High-speed Framing Camera Research Articles

Dynamic Wall Shear Stress Measurement Using Event-Based 3D Particle Tracking

We describe the implementation of a 3d Lagrangian particle tracking (LPT) system based on event-based vision (EBV) sensors and demonstrate its application for the near-wall characterization of a turbulent boundary layer (TBL) in air. The viscous sublayer of the TBL is illuminated by a thin light sheet that grazes the surface of a glass window inserted into the flat wall of the wind tunnel wall. The data simultaneously captured by three synchronized EBV-cameras is used to reconstruct the 3d particle tracks within 0.4 mm of the wall on a field of view of 12.0 mm by 7.5 mm. The velocity and position of particles within the viscous sub-layer permit the estimation of the local, unsteady wall shear stress vector under the assumption of linearity between particle velocity and wall shear stress. Thereby, 2d distributions and higher order statistics of the unsteady wall shear stress are obtained. The systematic underestimation of the spanwise shear stress fluctuation can be explained on the basis of DNS data; a methodology for its mitigation is proposed. The employed EBV hardware coupled with suited LPT algorithms provide data quality on par with currently used, considerably more expensive, high-speed framing cameras.

Ultra-high temporal resolution images of a homogeneous electric field short air gap negative streamer at overvoltage and atmospheric pressure

Air gap discharge is one of the most basic scientific problems in the field of high-voltage engineering. The homogeneous electric field 1.5 mm air gap negative streamer at overvoltage and atmospheric pressure is observed by a high-speed 4-channel framing camera. The ultra-high temporal resolution images of a single negative stream are captured (the exposure time is 5 ns, and the inter-frame delay is no more than 0.1 ns). It is observed that the negative streamer formed in the middle of the air gap and grew bidirectionally towards both electrodes. At the same time, the electrical measurement is also carried out.

Experimental investigation into lock-in characteristics of the composite hydrofoils

Abstract This study experimentally investigates the lock-in characteristics of structural vibration of hydrofoils made of CFRP composite material. The synchronous experimental measurement system constituted of high-speed framing camera, LDV, velocity and pressure regulation systems are set up. It can be found that the Com+45° hydrofoil accelerates the cavitation inception, and the Com-45° hydrofoil defers it. The evolution of the dominant vibration frequency and cloud cavity shedding frequency are semblable. As the cavitation evolution gradually develops, the larger dominant frequency reduces, while the lower frequency remains unchanged. The scope of frequency lock-in meets the magnitude relationship Com+45° hydrofoil> Com-45° hydrofoil> Com0° hydrofoil. The wavelet transformation of structural vibration displacement and non-dimensional cavity area shows that both f shed and f shed,lock-in exist in the lock-in condition.

Stability of breakdown phenomenon in N2, SF6, and their mixture under impulse voltages

In the self-breakdown experiment, it is demonstrated that the breakdown stability of 15% SF6/N2 was higher than that of pure SF6 and N2 in the non-uniform field under negative impulse voltages. In this paper, the stable breakdown phenomenon of the gas mixture is studied at the nanosecond scale. The corona process and streamer process of these three gases are investigated by using a high-speed framing camera. The stabilized corona and the abnormal streamer phenomena observed in the gas mixture discharge have a relation to the stable breakdown phenomenon. The stabilized corona is supposed to be the main reason that obstructs the development of negative steamer and stabilizes the supply of photons to the anode. Furthermore, the captured images of the streamer process in the gas mixture show that there is a negative ion sheath between the electrodes. The sheath keeps the corona stabilized near the cathode tip. In addition, photons emitted by the stabilized corona can ionize neutral particles near the anode. The generated photoelectrons and positive ions accumulate near the anode surface. The positive streamer occurs once the accumulation number reaches a certain value. In addition, the photon emission intensity and stability also have an influence on the stability of the positive streamer.

Dynamic characteristics of axial load bi-stable energy harvester with piezoelectric polyvinylidene fluoride film

In this paper, a nonlinear bi-stable energy harvester with a bulking beam is proposed. Its bi-stable characteristics can be realized by the structure. The characteristics of the piezoelectric energy harvester are tested under different conditions. In the experiment, high speed framing camera is used to capture the details of movement. It is found that similar characteristics occur at low frequency and high frequency, but the principles are not the same. The vibration amplitude is insufficient former when the frequency is low. Under high frequency conditions, the system is hindered by the reverse excitation when the system is about to enter the second steady state due to insufficient response time. In the experiment, the axial load is measured by the length of the buckling beam and the electronic scale. It is found that the axial load has great influence on the vibration amplitude of the harvester. The results show that the harvester changes from inter-well vibration to intra-well vibration when axial load changes from 0.4 N to 1.5 N. The inter-well vibration of the harvester occurs when the acceleration is 3.5 g, the vibration frequency is 15 Hz and the axial load is 0.4 N.

