Streak Photography Research Articles

Dynamics of the plasma induced by laser on a cryogenically cooled aluminum target for application in space propulsion

In this work, we investigate the properties of the plasma induced by focusing a high-power laser beam on an aluminum target that was cooled by a helium refrigerator from room temperature down to 20 K. Fast, streak photographs of the plasma were taken at different temperatures and laser energies. From the images obtained, position-vs-time plots were made for each experiment, and from them, the speed was calculated. Additionally, narrowband interference filters were employed to image the dynamics of ions and neutrals separately. It was found that the plasma plume has two distinct speeds: that of its center and that of the outer edge. For unfiltered images, the former has values within the interval 6.2 to 9.1 km/s, while the latter can reach speeds of the order of hundreds of km/s. It was found that the plume of a target cooled to 20 K has a length that is 8%–12% less than the corresponding size at room temperature. Chilling the target did not seem to affect significantly either the plume's speed of expansion or the size of the crater produced. Lower bounds were estimated for the momentum imparted to the ejecta and the specific impulse. The latter can reach a 920-s level, nearly twice as much the amount obtained with chemical rocket fuel.

Extended JCZ3 Equation of State and JAGUAR Implementation for High Explosives

AbstractAn extended JCZ3 based equation of state for the modelling of high explosive detonation products has been implemented in the JAGUAR computer model. This development includes the parameterization of EXP‐6 potentials for individual products species, as well as mixing rules for the combination of species. Non‐linear variable metric optimization was used for the species parameterization, as well as for the solution of free energy minimized states. Explosive detonation and products expansion are typically modelled for JWL and JWLB high explosives thermodynamic equation of state development. An analytic c ylinder test model was developed for the comparison to experimental cylinder expansion test data and for the characterization of explosive products expansion and work output. Examples and comparisons to data are presented. Historically, streak photography was used for cylinder testing, whereas photon‐doppler velocimetry (PDV) is now standard. This has resulted in some changes in cylinder testing data reduction. A JAGUAR MSIAC web‐based application is currently under development for use by the MSIAC member nations.

Observation of Debris Clouds Generated by Hypervelocity Impact of Copper Jets with Thin Aluminum Plats

With high kinetic energy and excellent penetration performance, metal jets have been widely used in the defence industry. When a thin plate is impacted by a metal jet with a velocity higher than 3 km/s, debris clouds will be generated at both sides of the plates. However, compared with the debris clouds generated by hypervelocity impact of projectiles, debris clouds generated by hypervelocity jets is rarely observed. In this paper, the utility of ultrahigh-speed simultaneous framing and streak photography technology is applied in the experiment of copper jets impact thin aluminum plates at hypervelocity. Debris clouds and front edges of shock waves are clearly observed and recognized after using newly developed technology to effectively restrain excessive exposure caused by intensive self-illumination in the experiment. From framing and streak pictures, the process of jets impact targets is comprehensively and completely recorded and the velocities of the jet and the debris cloud are obtained using the methods of data processing developed in present paper.

High velocity outflows along the axis of pulsed power driven rod z-pinches

We report on initial observations of high velocity outflows from the ends of a rod compressed using pulsed power. 1 mm and 2 mm diameter copper rods were placed in a water bath and driven by ∼0.6 MA currents with rise times of ∼700 ns. Laser backlit framing images and streak photography showed an outflow of the material from the ends of each rod, of the initial velocity of up to 7 km/s, which began ∼500 ns after the start of the current pulse and continued throughout the experiment. Ballistics gel was used to help separate low density gas/plasma from any solid/liquid component in the outflow, successfully capturing the material from larger diameter rods (enabling an estimate of its energy) and tracing the path of the material that passed straight through the gel with smaller rods. Experimental results were compared to 1D and 2D MHD simulations performed with the Gorgon code. These suggested that the outflow had two different components, resulting from two different physical processes. Differences in the resistivity between the copper rod and stainless steel anode result in the opening of a small gap between them and ablated stainless steel being projected above the rod, which is captured in framing and streak images. Later in time, a dense copper material, pinched by the magnetic pressure, is launched—explaining the ballistics gel results. The simulations also suggest that the tamped explosion of the rod surface plays a small role in any outflow.

