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Purpose The purpose of this study is to understand experimentally the mixing characteristics of a two-stream exhaust system with a supersonic Mach 1.5 primary jet that exits the rectangular C-D nozzle surrounded by a sonic secondary jet from a convergent rectangular nozzle by varying the aspect ratio (AR = 2 and 3) similar to those that can be available for future high-speed commercial aircraft. Design/methodology/approach This paper focuses on the experimental results of effects of AR at various expansion levels of jets issued/delivered from a central rectangular convergent-divergent nozzle of AR 2 and 3 surrounded by a coflow from a convergent rectangular sonic nozzle. The lip thickness of the primary nozzle is 2.2 mm. various nozzle pressure ratios (NPRs) ranging from 2, 3, 3.69 and 4 were chosen for pressure measurements. Findings For all the NPRs, AR 3 had a shorter core than AR 2. Also, AR 3 was found to decay faster in the transition and fully developed zones. The lateral plots show that the AR has an influence on the jet spread. Originality/value The structure of waves existing in the potential core of the rectangular coflow jet along with the major and minor axis planes was visualized by the shadowgraph technique.

To establish an effective recycling process for waste photovoltaic (PV) panels, a wire explosion method using a high-voltage pulsed discharge was used to separate silver (Ag) from an ethylene-vinyl acetate (EVA) copolymer resin sheet. The cell used in the experiment was prepared by removing the aluminum frame and the glass cover plate from the waste PV panel. Ag particle recovery experiments were conducted in water. Electrodes were installed on each copper busbar of the cell. A 40.8- $\mu \text{F}$ capacitor was charged to a maximum of 15.0 kV and then discharged using a mechanical switch. Wire explosions occurred through the 102 Ag wires connected in parallel between the busbars. The particles recovered after the wire explosion were sieved and then characterized by inductively coupled plasma atomic emission spectroscopy and X-ray fluorescence. Scanning electron microscope and energy dispersive spectroscopy analysis revealed that the majority of separated Ag particles were attached to the silicon particles. The Ag recovery rate was 69% at a charging voltage of 15.0 kV. Higher discharge energies increased the Ag recovery rate; however, the recovered particles had lower Ag concentration. A circuit simulation was performed to analyze the current waveforms and calculate the current distribution. Shadowgraph imaging was used to observe the separation process of Ag particles and the EVA layer by wire explosion during pulse discharge. The shadowgraph images indicated that plasma generation and gas expansion have important roles in the separation process.

Numerical simulation of the motion of a cuboidal solid body induced by a propagating shock wave was conducted by using the overset grid functionality of OpenFOAM (v1712). A body-fitted grid associated with the moving body was overlapped with a background Cartesian grid fixed in space, and transient, three-dimensional, inviscid flows around the moving body were simulated coupled with the six-degrees-of-freedom motion of the solid body. The results from the present simulation were compared with those from shock tube experiments, in which the shock-induced motion of a cuboidal solid body floating in air and the flow field around the solid body were visualized using the shadowgraph technique and recorded by a high-speed video camera with a frame rate of 20,000 frames per second. The good agreement found between the numerical and experimental results for the translational and angular displacements of the shocked solid body indicates the capability of OpenFOAM to predict the shock-induced motion of a solid body.

Full-cone spray is quite important in spray cooling and catalytic combustion applications; however, it is not extensively studied. Besides, the liquid spray is relatively a non-uniform structure especially along longitudinal axis which includes different sizes and distribution of droplets. The few published experimental studies are limited to calculate some of the spray characteristics on a certain plane located downstream of the nozzle exit. Therefore, the spray parameters representing fluid structure, droplets mean diameter, and their distribution in different cross sections from nozzle exit are considered in this study. Accordingly, a jet-swirl atomizer with pressure-swirl full-cone spray is investigated where all important full-cone spray characteristics are considered at different planes from nozzle exit. The spray images are obtained with a shadowgraph technique and are analyzed to obtain the Sauter mean diameter (SMD), D10, and droplet size distribution along with the spray structure, spray cone angle, and discharge coefficient. The experimental results are verified based on the pre-published numerical studies on the same atomizer. The experimental and numerical results show good agreement. Moreover, the results show that the SMD is increased by moving away from center of spray to its edges, and the droplets number density is increased in central regions. The increased droplets number density leads to the greater external forces which create smaller droplets. In contrast, larger particles exist in peripheral parts due to the less droplets concentration. Furthermore, and far away from the exit nozzle, the SMD values are decreased due to the increased aerodynamic forces and oscillations. The droplets dispersion including spray density in radial and axial directions is also observed using spray density images.

A modified shadowgraph imaging setup equipped with a high-speed camera is utilized to achieve a numerical evaluation of temperature, number densities of molecules, ions and electrons to the refractive-index of negative non-breakdown streamer in rapeseed insulating oil for revealing the dynamics of streamer propagation. The non-breakdown streamer is found reaching fairly high speed of 3.01–4.66 km/s during $0{-}1.7 \ \mu\mathrm{s}$ . The following 10 µs is dominated by streamer propagation with the grayscale decreasing continuously for the entire streamer. The dissipation is identified starting at ca. $11.7 \ \mu\mathrm{s}$ in the stem region while the head of streamer remaining further propagation. This is followed by a dissipation expansion from the stem to the head regions. The double-unsaturated triglycerides are evidenced as the main ionization components in the non-breakdown streamer, which aid to identify the temperature variations, instead of electrons, positive ions, or dissociation of molecules, contribute mainly to the refractive index drop of the streamer. The mapping for the temperature distribution of streamer is resultantly possible and provides the insight to ascertain the inhomogeneous migration of electrons the kinetic superiority for the streamer evolution dynamics.

Experimental investigations have been carried out on a suddenly expanded duct in a view to determining buzz phenomenon for different Mach numbers (M), nozzle pressure ratios (NPR), and mass flow rates (MFR). The experimental facility was designed with sensitive and high-frequency pressure controls. The experiments were conducted for Mach numbers ranging from 2.0 to 3.0, with multiple NPRs of 3, 5, 7, and 9. In combination with the high-speed camera, shadowgraph mapping techniques were employed to provide an optical depiction of the shock system in front of the inlet and to study the buzz features. Displacement of the shock and buzz frequency was calculated by shadowgraphs. The information gathered from the shadowgraphs was assessed against the pressure measurements and the values were in good agreement with each other. The results observed that for nominal values of MFR, the buzzing frequency decreased with increasing Mach number. Furthermore, for an increasing NPR, the shock displacement further increases. The results also showed that there were significant pressure fluctuations in the expanded duct for lower Mach numbers.