Nonlinear characteristics and corrections of near-field underwater explosion shock waves

The shock wave characteristics within the near-field are one of the most challenging aspects of understanding an underwater explosion. The latest numerical and experimental techniques were utilized to investigate the near-field pressure distribution and decay features after a shock disturbance. The governing equations in the numerical simulation were discretized with a fifth-order weighted essentially non-oscillatory scheme in space and a third-order Runge–Kutta scheme in time, and multi-medium interactions were defined and resolved via the modified ghost fluid method. The test system consisted of a synchronized high-speed framing camera and polyvinylidene fluoride (PVDF) sensors. Three identical spherical composition B charges were examined under the same test conditions, and the raw data from the high-speed camera were processed with edge detection and circle fitting techniques. The comparison showed that the high-speed camera image data, the PVDF signals, and the numerical computation results were highly consistent with each other. Higher-order correction terms were added to the pressure peak distribution model and the pressure decay model as nonlinear corrections based on further comprehensive and insightful analysis of the verified results. The corrected models not only fit with the near-field data but had better accuracy under the far-field condition as well.

A four-channel ICCD framing camera with nanosecond temporal resolution and high spatial resolution

In this paper, a high spatial resolution, high gating speed framing camera capable of four separate two-dimensional images is designed and tested. A mirror-based image splitter has been designed and developed to provide a separate image for each arm with equal path length and equal intensity splitting while having a spectral flat response. Fast gating of a single frame is accomplished by using a proximity-focused microchannle-plate image-intensifier (MCPII) gated by a MOSFET-based pulse module with adjustable gate width from 3 ns to D.C. operation. Multiple frames are obtained by using multiple gated tubes with time intervals between consecutive frames can be adjusted flexibly. Its spatial resolution reaches 36.5 lp/mm with each frame having a 4.6-ns gate width. Furthermore, a three-dimensional MCPII model is developed to investigate the transit time and spatial resolution of the proximity-focused MCPII based on the Finite Integral Technique (FIT) and Monte Carlo method (M–C method).

Damage mechanisms and energy absorption of aluminosilicate glass under compression/tensile loading

The transparent laminated structures having annealed and tempered glass layer with adjoining interlayer are used increasingly in aircrafts, high-speed trains, and important installations. In a pursuit to realize the fracture initiation, propagation, speed, path, and catastrophic failure to dynamic impact event in brittle materials like (annealed and tempered) silicate glass are experimentally analyzed in this research article undergoing tensile and compression loading. The Kolsky bar system linked with high-speed framing camera is applied to record the deformation images of glass samples for compression/tensile dynamic loading. The captured images are studied to identify the damage development and final rupture of silicate glass specimens. The stress-time history curves were related with recorded high-speed stills to enlighten the load-bearing capacity and damage accumulation in glass specimens. The stress–strain relations from two types of loading were used to envisage the energy absorption capability of annealed and tempered glass. The collected debris after test suggested that both glass types are loading-rate sensitive and the tempered glass dices in fine particles than annealed glass. In compression, and tensile loading mode the maximum energy was absorbed in by tempered glass which supports the use of this type of glass in impact resistant transparent laminated structures. The microscopic and scan electron microscope (SEM) pictures of collected fragments were also analyze to observe the failure mechanism and fracture surfaces in two types of glasses.

Measurement of time-varying electron density of the plasma generated from a small-size cylindrical RDX explosion by Rayleigh microwave scattering

It is challenging to measure the electron density of the unsteady plasma formed by charged particles generated from explosions in the air, because it is transient and on a microsecond time scale. In this study, the time-varying electron density of the plasma generated from a small cylindrical cyclotrimethylenetrinitramine (RDX) explosion in air was measured, based on the principle of microwave Rayleigh scattering. It was found that the evolution of the electron density is related to the diffusion of the detonation products. The application of the Rayleigh microwave scattering principle is an attempt to estimate the electron density in explosively generated plasma. Using the equivalent radius and length of the detonation products in the bright areas of images taken by a high-speed framing camera, the electron density was determined to be of the order of 1020 m−3. The delay time between the initiation time and the start of variation in the electron-density curve was 2.77–6.93 μs. In the time-varying Rayleigh microwave scattering signal curve of the explosively generated plasma, the electron density had two fluctuation processes. The durations of the first stage and the second stage were 11.32 μs and 19.20 μs, respectively. Both fluctuation processes increased rapidly to a peak value and then rapidly attenuated with time. This revealed the movement characteristics of the charged particles during the explosion.