An experimental and simulation comparison of a 3-D abrupt contraction flow using the molecular stress function constitutive model

ABSTRACT The Molecular Stress Function (MSF) constitutive model with convective constraint release (CCR) mechanism has been shown to accurately fit a large range of viscometric data, and also shown to give strong vortex growth in flows of LDPE through planar and axisymmetric contractions. This work compares simulation and experimental results for 3-D flows of Lupolen 1840H LDPE through a contraction slit. Comparisons are made with vortex opening angles obtained from streak photography, and also with stress-birefringence measurements. The comparisons are made with two versions of the CCR mechanism. The simulated vortex angles for one version of the CCR mechanism are found to approach what is seen experimentally. The best-fit value for the stress optical coefficient was found to vary between CCRs and to decrease with flow rate. This is partially explained by different centreline elongational rates with the two CCRs, which in turn is related to different opening angles.

Applying visualization techniques to study the fluid flow pattern and the particle distribution in the casting of metal matrix composites

Abstract For a past few decades, metal matrix composite has a significant role in the various industrial and automobile applications. Production of metal matrix composite using stir casting technique is one of the most effective and economical technique. Achieving the uniform distribution of the reinforced particles was the major problem facing during the production of particle reinforced metal matrix composites and also achieving the effective fluid flow pattern was the greatest challenge to the research community. In this research, some of the major stir casting process parameters such as blade geometry, stirring speed and impeller position were optimized based on the vortex height of the fluid and settling time of the reinforced particles using visualization experiments and Grey relation analysis. Flow pattern of the fluid and the trace of the particles was analysed using a new technique called streak photography. Streak photography was one of the visualization experiments using glycerol water based model. Al8011/SiC metal matrix composites were manufactured based on the optimized stir casting process parameters. Finally, the fabricated composites were carried out for OM, SEM analysis and Brinell hardness testing for confirming the uniform distribution of the reinforced particles. Based on the confirmation results, 45ᴼ stirrer blade angle, impeller position 40 % from the base and 250 rpm stirring help to produce uniform distribution of the particles.

Cavitation Inception from Transverse Waves in a Thin Liquid Gap

It is well known that dielectric breakdown in a liquid generates cavitation bubbles and shock waves. Here we demonstrate that when the liquid is bounded by two solid glass boundaries (10--20-$\ensuremath{\mu}\mathrm{m}$ separation), rings of microscopic bubbles can be nucleated around the laser-induced cavitation bubble. While generally acoustic nucleation is achieved with longitudinal waves of sufficient tension, this work demonstrates that acoustic cavitation can also be generated from transverse waves. Our experiments identify three waves originating at the boundaries: the fastest is the bulk wave in the solid, followed by a leaky Rayleigh wave at the liquid-solid contact, which is trailed by a Lamb-type wave. For the latter, the two solid boundaries act as a wave guide and generate intense and short-lived cavitation activity within the gap. Streak photography and high-speed photography reveal the microsecond-duration cavitation-bubble dynamics, and subpicosecond strobe photography visualizes the mechanism of bubble nucleation from the accelerated surface. Simulations coupling the solid mechanics with the acoustics support the experimentally observed mechanisms of transverse-wave-induced cavitation inception.

The Use of Streak Photography, X-Ray Radiography, and Radiometric and Spectrometric Measurements to Study Plasma Bunches Generated under Gyroresonant Interactions

Results of experimental studies of relativistic plasma bunches obtained under gyromagnetic autoresonance (GA) and confined in a magnetic mirror trap are provided. The main attention is paid to observations of the radiation generated by plasma bunches at different stages of their lives in different spectral ranges using streak photography and X-ray imaging techniques. The original method of accounting for the instrument function in X-ray spectroscopic analysis and radiometric measurements is applied within the quantum energy range of 0.2–2 MeV to take account of considerable distortion caused by the Compton scattering in the detector.