Jet fuels with various properties are desirable for different operational or environmental advantages, necessitating fuel-flexible technology. For example, JP-5 (A-3) and alternative jet-fuels with high flash points are favored for safe handling and other advantages. The study investigates the effects of fuel properties on the near-field spray characteristics of the relatively viscous A-3 and a more viscous alternative fuel blend (C-3) compared to the baseline fuel—Jet A (A-2) with the least viscosity, using the relatively novel twin-fluid flow blurring (FB) injection. Prior studies have acquired fine FB atomization with high viscosity tolerance compared to conventional jet-based injectors, where atomization is suppressed by high viscosity. The study aims to further quantify the FB fuel flexibility using shadowgraph and particle image velocimetry to potentially enable effective utilization of the desired jet fuels. Results show that the FB injection achieved fine sprays immediately at the injector exit rather than typical jets by conventional atomization. Nearly the same atomization completion length, within 4 D downstream the injector exit with the diameter D of 1.5 mm, was acquired for all three fuels regardless of the varying properties. Compared to A-2, the final droplets of the viscous A-3 are only 6% larger and the most-viscous C-3 are only 10% larger. It is also observed that the density of fuel affects the droplet travel distance from the injector exit to a downstream location before achieving a terminal or peak velocity. Droplet size distribution, velocity distribution, and Weber number estimate consistently reveal more turbulent bag breakup mode at the spray periphery resulting in smaller droplets, compared to the oscillatory mode yielding larger ones in the center.

In this study, the effect of the pressboard barrier (PB) on streamer pattern in natural ester under the non-uniform electric field is investigated by experiment and simulation. The propagation patterns and characteristics of the streamer in the oil with/without a pressboard barrier are obtained by the shadowgraph technique. The mechanism of the pressboard barrier effect on the streamer pattern and propagation is revealed using a multi-streamer branch simulation method. Results show that the positional relationship between the needle electrode and the pressboard barrier significantly affects the streamer propagation and branching. The barrier effect of the pressboard, the distribution of space electric field affected by the pressboard barrier, and the image force of charge on the pressboard surface are all critical factors for affecting the propagation and branching of the streamer. The greatest threat to the pressboard failure is the z-axis streamer or those branches close to the z-axis that is severely inhibited by other streamer branches. It raises the risk of insulation failure caused by the breakdown of the insulating pressboard.

No abstract available.

This paper presents a numerical, and experimental study on atomization characteristics and droplet distribution of a twin-fluid two-phase internal mixing atomizer. Using the discrete phase model and Eulerian–Lagrangian numerical study, internal and external flow fields are simulated to describe two-phase flow in mixing chamber. The exterior region near the exit nozzle in the combustion chamber is simulated by particle trajectory method. Internal-mixing atomizer is manufactured and tests are carried out to verify the numerical results by applying shadowgraph method visualizing flow near the exit nozzle of the atomizer and downstream in combustion chamber. In order to study atomization characteristics including droplet dimension and velocity distribution in different gas to liquid ratio, an advanced twin-fluid test stand for two-phase flow atomizers is designed and built. By changing operating parameters of gas phase pressure and liquid flow rate a reasonable agreement for spray cone angle, maximum penetration depth and droplet diameter and velocity distributions is found between numerical and experimental result. As result, turbulence modeling assists to estimate atomization characteristics of complicated twin-fluid two-phase atomizers precisely to avoid time-taking and costly experimental tests although an advanced test stand is developed and built to cover all types of atomizer.

The separation behavior of two bodies was experimentally investigated in a shock tunnel. Simple-shaped bodies, including a cylinder, cube, and sphere, with a size of 3 or 5 mm were used as the test models. The two contact models with a plane-exposed configuration were mounted using a thin wire, and the separation behavior was observed and analyzed using the shadowgraph technique. The surface roughness effect on the separation of two spheres was also considered. Particularly, attention was given to the lateral velocity. It was found that the spheres had a constant final lateral velocity after the shock interaction, whereas the velocities of the cylinders and cubes were determined by the rotational motion of the body. Based on the experimental data, a reentry trajectory analysis was conducted using Monte Carlo simulation. The results showed large ground footprint and downrange differences depending on the characteristics of the body, implying the importance of separation behavior for a reentry analysis.

Impinging gas jets can induce depressions in liquid surfaces, a phenomenon familiar to anyone who has observed the cavity produced by blowing air through a straw directly above a cup of juice. A dimple-like stable cavity on a liquid surface forms owing to the balance of forces among the gas jet impingement, gravity and surface tension1,2. With increasing gas jet speed, the cavity becomes unstable and shows oscillatory motion, bubbling (Rayleigh instability) and splashing (Kelvin-Helmholtz instability)3,4. However, despite its scientific and practical importance-particularly in regard to reducing cavity instability growth in certain gas-blown systems-little attention has been given to the hydrodynamic stability of a cavity in such gas-liquid systems so far. Here we demonstrate the stabilization of such instabilities by weakly ionized gas for the case of a gas jet impinging on water, based on shadowgraph experiments and computational two-phase fluid and plasma modelling. We focus on the interfacial dynamics relevant to electrohydrodynamic (EHD) gas flow, so-called electric wind, which is induced by the momentum transfer from accelerated charged particles to neutral gas under an electric field. A weakly ionized gas jet consisting of periodic pulsed ionization waves5, called plasma bullets, exerts more force via electrohydrodynamic flow on the water surface than a neutral gas jet alone, resulting in cavity expansion without destabilization. Furthermore, both the bidirectional electrohydrodynamic gas flow and electric field parallel to the gas-water interface produced by plasma interacting 'in the cavity' render the surface more stable. This case study demonstrates the dynamics of liquids subjected to a plasma-induced force, offering insights into physical processes and revealing an interdependence between weakly ionized gases and deformable dielectric matter, including plasma-liquid systems.

