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Digital maskless lithography is gaining popularity due to its unique ability to quickly fabricate high-resolution parts without the use of physical masks. By implementing controlled grayscaling and exposure control, it has the potential to replace conventional lithography altogether. However, despite the existence of a theoretical foundation for photopolymerization, observing the voxel growth process in situ is a significant challenge. This difficulty can be attributed to several factors, including the microscopic size of the parts, the low refractive index difference between cured and uncured resin, and the rapid rate of photopolymerization once it crosses a certain threshold. As such, there is a pressing need for a system that can address these issues. To tackle these challenges, the paper proposes a modified Schlieren-based observation system that utilizes confocal magnifying optics to create a virtual screen at the camera's focal plane. This system allows for the visualization of the minute changes in refractive indices made visible by the use of Schlieren optics, specifically the deflection of light by the changing density-induced refractive index gradient. The use of focusing optics provides the system with the flexibility needed to position the virtual screen and implement optical magnification. The researchers employed single-shot binary images with different pixel numbers to fabricate voxels and examine the various factors affecting voxel shape, including chemical composition and energy input. The observed results were then compared against simulations based on Beer-Lambert's law, photopolymerization curve, and Gaussian beam propagation theory. The physical experimental results validated the effectiveness of the proposed observation system. The paper also briefly discusses the application of this system in fabricating microlenses and its advantages over theoretical model-based profile predictions. [ABSTRACT FROM AUTHOR]

The disturbance flow field arises naturally with hypersonic target flying in near space. In situations where traditional infrared and radar systems lose effectiveness, space-based optical detection of this surrounding flow can serve as an alternative method for detecting high-speed targets. This paper presents a remote sensing imaging analysis of the disturbance flow field surrounding a hypersnoic target at different flight altitudes and Mach numbers. Utilizing Fourier Optics and Background-Oriented Schlieren, in conjunction with the fourth-order Runge-Kutta ray tracing algorithm, the imaging blurring and imaging deviation of three typical backgrounds under the influence of the disturbance flow field are obtained. Additionally, the study analyzes the influence of flight conditions and parameters of the imaging system on the imaging characteristics, and provides optical design recommendations. The results indicate that the presence of disturbance flow fields leads to varying degrees of visually apparent blurring effects and indiscernible deviation effects on the background images. Furthermore, the profiles of the disturbance flow field are extracted, in agreement with current experimental research. This study verifies the feasibility of space-based optical detection of hypersonic targets through disturbance flow field remote sensing imaging and contributes to the advancement of imaging research in this field. [ABSTRACT FROM AUTHOR]

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The jets produced on a microsecond x-pinch (quarter period T1/4 ∼ 1 μs, dI/dt ∼ 0.35 kA/ns) have been studied through light-field schlieren imaging and optical framing photographs across 4 different materials: Al, Ti, Mo, and W. The axial velocity of the jets was measured and exhibited no dependence on atomic number (Z) of the wire material. There may be a dependence on another factor(s), namely, the current rise rate. The average axial jet velocity across all four materials was measured to be 2.9 ± 0.5 × 106 cm/s. The average jet diameter and the average radial jet expansion rate displayed inverse relationships with Z, which may be attributed to radiative cooling and inertia. Asymmetry between the anode and cathode jet behavior was observed and is thought to be caused by electron beam activity. The mean divergence angle of the jet was found to vary with wire material and correlated inversely with the thermal conductivity of the cold wire. Optical images indicated a two-layer structure in Al jets which may be caused by standing shocks and resemble phenomena observed in astrophysical jet formation and collimation. Kinks in the jets have also been observed which may be caused by m = 1 MHD instability modes or by the interaction of the jet with the electrode plasma. [ABSTRACT FROM AUTHOR]

Turbulent burning velocity is one of the most relevant parameters to characterize the premixed turbulent flames. Different correlation has been proposed to estimate this parameter. However, most of them have been obtained using experimental data at atmospheric pressure or higher. The present study is focused on obtaining a correlation for the turbulent burning velocity using data at sub-atmospheric pressure. The turbulent burning velocity was experimentally calculated using the burner method, where turbulent premix flames are generated in a Bunsen burner. Stoichiometric and lean conditions were evaluated at a pressure of 0.85 atm and 0.98 atm, whereas the turbulence intensity was varied for each condition. Perforated plates and a hot-wire anemometer were used to generate and measure the turbulence intensity. Schlieren images were used to obtain the average angle of the flame and calculate the turbulent burning velocity. Experiments and theory show that the turbulent deflagration rate decrease as pressure decrease. The turbulent deflagration speed decreased by up to 16 % at 0.85 atm concerning atmospheric conditions for the same turbulence intensity, discharge velocity, and ambient temperature, according to the experimental results. The comparison among the experimental results at sub-atmospheric conditions and the correlations reported in the literature exposes prediction issues because most of them are fitted using data at atmospheric conditions. A general correlation is raised between turbulent burning velocity (ST), laminar burning velocity (SL) and turbulence intensity (u′) proposed from the experimental data. This correlation has the form ST/SL ∝α [ur/SL]n. For sub-atmospheric and atmospheric conditions, the coefficients were determined. [ABSTRACT FROM AUTHOR]

A nanosecond pulsed laser that can induce the breakdown of air in the focal region was used to irradiate a transparent PE film having a thickness of 0.6 mm. A high-speed Schlieren optics system was used to record the ablation process. The results indicate that the irradiation process was sensitive to the specific positions of the PE film. When the front surface of the PE film was positioned slightly away from the beam waist, the initiated plasma, the generated plume and the crater on the surface of PE film differed significantly with the position of PE film in relation to the beam waist. [ABSTRACT FROM AUTHOR]