Micro-machining with abrasive slurry-jets: effects of dissolved polymer concentration and nozzle design

The resolution of features machined with abrasive slurry jets is dependent on the diameter of the erosion footprint on the workpiece and on the stability of the jet. Previous work has shown that the footprint size decreases, but the jet oscillation increases, with increasing polymer concentration. The present study investigated the effect of nozzle design on jet stability in abrasive slurry-jet micro-machining of glass using slurries with dissolved polymers. A variety of transparent acrylic nozzles were made with different geometries and a 180-μm sapphire orifice to study the oscillation of jets of water mixed with high molecular-weight polyethylene oxide. Fluorescent streak photography of the flows within the nozzles confirmed the presence of unstable vortices that can lead to jet oscillation. However, the dependence of these vortices on nozzle geometry and PEO concentration was unclear. Jet oscillation was then measured photographically and found to increase nonlinearly with polymer concentration for all nozzle geometries. These results were used to design and manufacture a new steel nozzle with a 30° tapered entrance region which reduced jet oscillation. Micro-channels were then machined in glass plates using a range of polymer slurries containing aluminium oxide particles. The polymer concentration that produced the best combination of a relatively small footprint while simultaneously minimizing the jet oscillation amplitude resulted in channels that were 11% narrower than reference channels machined with a pure water slurry and a standard nozzle.

Flow past a Sphere

A new theoretical framework is applied to the steady fluid flow past a solid smooth sphere. Bernoulli’s law along a streamline is combined with the cross-stream force balance: centrifugal force on the curved flow equals a pressure gradient. When compared with the standard potential theory for flow past a sphere in a text book, the prospect of a major discrepancy is found. Whereas the decay rate of the velocity perturbation away from the sphere goes as the inverse cube of the distance in the text book, the decay rate computed here is in all likelihood very different, and it depends on an unknown constant function, the radius of curvature of the streamlines versus distance from the sphere. When that function is supplied either from another theory or from detailed observations (probably streak photographs), then the new approach can be solved completely. In any case, accurate measurements of flow rates at different positions with respect to the solid are badly needed.

Experimental and numerical methodology to analyze flows in a coronary bifurcation

Atherosclerosis is frequently associated to disturbed flow patterns in regions of the arteries with geometrical complexity. To understand atherosclerosis development, the hydrodynamic characteristics of the blood flow in arteries, such as the velocity and wall shear stress, must be studied and quantified. To investigate the local hemodynamics in the Left Coronary Artery (LCA) bifurcation, a methodology combining numerical (CFD) and experimental methods (μPIV and streak photography) was designed. A whole volume μPIV experimental method was developed and applied to a simplified 3D phantom of the LCA bifurcation. The experimental and numerical results prove to be in a good agreement. Implications of a deviation from Murray’s law and the effect of different Reynolds numbers in flow behavior were also explored. A streak line photography method was used to complement the methodology and analyze the flow patterns. When the deviation from Murray’s law is significant, secondary flows are observed and they intensify as the Reynolds number increases. The secondary flows are visible in regions of low wall shear stresses (WSS). The present work reinforces, with numerical and experimental support, the importance of flow patterns that are relevant for prevention and treatment of atherosclerosis.

Generation of highly symmetric, cylindrically convergent shockwaves in water

We report on pulsed power driven, exploding copper wire array experiments conducted to generate cylindrical convergent shockwaves in water employing μs risetime currents >550 kA in amplitude and with stored energies of >15 kJ—a substantial increase over previous results. The experiments were carried out on the recently constructed Mega-Ampere-Compression-and-Hydrodynamics facility at Imperial College London in collaboration with colleagues of Technion, Israel. 10 mm diameter arrays consisting of 60 × 130 μm wires were utilized, and the current and voltage diagnostics of the load region suggested that ∼8 kJ of energy was deposited in the wires (and the load region close to the wires) during the experiments, resulting in the formation of dense, highly resistive plasmas that rapidly expanded driving the shockwaves in water. Laser-backlit framing images of the shockfront were obtained at radii <0.25 mm for the first time, and there was strong evidence that even at radii <0.1 mm this front remains stable, resulting in a convergence ratio of >50:1. Framing images and streak photographs showed that the velocity of the shockwave reached ∼7.5 km s−1 at 0.1 mm from the axis. 2D hydrodynamic simulations that match the experimentally obtained implosion trajectory suggest that pressures >1 Mbar are produced within 10 μm of the axis along with water densities of 3gcm−3 and temperatures of many 1000 s of Kelvin. Under these conditions, Quotidian Equation of State suggests that a strongly coupled plasma with an ionization fraction of ∼0.7 would be formed. The results represent a “stepping stone” in the application of the technique to drive different material samples into high pressure, warm dense matter regimes with compact, university scale generators, and provide support in scaling the technique to multi-mega ampere currents.

