting attached to them which holds the adjustable gap electrode. The end of this electrode is coupled by means of a short piece of rubber tubing to a phenolic resin insulat-ing rod so that the gap can be adjusted while the high-voltage is on. The third and fourth plates (lucite) support the 20 megohm charging resistors (Sprague 704 E Spirameg 5 watt x 25 KV, Sprague Products Co., North Adams, Mass).

A central tube (phenolic resin, 1-1/2 in. o. d. x 1/8 thick) runs between the second (aluminum) and fourth (lucite) plates. The tube houses

the steady source. It also carries 1/16 in. thick baffle plates (phenolic resin)

to physically isolate and shield the gaps from each other. The fourth plate also contains a copper collector ring which holds the high-voltage lead from the power supply. One end of the charging resistor is fastened directly to the collector ring, while the other end delivers the high voltage to the condenser through a semi-circular contactor . The aluminum plate also has 8 x 1/4 in.

dia. holes which permit the ,high-voltage leads from the pulse transformers to

pass to the fixed electrode. The entire unit is mounted on a standard drill-press pedestal for rigidity, ease of alignment, and economy.

b) The Camera Unit

The camera unit appears in Fig. 6 and consists essentially of the framework of a standard Graphic View Camera. This unit was selected because of its adaptability to the present schlieren system. The usual

single-lens plate was replaced by a new single-lens plate as noted previsouly, consisting of 8 lenses 0.87 in. dia. x6. 50 in. focallength, equally spaced'on a 2-7/8 in. dia. pitch circle to exactly match the 8 spark sources. A riinth lens is located on the central optical axis for use with the steady alignm ent sourcé . . The lenses are housed in snug-fitting circular cut-outs in a 1/4 in.' thick plywood plate. An aluminum cover plate (1/8 in. thick) containing all of the 9 knife edges (individually adjustable) is fastened to the plywood lens holders. The knife edges are 1 in. x 5/16 in. x 1/32 in. steel plates with one sidè chamfered to give a sharp edge. The plates are slotted top and bottorn to accom modate retaining screws that provide for lateral and some rotational adjustment. Some of the knife-edges are also backed by wire springs that are retained by the shanks of the screws, in order to permit finer and more reproducible knife-edge cut-off adjustments (see Fig. 6).

. A very desirable feature of the Graphic View is th at thé lens plate and the film holder are individually adjustable along the optical axis. Also, as a result of the connecting bellows, these units can be moved from a 5 in. separation distance to one of about 14 in. This feature is very helpful in cut-off positioning and for focussing on the test plane or model. A ground glass screen and cut-film holder is part of the framework. The camera unit like the spark source unit is mounted on a drill-press pedestal for the rea-sons noted previously.

Figure 7 illustrates the use of the camera with the underwater explosion facility. The spark source, schlieren mirrors, and shock sphere are readily seen on the photograph.

2.3 Spark Circuitry and Multi-Channel Delay Unit

The schematic layout of the circuit for the spark light source is shown in Fig. 5. The light is produced by an almost totally confined spark taking place in the light gap. The light gap has as one electrode the front plate of the light source unit with a 1/16" hole serving as the effective light source. The gap distance is approximately 3/16" and is therefore not capable of holding.off the full 10, 000 V of the capacitor voltage. For this purpose a series hold-off gap is used, which is adjustable and set to the required gap size to prevent breakdown. Each of the two electrodes forming the light source gap is attached to one of the secondary terminals of the high voltage pulse

transformer. Since, as is indicated in Fig. 5 the front electrode is grounded, this means that normally the middle electrode is also at ground potential. In other words, the hold-off gap alone is subjected to the fuU capacitor voltage. To trigger the spark, a thyratron switched pulse of approximately 300 V is supplied by the controlling multi-channel delay unit at the appropriate instant to the primary winding of the pulse transformer. The high-voltage pulse in the secondary winding which is of a short-duration, strongly damped osciUatory nature temporarily increases the voltage across the hold-off gap to above the breakdown value and the spark gap fires. It is desirable to ar range the polarity of the high voltage pulse from the pulse transformer such that its initial volt-age rise is of a sign to increase the voltvolt-age across the hold-off gap. This

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ensures faster and more dependable triggering. In the first place it avoids the delay associated with the characteristic time of the trigger pulse (approx.

1 microsec. ) and in the second place since the pulse signal is strongly damped it increases the reliability when the first and therefore strongest peak is

adding to the capacitor voltage across the gap. One of the serious problems in the operation of the unit is to prevent spurious triggering of a spark by interference when one of the other sparks fires. The light shields are intro-duced for this purpose to prevent the ultraviolet light emitted by one spark from firing the adjacent ones through the photoemission of electrons. Direct electrical interference is also troublesom e and it was found necessary to connect the ground terminal of the secondary winding of each pulse trans-form er to a point on the electrode plate close to the spark gap it is serving. In addition the performance was improved by providing the primary terminals with completely separate, shielded cables to the multi-channel delay unit.