To optimize a laser ignition scheme, absorption rate measurements and Schlieren visualizations are performed on dual-pulse laser-induced breakdowns (LIBs) at incident energies from 50 mJ to 200 mJ and pulse intervals that range from 20 ns to 250 µs in quiescent air at atmospheric pressure. For comparison, experiments on single-pulse LIBs are also conducted. The shock loss is determined using a semi-empirical model (Jones' model), and quantitative information on the spatial distribution of the hot plume is extracted from Schlieren images using in-house code. The results reveal that multi-location laser ignition can be achieved without reducing the energy absorption or strengthening the shock loss only when the energy of each laser pulse exceeds 200 mJ. This requirement is because the absorption rate of single-pulse LIB decreases significantly when the laser energy is lower than 200 mJ, and the shock loss of single-pulse LIB invariably accounts for approximately 80% of the absorbed laser energy at various incident energies. Compared with single-pulse LIB, dual-pulse LIB with a pulse interval of less than 200 ns is slightly inferior in terms of energy absorption and shock loss; however, the advantages of a larger initial plasma volume and lower energy dissipation can compensate for this deficiency. Therefore, dual-pulse laser ignition is a promising alternative to single-pulse laser ignition. Moreover, ignition reliability can be enhanced by initially releasing the laser pulse with higher energy when the energies of the successive pulses are not the same because of higher energy absorption and lower shock loss. In addition, the spatial distribution of the resulting hot plume is relatively centralized, which helps to reduce energy and radical dissipation. However, a pulse interval longer than 200 ns should be avoided for dual-pulse LIB because the laser energy cannot be utilized efficiently. [ABSTRACT FROM AUTHOR]

This work explores several low-cost methods for the visualization and analysis of pulsed synthetic jets for cooling applications. The visualization methods tested include smoke, Schlieren imaging, and thermography. The images were analyzed using Proper Orthogonal Decomposition (POD) and numerical methods for videos. The results indicated that for the specific nozzle studied, the optimal cooling occurred at a frequency of 80 Hz, which also corresponded to the highest energy in the POD analysis. The combination of Schlieren photography and POD is a unique contribution as a method for the optimization of synthetic jets. [ABSTRACT FROM AUTHOR]

This work investigates the hydrodynamic characteristics of a coaxial double-ring electrode helium plasma jet by means of a "Z-type" Schlieren imaging system. The Schlieren images and visual optical photographs made show that a transition point from a laminar region to a turbulent region exists for gas flow without plasma when the helium flow rate exceeds a certain value. After plasma ignition, the laminar region shrinks with voltage increases, and the maximum length of the plasma plume is confined to the laminar region. The heat transfer equation and the spectral broadening of the He I 667.8 nm were used to estimate the increased gas temperature in the plasma jet, and the change in gas velocity by ionic momentum transfer was found by application of a double sphere collision model. As a result, gas heating is considered to be the dominant factor for the earlier onset of turbulence after plasma ignition, whereas the role of ion momentum transfer to neutral gas molecules is comparatively weak. The hydrodynamic behaviors of the plasma jet at the impact region for organic glass and silicon substrates are also researched. The ionization front propagates along the organic glass surface and contracts at the impact point on the silicon surface. More visible vortices are observed from Schlieren images with silicon substrates than with organic glass substrates. Possible mechanisms related to the different treatment effects are discussed. [ABSTRACT FROM AUTHOR]

An experimental study aimed at the characterization of energy deposition of nanosecond Dielectric Barrier Discharge (ns-DBD) plasma actuators was carried out. Special attention was given on the effect of the thickness and material used for dielectric barrier. The selected materials for this study were polyimide film (Kapton), polyamide based nylon (PA2200), and silicone rubber. Schlieren measurements were carried out in quiescent air conditions in order to observe density gradients induced by energy deposited. Size of heated area was used to qualify the energy deposition coupled with electrical power measurements performed using the back-current shunt technique. Additionally, light intensity measurements showed a different nature of discharge based upon the material used for barrier, for a fixed thickness and frequency of discharge. Finally, a characterisation study was performed for the three tested materials. Dielectric constant, volume resistivity, and thermal conductivity were measured. Strong trends between the control parameters and the energy deposited into the fluid during the discharge were observed. Results indicate that efficiency of energy deposition mechanism relative to the thickness of the barrier strongly depends upon the material used for the dielectric barrier itself. In general, a high dielectric strength and a low volumetric resistivity are preferred for a barrier, together with a high heat capacitance and a low thermal conductivity coefficient in order to maximize the efficiency of the thermal energy deposition induced by an ns-DBD plasma actuator. [ABSTRACT FROM AUTHOR]

The elongation and branching of long positive spark discharges in the laboratory and in lightning have been attributed to the formation of thermalized channels inside a diffuse, glow-like streamer section at the leader head. It is experimentally shown here that the structured morphology of streamers produce low-density stem channels that elongate and branch well before a new leader channel section is formed. These non-thermalized stems are also shown to develop ahead of a developing leader channel. These findings are based on high-speed photography and Schlieren imaging used to visualize both the morphology of streamer filaments and stem channels. Numerical analysis is also performed to estimate the axial temperature and density of the stem channels. A stem-driven mechanism for the propagation and branching of positive long air gap discharges is proposed and discussed based on the presence of not-yet thermalized, low density channels formed by streamer ensembles at the leader head. [ABSTRACT FROM AUTHOR]