Evaluation of a Methodology for Automated Cell Counting for Streak Mode Imaging Flow Cytometry

Identification of Circulating Tumor Cells (CTCs) has shown promising clinical applications, but since CTCs are found in very low concentration in blood large sample volumes are needed for meaningful enumeration. This issue impedes the analysis of CTCs using standard flow cytometry due to its low throughput. To address this issue, a high throughput microfluidic cytometer was recently developed using a wide field flow- flow cell instead of the conventional narrow hydrodynamic focusing cells (used in traditional flow cytometry) enabling analysis of large volumes at lower flow rate. This wide-field flow cytometer adopts a technique known as “streak photography” where exposure times and flow velocities are set such that the particles are imaged as short “streaks”. Since streaks are imaged with large number of pixels, they are easily distinguished from the noise which appears as “speckles” increasing the detection capabilities of the device, making it more suitable for analysis using current low sensitivity, high noise webcams or mobile phone cameras. The non-stationary nature of the high noisy background found in streak cytometry introduces additional challenges for automated cell counting methods using traditional cell detection techniques such TLC, CellProfiler, CellTracker and other tools based in traditional edge detection (e.g., Canny based filters) or manual thresholding. In order to address this issue, we developed a new automated enumeration approach that does not rely on edge detection or manual thresholding of individual cells, rather is based in image quantizing, morphological operations, 2D order-statistic filtering and decisions rules that take into account knowledge of the structure and expected location of the streaks in consecutive frames. We evaluated our approach comparing it with two current methods representing the major computational modalities for cell detection: CellTrack (based in edge detection) and MTrack2 (based in manual thresholding). Samples of 1 cell/mL nominal concentration were analyzed in batch size of 30 mL at flow rate of 10 mL/min and imaged at 4 frames per second (fps), the files were saved in uncompressed AVI format files. The cells were annotated and the signal to noise ratio (SNR) was calculated. For samples with average SNR greater than 4.4 dB, our method achieved a sensitivity of 91% compared to CellTrack (60%) and MTrack2 (71%). The True Positive Rate (TPR) of cells detected was 0.93 for our method compared with 0.80 for Mtrack2 and 0.29 for CellTrack. This demonstrated the ability of the algorithm to count rare cells with high accuracy for concentrations of 1 cell/mL with SNR greater than 4.4 dB. This cell counting capability will enable to automate low cost imaging flow cytometry based on CCD detector and the expansion of cell-based clinical diagnostics in resource-poor settings.

Measuring of high current channel parameters in high pressure gas by combined using of magnetic probe and high speed streak photography

Experimental results for discharge in hydrogen with current amplitude up to 1 MA, current rise rate of ∼ 1010 A/s, and at initial pressure up to 30 MPa are presented. A series of channel contractions was observed at a current rise stage. Estimation of plasma channel parameters was made for equilibrium state at the channel diameter oscillations. The speed of the discharge channel contraction was determined by the specially developed magnetic- probe technique. Comparison of these magnetic probe measurements with high-speed optical photostreaks was carried out.

Visualization of the trapping of inertial particles in a laminar mixing tank

We report on new methods and results for the trajectory visualization and tracking of large inertial particles in a laminar (chaotic) mixing tank. Using specially designed light-emitting capsules, particle orbits were visualized using streak photography to reveal pathlines of capsules moving in the laminar fluid flow. We present visualization results on unexpected trapping of particles whereby finite-sized particles/capsules can spontaneously localize into the vortex tubes in the laminar stirred system. Once into tubes, large inertial particles move in helical orbits as do passive particles. Mixing tanks have been in use over a number of centuries but this trapping phenomenon has been consistently ignored. We also measure the three-dimensional position of large inertial particles over time using an externally positioned camera and planar mirror in a calibrated setup. As the light capsules used in this study give very high contrast even when the fluid is murky, the work provides a sound basis for further research development of measurement techniques for large (industrial) scale flows.