A picture of the multi-channel delay unit is shown in Fig. 8 and

its operation is illustrated by the block diagram in Fig. 9. Upon receipt of an

input signal derived fr om the phenomenon to be photographed, the input pulser unit supplies a start pulse simultaneously to a series of identical delay units.

It also produces a positive pulse at a front panel term inal to be available as

the starting pulse for a chronograph. The time delay for each of these delay units can be adjusted individually from a minimum of 7 to a maximum of 4000 microseconds. At the end of their particular time delay, each of these units provides a short, high-current pulse to the prim aries of the corresponding high-voltage pulse transformer on the light source unit. As described pre-viously the voltage in the secondary of the transformer is instrumental in

breaking down the hold-off gap of an individual spark gap. It was decided to

start all delay units at the same instant on the basis of the experience of North, Ref. 5, who found this mode of operation to result in a reliability superior to that obtainable from a system in which each time delay unit at the end of its delay period starts the next unit.

As is shown in Fig. 9 it is possible to delay the common trigger pulse to the delay units by switching in a common delay unit, which can hold back the trigger for a period also adjustable from 7 to 4000 micro-seconds. This feature is very useful when a number of pictures are required in rapid succession but at a relatively long time af ter the basic starting signal is received. Such a situation could happen for instance in a hypersonic shock tunnel when the establishment of the flow over a model has to be photographed. In many cases a convenient starting signal has to be obtained from the shock

wave passing an available detection station which·may be located at some

dis-tance upstream from the test section.

In addition to the pulse to the high voltage transformer each delay unit produces a negative pulse. A switch selects the signal from one of the units and directs it to the chronograph stop circuit which is incorporated with the input pulser unit. This circuit inverts the pulse and makes it

avail-able at a front panel connector for use as the stopping pulse for a chronograph.

chronograph receives the start pulse from the input pulser, which for th is purpose can also be triggered manually from the front panel and the time de-lay of each unit and the common dede-lay unit is set to the desired value by selecting the units in turn with the switch and adjusting the delay until the chronograph indicates the correct value. When the time delays are close to their minimum values a significant error in the timing of the sparks can

occur as a result of the delay between the time the light source spark actually fires and the time the trigger pulse is supplied to the primary of the pulse transformer. This delay can under unfavourable circumstances amount to several microseconds. In such cases it is therefore necessary to use an alternative method of establishing the spark timing. Since the spark break-down delays are usually of a random nature, a suitable method consists of detecting the firing instants of all individu al sparks by m eans of inductive pick-up signals in a common lead, which can be displayed on an oscilloscope. An example of such inductive pick-up signals is given in Fig. 10. In this way the timing can be determined quite accurately. The order of firing of the sparks can in almost all cases be determined unambiguously fr om the developing phenomenon in the actual photographs.

The design of the detailed circuitry and the actual construction of the Multi-Channel Delay Unit to specifications laid down by the Institute of Aerophysics was done by Electronic Associates, Willowdale, Ontario.* A brief description of the function of the various units is given in the following paragraphs.

Figure 11 shows the circuit diagram for the input pulser unit and the amplifier for the chronograph stop pulse. In the original design the input signal was required to be a positive pulse, which raised the normally negative grid potentialof the 884 thyratron sufficiently to fire the thyratron.

This discharges the O. 1

JA

f condenser and the positive pulse of approximately

150 V across the IK cathode resistor is led to a front panel connector to

serve as the chronograph start pulse as well as to a Jones connector from where it is distributed to the various individual delay units or the common delay unit. The thyratron can also be fired manually by depressing the start button which essentially shorts out a normally negative potentialof approximately 75V resulting in a net positive pulse to the grid of the thyratron.

It should be pointed out at th is stage that it was found

fre-quently more convenient to obtain a starting signal by breaking a circuit rather than producing a positive pulse. For such cases an alternative trigger mode was used in which essentially the negative hold-off voltage on the grid of the 884 was supplied through an external conductor in the experi-mental arrangement under investigation. Af ter the breaking of this conductor the grid rises towards ground potential and the thyratron is triggered in this manner. The method is described in some more detail in Section 3.2.

The. sw~tch-selected negative timing pulse from one of the

delay unit enters through the Jones connector and is inverted and amplified

* We wish to acknowledge with thanks the use of their circuit diagrams

shown in Figs. 9, 11 to 14.

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•

in the pentode part

.o

f-

the 6U8 tube. The triode part serves as a cathode follower and supplies a positive pulse to'the chronograph stop' connector on the front panel.

