Streak Photography Research Articles

Liquid ejection and possible nucleate boiling mechanisms in relation to the laser drilling process

The laser drilling process is a complex phenomenon. This is especially true after the evaporation process starts. It is experimentally evident that liquid ejection occurs due to drag forces developed around the solid cavity and/or explosion resulting from nucleation of vapour bubbles in the liquid zone. Therefore, study into the ejection of liquid due to vapour bubble formation is necessary. Consequently, the present study examines the liquid ejection mechanism experimentally and possible saturated nucleate boiling is treated theoretically. In the experimental study, streak photography is introduced while a kinetic theory is adopted for the heat transfer model. This enables us to obtain the surface and internal temperature rise due to the laser heating pulse. It is found that the time measured for the liquid expulsion from the heated zone is identical with the time computed corresponding to possible saturated nucleate boiling.

Sensory contributions to nocturnal prey capture in the dwarf scorpion fish (Scorpaena papillosus)

Dwarf scorpion fish live in broken rocky reef areas at depths of 5–25 m. By day they are found in crevices and beneath rock overhangs, whereas at night they are observed sitting in the open. Gut samples show that feeding occurs predominantly at night with the most common prey items being crabs and brittlestars. In the laboratory experiments, under an artificial day/night light regime (LD 12:12) the prey species show a strong daily rhythm of activity, with activity confined to the period of darkness. Activity is controlled by light level. An artificial “sunset”; in the middle of the day has both crabs and brittlestars emerging from cover at ambient light levels of about 4 × 10‐2μE.m‐2.s‐1 but not becoming fully active until dark. Under similar conditions, the dwarf scorpion fish's visual threshold for response to a moving visual target occurs at light levels of about 7 × 10‐3μE.m‐2.s‐1. Field measurements show light levels between these two values occur only for a limited period after sunset, and that light levels at night will probably be too low for visual feeding. The anterior lateral line system is described and its contribution to feeding shown in experiments where fish in the dark, or blinded fish, precisely locate stationary or moving crabs out to a range of about 10 cm. Particle streak photography shows that the respiratory currents generated by the crabs extend out to at least this range. Brittlestars are most commonly taken when they come into direct contact with the fish, particularly with the modified ventral edge of the pectoral fin. It is concluded that in the dwarf scorpion fish non‐visual senses make an important contribution to prey detection and capture.

Visualization of convection in Czochralski melts using salts under realistic thermal boundary conditions

NaClCaCl 2 was used to visualize convection in Czochralski melts under realistic high-temperature boundary conditions. The side view of the NaClCaCl 2 melt at T = 600°C was obtained by rapidly lowering the tubular heater, in 10 s intervals. The Prandtl number of the melt is Pr = 0.58. Streak photography revealed asymmetric, unsteady, convection. Temperature fluctuations were present in almost all flow regimes.

Laser heating process and experimental validation

Laser machining of engineering materials requires deep investigation into the laser-workpiece interaction mechanism, which is generally complicated and depends on the laser and workpiece properties. The present study examines the heat transfer mechanism, including conduction, phase change and convection processes taking place during Nd YAG laser irradiation of steel workpieces. Gaussian profile is considered for the spatial distribution of the laser output power intensity, while a time-dependent profile resembling the actual laser output pulse is introduced in computation. The thermal properties of the substance is considered as temperature dependent. Mass removal from the laser irradiated spot is modeled considering momentum and continuity equations. To validate the theoretical predictions, experimental measurements of surface temperature and evaporating surface velocities are carried out. Optical method and streak photography techniques are employed for this purpose. It is found that surface temperatures and evaporating front velocities are in good agreement with the experimental results.

Electrode phenomena and measurements of the erosion rate in high-current pseudospark switches

For microsecond pulses and peak currents up to 105 kA, the erosion rate, electrode profile, and electrode surface of molybdenum electrodes were investigated. The direction of electrode material transport was verified. The discharge behavior of molybdenum electrodes was studied by streak photography. A contraction of the plasma column occurs at the cathode for peak currents above 20 kA. A bright luminous anode flare appears at a peak current of about 40 kA, correlated with an evident anode melting. A further constriction of the anode flare was observed for currents exceeding 90 kA. Opposite to lower currents, an increase in the weight of the cathode was noticed as well.