In this study, the characteristics of side-projected planar shock waves propagating through grid turbulence were investigated using a counter-driver shock tube. At the location of visualization of the shock–turbulence interactions, the range of the shock Mach numbers was $$M_{\mathrm {s}} = 1.01{-}1.15$$ and the representative values of the turbulent Mach numbers $$\widetilde{M_{\mathrm {t}}}$$ were $$ 0.005{-}0.014$$ . The interaction length between the shock wave and grid turbulence was in the range approximately from $$-50$$ to $$300$$ times the integral length scale of the turbulence. For the interaction involving the strongest turbulence with $$\widetilde{M_{\mathrm {t}}} = 0.014$$ , the weakest planar shock wave with $$M_{\mathrm {s}} = 1.01$$ was largely deformed and could not be detected on the projected shadowgraph and schlieren images. The density changes on the grayscale in the projected images of the shock waves weakened and expanded multidimensionally. This undetectable profile of the shock wave could indicate that the shock wave locally lost discontinuous property change profile. The shock waves with $$M_{\mathrm {s}} \ge 1.05$$ did not show the undetectable profile in the projected image. In these cases, the projected thickness of the shock wave increased with an increase in the interaction length. The increase in the projected thickness became larger as the turbulent Mach number increased, and the shock Mach number decreased.

The diffusion process and spatial distribution of a liquid jet in supersonic crossflow (Mach number of 2) in a model combustor with an expanded section were investigated on the basis of experiments and simulations. Experiments were performed using high-speed photography, high-speed shadowgraph and laser-based particle size analyzer to investigate the spray structures, flow fields and droplet diameter, respectively. The Eulerian-Lagrangian method coupled with the Kelvin-Helmholtz/Rayleigh-Taylor breakup model were used in the simulations. Moreover, delayed detached eddy simulation numerical method based on the two-equation shear stress transport turbulence model was applied. The spray distributions and flow fields predicted in the simulation, including the spray penetration, cross-sectional distribution, droplet size in the far-field, the bow shock, large-scale vortices and recirculation zones were in good agreement with experiments. The evolution of spray in supersonic crossflow could be divided into three stages, aerodynamic induced liquid column fracturing, expansion wave promoted spray accelerating, and compression wave disturbed spray blending. Overall, the channel configuration (with expended section) changed the direction and strength of the supersonic airflow. The force of the crossflow determined the spray diffusion.

The present experimental investigation explores the ability of the corrugated actuator-tabs (or simply, the corrugated actuators) of aspect ratio 2 in encouraging the mixing of Mach 1.73 circular jet at different levels of expansion conditions. Essentially, rectangular, triangular, and semi-circular corrugated actuators are deployed at the opposite positions across the diameter of the nozzle outlet and compared with the plain or simple actuators located at the same positions. Quantitative and qualitative observations were conducted by pitot pressure measurements and shadowgraph flow visualization. Interestingly, the characteristic decays are higher for the semi-circular and triangular corrugated actuators, exhibiting superior mixing than the rectangular corrugated actuator in the far-field. However, the core length reduction for rectangular corrugation is greater than for triangular and semicircular corrugation geometry. As high as 96.5 % core length reduction was obtained for the rectangular corrugated actuators operating at the overexpansion condition. The pressure profiles along and perpendicular to these actuators confirmed that the deployment of the corrugation geometry significantly reduces the flow asymmetry. The shadowgraphic flow visualizations reveal that the plain and corrugated actuators weaken the waves and shorten the cells when compared to the uncontrolled or free jet.

The combustion instabilities of supersonic combustion were investigated experimentally in a laboratory-scale scramjet combustor with a cavity flame holder. Ethylene was injected transversely from an orifice to the supersonic flow of Mach 2 with a stagnation temperature of 1900 K and a total pressure of 0.37 MPa. The dynamic pressure, CH* chemiluminescence and shadowgraph images were measured with a pressure sensor and a high-speed video camera. Dynamic pressure was analyzed by fast Fourier transform, and time-resolved CH* chemiluminescence images were modally decomposed by the sparsity-promoting dynamic mode decomposition (SP-DMD). The results indicated that two combustion instabilities were observed for cavity shear-layer stabilized combustion and the oscillation between jet-wake stabilized and cavity shear-layer ram combustions for the power spectral density (PSD) of pressure. In the case of the combustion instability of cavity shear-layer stabilized combustion, a dominant peak of approximately 128 Hz was observed for the PSD of pressure. This instability corresponded to an entire flame oscillation of the cavity shear-layer stabilized combustion, which was validated by the SP-DMD and a low rank reproduction with 10 modes. This was driven by a fuel injection oscillation in the injection orifice. In the case of oscillation between the jet-wake stabilized and the cavity shear-layer ram combustions, peaks around 1600 Hz were observed for the PSD of pressure. This mechanism was also explained by the SP-DMD modes and a low rank reproduction using within 10 modes. The DMD and shadowgraph images indicated that the vortex formed by a separation of the boundary layer induced a strong jet-wake flame, resulting in the temporal thermal choke followed by cavity shear-layer stabilized ram combustion. The data-driven approach with SP-DMD clarified the combustion instability mechanisms of the supersonic combustion in detail.

Understanding the flow characteristics over a double compression ramp is crucial for high-speed vehicle design. Leading edge bluntness is a key factor influencing the formation of a separation region on a double compression ramp flow. In the present study, the effect of bluntness on a double compression ramp is investigated experimentally at a nominal Mach number of 4. The test model has 13° and 40° inclinations with respect to the freestream. Five different levels of leading-edge radius, varying from 0.0 to 2.0 mm, were subjected to supersonic wind tunnel tests. Shadowgraph and infrared thermography techniques were employed to visualize the flow features of the double ramp model. Measurements of surface heat-transfer along the centerline of the test model were obtained from the acquired infrared images. It is shown that the leading-edge radius alters the separation characteristics as well as the surface heat-transfer. Possible reasons for such flow characteristics are discussed.