The circuits for the delay units and the common delay unit are identical and are shown in Fig. 12. The positive starting pulse from the input pulser is inverted by the first 6AU6 tube af ter which it is transmitted to the positively biased grid of the third 6AU6 which constitutes the normally conducting half of a monostabie multivibrator unit consisting of the second and third 6AU6's. This negative pulse causes the current in the third 6AU6 to decrease, raising the plate potential, which through the coupling to the negative biased grid of the second 6AU6, switches this latter tube in a tem-porarily stabie conducting state. The grid of the third 6AU6 leaks back up to a positive voltage at a rate determined by the time constant of the switch selected capacitance and the continuously variabie resistance to the B+ line.

In this way it is possible to vary the time delay continuously from 7 to 4000 microseconds in three slightly overlapping ranges. By chang-ing the values of the capacitors it is easy to choose different delay values. For instance, the units have been used for delays up to 100 milliseconds.

When the grid voltage of the third 6AU6 has risen to a certain level this tube will again start to conduct and the situation switches back

extremely rapidly to the normal quiescent state. The resulting positive pulse at the plate of the second 6AU6 reaches the grid of the 884, which normally is at a sufficiently negative potential to keep the 884 in the non-conductive state. This positive pulse at the termination of the delay period triggers the 884 and it discharges the 0.5 }.J. f condenser through the primary winding of the high voltage pulse transformer which is connected to the connector UG-2 90

Iu.

It may be pointed out that the IK resistor in the cathode circuit is shorted out in the Jones plug when the unit is used as an ordinary delay unit, as shown on Fig. 13, which represents the actual inter-wiring of the various units. Consequently the dis charge circuit comprises only the 0.5)-A f capacitor and the primary winding with the thyratron acting as the switch. On the other hand, when a delay unit is in operation as the common delay unit, the IK cathode resistor is left in the circuit and it is the positive pulse

across this resistor that is distributed to all the other delay units as the starting pulse. The negative pulse across the primary winding of the pulse transformer is fed to the Jones plug for connection via the selector switch to the 'chronograph stop' amplifier.

The power supply makes available a positive voltage of 300V and a negative voltage of -150V, in addition to filament power for all units. The circuit is given in Fig. 14.

Finally, a few remarks are in order regarding the interference difficulties that were experienced in putting the unit into operation. It was found th at no trouble occurs when the multi-channel delay unit is operated without being connected to the light source unit. However, with the light

source unit in use, interference did occur even from the high voltage pulses produced by the pulse tra~sformers. With the main sparks firing, the pro-blem was considerably more severe. Interference occurred in two different ways. In the first place the thyratron in the delay units would be triggered in a spurious manner by the interference. In the second place the timing of the delay unit would be influenced. The sensitivity of the thyratron to interference was diminished by installing a small capacitor between grid and cathode. In addition to th is theentrance of spurious signals into the unit was made more difficulty by the introduction of O. 01 and O. 05

fl.

f capacitors between the lead to the pulse transformer and ground. These capacitors provide adequate pro-tection against the undesirable, high-impedance interference signais.

As a consequence of these features, the unit works well, except

that some residual influence takes place when the selected time delays of some units have almost the same value. In such cases, the monostabie multivibra-tors are all very close to their switch-over conditions at the same time and even a very small disturbing signal from the first spark can produce enough of a change in the timing that the other sparks will occur essentially simul-taneously. To counteract this tendency it is sometimes possible to use the common time delay unit to start all delay units involved at an instant chosen such that the various times for the sparks can be obtained by using individu al time delays with greater relative differences.

3. OPERATING . PROCEDURES 3. 1 Optical Alignm ent

The alignment of the multi-spark source schlieren system pre-sents considerable more difficulty than that which is encountered when using a standard two-mirror, parallel-light, continuous-source system. The fact th at eight schlieren systems must be aligned simultaneously, accounts for only a small portion of the difficulty. The major problem arises from the fact that the multiple -spark light source is not equipped to produce continuous illumination at the exact positions of each spark. Normally, in a continuous light-source schlieren system it is a straightforward procedure to adjust the single knife-edge so that it is precisely at the focal point of the second para-bolie mirror and intercepts the proper porti on of the light in order to produce the desired uniform and sensitive background for good schlieren photography. However, in this case, where the light sources are short duration sparks, the knife-edge adjustment procedures which are usually used for a continuous light source become very inconvenient and exceedingly difficult.

The procedure for setting up the multiple-spark schlieren system can essentially be divided into two distinct phases. The first phase consists of arranging the components of the system, i. e. source, camera, m irrors and test section on an optical axis according to the optical system layout shown in Fig. 1. This portion of the alignment presents no difficulty as the procedure is identical to the techniques used in the alignment of any

continuous source schlieren system. It is completed by means of the central,

•

steady, point-light source. This central light enables the spark source to be positioned in the focal plane of the first parabolie mirror and thereby produc-ing a collimated beam of light through the test section on to the second para-bolie mirror. The camera is then adjusted so that the knife-edge of the central lens is in the focal plane of the second parabolie mirror, and the

image of the test object is sharply focussed on the ground glass of the camera. These two adjustments are accomplished by translating the camera parallel to the optical axis by means of the two large adjustable and locking knobs on the triangular track which individually control the position of the lens holder

and the film holder (see Fig. 6). The final step in this first phase is to adjust

the central knife-edge itself so that it intercepts a portion of the image of the continuous point-light source. This adjustment is satisfactorily completed when the prevailing atmospheric heat fluctuations in the test section are uni-formly discernable on the ground glass.