流し写真から見たロケット誘雷前駆放電の進展過程

流し写真から見たロケット誘雷前駆放電の進展過程

Electrode architecture related to surface flashover of solid dielectrics in vacuum

A lateral test structure with metalized film electrodes is shown to have distinct advantages over a conventional solid electrode structure for studying the HV characteristics of solid insulators in vacuum. The paper presents results of streak photography and spectral analysis of surface discharge luminosity, and of preflashover activity using time-coordinated current, luminosity, and imaging of light activity emanating from the surface of the stressed dielectric. The preflashover activity, consisting of bursts of time-correlated current and light emission, is associated with randomly distributed emission spots on the surface. It is proposed that the above observations are related to charge trapping and detrapping processes at localized energy levels associated with insulator surface defects.

High rate deformation of metallic liner and its dislocation description

The dynamics of deformation in cylindrical liners are studied experimentally and theoretically in -pinch geometry, where the cylinders are deformed by a magnetic field created by a current flowing along the axis. This method allows one to obtain one-dimensional deformation and a reliable recording of magnetic field and cylinder deformation. The experiments are performed with a current amplitude of 0.8 - 3 MA and a current rise time of 2.5 - . Aluminium and copper tubes, from 4 to 6 mm in diameter and 0.25 - 1 mm wall thick, are compressed. The deformation rates under study are in the range of . The time dependence of the radii of the copper and aluminium tubes are measured with a streak camera and by the pulsed x-ray technique. The time resolution of the streak and x-ray photographs is 10 - 15 ns, their spatial resolution is . A rheological model describing the dynamics of compression is developed. The model includes the description of the metal as a plastic medium with moving dislocations in the solid state, and as a viscous medium in the liquid state. The one-dimensional solution to magneto-hydrodynamical equations of the liner dynamics is compared with the experimental results and thus the following rheological parameters of the metal are obtained: , the probability of dislocation generation in plastic deformation; and , the drag stress, the parameter which characterizes a drag force acting on the dislocation.

Experimental simulation of shock focusing in a collapsing cavity

Recent models have highlighted the effects of shock-focusing in a spherically symmetric collapsing cavity. Several of these simulations have predicted extremely high temperatures in the final stages of the collapse. In order to investigate the feasibility of such a mechanism, an idealized one-dimensional experiment was designed in which a shock wave was introduced by a piston launched from a gas gun into a closed cylindrical cavity. The compressed gas contained within had a quantity of argon introduced. Light was monitored using a fast photodiode and a gated spectrometer capable of resolving nanosecond duration pulses. The piston motion was measured by streak and framing photography using an Ultranac high-speed camera. A threshold in light emission was observed at a piston velocity of ca. 20 m s−1. It is shown that the light emission shows some of the features of the observed luminescence. The effects of colliding waves are discussed.

Interaction of detonation with inert gas zone

The results of experimental study on detonation interaction with the regions of low reactivity, generated by the injection of an inert gas into reactive mixture, are reported. A square cross-section 60×60 mm, 3.6 m long detonation channel was used. The experiments were done for stoichiometric H2−O2 mixture at 0.3 bar and 0.5 bar initial pressure and room temperature. The inert gas (Ar, He, N2 or CO2) was injected from 0.523 dm3 container into the main channel 1 s before ignition. The size of the inert zone was controlled by inert initial pressure. The behavior of detonation was studied using direct streak photography and pressure transducers. The study has shown that at low pressure of Ar, N2 and CO2 injection only a slight decrease of detonation velocity occurs. At higher injection pressures complete damping of detonation and flame extinguishment occur, followed by flame reigniton and DDT outside the inert zone. For low He injection pressures an increase in detonation velocity was recorded. For higher injection pressures, detonation damping occurred, followed by DDT process. The results have shown that CO2 has the strongest effect on damping 2H2+O2 detonation, with N2 and Ar in the next places, and He very far behind. The effectiveness of inert gas in detonation damping was found proportional to its molecular weight and reciprocal to its specific heat ratio. The numerical simulations of detonation propagation through inert gas zone were also performed using the one- dimensional Detonation Lagrangean code with simple energy release model. The results of simulations are in good qualitative agreement with experimental tendencies. In particular, the model has shown that the re-initiation of detonation is enhanced by smooth concentration gradients at inert/reactive interface.

Velocity analysis of ablated particles in pulsed laser deposition of NiO film

Optical emission of the ablated particles in pulsed laser deposition of NiO films was measured dynamically. The emission spectra showed species of the ablated particles. There are three types of light emitting particles, that is, the laser induced plasma near the target, two groups of the depositing ionized particles or the neutrals. The streak photography displayed their kinetic motions. Velocities of the ionized particles were almost proportional to the square root of the incident laser fluence ranging from 5 to 150 J cm −2. It is possible to control the kinetic energy of particles incident onto a substrate by adjusting the laser fluence.