High-resolution x-ray radiography with Fresnel zone plates on the University of Rochester's OMEGA Laser Systems.

Experiments performed at the Laboratory for Laser Energetics with a continuous-wave (cw) x-ray source and on the OMEGA and OMEGA EP Laser Systems [Boehly et al., Opt. Commun. 133, 495 (1997) and Waxer et al., Opt. Photonics News 16, 30 (2005)] have utilized a Fresnel zone plate (FZP) to obtain x-ray images with a spatial resolution as small as ∼1.5 μm. Such FZP images were obtained with a charge-coupled device or a framing camera at energies ranging from 4.5 keV to 6.7 keV using x-ray line emission from both the cw source and high-intensity, laser-beam-illuminated metal foils. In all cases, the resolution test results are determined from patterns and grids backlit by these sources. The resolutions obtained are shown to be due to a combination of the spectral content of the x-ray sources and detector resolution limited by the magnification of the images (14× to 22×). High-speed framing cameras were used to obtain FZP images with frame times as short as ∼30 ps. Double-shell implosions on OMEGA were backlit by laser-irradiated Fe foils, thus obtaining a framing-camera-limited, FZP-image resolution of ∼3 μm-4 μm.

Effect of Fuel Mass Flow at the End of Injection on Cavitation and Gas Ingestion in the Nozzle

The fuel flow in the diesel engine nozzle has a vital impact on the fuel atomization and spray, and the fuel mass flux affects the internal flow of the nozzle. The visual experimental platform for a transparent nozzle was built to obtain the image of fuel flow in a nozzle with a small sac combining the back-light imaging technology and a high-speed framing camera. A two-phase three-component numerical model, based on the OpenFOAM solver, was calculated to quantitatively analyze gas ingestion and cavitation in the nozzle. The results indicate that at the end of injection (EOI), fuel cavitation and external air backflow (gas ingestion) occur successively in the nozzle, and both phenomena first appear in the orifice and then transition to the sac. Cavitation collapse is the major factor of gas ingestion, and the total amount of gas ingestion and cavitation mainly depends on the sac. The outflow of fuel largely depends on the total amount of cavitation and the inertial outflow of fuel at the EOI. The type of cavitation in the nozzle mainly presents annular and bulk cavitation, the former primarily exists in the sac, while the latter is established within the orifice. Therefore, larger mass flows will contribute to stronger cavitation and gas ingestion.

Free-field schlieren measurements of laser-induced breakdown shock propagation

Measurements of free-field shock propagation are fundamental to understanding source characteristics, and for transfer functions through complex environments. Probe interference and undesirable high-frequency response plague typical approaches with acoustic microphones, which are also limited to resolving the pressure field at a single position. Measurements made with optical methods do not have such drawbacks, and schlieren measurements are particularly well suited to measuring both the spatial and temporal evolution of nonlinear pulse propagation. A laser induced breakdown source is used to generate spherically symmetric weak shockwaves. A z-type schlieren instrument and high-speed framing camera are used to measure schlieren images of passing shock waves. Quantitative measurements relate schlieren image intensity values to light refraction angles, and by the inverse Abel transform the density (and pressure) fields induced by the shockwave are recovered. Comparisons of analytical predictions, assuming spherically symmetric and inviscid propagation, with experimentally measured shockwaves are presented.

Striped superlattice pattern in dielectric barrier discharge

We report on the striped superlattice pattern in dielectric barrier discharge for the first time. The spatiotemporal structure of the striped superlattice pattern is investigated by a high-speed framing camera. The result shows that the striped superlattice pattern consists of three different transient sub-lattices which are striped-dots, stripes, and small-dots surrounding a striped-dot, respectively. Images of a single frame indicate that the stripes which look like they are diffused are actually made up of individual filaments. The optical emission spectra of different sub-lattices are collected and investigated; it is found that plasma parameters of the three different transient sub-lattices are different. The formation mechanism of the striped superlattice pattern is discussed. And a tunable plasma photonic crystal with one and two-dimensions structures which has the dynamic controllability based on the striped superlattice pattern is present.