The work is dedicated to the experimental and numerical study of the mechanisms of gaseous detonation initiation in a stoichiometric hydrogen–oxygen mixture due to the reflection of a shock wave from a complex-shaped end wall. Several elliptic surfaces of different geometries, including distributed ones, were considered. We refer to such reflectors with multiple elliptical surfaces as multi-focusing systems. The experiments were carried out in a shock tube. Visualization of the process was carried out with a high-speed schlieren system. In the experiments, the ignition delay times and the critical incident shock wave Mach number for detonation initiation were measured. Two-dimensional Euler simulations, on a fully unstructured computational grid, were carried out to determine the mechanism of detonation initiation. The numerical approach was verified using the experimentally measured ignition delay times. Reasonable agreement between the simulations and experiments for the critical Mach number of detonation initiation and for the efficiency of various multi-focusing systems for detonation initiation was achieved. Different regimes of detonation initiation depending on the incident shock wave Mach number were observed. [ABSTRACT FROM AUTHOR]

We theoretically analyze the effect of density/refractive index gradients on the measurement precision of background-oriented schlieren (BOS) experiments by deriving the Cramer–Rao lower bound (CRLB) for the 2D centroid estimation process. A model is derived for the diffraction limited image of a dot viewed through a medium containing density gradients that includes the effect of the experimental parameters such as the magnification and f-number. It is shown using the model that nonlinearities in the density gradient field lead to blurring of the dot image. This blurring amplifies the effect of image noise on the centroid estimation process, leading to an increase in the CRLB and a decrease in the measurement precision. The ratio of position uncertainties of a dot in the reference and gradient images is shown to be a function of the ratio of the dot diameters and dot intensities. We termed this parameter the amplification ratio ( A F ), and a methodology for reporting position uncertainties in tracking-based BOS measurements is proposed. The theoretical predictions of the dot position estimation variance from the CRLB are compared to ray tracing simulations, and agreement is obtained. The uncertainty amplification is also demonstrated on experimental BOS images of flow induced by a spark discharge, where it is seen that regions of high amplification ratio correspond to regions of density gradients. This analysis elucidates the dependence of the position uncertainty on density and refractive index gradient-induced distortion parameters, provides a methodology for accounting its effect on uncertainty quantification and provides a framework for optimizing experiment design. [ABSTRACT FROM AUTHOR]

Experiments have been conducted in a supersonic rectangular duct (Mach 1.4), with a normal shock wave/boundary layer interaction. The duct is designed in a modular fashion so that its aspect ratio can be varied without a change in the flow geometry. The shock location, duct height, and Mach number are kept constant, while varying the aspect ratio. Conventional and inclined schlieren techniques have been used to visualize the normal shock. The height and width of the "normal part" of the normal shock have been measured. A parameter termed area fraction of the normal shock is used to quantify the extent of shock bifurcation, and the effect of the duct aspect ratio on this parameter is studied. It has been found that a nearly square duct is the preferred geometry for maximizing the area fraction of the normal shock. [ABSTRACT FROM AUTHOR]

This paper presents a focusing schlieren system designed for the investigation of transonic turbine tip-leakage flows. In the first part, the functional principle and the design of the system are presented. Major design considerations and necessary trade-offs are discussed. The key optical properties, e.g., depth of focus, are verified by means of a simple bench test. In the second part, results of an idealized tip-clearance model as well as linear cascade tests at engine representative Reynolds and Mach numbers are presented and discussed. The focusing schlieren system, designed for minimum depth of focus, has been found to be well suited for the investigation of three-dimensional transonic flow fields in turbomachinery applications. The schlieren images show the origin and growth of the tip-leakage vortex on the blade suction side. A complex shock system was observed in the tip region, and the tip-leakage vortex was found to interact with the suction side part of the trailing edge shock system. The results indicate that transonic vortex shedding is suppressed in the tip region at an exit Mach number around M2,is = 0.8. [ABSTRACT FROM AUTHOR]

Spurious displacements (SDs) associated with high-framing-rate background-oriented Schlieren (BOS) measurements were investigated experimentally in this study. To obtain the SDs, a multiple-pass cross-correlation algorithm was applied to reference/data images on a background printed with random dot pattern. Four sets of image pairs were dedicatedly selected to characterize the SDs. To isolate the SDs unique to the high-framing-rate BOS, measures were taken to minimize the contributions from other sources. Results showed that the SDs distinctive in high-framing-rate BOS measurements can be categorized according to their characteristic time: spatially uniform and fast oscillating SD (F-SD) that could be resolved during one recording event and non-uniform slowly varied SD (S-SD) that only become identifiable after several minutes time intervals between the two subsequent recordings. The proper boundary conditions (BCs) were proposed for solving the Poisson equation of the displacement fields contaminated by the SDs. A method allowing in-situ mitigation of both SDs was also proposed by introducing one additional light path into the BOS setup. Experiments on quasi-two-dimensional helium jets were conducted with and without acoustic excitations. The results confirmed the effectiveness of the proposed method to remove the SDs in both steady and unsteady flows. [ABSTRACT FROM AUTHOR]

The objective of this study is to understand how internal engine corrugated seals affect the aerodynamic performance of a convergent/divergent supersonic nozzle all the while promoting mixing. These corrugated seals are designed to protrude into the flow to alter the design pressure ratio of the nozzle through changes in area ratio. Measurements comprise axial thrust for nozzle pressure ratios ranging continuously from overexpanded to underexpanded nozzle flow states. Optical flow measurements using a z-type schlieren system are also used to qualitatively visualize the affect of protrusion depth on shock strength and boundary layer thickness at the nozzle exit. Findings reveal the effect of the corrugated seals on the double-shock pattern that forms from the imperfect expansion of the gas through this sharp throat nozzle. As much as a 2% increase in thrust coefficient is observed with one of the corrugated seal concepts and over a broad operating envelop corresponding to both overexpanded and underexpanded nozzle states. [ABSTRACT FROM AUTHOR]