Background: Despite a wealth of literature on medical high-intensity focused ultrasound (HIFU), there are knowledge gaps related to therapeutic ultrasound in the presence of bone obstructions. In particular, the literature on detailed imaging of ultrasonic wave-scattering at bones to validate numerical models is still sparse. Objective: Experimentally investigate the ultrasonic energy deposition in areas surrounding flat and curved bone-like obstructions. Methods: Imaging from a diffraction-based shadowgraph method is used to visualize ultrasound wave propagation in laboratory water tank experiments. The experiments are performed with no obstruction, varying partial obstruction, and with complete obstruction by bone-like constructs. In addition, hydrophone measurements and numerical simulations are performed for similar bone obstruction configurations, and the results are compared. Results: The shadowgraph technique produces high-resolution images of the wave transmission through and reflection from bone-like constructs. There is generally good agreement between experimental findings and numerical results. In particular, hydrophone measurements match closely with simulation data for both obstructed and unobstructed cases. Also, shadowgraph images qualitatively confirm simulation and hydrophone measurements. Conclusions: Overall, the results of this work demonstrate the utility of shadowgraph imaging in the development and evaluation of novel treatments in the field of therapeutic ultrasound.

Fragmentation and the subsequent heat transfer from solvated gold nanoparticles with sizes 50, 100, 150, and 250 nm to water are studied by time-resolved shadowgraph photography. The measured fragm...

The interactions of the bubbles in a loosely packed bubbly flow in a high viscous fluid approaching a pore space are studied using a shadowgraph imaging technique. The motion of the bubbles has been evaluated by considering shape analysis of their deformation and the variation in the velocity and pressure distribution of the phase. A comparison of two cases of a linear array and a matrix of bubbles emphasizes the importance of the arrangement on the deformation and motion of the dispersed phase in the pore space. The deformation of the bubbles in both cases results in a deceleration and acceleration process of the dispersed phase in the pore region. This process was a function of size, number of the bubbles competing in the pore throat and the arrangement of the competing bubbles. The variation in the motion of the dispersed phase will ultimately lead to different flow motion and phenomena at the entrance of the pore throat. The results also highlight that although bubbles had different motion approaching the pore throat, they follow similar deformation transition as they enter and exit the pore throat. This work contributes to existing knowledge of multi-phase flow in pore space by providing further understanding the effect of the interaction of phases based on the arrangement and their motion in a porous geometry.

Data Availability Statement: Data available upon request at panos.sphicas@gmail.com. Copyright © 2023 by the authors. Engine efficiency and emissions depend on the fuel atomization and dispersion. The fuel atomization and dispersion depend heavily on the ambient pressure and temperature. In this work, to study Diesel sprays in engine conditions, an electrically heated, constant-volume, pressurized vessel was designed and manufactured. The controlling electronics and software were developed and tested to ensure safe and precise operation. A commercial Bosch six-hole automotive Diesel injector was used. The spray spatial and temporal development were studied. In the literature, spray liquid length and cone angle are extensively used to quantify fuel dispersion. In this work, these parameters were quantified using a high-speed shadowgraph technique. Models were derived to describe the temporal evolution of the liquid core. Such models can be used to predict the Diesel spray behaviour and the engine performance. This research received no external funding.

Understanding the size of oil droplets released from a jet in crossflow is crucial for estimating the trajectory of hydrocarbons and the rates of oil biodegradation/dissolution in the water column. We present experimental results of an oil jet with a jet-to-crossflow velocity ratio of 9.3. The oil was released from a vertical pipe 25 mm in diameter with a Reynolds number of 25 000. We measured the size of oil droplets near the top and bottom boundaries of the plume using shadowgraph cameras and we also filmed the whole plume. In parallel, we developed a multifluid large eddy simulation model to simulate the plume and coupled it with our VDROP population balance model to compute the local droplet size. We accounted for the slip velocity of oil droplets in the momentum equation and in the volume fraction equation of oil through the local, mass-weighted average droplet rise velocity. The top and bottom boundaries of the plume were captured well in the simulation. Larger droplets shaped the upper boundary of the plume, and the mean droplet size increased with elevation across the plume, most likely due to the individual rise velocity of droplets. At the same elevation across the plume, the droplet size was smaller at the centre axis as compared with the side boundaries of the plume due to the formation of the counter-rotating vortex pair, which induced upward velocity at the centre axis and downward velocity near the sides of the plume.

Tomographic shadowgraph imaging is applied to reconstruct the instantaneous three-dimensional spray field immediately downstream of a generic aero engine fuel injector. Within the swirl passage of the injector model, a single kerosene jet undergoes air-blast atomization in a cross-flow configuration at Weber numbers of $$\text {We}=360-770$$ We = 360 - 770 , air pressures of $$p_a=4-7\,\text{ bar }$$ p a = 4 - 7 bar and air temperatures of $$T_a=440-570\,\text{ K }$$ T a = 440 - 570 K . High-speed, high magnification shadowgraphy is used to visualize the initial fuel atomization stages within the fuel injector before the spray enters the spray chamber. The 4-camera tomographic measurement setup is described in detail and includes a depth-of-field analysis with respect to droplet size based on Mie simulations and calibration data of the point-spread function. For a volume size of $$16\times 13\times 10\,\text{ mm}^3$$ 16 × 13 × 10 mm 3 , the smallest resolvable droplet diameter is estimated to be $$d=10\,\mu \text{ m }$$ d = 10 μ m within the focal plane and increases to $$d \approx 20\,\mu \text{ m }$$ d ≈ 20 μ m toward the edges of the volume. Droplet velocities above the resolution limit were retrieved by 3-d cross-correlation of two volumetric reconstructions recorded at two consecutive time-steps. This is accompanied by an error analysis on the random error dependency on the camera viewing geometry. The results indicate increasing motion and fluctuations of the spray tail with increasing temperature and Weber number. Validation against PDA data further downstream of the burner plate revealed consistency for size classes $$d=10\,\mu \text{ m }$$ d = 10 μ m and $$d=15\,\mu \text{ m }$$ d = 15 μ m . Deviations from PDA occur in regions with strong velocity gradients due to different spatial resolutions, the presence of reconstruction ambiguities (ghost particles), uncertainties inherent to the two-frame cross-correlation of spray volumes and the finite LED pulse duration. Graphical Abstract

The microphysical properties of cloud droplets, such as droplet size distribution and droplet number concentration, were studied. A series of field experiments was performed in the summer of 2019 at the Umweltforschungsstation Schneefernerhaus (UFS), an environmental research station located just below the peak of the Zugspitze mountain in the German Alps. A VisiSize D30 manufactured by Oxford Laser Ltd., which is a shadowgraph imaging instrument, was utilized for the first time to measure the size and velocity of cloud droplets during this campaign. Furthermore, a phase Doppler interferometer (PDI) device, manufactured by Artium Tech. Inc., was simultaneously measuring cloud droplets. After applying modifications to the built-in software algorithms, the results from the two instruments show reasonable agreement regarding droplet sizing and velocimetry for droplet diameters larger than 13 µm. Moreover, discrepancies were observed concerning the droplet number concentration results, especially with smaller droplet sizes. Further investigation by applying appropriate filters to the data allowed the attribution of the discrepancies to two phenomena: the different optical performance of the sensors with regard to small droplets and high turbulent velocity fluctuations relative to the mean flow that result in an uncertain estimate of the volume of air passing through the PDI probe volume.