The second phase of the optical alignment is the adjustment of the eight knife-edges associated with the off-axis lenses and as already indi-cated presents the greatest alignment difficulty. Since both the sparks of the source and the centres of the lenses in the camera are on a 2-7/8 in. dia. circle, and since the optical system is completely symmetrical, theoretically, it should be possible to obtain good schlieren on all eight systems by merely adjusting each knife-edge to correspond exactly to the central knife-edge. However, in practice, this is not applicable for several reasons. In the first place, small unavoidable misalignments are always inherent in the system such that the plane of the sparks in the source unit or the plane of the lenses in the camera are not truly perpendicular to the optical axis of the system.

It is also possible that the actual location of each point light source differs

from th at of the spark source plate. In addition, this condition probably varies with time and usage. Thus the adjustment of all knife-edges as a single

entity is an impossibility and in order to align the system correctly each knife -edge must be individually adjusted.

The first method attempted to obtain these individual adjust-ments was simply to observe the image on the knife-edges by repeatedly firing the multiple spark source. This technique produces a quick adjustment of the knife-edge with respect to the amount of cut-off. However, this adjustment

is very coarse and in addition it is not possible to determine from an

observa-tion of the pulsating images if each knife-edge is at the exact focal point of its own system. In an effort to create a continuous image on the knife-edge a

Teslà coil was applied to each of the light source gaps in the source. This

was unsuccessful as the resulting image was very weak and not a true simula-tion of the actual sparks. Another attempt to obtain a steady image on the knife-edges was accomplished by first removing the condensers and the light source insulations (see Fig. 4) and placing a flashlight bulb behind the spark source holes. This method besides being inconvenient was very unsatisfactory

because the images produced wer./e very sensitive to the position of the light

A more elegant technique, in which the images of the sparks are actually photographed was also attempted. In this method a film is placed in the plane of the knife-edges and this records the exact shape and position of the spark images. This film can be developed and a scribe line etched through the middle of each image. The film is then placed back in the plane of the knife-edges in the same position as it occupied before. The

knife-edges can then be adjusted to coincide with the scribe lines. It was found that this system gave excellent results with regard to the proper cut-off adjust-ments but it was not able to indicate the correct focal point setting for each knife-edge.

The method which has been adopted as the most acceptable is the obvious trial and error system. This technique has the advantage that adjustments are made with the source and camera functioning under their normal operating conditions and therefore when the knife-edge settings are satisfactory, the alignment can be considered complete.

A steady flow phenomena is created in the test section by an electrical space heater, which forms an ideal background because the stria-tions fill the entire 12-ineh diameter field of view. With this condition exist-ing in the test section an actual multi-spark schlieren record is made of the heat-flow phenomenon. After the development of the film each of the eight images recorded can be inspected to see if they possess the required

schlieren effeets. If a knife-edge is eorrectly set, the assoeiated image on the film will c1early display the density fluctuations in the heat flow uniformly over the complete extent of the field of view. If th is is not the case then the knife-edge must be adjusted. However, the nature of this adjustment can be interpreted from the unaeeeptabie image. For example, if the image on the emulsion side of the negative is sueh that the left hand side appears dark and the right side light, then the knife -edge must be ahead of the true focal point. Thus it is necessary to move the knife-edge (parallel to the optical axis) backwards, that is further away from the second parabolic mirror. If the light and dark eonditions were reversed a forward knife-edge adjustment would be indieated. If the image appears uniformally too light or too dark the vertical knife -edge must be adjusted in a horizontal direction (perpen-dieular to the optical axis) so as to intereept less light or more light respee-tively. When each of the eight im ages have been thus interpreted and the appropriate adjustments made on eaeh knife-edge in turn, a new multi-spark schlieren record is made and compared with the first. If necessary, the knife-edges are aqjusted again accordingly and another schlieren film is recorded. This process continues until all eight frames possess a uniform schlieren background. It was found in practice that if care was taken, the process converged quite rapidly. A final acceptable result obtained from such an alignment procedure, showing the eight schlieren records of the forced heat flow in the test section is presented in Fig. 15.

The final step in the alignment is to check that the image of the central-point-light source is still correctly aligned. When this condition is assured the system is completely and correctly aligned. This final check

(13)

is necessary because in day' to day operation it is possible, for example,

that a mirror might accidentally be bumped. The whole system can be quickly realigned then by turning on the point-light source and rotating the second parabolic mirror until the schlieren quality is restored on the central

lens which will usually also guarante.e the schlieren quality of the eight other

systems.