Hydrodynamics and mass transfer of oscillating gas‐liquid flow in ultrasonic microreactors

Ultrasonic microreactors were used to intensify gas‐liquid mass‐transfer process and study the intensification mechanism. Fierce surface wave oscillation with different modes was excited on the bubble. It was found that for slug bubbles confined in smaller microchannel, surface wave oscillations require more ultrasound energy to excite due to the confinement effect. Cavitation microstreaming with two toroidal vortices was observed near the oscillating bubble by a streak photography experiment. Surface wave oscillation at the gas‐liquid interface increases the specific surface area, while cavitation microstreaming accelerates the interface renewal and thus improves the individual mass‐transfer coefficient. With these two reasons, the overall mass‐transfer coefficient was enhanced by 3–20 times under ultrasonication. As for gas‐liquid flow hydrodynamics, ultrasound oscillation disturbs the bubble formation process and changes the initial bubble length and pressure drop. © 2015 American Institute of Chemical Engineers AIChE J, 62: 1294–1307, 2016

Comparison of measured and computed radial trajectories of plasma focus devices UMDPF1 and UMDPF0

In published literature, there has been scant data on radial trajectory of the plasma focus and no comparison of computed with measured radial trajectory. This paper provides the first such comparative study. We compute the trajectories of the inward-moving radial shock and magnetic piston of UMDPF1 plasma focus and compare these with measured data taken from a streak photograph. The comparison shows agreement with the measured radial trajectory in terms of average speeds and general shape of trajectory. This paper also presents the measured trajectory of the radially compressing piston in another machine, the UMDPF0 plasma focus, confirming that the computed radial trajectory also shows similar general agreement. Features of divergence between the computed and measured trajectories, towards the end of the radial compression, are discussed. From the measured radial trajectories, an inference is made that the neutron yield mechanism could not be thermonuclear. A second inference is made regarding the speeds of axial post-pinch shocks, which are recently considered as a useful tool for damage testing of fusion-related wall materials.

An ultrafast camera films light at light speed

Exploiting a strategy known as compressed sensing, the camera can capture video at 100 billion frames per second.

Flow past a Groove

Based on available evidence, it is hypothesized that the net force of friction on a flat solid wall, when fluid flows steadily along it, is reduced by putting one or more grooves in the wall’s surface oriented perpendicular to the mean flow. Among the convincing observations are the existence and history of golf balls which show that golf balls with dimples travel farther than golf balls without dimples. Also there is a laboratory experiment using streak photography of low Reynolds number flow along a straight wall with a square cavity in it, illustrating that the flow jumps right across the cavity’s opening, strongly suggesting that there is no friction of the fluid on the wall in the region of the cavity. One forecast is that if grooves or dimples are made on the inside surface of pipes, the discharged rate of the pipe for fluid flow should become increased.

Influence of flow rate variation on the development of Escherichia coli biofilms

This work investigates the effect of flow rate variation on mass transfer and on the development of Escherichia coli biofilms on a flow cell reactor under turbulent flow conditions. Computational fluid dynamics (CFD) was used to assess the applicability of this reactor for the simulation of industrial and biomedical biofilms and the numerical results were validated by streak photography. Two flow rates of 374 and 242 L h(-1) (corresponding to Reynolds numbers of 6,720 and 4,350) were tested and wall shear stresses between 0.183 and 0.511 Pa were predicted in the flow cell reactor. External mass transfer coefficients of 1.38 × 10(-5) and 9.64 × 10(-6) m s(-1) were obtained for the higher and lower flow rates, respectively. Biofilm formation was favored at the lowest flow rate because shear stress effects were more important than mass transfer limitations. This flow cell reactor generates wall shear stresses that are similar to those found in some industrial and biomedical settings, thus it is likely that the results obtained on this work can be used in the development of biofilm control strategies in both scenarios.