Effect of sample morphology on the spectral and spatiotemporal characteristics of laser-induced plasmas from aluminum

Species stratification and local plasma composition can affect microsecond-timescale oxidation reaction rates of metals such as Al in an oxidizing atmosphere. Here, we utilize fast, gated emission spectroscopy and a high-speed framing camera to determine the intensity and spatiotemporal evolution of various Al ablation products within a laser-induced plasma. Using a high-purity Al plate, micron- and nano-sized Al powders, and inert micron Al2O3 powder, we studied the effect of Al morphology and reactivity on the oxidation characteristics and plasma hydrodynamics in air at 1 atm over two temporal regimes (2–10 μs and 20–100 μs). We observed an increase in the spatial distribution and intensity of emission from vaporized Al within the plasma for powder-based samples compared to plate Al due to enhanced material dispersion. In nano-Al, AlO emission forms at, and propagates along, the surface of the powder bed from 2–10 μs, whereas for micron powder, there is a delay in AlO formation within the bulk of the plasma until tens of microseconds. We measured electron densities from a variety of spectral lines, which can range from ~ 2 × 1015 to 2 × 1018 cm−3, and which scale inversely with the rate of plasma expansion across morphologies. The Al I and Al II species temperatures from 2 to 10 μs calculated via Boltzmann plots are similar (from ~ 10,000 to 14,000 K), and we performed a suite of local thermodynamic equilibrium (LTE) validity calculations to establish that these two species are in LTE, while H is not. Using image co-registration, we calculated the thickness of the AlO layer surrounding the expanding Al cloud at times > 20 μs, which can range from ~ 50 to 300 μm. These results allow us to begin to understand the complexities of laser ablated metal powder reactions at microsecond timescales.

Deformation, fragmentation and acceleration of a controlled fragmentation charge casing

Abstract Two different finite element software, LS-DYNA and Impetus, have been evaluated to test their ability to predict the deformation, fragmentation and acceleration of a controlled fragmentation charge casing. The general-purpose program LS-DYNA was used with a multi-material ALE formulation and a mass-preserving erosion criterion coupled to a Johnson-Cook fracture criterion. In the Impetus simulations, a third order Lagrangian element formulation was used for the casing and a node-splitting element erosion treatment coupled to a Cockcroft-Latham failure criterion was used to describe casing fracture. The high-explosive gases were described by a discrete particle formalism. In order to acquire data to validate our computational tools and constitutive models, a series of experiments have been performed using a laboratory charge with an internal grooved casing. In the test series, the charge geometry was fixed except that the groove depth were varied from very shallow to very deep resulting in different deformation patterns, fracture modes and terminal velocities. Various diagnostic tools captured the different stages of the expansion and fragmentation of the casing. A high-speed framing camera depicted the deformation pattern before fragmentation and was used to determine the moment when the casing failed. Three different complementary techniques were used to follow the acceleration of the fragments; a Photon Doppler velocimetry to determine the initial acceleration of the casing, double exposed radiographs to estimate the fragment velocity after break-up and a high-speed video to determine the terminal velocity of the fragment after leaving the fireball. In addition, the fragments were soft recovered in a set of sawdust pit tests and their final shape and weight were measured. A SEM was used to characterise the fracture surfaces and to determine the modus of fracture (tensile or shear failure). Comparisons to experiments show that both software can predict the change in deformation behaviour when the groove depth increases, from tangential necking for shallow grooves to radial punching for deep groves. Both software could also reasonable well predict the acceleration of the fragments, though both overestimates the terminal velocity for the charge with the deepest grooves.

Criteria and propagation processes of electrode-initiated and electrodeless-initiated discharge in SF6

The differences between the gas gap discharge and surface flashover have not been clarified because of the complexity of the flashover inception process at the solid–gas interface. In this paper, epoxy resin insulators with an adjustable inserted electrode for electric field regulation were employed to investigate the inception and propagation of electrodeless-initiated discharge (ELID) under the standard lightning impulse in SF6, and the differences from the breakdown process of an equal-sized gas gap (electrode-surface-initiated discharge, ESID) are discussed. Both a photomultiplier tube and a high-speed framing camera were used to observe the luminous strength and configuration of the initiation and propagation processes of the discharge. The results show that various luminous pre-signals can be detected before breakdown, indicating different physical processes during discharge. In ESID, the discharge begins with several streamer bursts followed by a step-by-step stem leader propagation and then breaks through the whole gas gap, with a submicrosecond time delay, whereas, in ELID, the streamer bursts are much weaker than those in ESID, and they turn into a bidirectionally propagated leader that breaks through the residual insulation faster, with a much shorter breakdown time delay, which is ≤300 ns. The criterion of streamer inception is also analyzed via the Raether–Meek criterion (K value), which is found to be higher in ELID (545 ± 20) than that in ESID (375 ± 25), which implies a higher requirement of an electric field for streamer inception and discharge initiation from the dielectric surface than that from the electrode surface.