3.2 Operating Techniques

The operation of the multi-source spark camera is relatively straight forward and the only problem in the normal day to day operation is the m aintenance of the original alignment. That is, it is necessary to con-tinually check that the system is in correct optical alignment and that all sparks are correctly firing in the time sequence preset on the multi-channel delay unit.

Since the central steady point-light source system was aligned in conjunction with the eight spark source systems the optical alignment can be quickly checked each day by inspecting the image of the central source system on the ground glass of the camera. In order not to disturb this

alignment during the operation of the camera the insertion and removal of the

4 x 5 sheet film holder must be done with great care. To facilitate this

operation two of the four retaining screws on the leaf-springs of the camera film holder were removed to decrease the pressure applied by the camera holder on the sheet film holder .

In the normal operation of the unit it was found that considerable

more attention must ,be given to the operation and behaviour of the multi-spark

source than to maintaining the camera in correct optical alignment. The performance of the sparks is greatly affected by the atmospheric conditions and therefore it is necessary each day and very of ten during a sequence of runs to readjust some of the hold-off gaps. The length of the hold-off gaps

is a very important setting. If the gap separation is too sm all then the

charge can be triggered just by the interference effect produced by the dis-charge from one of the other systems. On the other hand, if the gap distance is too large, then the voltage pulse from the induction coil will be inadequate to trigger the discharge. This latter type of failure can be detected by simply observing the multi-spark-source during the actual firing of the spa:rks.

However, a misfire of the type where the discharge of one system initiated the discharge of another can only be detected by means of a chronograph. A proper gap setting is anywhere in.the rang.e where the above types of misfires do not occur and this is determined by trial and error. Wiih continued usage it is found that this operating range becom es very narrow until the point

where controlled discharges are no longer possible. It is then necessary to

disassemble each unit in turn and clean the electrodes of both the hold-off gap and the light-source gap with very fine emery paper.

Although not as convenient, it is also possible to adjust the

light-source gap. From experience it' was found that the optimum setting

was a gap distance of 6/32" to 7/32"; howe'ver, in the event that controUed

discharges cannot bé obtained by the adjustment of the hold-off gap, it may be necessary to change the setting of the light-source gap. For example,

if this gap is too narrow then the voltage pulse from the in'ductio~coil will '

not reach its peak, owing to an early discharge across the light source gap, and so the effectiveness of the triggering pulse will be decreased and there-fore the operating range of the hold-off gap will be narrower. On the other hand, if the light source gap is too large, that is, larger than the hold-off gap, no discharge wiU occur at all.

Failure of the multi-spark source to operate correctly, other than from improper gap settings, can also result from a capacitor

break-down. This type of failure occurs periodically since the hold-off voltage' on

the source is 10 KV whereas the capacitors are only rated for 6 ~V. Since all

the eight capacitors of the source are in parallel, the break down of one capacitor will reduce the voltage on all the other spark gaps and therefore a discharge wiU be impossible. This condition can be quickly detected as the voltmeter on the power supply wil! show a definite decrease in voltage and its ammeter wiU display a finite current. Since the defective condenser must be passing this finite current to ground and heating the corresponding resistort

therefore, the defective condenser can be identifi~d by checking for á. warm

resistor.

An additional requirement in the operation of the multi-spark source camera is the development of a trigger system which will relate with

regard to time the phenomena of a flow event to the firing of the sparks. This has been accomplished by two different methods: one which is essentiaUy

the "m aking a circuitfl and the other the'breaking a circuit". For example.

in the case of the study of s'pherical explosions generated by shattering

pressurized glass spheres (Refs. 3, 7, and 8) a strip of aluminum foil was glued to the base of the sphere and in turn was connected to the input of the

com mon trigger unit (this is essentially the grid of the thyratron in this unit)

in the multi-channel delay system. The sphere is then ruptured by means of

a mechanical breaker, which in the process of initiating the explosion

simul-taneously hits the aluminum foil and connects it to ground. This ground pulse causes the thyratron of the common trigger unit to conduct and in turn starts

the eight del~y units and the chronograph.

However, in the study of implosions and explosions where the

glass spheres break as a result of overpressure and the mallet breaker is not used, the method of genérating the required pulse to the trigger unit must be done by the flbreaking a circuit". In this system a circuit is painted on the surface of the glass diaphragm with conductive paint. This painted circuit is used in conjunction with a 300 V power supply such that when the circuit is broken the grid of the thyratron, in the common trigger unit receives a posi-tive voltage pulse. In order to incorporate this system of triggering into the m ulti-channel delay unit, an additional input plug was instaUed in the com mon

trigger unit which allows either triggering system to be conveniently used without further modification to the triggering unit itself. The circuitry illustrating these two triggering methods is shown in Fig. 16.