A method of measuring density second derivative (shadowgraph) as a variant of background oriented schlieren (BOS) is presented. Pairs of images of a moving background with a small time difference are analysed using moving-window cross-correlation, giving the second derivative of density in the axis of background travel. The advantage over standard BOS is that no reference image is required. In contrast to the existing "reference-free BOS" method, no double-image is produced, making the data interpretation simpler. The sum-of-correlation (ensemble correlation) method can be used on an image series, where the flow is constant over the series time, to significantly reduce the size of the cross-correlation window. The reduction in the window size and sharpening of the resulting image is similar to that achieved by optical flow, but this method is easily automatable, without knowledge of the flow, and can be used with any existing BOS analysis software. [ABSTRACT FROM AUTHOR]

For stoichiometric C2H4–O2 and C2H2–O2 mixtures with or without argon dilution, the processes of detonation diffraction have been investigated in a two-dimensional setup through high-speed schlieren imaging, with the characteristic length and the stability of detonation varied by regulating the initial pressure and argon mole fraction of the mixture. In particular, a length relevant to the process of supercritical diffraction (i.e., distance from the channel end corner to reflection point of the transverse detonation on the channel end face, reflection point distance in short) was deduced from obtained sequential schlieren images and analyzed. The reflection point distance can be idealized for the infinitely wide donor channel, and thus, it can be a parameter in which properties intrinsic to each detonable mixture are manifested. Experimental results showed that the reflection point distance was roughly inversely proportional to the initial pressure for identical mixtures and independent of the width of the donor channel at high initial pressures. For a certain combination of the fuel and oxidizer, correlations between the reflection point distance and the initial partial pressure of fuel were very similar regardless of the argon mole fraction. Critical conditions of the diffraction problem could be given for the ratio of the reflection point distance to the channel width, and it was suggested that the critical value lies in a range of 3–5 and does not significantly depend on the stability of the mixture. [ABSTRACT FROM AUTHOR]

A two-wavelength quantitative Schlieren technique that allows inferring the electron and gas densities of axisymmetric arc plasmas without imposing any assumption regarding statistical equilibrium models is reported. This technique was applied to the study of local thermodynamic equilibrium (LTE) departures within the core of a 30 A high-energy density cutting arc. In order to derive the electron and heavy particle temperatures from the inferred density profiles, a generalized two-temperature Saha equation together with the plasma equation of state and the quasineutrality condition were employed. Factors such as arc fluctuations that influence the accuracy of the measurements and the validity of the assumptions used to derive the plasma species temperature were considered. Significant deviations from chemical equilibrium as well as kinetic equilibrium were found at elevated electron temperatures and gas densities toward the arc core edge. An electron temperature profile nearly constant through the arc core with a value of about 14000-15000 K, well decoupled from the heavy particle temperature of about 1500 K at the arc core edge, was inferred. [ABSTRACT FROM AUTHOR]

Plasma temperature and radial density profiles of the plasma species in a high energy density cutting arc have been obtained by using a quantitative schlieren technique. A Z-type two-mirror schlieren system was used in this research. Due to its great sensibility such technique allows measuring plasma composition and temperature from the arc axis to the surrounding medium by processing the gray-level contrast values of digital schlieren images recorded at the observation plane for a given position of a transverse knife located at the exit focal plane of the system. The technique has provided a good visualization of the plasma flow emerging from the nozzle and its interactions with the surrounding medium and the anode. The obtained temperature values are in good agreement with those values previously obtained by the authors on the same torch using Langmuir probes. [ABSTRACT FROM AUTHOR]

Nondisturbing laser-probing polarimetry (based on the Faraday and Kerr effects) and Schlieren diagnostics were used in the investigation of underwater electrical wire explosion. Measuring the polarization plane rotation angle of a probing laser beam due to the Faraday effect allows one to determine an axially resolved current flowing through the exploding wire, unlike commonly used current probes. This optical method of measuring current yields results that match those obtained using a current viewing resistor within an accuracy of 10%. The same optical setup allows simultaneous space-resolved measurement of the electric field using the Kerr effect. It was shown that the maximal amplitude of the electric field in the vicinity of the high-voltage electrode is ∼80 kV/cm and that the radial electric field is <1 MV/cm during the wire explosion. Finally, it was shown that the use of Schlieren diagnostics allows one to obtain qualitatively the density distribution behind the shock wave front, which is important for the determination of the energy transfer from the discharge channel to the generated water flow. [ABSTRACT FROM AUTHOR]

Recent experiments [N. F. Declercq and E. Lamkanfi, Appl. Phys. Lett. 93, 2 (2008)] showed evidence of the relationship between the generation of leaky Rayleigh waves and the formation of the two reflected lobes and a null strip for bounded beam incidence at the Rayleigh angle. The evidence was based on an experimental setup using a sound barrier on a liquid solid interface. The current paper presents a finite element investigation of the experiments and confirms the earlier conclusions. In addition, unexplained experimental observations are clarified. Furthermore, a detailed investigation of the influence of the bonding properties between the barrier and the solid on the observed radiation patterns is achieved, which is of broader importance to the nondestructive characterization of bonding in general. [ABSTRACT FROM AUTHOR]

We propose a general model for the spherically averaged exchange hole corresponding to a generalized gradient approximation (GGA) exchange functional. Parameters are reported for several common GGAs. Our model is based upon that of Ernzerhof and Perdew [J. Chem. Phys. 109, 3313 (1998)]. It improves upon the former by precisely reproducing the energy of the parent GGA, and by enabling fully analytic evaluation of range-separated hybrid density functionals. Analytic results and preliminary thermochemical tests indicate that our model also improves upon the simple, local-density-based exchange hole model of Iikura et al. [J. Chem. Phys. 115, 3540 (2001)]. [ABSTRACT FROM AUTHOR]