This paper shows a numerical comparison between DBI and focused shadowgraph imaging techniques applied in the characterization of macroscopic parameters and optical thickness of a multi-hole GDI spray. Simulated experiments were designed around a synthetic spray, whose jets have been modelled through the Musculus and Kattke model. The DBI configuration was in line with the experimental constraints for beam steering suppression reported in the literature. A simple ray-tracing programme based on geometrical optics and radiometric laws was developed, and the attenuation of light through the spray was evaluated according to Lambert-Beer law. The results show that diffuse backlighting can appreciably bias the measurement of the optical thickness and the macroscopic spray parameters, net of any beam steering effects. Significant discrepancies between techniques occur when the spray develops out of the focal plane and away from the optical axis, especially when the spray develops asymmetrically to the focal plane.

This paper is a comparative study on the characteristics of high-speed liquid jets injected in surrounding water and air using shadowgraph technique. One of the main objectives is to investigate the effects of liquid’s physical properties, used to generate the high-speed liquid jets, on jet generation’s characteristics. Moreover, comparative investigations on effects of those liquid jets after injected in water and air are reported. The high-speed liquid jets were generated by the impact of a projectile launched by a horizontal single-stage power gun. The impact-driven high-speed liquid jets were visualized by shadowgraph technique and images were recorded by a high-speed digital video camera. The process of impact-driven high-speed liquid jet injection in air and water, oblique shock waves, jet-induced shock waves, shock waves propagation, the bubble behavior, bubble collapse-induced rebound shock waves and bubble cloud regeneration were clearly observed. It was found that different properties of liquid (surface tension and kinematic viscosity) affect the jet maximum velocity and shape of the jet. Bubble behaviors were only found for the jet injected in water. From the shadowgraph images, it is found that the maximum average jet velocity, expansion and contraction velocities of bubble in axial direction increase when the value of the multiplied result of surface tension by kinematic viscosity increases. Therefore, surface tension and kinematic viscosity are the significant physical properties that affect characteristics of high-speed liquid jets.

Arc welding of aluminium and steel joins are commonly inundated by the formation of brittle intermetallic compounds (IMC) at the aluminium-steel interface. As the formation of IMC is closely related to the heat input into the base metals, it is hypothesised that the ability to regulate the heat input would enable the control of the thickness of IMC. This paper proposes the use of a pulse plasma MIGW system to regulate the heat input during the welding process. A shadowgraph method was used to compare the weld droplet stability of the pulse plasma MIGW with a pulse MIGW system. It was found that the plasma MIGW generated a regular heat input into the base metal due to the smoothness of the droplet detachment and resulted in the formation of IMC with uniform thickness. Al/steel joining was then conducted using both pulsed plasma MIGW and pulse MIGW systems, and the resulting IMC formations and thicknesses were evaluated. It was found that the formation and thickness of IMC in Al/steel dissimilar welding can be controlled by using pulse plasma MIGW. Its lower droplet temperature and decreased cooling rate during welding would suppress the formation of Fe2Al5 and FeAl3, and thus decreasing the overall thickness of IMC at the joining interface.

Experiments are performed for outwardly propagating spherical flames of R-32/air mixtures at constant pressure. A high-speed video camera captures shadowgraph images from which the flame radius as ...

A high-speed laser shadowgraph observation of the particles flow during cold spraying enables a good phenomenological characterization. The particles in-flight behaviour develops a kinematics with two regimes regardless the powder nature. When leaving the nozzle, the particles form a regime of uniform jet over a certain distance along the flow direction, and then a regime of sparse jet while particles deviate in the radial direction. This dispersion increases away from the nozzle exit and exhibits thereby an event of oblique collision during an additive route process. Under such conditions, kinematic deficiency prevails since oblique collision has shown to alter the deposition efficiency, in the literature. Albeit these particles flow regimes represent a generic behaviour, there are differences in the powder’s response depending of their features that can be combined using the parameter ρ p D p . The variance ρ p D p in cold spraying is scaled through a few cases of powders (copper, aluminium, magnesium, cermet, and PEEK). The powders dispersion becomes more and more important as ρ p D p decreases from high value (large Cu powders) down to low value (case of Mg powders). However, this tendency cannot be extrapolated to lightweight powders represented by the fine PEEK powders (low ρ p D p ) which are more stable over a larger distance due to a higher sensitivity to the axial component of the gas velocity. They get unstable later in the jet since they become sensitive to the radial component of the gas velocity due to a progressive decrease of the axial gas velocity. The fine WC-Co powders adopt similar behaviour since they are lightweight as the PEEK powders due to their porous structure.

A new design is developed for compact-size and lightweight laser beam diffuser of two rad divergence (± 1 rad ≈ ± 60°). The design combines diffractive–refractive beam shaping optics, and it consists of a 532 nm military laser pointer, equipped with a laser dot matrix beam shaper and a rod lens. The non-Gaussian laser beam is spread into a relatively homogenous large-field illumination. This design is a low-cost optical configuration, which enabled large-field automotive laser shadowgraphy and was found to be very useful in similar practice situations, where a laser beam of high divergence is required. The diffuser setup was tested successfully in composing laser shadowgraph of 2 m height and 18 m width for a test car interior, where both direct and indirect driver’s fields of views are presented.