The experimental procedure for obtaining multiple-spark schlieren records of a flow phenomenon or some other short duration event can be summarized as follows.

(a) Consider the phenomenon~to be photographed with regard to its development versus time and select the correct timing se-quence or framing rate in order to record that part of the event which is of interest.

(b) By means of the chronograph preset each of the eight delay units according to the above estimated times and then with the operating hold-off voltage of 10 KV recheck the times to be sure that all sparks are firing in the correct time sequence and not as a result of an electrical interference effect.

(c) Choose one of the two triggering systems described previously to produce the correct initiating pulse to the com mon trigger unit.

(d) Check the triggering system by simulating the grounding of the foil or the breaking of the painted circuit.

(e) Check the performance of the spark source visually. Check the alignment of the camera by using the steady source.

(f) Load the camera carefully with the sheet film holder, shut all the laboratory lights, remove the shutter and then initiate the event which is to be recorded.

(g) Af ter the sparks have fired close the shutter, open the labora-tory lights and develop the record.

Various multi-source spark schlieren records from different experimental studies which were obtained by using the above procedure are presented in the next section.

4. APPLICATIONS

The present high-speed, multi-source, spark came.ra has proved to be a very useful instrument in the investigation of nonstationary phenomena in gases, liquids, and solids. A few examples will be described briefly in the following subsections but further details can be found in the cited references.

4. 1 Explosions and Implosions in Gases

, The camera has b,een used in the study of gaseous explosions and implosions t'hat were generated with the aid of spherical or cylindrical glass "diaphragms". Figure 17 shows the blast produced by bursting a

pressurized glass sphere. The development of the spherical shock wave (Sl), the motion of the contact front (C), and the glass fragments (F) are well

illustrated in an enlargement of frame No. 3 (Fig. 18). The results shown in Fig. 19 are similar to those of Fig. 17, except the motion of the second shock front (S2) is clearly shown in frame 6. Combustible mixtures of oxygen and hydrogen diluted with helium are also very useful for producing even stronger explosions. Figure 20 shows such a record. It can be seen that the luminosity

due to com~ustion exposes all of the frames at once so that the centres appear

bright. Further details may be found in Refs. 3, 8, and 9.

Two sing~e explosions can be applied to the investigation of the

head-on collision of spherical shock waves (Ref. 7). This problem is of im portance in the study of the reflection properties of spherical shocks. Figures 21 and 22 are two representative photographs of this type of wave interaction. The approach, collision, and penetration of the shocks appear

clearly on both records. An enlargement of frame No. 6 (Fig. 22) is

included to show the excellent resolution that can be obtained with the present cam era. Addit.ional photographs and details can be found in Ref. 7.

By evacuating glass spheres in a high pressure atmosphere

it is possible to produce spherical implosions . The photographic records,

however, show that these experiments are not as successful as explosions owing to the asymmetrical breaking of glass spheres under compressive loading. Figures 24 and 25 illustrate the foregoing remark rather well, and

indicate the usefulness of such a camera in showing existing asymmetries and nonuniformities. Figure 24 shows the development of the expansion

wave, which is moving into the air at rest, as well as the Mach waves induced by the inflow into the sphere. Figure 25 is a similar run in which the main shock wave or compression zone after presumably imploding on the origin

is now moving away from the broken fragments (frame No. 4). This problem

is discussed in some detail in Refs. 3, 10, and 11.

Cylindrical explosions and implosions can be produced and

photographed in the same manner previously described for the spherical cases. A representative record showing the initial asymmetry of a cylindrical

in Fig. 26. An overchoked sonic jet develops at th at point and it can be seen to persist for the duration of the run. A cylindrical combustion-driven

explosion appears in Fig. 27. It is worth noting th at the combustion perturba-tions which are noticeable in the shock front and in the glass fragm ents are smoothed out and the shock wave and the fragments eventually attain symmetry. Examples of cylindrical implosions can be found in Ref. 11.

4.2 Underwater Explosions

The usefulness of a high-speed multi-source spark camera in the study of underwater explosions can also be illustrated with a few schlieren records. Figure 28 shows the underwater blast generated by a pressurized glass sphere in a small, commercially-available, water tank, The motion of the shock wave (S), its reflection at the water-air contact surface as a rare-faction wave (R), and the growth of the gas bubble (B) are clearly illustrated. Figure 29 shows a similar run at very early times in order to illustrate the

initial development of the primary blast wave. Some additional details are given in Ref. 3. However, this problem is presently under investigation and the research results wil! be published in a separate report.

4. 3 Stress and Impact Experiments

4.3. 1 Photo-Elastic Records of Stress Waves and Stress Concentrations The use of a Cranz-Schardin type of camera for investigating stress waves and concentrations under impact loading by using photo-elastic effects and properties of elastic waves in transparent solids wil! be illustrated by the following results.