Tunable acoustic gradient index (TAG) lenses create tunable multiscale Bessel beams. These lenses are fluid-filled cylindrical cavities within which an acoustic radial standing wave is excited. This standing wave modulates the density, and thereby the refractive index within the lens. Spatial gradients in the refractive index can be used for lensing. A predictive model for the steady-state fluid mechanics behind TAG lenses driven with a sinusoidal voltage signal is presented here. The model covers inviscid and viscous regimes in both the resonant and off-resonant cases. The density fluctuations from the fluidic model are related to refractive index fluctuations. The entire model is then analyzed to determine the optimal values of lens design parameters for greatest lens refractive power. These design parameters include lens length, radius, static refractive index, fluid viscosity, sound speed, and driving frequency and amplitude. It is found that long lenses filled with a fluid of high refractive index and driven with large amplitude signals form the most effective lenses. When dealing with resonant driving conditions, low driving frequencies, smaller lens radii, and fluids with larger sound speeds are optimal. At nonresonant driving conditions, the opposite is true: High driving frequencies, larger radius lenses, and fluids with low sound speeds are beneficial. The ease of tunability of the TAG lens through modifying the driving signal is discussed, as are limitations of the model including cavitation and nonlinearities within the lens. [ABSTRACT FROM AUTHOR]

Thin films with sinusoidally varying refractive index profiles display photonic band gap effects. Intentional deviations from the periodic index profile can be used to tailor the optical properties of the resulting thin film. We present experimental characterization of TiO2 films with periodic index profiles fabricated using a deposition technique known as glancing angle deposition (GLAD). The resulting porous thin films have a microstructure consisting of vertically aligned columns. Sinusoidal porosity gradients, and therefore sinusoidal index profiles, can be introduced in the direction of the substrate normal by fabricating columns with a periodically varying diameter. Local modifications of the index profile are achieved by inserting thin layers of constant porosity into the center of the film, or by discontinuously changing the phase of the sinusoidal gradient. The introduction of these structural defects creates a narrow optical passband inside the larger band gap, and we demonstrate how the properties of this passband can be controlled through modification of the defect parameters. The magnitude of the phase shift constituting the defect is shown to control the location of the resulting passband. By inserting a layer with in-plane birefringence, we show that it is possible to create two separate passbands, one for each polarization, within a single stop band. The results illustrate how nanoscale porosity engineering using GLAD is a precise technique for fabricating one-dimensionally periodic films with a variety of optical characteristics. [ABSTRACT FROM AUTHOR]

Experiments using the schlieren technique to image sound incident on a corrugated, water-brass interface show a backward displacement of the reflected beam at an angle of 22.5°, confirming the observations of Breazeale and Torbett [Appl. Phys. Lett. 29, 456 (1976)]. However, the present theory hypothesizes that this beam displacement results from excitation of a type of leaky surface wave. Further experiments with a sufficiently narrow incident beam reveal a backward displacement also at angles around 44°, resulting from excitation of Rayleigh surface waves, as predicted by the theory of Tamir and Bertoni [J. Acoust. Soc. Am. 61, 1397 (1971)]. Thus, a wide beam gives a backward displacement at 22.5° only. A narrow beam gives a backward displacement also at angles around 44°. [ABSTRACT FROM AUTHOR]

A study by means of the Schlieren technique for the visualization of the ultrasonic beams has revealed that when the leaky Rayleigh waves propagating along the horizontal edge of a thick fluid-loaded solid plate are scattered at the extremity of the plate, they travel around the corner and start leaking into the liquid along the Rayleigh angle measured from the normal to the vertical edge of the plate. Furthermore, the study reveals that the leaky Rayleigh waves are stimulated by the border of an incident ultrasonic-bounded beam more than by the interior of the beam. A comparison with an earlier work shows that the characteristics of the scattering of the leaky Rayleigh waves at the edge of the plate is very different from that of the Scholte–Stoneley waves. [ABSTRACT FROM AUTHOR]

Gradient index lenses were formed in a liquid-filled cavity supporting an ultrasonic standing wave. The constructed devices acted as diverging lenses or axicon lenses, depending on whether the center or edge region is interrogated. Experiments and models suggest that the primary process contributing to lensing is the steady-state density component of the finite-amplitude standing wave. sound amplitudes up to 150 MPa were calculated in glycerin, corresponding to a maximum contrast in the refractive on the order of 0.1%. This amplitude was also sufficient to move high index nanometer-scale particles via an acoustic radiation force and thereby create larger refractive index gradients. Concepts are also explored to manipulate the particle distribution in order to create converging lenses and/or other desirable optical components. [ABSTRACT FROM AUTHOR]