As the field of biomedicine continues to grow, so will the need for the quick and efficient manufacture of high-quality micro-textures, such as microfluidic systems, for biomedical functions. The utilization of a nanosecond-pulsed laser enables the realization of an efficient ablation rate in the preparation of a microscaled texture for biomedicine applications. To obtain high-quality ablated features, it is necessary to understand the interaction regime between the pulse laser and cermet. In this work, the effects of the scanning speed on formation mechanisms of microgrooves in pulse laser scanning ablation of cermet are investigated. The relative ablation processes in terms of the particle ejection and the morphology of the ablation traces under various scanning speeds are characterized. In addition, pump-probe shadowgraph imaging is used to observe the ablation dynamics and laser-plume interaction. The results demonstrate that the plume shielding effect, which stems from the intensive pulse energy overlap in the irradiated region, tends to occur at a low scanning speed and results in an intermittent ablation regime. Moreover, there is a critical scanning speed for overcoming the plume shielding effect when other laser parameters are kept constant. To realize an even and continuous ablation process, a high scanning speed that exceeds this critical value should be applied.

Early stage of underwater electrical wire explosion was observed by shadowgraph. By decreasing initial energy, the early stage of wire explosion was slowed down and possible to capture shadow photos during phase transitions. The vaporization process of wire is usually axially non-uniform due to random local micro explosions along the wire. The degree of non-uniform will be significantly decreased when increasing the initial energy. During explosion of the wire, more than one shock waves were generated not only from phase transition, but also from different pulses of deposition power. A modified piston model was introduced to explain the phenomenon.

In this study, we apply the optical flow method to the time-series shadowgraph images of impinging jets using a high-speed video camera with high spatial and temporal resolution. This image analysis provides quantitative velocity vector fields in the object space without tracer particles. The analysis results clearly capture the details of the coherent vortex structure and its advection from the shear layer of the free jet. Although the results still leave challenges for the quantitative validation, the results show that this analysis method is effective for understanding the details of the physical phenomenon based on the quantitative values extracted from the shadowgraph images.

Double-pulse laser-induced breakdown spectroscopy (DP-LIBS) has been demonstrated to enable efficient elemental analysis of bulk water. Its applications are based on the fundamental understanding of the characteristics of the secondary plasma. However, due to the poor stability of the secondary breakdown, the inherent formation and evolution processes of the secondary plasma within the first laser formed bubble (LFB) remain unknown. By post focusing the second laser beam at the rear boundary of the first LFB (post-focus arrangement), the generated secondary plasma can provide a stable system for understanding its spatiotemporal behaviors. Based on this post-focus arrangement, the secondary plasma was characterized by using temporally resolved spectroscopy, monochromatic fast imaging and shadowgraph techniques. It was shown that the emission of underwater DP-LIBS presented transient increments within 1 μs. Such a distinctive temporal behavior was closely related to the back and forth movement of the secondary plasma within the first LFB, due to its spatial confinement effect. Species-specific imaging studies reveal that the interactions between the secondary plasma and the surrounding water vapor can give rise to the dissociation of water molecules, as well as the excitation of hydrogen and oxygen atoms. Meanwhile, the molecular emissions (CaOH) appeared only after 2 μs, but lasted for 32 μs, suggesting a slower plasma relaxation within the first LFB. Further shadowgraphic investigations highlight the quasi-static, low-pressure environment of the first LFB, which can reduce energy losses and enhance optical radiation from the secondary plasma.

Light rays refract when passing through pockets of transparent fluids with different indices of refraction such as ordinary air pockets of varying temperature. This phenomenon makes night stars twinkle, distorts views above hot asphalt roads and hot barbeque grills, and provides an opportunity for visualizing the normally invisible movement of transparent gases and liquids. Schlieren and shadowgraph imaging allows for visualizing transparent gas density variations, allowing ordinarily unseen phenomena to be imaged and experimented with in real time. Traditional Schlieren systems require expensive equipment (DSLR camera, telephoto lens, several high-quality mirrors, an adjustable knife edge) and are difficult to align, while shadowgraph systems trade simplicity for image sensitivity. We outline a simplified classroom shadowgraph system that uses only a single telescope mirror, an LED flashlight, and an iPad to teach topics in high school science.

Combustion of heavy fuels is one of the main sources of greenhouse gases, particulate emissions, ashes, NOx and SOx. Gasification is an advanced and environmentally friendly process that generates combustible and clean gas products such as hydrogen. Some entrained flow gasifiers operate with Heavy Fuel Oil (HFO) feedstock. In this application, HFO atomization is very important in determining the performance and efficiency of the gasifiers. The atomization characteristics of HFO (Mazut) discharging from a pressure-swirl atomizer (PSA) are studied for different pressures difference (Δp) and temperatures in the atmospheric ambient. The investigated parameters include atomizer mass flow rate ( m ), discharge coefficient (CD), spray cone angle (θ), breakup length (Lb), the unstable wavelength of undulations on the liquid sheet (λs), global and local SMD (sauter mean diameter) and size distribution of droplets. The characteristics of Mazut sheet breakup are deduced from the shadowgraph technique. The experiments on Mazut film breakup were compared with the predictions obtained from the liquid film breakup model. Validity of the theory for predicting maximum unstable wavelength was investigated for HFO (as a highly viscous liquid). A modification on the formulation of maximum unstable wavelength was presented for HFO. SMD decreases by getting far from the atomizer. The measurement for SMD and θ were compared with the available correlations. The comparisons of the available correlations with the measurements of SMD and θ show a good agreement for Ballester and Varde correlations, respectively. The results show that the experimental sizing data could be presented by Rosin-Rammler distributions very well at different pressure difference and temperatures.

The prevention of erosion caused by hydrodynamic cavitation plays an important role in the design of hydraulic components, such as fuel injection systems. A high pressure level and the corresponding flow speeds at a very small scale of several micrometers limit the optical accessibility in real fuel injectors. For this reason, flat micro-channels of different geometry, covered by sapphire windows at the front and the back side are used in this paper. Calibration fluid ISO 4113 and dodecane serve as flow media. The aim is to investigate the influence of pressure, temperature and the geometry of the micro-channels on the cavitation extent and to resolve the cavitation dynamics such as cloud shedding. Detached clouds will collapse if the static pressure exceeds the vapor pressure and may damage channel walls of hydraulic components. Shadowgraph images visualize the cavitation extent and flow instabilities, Computational Fluid Dynamics (CFD) results reveal detailed pressure and velocity distributions. Particle Image Velocimetry (PIV) measurements show the motion of the cavitating structures. With high-speed (HS) visualization, cloud detachment is investigated. It was found that the normalized cavitation length can be described as an S-shaped curve depending on the normalized cavitation number. The cavitation length significantly increases in the parallel and diverging channels when reaching the cavitation transition point. The optical setup reveals a deeper insight into the cavitation inception in the shear layer near the throttle inlet. Kelvin–Helmholtz vortices downstream the shear layers could be observed. The calculated pressure and velocity distributions help to validate the plausibility of the cavitation inception for different tapers. For a deeper understanding of the overall flow conditions, basic velocity measurements of the phase boundary in the diverging part of a converging-diverging flow channel were carried out. No seeding particles are used as cavitation structures serve as tracers for the image evaluation. It was found that the velocity of phase boundary between liquid and vapor is lower than the mean fluid velocity derived from the measured mass flow because of the unstable behavior of cloud shedding. A recirculation flow and eddy formation at the cavitation tip is visible. With the help of high-speed visualization, the re-entrant motion which occurs in connection with cloud shedding could be observed. The investigation of the shedding frequency reveals its dependency of pressure, temperature and geometry.