*

Figure 30 shows a plastic test specimen (CR-39, Homolite, specific gravity = 1. 31 at 250_{C and modulus of elasticity E = 250,0-00} -330,000 psi at 250_{C, refractive index nD}

₌

_{1. 50398) 0.185 in. x 1. 233 in. x} 3. 163 in. mounted between two crossed-polaroid plates of 4 in. dia. One end of the specimen was rigidly fixed and the other received an impulsive blow from a 1/2 in. dia. spring loaded mallet. Both ends of the specimen were rounded so that the axial load was practically a concentrated load. The pro-gression of the stress fringes may be readily seen in frames 1 to 8 from about

10 to 116

J.l.

sec. as noted.

Enlarged views of the eight frames appear in Fig. 31. A plot of the advance of the white fringe along the axial direction yields a velocity of 3960 ft/sec (0.0475 in/).J. sec), and the black fringe advances at 2570 ft/sec (0.0308 in/).J- sec). It is seen that the fringes spread over a greater area with time and is indicative of the manner in which a large stress at a point is in time dissipated until the stress reaches a low value, but over a large area. The process is somewhat analogous to an explosion (Ref. 3). The

*

The results were obtained at ,the request of Professor 1. W. Smith and Dr. M. A. Dokainish, Mechanical Engineering Department, Universityof Toronto.

velocity of 2960 ft/ sec for an elastic :,rJve a~rees quite well with the com-put'ed speed of propagation"based on E/? . The reflection of the com-pressive stress wave as a tensile stress wave, at the free surface, at the opposite end of the specimen is seen in frame 6 and subsequent frames.

A similar result for Hysol 4485 (precast sheet, specific gravity 1. 1, modulus of elasticity 444 psi) is shown in Fig. 32. This specimen has rubber-like characteristics so that a low stress propagation might be expected. The specimen was 1/2 in. x 1-1/4 in x 3-1/8 in., and was loaded as noted above. The white fringe advances at 500 ft/ sec (0. 006 in/

J1

sec) and the black fringe at 387 ft/sec (0.00465 in/

fA

sec). The pro-gression of the fringes in this specimen are such that they tend to become plane quite rapidly, whereas for the Homolite CR-39, they initially started " as circles tangent to the point of impact (Fig. 31).

The propagation of the elastic compression wave noted above agrees quite wel! with the value computed on the basis of

IE/p' ,

when account is taken of the fact that the Hysol modulus of elasticity increases with the rate of loading. For example, the static modulus is nearly doubled at a loading rate of 64 in/sec and would be considerably higher under impact. The annular specimen of Homolite, CR-39 is included in Fig. 33 to show a more complex type of stress distribution obtained under similar loading con-ditions . Additional details concerning such problem s may be found in Refs. 12 and 13.

4.3.2 Shattering Characteristics of Tempered and Plate Glass Panels

It was shown in subsections 4. 1 and 4. 2 that glass spheres and cylinders had excellent shattering characteristics when subjected to tensile stresses under an overpressure loading. It immediately suggests the possibility of using glass plate panels for large blow-out sections or diaphragms in case of an emergency arising from a blast in order to reduce the hazards of shock pressure build-up arising from the possible failure of a commercial high-pressure vessel, for example. Some runs illustrating the manner in which a 12 in. x 12 in. x 1/4 in thick glass panel shatters are shown in Figs. 34 and 35. The panels were under an overpressure of about 1. 5 psi and were ruptured by a slug fired from a 1/4 in. Ramset gun (a commercial device used to drive bolts into concrete by employing

explosive cartridges).

Figure 34 is a head-on view of the glass panel. Frame (a) shows the propagation of the shock front (to the left) in a 1/4 in. thick

tempered-glass plate, 9.6 jJ. sec. af ter the glass was hit by the Ramset slug. The characteristic fracture pattern of tem pered glass behind the front is clearly indicated. The ever increasing open area (which means reduced resistance to the unsteady flow development) is shown in the remaining three frames up to 30 milliseconds af ter the slug hit the panel.

Figure 35 shows a similar configuration, but this tin)e in an edge-on view. In frame (a) only the fragments produced from the direct hit by the Ramset slug are seen to move to the left. The subsequent development of the shattering process appears in the remaining three frames.

Head-on views of the breaking process were made possible through the use of an arrestor mechanism which stopped the Ramset bullet after it had travelled about 2 inches. The bow shock wave produced by the slug and the continued advance of the shock after the slug was stopped appears in Fig. 36. The separation of a plastic cap is' clearly se en in the last frame. The maximum speed attained by the slug is still subsonic and is about

760 ft/sec (between 30 and 80}J. sec); between 150 and 200 jJ- sec it has

decelerated to 560 ft/sec. Consequently, the shocks noted in the photographic records must have been produced in the gun barrel ·rather than by the slug in the free atmosphere. Some very interesting studies could be done of this type of non-stationary supersonic flow arising from accelerating and decelerating bodies using a high-speed camera of the present design. Additional details are given in Ref. 14.