The impact and penetration of a water surface by a liquid jet

It is well known, that a high velocity jet threads a spherical bubble during asymmetric collapse. The early phases of jet formation and travel have been investigated by several workers. The final stage of collapse, in which a microjet impacts on the rear wall of the cavity and penetrates it are less well studied and difficult to model accurately. A series of experiments were thus conducted in which liquid jets of varying velocity were fired at semi-infinite water targets. The penetration was monitored using high-speed streak and framing photography. An unexpected feature of the penetration was found to be the appearance of an axial cavitation cloud ahead of the jet. This cloud was subsequently collapsed by what is believed to be an entrapped air bubble collapsing at the jet-target interface. The subsequent motion of this interface showed periodic acceleration and deceleration. The results have interest to a range of practical situations involving cavities and their collapse.

Laboratory studies of a round, negatively buoyant jet discharged horizontally into a rotating homogeneous fluid

Results of laboratory experiments are described in which the motion of a round negatively buoyant jet discharged horizontally into a rotating homogeneous fluid has been investigated. Particle streak and dye photographs are presented (i) to demonstrate the complex three-dimensional flow fields generated by the discharge and (ii) to illustrate how the structure and time development of the flow field is controlled by the momentum ( M) and buoyancy ( B) fluxes of the discharge, the lateral location y ∗ of the discharge source from the container wall and the rotation rate Ω of the system. The effects of background rotation are studied by contrasting flow patterns for rotating and non-rotating cases, with all other parameters being kept fixed. For cases in which the system is rotating, measurements of momentum-dominated and buoyancy-dominated discharges reveal that the downstream position x p and streamwise dimension L p of the primary anticyclonic eddy corner circulation (formed by the deflection to the right of the descending jet) depend primarily upon the dimensionless distance y ∗ W (where W is the width of the channel). The influences of the dimensionless time Ωt and the relevant Coriolis parameter MΩ B on x p and L p are shown to be relatively weak. The counterpart dimensions L s and x s of the secondary cyclonic eddy (generated by the shear associated with (i) the primary eddy and (ii) a boundary current formed along the right side wall of the channel) increase with time. Both dimensions are shown to scale satisfactorily with the inertial scale u 0 Ω and the buoyancy/Coriolis scale g′ Ω 2 , where u 0 and g′ are the source discharge velocity and the modified gravitational acceleration, respectively. The speed u N of the nose of the wall boundary current is shown to be determined primarily by the dimensionless parameter MΩ B ; for values of MΩ B greater than about 5, the quantity u N is determined primarily by the discharge velocity at the source. For values of MΩ B less than about 5, the dimensionless nose velocity u N Ω g′ increases monotonically with increasing MΩ B for all values of y ∗ W .

Deflagration-to-detonation transition in granular pentaerythritol tetranitrate

The deflagration-to-detonation transition process has been observed in pressed granular columns of the explosive pentaerythritol tetranitrate. Charges were confined within a steel housing which had been fitted with a polycarbonate window to allow direct recording by high-speed streak photography. The explosive was thermally ignited by a gasless pyrotechnic mixture to minimize pre-pressurization of the charge. The results indicate that upon ignition, early choked flow of the combustion gases prevents the continued propagation of the combustion via a convective heat transfer mechanism and that the propagation of reaction becomes governed by a leading compaction wave which causes ignition through the mechanical formation of hot spots. Detonation finally occurs when the leading front of an accelerating plug of highly compacted material (density close to the theoretical maximum) formed between the reaction front and the leading compaction wave, attains the critical pressure necessary for shock-to-detonation transition.

A simple digital multilevel streak image of combustion

A method of a digital streak photography of combustion or a detonation process is described. The streak image is reconstructed from binary words produced by an ordered set of sensors which monitor the process. The temporal and spatial resolution of the process is increased by a quick periodic changing of threshold levels used for the analog–digital conversion of the sensor signals. The multilevel digitization gives a measure of signal intensity with time. The proposed streak-photograph-like projection of the flame position has been used to visualize the condition of the burning fuel in long pipes.