Vibrational relaxation has been seen in shock waves in propane, isobutene, isobutane, neopentane, and toluene dilute in krypton with the laser–schlieren technique. These experiments cover about 600–2200 K and post-shock pressures from 5 to 29 Torr. The process cannot be resolved in any for T<600 K, or in any for large molecule fraction. All the ultrasonic measurements of relaxation in these at room temperature show characteristic times in the 1–5 ns atm range, corresponding to fewer than five collisions, whereas the relaxation times in the shock waves range from 20 to 200 ns atm, with a clearly defined negative or “inverted” temperature dependence. It would seem the observed slowdown of relaxation with increasing T is simply a consequence of the much increased energy transfer required at high temperature in such large polyatomics when this is combined with a collision efficiency, here interpreted as 〈ΔE〉[sub down], already so large it cannot much increase. The simple method for the extraction of a 〈ΔE〉[sub down] from relaxation data offered here by consideration of the energy relaxation equation for E[sub vib]=0 appears to be original and should prove quite useful in connecting thermal relaxation data to values obtained from spectroscopy and master-equation analyses. Here it is found that the derived 〈ΔE〉[sub down] extrapolate well to room temperature ultrasonic measurements, showing a slight increase with temperature. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract In search for a feasible replacement for fossil fuels, there has been extensive research into biofuels in the past few decades. Oxygenated compounds, such as anisole, in particular have shown some promising characteristics. Anisole is an oxygenated biofuel widely adopted as a solvent and seems to be an apt fuel for the combustion application. Although being a potential biofuel, only a few investigations have been conducted for anisole; therefore, a lack of supplementary experiments in high temperatures and engine-relevant conditions is obvious. The experimental results are also highly demanded as a source of verification and update for the available chemical kinetic mechanisms. In this research, performance characteristics of anisole are studied; specifically, the Laminar Burning Speed (LBS), flame structure, and flame stability have been experimentally and theoretically discussed. The calculations have been produced using a constant volume combustion chamber (CVCC) and a z-type schlieren optical technique accompanying a differential-based multi-shell thermodynamic model. The data has been reported in a wide range of temperatures (T = 460–550 K), pressures (p = 0.5–5.5 atm), and equivalence ratios (ϕ = 0.8–1.4). In order to provide a convenient way for other researchers to access LBS for anisole in a wider range, a power-law correlation has been presented for LBS as a function of p, T and ϕ. Regarding the flame structure, the hydrodynamic sources have been shown to be the main influential root to spherical flame instabilities which is directly related with ϕ and reversely to pressure. The experiments demonstrated that anisole has a relatively high laminar burning speed, with a maximum at ϕ = 1.1 for different p and T conditions. As expected, the LBS increases by increasing the temperature and reducing the pressure. The two available chemical mechanisms for anisole combustion have been compared with the experimental results and a need for modification in the mechanisms are observed. A sensitivity analysis is performed to investigate the sources of discrepancies. [ABSTRACT FROM AUTHOR]

Highlights • Six-frame laser probing system with spatial resolution of 34 µm is developed. • System allows simultaneous recording of interference, shadow and schlieren images. • High-voltage generator is triggered by 70 ps, ∼ 109 W laser beam with ∼ 1 ns jitter. • Setup is tested by studying fast (∼ 1 ns) spark formation in atmospheric discharge. Abstract We present the setup that combines the diagnostics involving a multi-frame laser probing system and a 25 kV generator providing a voltage pulse with a rise time of ≈ 4 ns. An intermediate spark switcher triggered by a ∼ 109 W picosecond laser ignition beam is employed to govern the high-voltage generator with jitter as low as ∼ 1 ns. In order to implement the laser probing, a practical six-frame optical system for simultaneous recording of interference, shadow and schlieren images of the investigated object is developed. The system has an exposure time of 70 ps and spatial resolution as high as 3–4 µm. The average interval between the probing beams is ≈ 2 ns. Our setup allows to investigate fast (∼ 1 ns) processes of the plasma formation with high temporal and spatial resolutions as well as to obtain quantitative data on the plasma parameters. The described approach provides a means to greatly simplify the design of the multi-frame optical system and also facilitates efficient collection and separation of the probing beams. The setup has been tested by studying the spark formation in an atmospheric discharge on a nanosecond time scale. [ABSTRACT FROM AUTHOR]

Synthetic schlieren tomography is a recently proposed three-dimensional (3D) optical imaging technique for studying ultrasound fields. The imaging setup is composed of an imaged target, a water tank, a camera, and a pulsed light source, which is stroboscopically synchronized with an ultrasound transducer to achieve tomographically stationary imaging of an ultrasound field. In this technique, ultrasound waves change the propagation of light rays by inducing a change in refractive index via the acousto-optic effect. The change manifests as optical flow in the imaged target. By performing the imaging in a tomographic fashion, the two-dimensional tomographic dataset of the optical flow can be transformed into a 3D ultrasound field. In this work, two approaches for acoustic pressure field estimation are introduced. The approaches are based on optical and potential flow regularized least square optimizations where regularization based on the Helmholtz equation is introduced. The methods are validated via simulations in a telecentric setup and are compared quantitatively and qualitatively to a previously introduced method. Cases of a focused, an obliquely propagating, and a standing wave ultrasound field are considered. The simulations demonstrate the efficiency of the introduced methods also in situations in which the previously applied method has weaknesses. [ABSTRACT FROM AUTHOR]

Abstract Experiments on single bubble-based pool boiling have been performed under saturated bulk conditions to understand the influence of suspended nanoparticles on bubble dynamics and the associated temperature gradients field. Water and two concentrations of silica-water nanofluid (0.005 and 0.01% (V/V)) have been subjected to isolated nucleate pool boiling on a ITO-coated borofloat heater by supplying constant heat flux. Experimental measurements have been made in a purely non-intrusive manner by employing refractive index-based rainbow schlieren deflectometry technique. Bubble dynamic parameters, such as bubble departure diameter and departure frequency, wait and growth times etc. and the temperature gradients in the bulk fluid have been mapped simultaneously in the form of schlieren images. Results on bubble dynamic parameters revealed that these parameters show more variability and assume skewed normal distribution in the case of nanofluids whereas water experiments showed insignificant variations in these parameters. Of the significant changes, the bubble departure diameter reduced, departure frequency and growth rate increased with increasing concentration of nanofluids. Schlieren images corresponding to 0.005 and 0.01% (V/V) nanofluids showed more spreading out of the superheat layer adjacent to the heater substrate than that observed in the case of water experiments. Time-averaged values of natural convection and evaporative heat transfer rates associated with single bubble-based boiling indicated that both these rates decrease in the presence of dispersed silica nanoparticles. Reduction in the overall heat transfer rate in the case of nanofluids has been explained on the basis of direct experimental measurements. In this direction, the observed phenomenon has been attributed to the role played by the suspended nanoparticles in diffusing the thermal gradients (reflected in the form of the broadening of the superheat layer) adjacent to the heater substrate along with substantially increasing the wait time of bubble inception in the case of nanofluids as compared to water-based experiments conducted under same wall superheat conditions. Highlights • Single bubble-based nucleate pool boiling experiments under saturated bulk conditions. • Influence of suspended nanoparticles on bubble dynamics and heat transfer rates. • Rainbow schlieren-based optical diagnostics. • Plausible reasons to explain the role(s) of nanoparticles on the phenomena studied. [ABSTRACT FROM AUTHOR]