4.3.3 Hypervelocity Impact Analogy*

When a metal cylinder strikes a relatively thin metal target at hypervelocity it penetrates the target. During penetration the collision

generates a shock in the target and in the cylinder. Enormous pressures and temperatures are generated in the cylindrical bullet such that it can be-come gaseous and expand into the free atmosphere. The conical-like expansion with its apex at the shock front is self-similar (Ref. 15), as long as the shock is strong enough to maintain gaseous conditions.

The present equipment was used to obtain some qualitativè

results by using a gas analogy. Figure 37 shows a plane shock wave(M s = 5.82 or 2620 ft/sec) in a glass tube containing SF6 (15 psi, 2950_{K). The glass tube}

shatters (at a fragmentation speed of 2700 ftl sec) and the high pressure gas (546 psi, 8050_{K) expands into a vacuum (air, 100 mm Hg, 295}0_{K), with the}

shock inclined at a self-similar angle of about 50 degrees. Figure 38 shows a similar breaking process but this time the expansion process is initiated by the reflected shock wave at the end of the glass cylinder.

Additional information on the theoretical analysis of this interesting problem can be found in Ref. 15.

* The experiments were performed at the request of Dr. G. V. Bull, Professor of Engineering Science, McGill University. The assistance re-ceived from Mr. G. F. Bremner in the design of the apparatus and in con-ducting the experiments is acknowledged with thanks.

5. CONCLUSIONS

The design, construction, operation and applications of a high-speed muiti-source spark camera of the Cranz-Schardin type are described. The camera is capabie of taking 8 frames, on a standard 4 in x 5 in. sheet of

film, in the range of 2 fJ. sec. inte.rvals to tens of milliseconds, at equal or

continuously variabie preset intervals. The framing is accomplished through short-duration puise techniques and consequently there are no moving parts. The camera essentially is an assembly of independent schlieren systems that

make use of a comm.on pair of parabolic 12 in. dia. mirrors. The camera

has proven itself as a very versatile and economical high-speed photographic recording instrument.

Some irnprovements in the details of the design wouid ease, or compietely remove, the few difficulties encountered in the alignment and operation of the camera. However, the illustrations provide ampie evidence of the usefuiness and definition that can be obtained with the present camera.

It has been applied successfully to the study of explosions, implosions and wave

interactions in gases, underwater blast;;, and to stress and impact experiments in transparent solids.

1. Ladenburg. R. W. Lewis, B. Pease, R. N. Taylor, 'H. S. 2. Holder, D. W. North, R. S. 3. Glass, 1. 1. 4. Cranz, C. Schardin, H. 5. North, R. J. 6. Glass, 1. 1. 7. Glass 1. I. Heuckroth. L. E. 8. Boyer, D. W. Brode, H. L. Glass, 1. 1. Hall, J. G. 9. Collins, R. 10. Boyer, D. W. 11. Heuckroth. L. E. 12. Frocht, M. N. 13,. Durelly. A. J. Dolly, J. W. RE'FERENCES I ' " . ~

Physical Measurements in Gas Dynam:l.cs and Combustion, Vol. IX, High Speèd Aero-dynamics and Jet Propulsion. Princeton Ufiiversity Press (1954).

Optical Methods for Examining the Flow in High Speed Wind Tunnels. NPL/AERO/300

(1956).

Aerodynamics of Blasts. UTIA Review No. 17, (1960).

Kinematographie auf ruhendem Film und mit extrem hoher Bildfrequenz. Zeits. f. Physik, Vol. 56, p. 147 (1930).

A Cranz-Schardin High-Speed Camera for

Use With a Hypersonic Shock Tube. NPL/Aero/ 399 (1960).

Design of a Wave Interaction Tube. UTIA Report No. 6 (1950).

Head-On Collision of Spherical Shock Waves, UTIA Report No. 59 (1960) and Phys. Fluids, 2, 5, p. 542 (1959).

Blast from a Pressurized Sphere, UTIA Report No. 48(1958).

Some Methods of Generating Cylindrical Explosions. UTIA Technical Note No. 43 (1960).

Spherical Implosions and Explosions. UTIA Report No. 58(1959).

Some Experiments on Cylindrical and

Spherical Implosions. UTIA Technical Note (to be published>..

Photoelasticity, Vols. 1 and 2, John Wiley Stress Concentration Factors Under Dynamic Loading Conditions, J. Mech. Eng. Sci, Vol. 1, No. 1, p. 1, June (1957).

14. Billington, 1. J. Favreau, R. F. Glass, 1. I.

Heuckroth, L. E. 15. Bull, G. V.

Some Experiments on the Use of Large Glass Panels as Emergency Pressure Relief Diaphragms. UT IA Tech. Note

(to be published).

On the Impact of Pellets with Thin Plates. McGill University Tech. Note 1-10/61

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