Recently, molecular emissions from the laser-induced plasma in ambient gas have gained increasing interest; however, very little is known about the case in water solutions. In this work, we investigated the spatiotemporal characteristics of molecular emissions, CaOH for instance, in underwater laser-induced breakdown spectroscopy (LIBS) by using time-resolved spectroscopy, spectral-resolved imaging, and shadowgraph techniques. It was shown that clear CaOH molecular bands can be observed in the spectrum at very early times after the laser pulse and presented a much longer lifetime and more homogeneous emission distribution compared with the Ca I and Ca II lines. Such unique characteristics of CaOH molecular emission inspired us to improve the performances of underwater LIBS by using the CaOH molecular bands instead of Ca I and Ca II lines. We demonstrated the excellent quantification results of CaOH with higher stability, less self-absorption, and reduced matrix effect. Meanwhile, the limit of detection (LOD) of Ca with the CaOH molecular band (2.46 ppm) is comparable to that with the atomic line of Ca I (2.07 ppm), and much lower than that with the ionic line of Ca II (13.81 ppm), indicating a good sensitivity of CaOH. This work gives not only some insights into the molecule formation mechanisms in underwater plasmas, but also provides new ideas to improve the analytical performances of underwater LIBS.

This research experimentally studied the effect of operational parameters on spray characteristics of flash evaporation system which is the key component of the discharge thermal energy combined desalination (DTECD) system. For this purpose, initially the effect of superheat degree, inlet pressure, and inlet temperature on the spray angle of three different types of full cone spray nozzles was investigated using a high speed camera. Moreover, a force balanced mathematical model was developed to calculate droplet sizes and the results were compared with experimentally measured values using shadowgraph technique. The results show that the spray angle was largely affected by the superheat degree regardless of nozzle type. In addition, an empirical equation was suggested to correlate inlet pressure, saturation pressure, and spray angle. Further, a model was proposed to estimate the variation of droplet sizes along the spray centreline and concluded that the spray capacity significantly influenced the droplets' lifetime.

The damage propagation and fracture process of monolithic annealed and chemically strengthened aluminosilicate (ALS) glass plates are characterized using Edge-on impact (EOI) testing technique. The EOI experiments on two types of ALS glass plates at a medium impact velocity of ∼180 m/s with spherical and cylinders steel projectiles were completed using pneumatic gas-gun. The high-speed images in shadowgraph mode during the impact process were recorded at the framing rate of 1,000,000 frames per second (fps) to visualize the fracture and damage front in annealed and strengthened glass plates. The real-time captured shadowgraphs in case of annealed glass (AG) showed that the damage front initially starts to develop from the impact end and then after traveling some distance arrested in few microseconds and measured average front velocity against cylindrical and spherical projectile was 2658.91 m/s and 1710.0 m/s, respectively. The tensile wave reflection in AG impact case against both types of projectiles was observed. In a chemically strengthened glass (CSG), the damage front shape and propagation against both types of projectiles were quite different than the AG, front traveled in a self-sustained mode due to the release of elastic stored energy with the velocity of 1746.92 m/s and 1690.12 m/s, respectively. The damage evolution curves plotted from captured high-speed images show that damage traveled slower in case of the spherical projectile which suggests that the shape of the projectile has a great influence on the fracture process of annealed and strengthened glass. The damage front shape in CSG against cylindrical projectile impact was more dominant along the glass boundaries and the spallation effect was not observed in CSG against spherical projectile impact. Additionally, the damage-free zones and fracture surfaces were studied using a scan electron microscope (SEM) from collected post-test debris to discuss the fracture process.

Reactive gaseous oxygen/kerosene spray was visualized by a shadowgraph imaging technique in a lab-scale rocket engine combustor. Dynamic behaviors of inner and outer shear layer were observed morphologically and analyzed quantitatively by image processing. Correlation methods were used to statistically analyze dynamic behaviors of each shear layer and interaction between two shear layers. Relationship between shear layer dynamics and combustion flow-field was investigated by analyzing periodicity of jet-core length, lump-detachment, and luminous flame intensity. The results of correlation and frequency analysis show that dominant frequency of behaviors and interaction of shear layers is 270 Hz and the degree of correlation becomes stronger along the flow direction. The dominant frequency of core length variation and luminous flame formation is around 270 Hz and, which is the same as that of behaviors and interaction of shear layers. This proposes that the surface behaviors and interaction of the inner and outer jets in the upstream affect the downstream combustion flow-field.

A theoretical and experimental study was performed to investigate the wavelength and frequency of axis-switching oscillations established over rectangular and elliptical liquid jets. Several injectors with aspect ratios ranging from 2 to 6 were manufactured for both rectangular and elliptical shapes. Shadowgraph technique was used for flow visualization and high speed photography was employed to capture the flow physics of the liquid jets. The theoretical model developed by Tadjfar and Jaberi (2019) was rewritten by looking at frequency analogous to the previous one for wavelength. The model rewrote the Rayleigh's classical analysis in terms of three segments identifying the influence of shape, aspect ratio, and Weber number. This model was modified to improve predictive accuracy by adding a correction term in terms of powers of aspect ratio. This time, instead of a constant power, a logarithmic function was used to evaluate the coefficients. The modified model improved the predictive accuracy of Rayleigh's model significantly. The results were analyzed from the viewpoint of frequency and wavelength.