Temporal variations in the gas density gradient occurring in an atmospheric-pressure streamer discharge are visualized by the schlieren method to investigate the effects of the background gas species on the temporal variations of the gas heating process after the discharge. The streamer discharge is generated in needle-to-plane electrode configurations in dry-nitrogen, humid-nitrogen, dry-air, and humid-air conditions. The discharge characteristics are measured by the voltage and current waveforms and the spatial distribution of the discharge emission of light. The results of the schlieren visualization show that the gas density gradient increases after the discharge and its rate of increase is accelerated by the presence of water molecules. In addition, the degree of the increase in the density gradient is higher in humid-nitrogen discharges than in humid-air discharges. These characteristics are subsequently explained by the vibrational to the translational energy-transfer process of nitrogen, oxygen, and water molecules. [ABSTRACT FROM AUTHOR]

Acoustic-triggered conditional sampling analysis can extract the broadband-noise-related intermittent fluctuations from complicated schlieren visualization movies around the source region, which is difficult when using conventional methods. In this study, an analysis of the Mach wave radiation from a supersonic jet was performed. The extracted results properly captured the known features of the Mach waves and their sources (i.e., wavepacket at the jet boundary) and clearly showed the correlation between these near-field fluctuations and the far-field intermittent acoustic events. This analysis can potentially be applied to other broadband, intermittent turbulent noise and will provide a better understanding of their generation mechanisms. [ABSTRACT FROM AUTHOR]

A quantitative interpretation of the schlieren technique applied to an atmospheric pressure, vortex-stabilized nitrogen thermal plasma jet generated in a direct-current nontransferred arc plasma torch (nitrogen gas flow rate of 25 NL/min, power level of 15 kW), discharging into ambient air is reported. A $Z$ -type, two-mirror schlieren system was used in the research. The technique allowed inferring the temporally averaged values of the temperatures and densities of different species present in the plasma jet in a wide range of radial and axial distances. Deviations from kinetic equilibrium in the calculation of the plasma refractive index were accounted for, but maintaining the assumption of the local chemistry equilibrium. The influence of several assumptions on the accuracy of the measurements was considered. The results have shown that for a distance of 3.5-mm downstream from the nozzle exit, the kinetic equilibrium is realized (being both electron and gas temperatures values around 11 000 K), but noticeable deviation from kinetic equilibrium appears toward the jet border. On the other hand, a marked deviation from the kinetic equilibrium was found in the whole far field of the plasma jet, where the electron temperature remains still quite high (about 10 000 K at 30-mm downstream of the nozzle exit), well decoupled from the gas temperature (about 7000 K at the same distance). The obtained results are in reasonable good agreement with those previously reported by some of the authors by using a double floating probe method in the same plasma torch. [ABSTRACT FROM AUTHOR]

The compressible accelerated mixing layer of a central injector was thoroughly investigated experimentally to provide a data set that can be used for validating numerical simulations. A drop-shaped central injector was mounted upstream of a rectangular convergent-divergent nozzle, through which air was accelerated to a Mach number of 1.7. The free-stream Reynolds number at the point of injection was 6.245 × 104. Four different measurement techniques—short-time illuminated schlieren imaging, laser schlieren, laser-induced thermal acoustics, and laser-induced fluorescence (LIF)—were applied to visualize the flow structures and to measure the predominant frequency of periodic flow features, the Mach number and temperature, and the injectant distribution. Instantaneous images show that the mixing layer was dominated by a series of alternating vortices. The mixing layer's self-similarity could be proven by means of injectant mass fraction profiles, which were derived from LIF measurements. The growth rate of the mixing layer was shown to approximately follow the 1 2-power law. It was concluded from comparison to literature data that the growth rate is primarily determined by the free-stream Reynolds number, whereas the free-stream Mach number (compressibility effects) and the injectant amount play a minor role. These experimental data were used to validate three-dimensional (3D) unsteady Reynolds-averaged Navier-Stokes simulations using the shear-stress transport turbulence model. It was shown that the vortex shedding frequency and the mixing layer growth rate as well as the wake velocity deficit were underestimated by the simulations. This indicates that the flow physics of vortex formation were not entirely reproduced. [ABSTRACT FROM AUTHOR]

When light is incident upon a diffraction grating, images of the grating appear at periodic intervals behind the grating. This phenomenon and the associated self-imaging distance were named after Talbot, who first observed them in the nineteenth century. A century later, this effect held new surprises with the discovery of sub-images at regular fractional distances of the Talbot length. In this paper, we show that water waves enable one to observe the Talbot effect in a classroom experiment. Quantitative measurements, of for example the Talbot distances, can be performed with an easy-to-use digital Schlieren method. [ABSTRACT FROM AUTHOR]

The results of investigation of gas and gas-powder flows in various cladding nozzles are presented and discussed. Schlieren-imaging, high-speed imaging and laser Doppler anemometry were used for optical diagnostics of the flows. It is demonstrated that diameter of the outlet in removable cyclone cap has significant impact on the powder flow pattern in an off-axis nozzle. It provides a convenient way to optimize focusing of the powder flow. It is found that stable vortices that appear near the surface of the substrate significantly reduce efficiency of powder material use when off-axis nozzle is applied. It is demonstrated that excessive shielding gas flow rate in four-stream and coaxial nozzles may lead to defocusing of gas-powder flows. A possibility to increase the efficiency of powder use in various laser cladding nozzles through modification of their design as well as shielding and carrier gases flow rates is discussed. [ABSTRACT FROM AUTHOR]