Time-and-Space Resolved Cathode Plasma Expansion Velocities in Magnetically Insulated Coaxial Diode With a Metal–Dielectric Cathode

Time-and-space resolved cathode plasma expansion velocities in magnetically insulated coaxial diodes (MICD) with edged cathodes made of bronze dielectric, bronze, and graphite have been compared through optical diagnosis. The voltage and current of the diode pulse of 260 ns are about 400 kV and 2.8 kA, respectively. The axial and radial expansion velocities of the cathode plasma were obtained through the angled lateral view by utilizing high-speed framing camera combined with the digital image processing methods. In addition, the evolution of the cathode and collector plasmas was visible firstly on the front view at the same time. The bronze-dielectric cathode not only showed low expansion velocity of about 0.5 cm/μs but also had other superior characteristics indicated by the strong emission capability, the fast plasma formation and the uniform plasma distribution. Thereby, the metal-dielectric cathode is a good candidate for applications in a long-pulsed MICD.

Characterization of flashover plasma across a large-scale ceramic vacuum interface initiated by explosive electron emission

Flashover plasma characteristics across a large-scale ceramic vacuum interface initiated by explosive electron emission (EEE) are investigated with the experimental and simulation methods. Driven by a negative high voltage pulse (-300 kV, 150 ns), flashover plasma luminescence processes were observed by a high-speed framing camera and the accompanied spectrum characteristics were measured by a spectrograph. Compared with flashover initiated by field electron emission (FEE), a faster light expansion velocity and higher electron temperature of flashover initiated by EEE were obtained which were 200 cm/μs and 4.57 eV, respectively. Radial and angular components characteristics of expansion velocity were analyzed as well and their maximum values were 200 cm/μs and 50 cm/μs. Furthermore, comparisons of flashover initiated by EEE and FEE were conducted by particle-in-cell methods and the results agreed with the experimental observations. From these results it can be concluded that due to higher primary electron energy and stronger secondary electron avalanche and gas ionization, flashover along the ceramic interface initiated by EEE has an easier and faster develop process compared with that initiated by FEE. This work can give a reference for evaluation and design of ceramic vacuum interfaces for high-current applications.

Spatio-temporal dynamics of calcium electrotransfer during cell membrane permeabilization.

Pulsed electric fields (PEFs) are applied as physical stimuli for DNA/drug delivery, cancer therapy, gene transformation, and microorganism eradication. Meanwhile, calcium electrotransfer offers an interesting approach to treat cancer, as it induces cell death easier in malignant cells than in normal cells. Here, we study the spatial and temporal cellular responses to 10μs duration PEFs; by observing real-time, the uptake of extracellular calcium through the cell membrane. The experimental setup consisted of an inverted fluorescence microscope equipped with a color high-speed framing camera and a specifically designed miniaturized pulsed power system. The setup allowed us to accurately observe the permeabilization of HeLa S3 cells during application of various levels of PEFs ranging from 0.27 to 1.80kV/cm. The low electric field experiments confirmed the threshold value of transmembrane potential (TMP). The high electric field observations enabled us to retrieve the entire spatial variation of the permeabilization angle (θ). The temporal observations proved that after a minimal permeabilization of the cell membrane, the ionic diffusion was the prevailing mechanism of the delivery to the cell cytoplasm. The observations suggest 0.45kV/cm and 100 pulses at 1kHz as an optimal condition to achieve full calcium concentration in the cell cytoplasm. The results offer precise levels of electric fields to control release of the extracellular calcium to the cell cytoplasm for inducing minimally invasive cancer calcium electroporation, an interesting affordable method to treat cancer patients with minimum side effects.

Spatiotemporal distribution of surface charges in square-grid state in a dielectric barrier discharge

The spatiotemporal distribution of surface charges in the square-grid state in a dielectric barrier discharge system is investigated at the sub-microsecond time scale using a spectrograph combined with a high-speed framing camera. With increasing driving parameter, the square-grid state transforms from a square pattern into one composed of three sub-lattices: a core sub-lattice with square symmetry and two marginal lattices with lower symmetry. In essence, this breaks the symmetry of the square-grid state—a highly symmetric state. From this perspective, the square pattern trifurcates to a square-grid state of lower symmetry. The electron density of the square-grid state changes periodically, both temporally and spatially, resulting in a periodic distribution of surface charges over the dielectric layer. The calculation of the surface-charge field further verifies that the surface charges have a significant influence on the formation of the square-grid state consistent with the experimental results.