Spray characteristics of a pressure-swirl atomizer are investigated using high-speed shadowgraph technique under different pressure drops (Δ P) and fuel temperatures ( T). An image processing method is developed using MATLAB. The results illustrate that the mass flow rate climbs with the increase of Δ P, while the discharge coefficient ( Cd) decreases firstly and then climbs with the increase of Δ P. Δ P has larger effect on the cone angle relative to fuel temperature. With the increase of Δ P, the shape of liquid film changes from ‘onion’ to ‘tulip’ and finally be fully developed spray cone. Meanwhile, the surface of liquid film becomes smoother with the increase of Δ P. The average breakup length climbs, then decreases to nearly a constant value with the increase of Δ P, which is induced by the “Impact wave,” surface wave, and turbulent energy. There are little differences on the shape of the liquid film under different temperatures, and temperature has different influence on breakup length under different Δ P. Both the fuel temperature and Δ P have significant impact on the surface wavelength ( λ) and velocities ( U, V) of surface wave. The width of fuel stream becomes larger with the increase of Δ P and fuel temperature. The results can further deepen the understanding of spray characteristics of pressure-swirl atomizer.

The study addresses the post arc ablation (PAA) of PTFE material used for High Voltage circuit breaker applications. The PAA phenomenon has to be understood in a reliable manner for the development and the optimization of the self-blast circuit breaker, especially for the accurate prediction of the dielectric recovery phase. The experimental activity consisted in generating a high current arc in a PTFE tube and in characterizing the post-arc ablation process. Arcs were generated in ambient air applying AC and DC current to reach current densities in the range 10 7 -10 8 A/m 2 . The characterization has been performed essentially based on standard optical measurement techniques. The shadowgraph technique enabled to show that the post-arc ablation flow is composed by a significant amount of soots, produced within the PTFE tube for several dozens of milliseconds after current interruption. The pyrometry technique enabled to estimate the soot temperatures which was found to be in the range 1400-2200 K. From the empirical data analysis, the post-arc ablation rates were inferred based on rough approximations.

The behavior of a diffusion flame under acoustic excitation was experimentally studied with the aid of simultaneous flow visualizations of the flame combining Mie scattering and schlieren/shadowgraph imaging. The Mie scattering imaging showed the periodic formation of a vortex street along the flame axis, while the schlieren/shadowgraph images indicated the formation of a high-amplitude density fluctuation near the burner exit. Furthermore, mean velocity and temperature along the flame axis were measured for evaluating the flame behavior by acoustic excitation. These experimental results showed that the acoustic excitation led to a density fluctuation near the burner and promoted the flame behavior near the burner. This behavior resulted in enhanced mixing and combustion in the flame near the burner, which grew as the oscillation amplitude of the acoustic excitation increased. In addition, the flame height, oscillation amplitude and luminosity decreased as the acoustic excitation amplitude is highly increased because of the formation of the increased velocity region with high-temperature near the burner associated with the initiation of laminar-to-turbulent transition of the flame.

Up to the present, no studies have been published to investigate the ignition process in air heater. In this paper, spray combustion flow field during ignition process in air heater was first visualized by high-speed shadowgraph technique. High-speed shadowgraph movies and pressure data indicated that ignition-quenching–reignition phenomenon occurred during one test run. The experimental results demonstrated that combustion of the accumulated propellant mixture during a very short period led to a peak pressure at the moment of last ignition. This process caused reverse flow of flame and ethanol spray into air injector annulus, which was one of the reasons leading to local ablation of injector. In order to obtain reliable ignition, effective improvements were necessary.

To evaluate the alternative fuel (n-heptane, ethanol) vapor- and liquid-spray characteristics of a direct-injection gasoline injector in a gasoline engine under various ambient and injection conditions, the spray angle, spray tip penetration (STP), and spray area were determined via high-speed shadowgraph and Mie-scattering techniques. Then, the fuel evaporation behavior during the injection process was studied by comparing the vapor- and liquid-phase spray characteristics to provide a theoretical basis for improving the performance of the spark ignition engine. Experimental results showed that temperature had little effect on the vapor-phase STP, spray angle, and spray area. Additionally, there was more evaporation in the radial direction than in the axial direction, and the difference of the evaporation ratio between the axial and radial directions increased with the temperature. Compared with ethanol, n-heptane showed better evaporation characteristics under high-temperature conditions owing to its lower latent heat of evaporation. Furthermore, the amount of evaporation of each plume was studied. As the axial distance from the nozzle increased, the amount of evaporation of the plumes increased exponentially and the evaporation of the plumes on both sides was greater than the evaporation of the middle plumes.

The behavior of oscillation modes in a supersonic flow of a Mach number of 1.58 over a cavity of L/D=2 and L/W=0.89 was studied using wind-tunnel experiments. Through instantaneous shadowgraph visu...

In vacuum circuit breakers, restrikes and nonsustained disruptive discharges are sometimes observed even up to several seconds after current interruption. These late breakdowns are triggered by a sudden increase in the field emission level and/or microparticle collision with the electrode during the application of recovery voltage. In this paper, we identify the late breakdowns triggered by the microparticles that emerged from the silver-tungsten carbide electrodes after current interruption. The combined use of a high-speed video camera and a pulse laser enabled us to conduct shadowgraph photography with a 150-ns time resolution, 10- $\mu \text{m}$ spatial resolution, and $\mu \text{s}$ framing rate for capturing the microparticle motion under the recovery voltage after current interruption. In this highly resolved photography, we observed many microparticles flying between the electrodes after current interruption. In our series of observations, we found two types of microparticle-inducing late breakdowns: those induced by small, fast microparticles whose diameter and velocity were approximately 20– $40~\mu \text{m}$ and 15 m/s, and those induced by large, slow microparticles whose diameter and velocity were approximately $100~\mu \text{m}$ and 1 m/s, respectively. Some of the large slow microparticles caused the sequential late breakdowns during the up-and-down motion between the electrodes. All the particle-induced late breakdowns occurred at the time of particle collision with the negative electrode defined by the polarity of the recovery voltage. The velocity of the large, slow microparticles was approximately 1 m/s, and these microparticles did not evaporate. Furthermore, the gradual decrease in the particle size was not observed in the continual occurrence of late breakdowns. We also observed many breakdowns caused by unknown factors. The electrode surfaces were also observed by using scanning electron microscopy and energy-dispersive X-ray spectroscopy. The microstructures and metal compositions on the electrode surfaces varied drastically with the observation positions.