Highlights • Trans-critical and flash-boiling sprays of propane show the similarity in spray morphology. • Shock structures near the nozzle are observed in trans-critical sprays of propane. • Decreasing the injection pressure of trans-critical spray can induce a gas-like jet. • Interaction of spray plumes increases Mach disk distance of trans-critical propane spray. Abstract This paper is to investigate the characteristics of trans-critical propane spray compared with the flash boiling spray ejected from a multi-hole injector in a constant volume chamber by using the Schlieren and backlit imaging methods. The fuel temperature (T f) is set from 30 °C to 120 °C, injection pressure (p f) from 60 bar to 120 bar and ambient pressure (p a) from 0.2 bar to 10 bar. The results show that the trans-critical spray has longer vapor penetration and shorter liquid penetration than the flash boiling spray, but those two sprays have similar curve liquid boundaries near the nozzle. Several discernable collapsed shock structures near the nozzle are observed at T f = 120 °C, p f =120 bar, p a = 1 bar, but they disappear when p f drops to 60 bar for the transition path may not enter the two-phase region, and the liquid phase hardly occurs. The Mach disk distance of trans-critical spray in this study is larger than that of the under-expanded ideal gas jet because of the collapse phenomenon. At T f = 120 °C, p f = 120 bar, the liquid length decreases when p a increases from 0.2 bar to 2.5 bar and increases when p a continues to rise. [ABSTRACT FROM AUTHOR]

Highlights • Simultaneous measurement of velocity and temperature fields using schlieren method. • The developed schlieren system is quite cost effective. • The velocity field has been measured by tracking the shadows of seeding particles. • The developed method has been successfully applied to a free convective flow. Abstract Many advanced schlieren systems have been reported in the literatures and a great number of it are associated in gas flow study, commonly air. Present research proposes an efficient use of Z-type schlieren technique for simultaneous measurement of ray-averaged velocity as well as temperature fields beside qualitative flow visualization. Experiments are conducted to study natural convective flow in de-ionized water in a differentially heated cubical enclosure. The same set of transient schlieren images has been used for simultaneous measurement of velocity and temperature field. The velocity field is also determined by using laser based particle image velocimetry (PIV) to compare with schlieren based technique. The developed schlieren system has been used to obtain ray-averaged quantities, such as, thermal and hydrodynamic boundary layer thicknesses, temperature gradient and temperature fields, distribution of local Nusselt number along the thermally active walls, average Nusselt number and velocity field. In this work, the above quantities have been simultaneously determined for the first time using halogen projector lamp based Z-type calibration schlieren system. The results inspire the use of calibration schlieren system for the study of natural convective flow in water-filled cubical enclosures. The above results also provide experimental data as new benchmark for researchers to compare results for high Rayleigh number free convective flow with water as the working medium. [ABSTRACT FROM AUTHOR]

Abstract The appearance of the squish flame is of great significance to accelerate burning progress and improve the combustion efficiency. In this paper, we experimentally studied the characteristics of the squish flame in a cylindrical constant volume vessel under different initial pressures and equivalence ratios by using high-speed schlieren photometry. Due to the compression of the main flame front, "squish flow" was induced in the analogous triangular vertebrae region besieged by the convex flame front, the concave wall and the flat optical windows, which provided the perturbation of large wavelength to promote the appearance of the squish flame. When the squish flames occur, as the initial pressure increases, the main flame propagation distance becomes shorter, the main flame propagation velocity increases first and then gradually saturates to a certain value; as the equivalence ratio increases, the main flame propagation distance becomes longer, the main flame propagation velocity rises first and then declines, and the maximum is obtained in the vicinity of Φ = 1.0. There exists a critical initial pressure at each equivalence ratio below which no squish flame appears, and it takes on a U-shaped trend with the increase of equivalence ratio. The hydrodynamic instability plays a key role in the formation of the squish flame. The squish flame tends to appear at higher hydrodynamic instability. The formation mechanism and the critical feature of the squish flame obtained in this paper can provide a theoretical guide to achieve fast controllable combustion. Highlights • The squish flame is caused by "squish flow" which is induced in the analogous triangular vertebrae region. • The critical initial pressure at which the squish flame appearing takes on a U-shaped trend with the increase of equivalence ratio. • The squish flame is inclined to be presented at higher hydrodynamic instability. [ABSTRACT FROM AUTHOR]

Abstract AWAKE develops a new plasma wakefield accelerator using the CERN SPS proton bunch as a driver Muggli et al. (2017). The proton bunch propagates through a 10 m long rubidium plasma, induced by an ionizing laser pulse. The co-propagation of the laser pulse with the proton bunch seeds the self modulation instability of the proton bunch that transforms the bunch to a train with hundreds of bunchlets which drive the wakefields. Therefore the plasma radius must exceed the proton bunch radius. Schlieren imaging is proposed to determine the plasma radius on both ends of the vapor source. We use Schlieren imaging to estimate the radius of a column of excited rubidium atoms. A tunable, narrow bandwidth laser is split into a beam for the excitation of the rubidium vapor and for the visualization using Schlieren imaging. With a laser wavelength very close to the D2 transition line of rubidium (λ ≈ 780 nm), it is possible to excite a column of rubidium atoms in a small vapor source, to record a Schlieren signal of the excitation column and to estimate its radius. We describe the method and show the results of the measurement. [ABSTRACT FROM AUTHOR]