Correlation between welding phenomena and weld defects before and after the start of upsetting: Welding phenomena and process control in flash welding of steel sheet (2nd Report)

Summary This paper describes an investigation of the variations in the welding process causing instability of weld quality during flash welding of steel sheet with a thickness of around 3 mm. The welding current, voltage, and platen travel distance are recorded, and photographs of the arc between specimen end surfaces are taken by streak photography during the period from the final flashing stage to the initial upsetting stage. When observed at any fixed point, the maximum time interval between flashing is determined to be over 100 msec Such a long time interval is found to cause oxidation and a temperature drop at the joint and to result in a loss of joint interface quality. Fast platen displacement at the initial upsetting stage, referred to as the ‘initial displacement (upsetting)’, is shown to be affected by both flashing at the final flashing stage and the voltage phase of the power source at the start of upsetting.

Investigation of the Process of Deflagration-to-Detonation Transition (DDT) in Granular Secondary Explosives with High-Speed Photography.

The deflagration-to-detonation transition (DDT) in confined charges of granular secondary explosives has been investigated with high-speed photography. Specially designed steel confinements, which incorporate slit windows to allow streak photography, were constructed to house pressed pentaerythritol tetranitrate (PETN) charges of known density. The results indicate that convective burning is only important during the initial stages of the DDTprocess in causing rapid pressure build-up, and does not govern the deflagration process up to the point of DDT. Following convection, the propagation of the combustion front was found to be governed by the compaction wave generation of hot-spot ignition sites, The transition to detonation is finally attained when the front of a plug of highly compacted material, essentially a second compaction waveformed at the combustion front, overtakes the first compaction wave and causes shock initiation of the uncompacted, and therefore. more highly shock sensitive, original-density PETN. The detonation then propagates to the end of the charge at constant velocity and a rearward shock propagates upstream through the highly compacted plug material and intersects the combustion front. The proposed mechanism is supported by the observed deformation of the internal surface of the steel confinement.

Fundamental Characteristics of Underwater Shock Wave due to Underwater Explosion of High Explosives.

Fundamental characteristics of underwater shock waves generated by underwater explosion of high explosives are investigated by both experimentally and by numerical simulation. The attenuation process of the underwater shock wave is also investigated theoretically using a method of characteristics. Slab explosives are used in the experiments. Streak photographs are taken by a high speed camera using a conventional shadowgraph system. When the detonation wave propagates into the explosive, the underwater shock wave is generated at the boundary between the slab explosives and the water. The incident angle of the underwater shock wave is the same as that obtained by the impedance matching method between the explosives and water. The angle then changes due to the effects of interaction with the expansion wave occurring due to the expanded product gas. The attenuation process of the underwater shock wave is well explained by the method of characteristics.

Luminosity characteristics of lightning M components

A high‐speed streak photograph of a natural cloud‐to‐ground lightning return stroke followed by two M components is analyzed. As opposed to the return stroke light pulse whose amplitude and waveshape vary markedly with height, the amplitude and waveshape of one M component light pulse is essentially invariant with height between the cloud base (about 1 km) and ground, while the other M component has a relatively constant light waveshape and a light amplitude that varies somewhat with height. The two M component light pulses, both occurring within about 0.6 ms of the return stroke pulse, exhibit a more or less symmetrical waveshape with a risetime and falltime of the order of many tens of microseconds. For one of the two M components a downward direction of propagation and a corresponding speed of the order of 108 m/s are inferred.

A high-speed photographic study of cavitation damage

Cylindrical cavities, viewed through the side as they collapsed onto solid surfaces, were studied using high-speed streak and framing photography. The cavities were collapsed asymmetrically using shock waves of varying amplitude so that the rear surface formed a high-speed jet which crossed the cavity and interacted with the target surface. Schlieren optics were used to visualise waves in the fluid and in the target. Two features of the collapsing bubble affected the damage to the target surface. The first was the impact of the high-speed liquid jet on either the rear wall of the cavity or the target itself. The second was the production of a strong compression wave on the rebound of the bubble after it reached minimum volume. Damage to the targets related to their material properties. Metals, with low compressive but higher tensile strengths, plastically deformed beneath the penetrating jet to form a pit. Brittle materials, with high compressive but low tensile strengths, deformed by cracking. The position of the cavity relative to the surface had a major effect upon the geometry of the damage. With the cavity close to the target, the penetrating jet dominated the damage leaving single pits. With the cavity at some distance, the rebound wave was more important than the jet giving rise to a circular damage mark. This mechanism can be used to re-interpret previous experimental observations [Y. Tomita and A. Shima, J. Fluid Mech. 119, 535 (1986)].