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An experimental investigation of thermocapillary deformations in a film of 10% ethyl alcohol solution in water, flowing down a plate with a heater of length 6.7 mm and width 68 mm, is performed. Heating of the film results in the formation of a horizontal liquid bump at the top edge of the heater. On the heater the flow divides into vertical rivulets with a thin film between them. Film deformations in the bump and the thin film between the rivulets are investigated. Local film thickness is measured by means of a double-fiber optical probe. The method is based on the dependence of the intensity of reflected light on the distance between the probe and the reflecting surface. The measurement results are compared to those previously obtained using the schlieren method. The experiment is controlled by three parameters. They are, with their respective values, the plate inclination angle (4?90°), the Reynolds number (0.15?62) and the heat flux density (0?4.5 W/cm 2). [ABSTRACT FROM AUTHOR]

Photopatterning metal and/or metal oxides in porous, silica-based glasses and xerogels followed by thermal consolidation to a nonporous glass yields refractive index gradients that,depending on their shape, guide, focus, and defract light. The optical performance of these structures depends on the species photodeposited in the matrix, the changes they undergo during consolidation of the matrix, and their effect on the consolidating matrix. Gradient index structures derived from Fe(CO) 5 are composed of nanometer diameter metal and/or metal oxide particles, whereas those derived from (CH 3 ) 3 SnI are composed of individual oxides or smaller aggregates within the SiO 2 matrix. Porous Vycor glass acts as a template for Fe o -Fe 2 O 3 particle growth during consolidation with the resulting gradient index structure consisting of 10±1 nm diameter particles composed of small aggregates of elemental iron dispersed in Fe 2 O 3 with an average inter-particle spacing equivalent to the correlation length of the porous glass. Consolidation of the xerogel produces equivalent changes in refractive index and similar changes in the electronic spectra, but the gradient index is composed of a mixture of octahedrally and tetrahedrally coordinated Fe 3+ present as individual molecular species, or as ≤1-nm diameter aggregates incorporated into the silica network. Photodeposition of tin offers a means to pattern porosity within a consolidated glass and a means to incorporate reagents unable to withstand the consolidation temperature of glass. Pattern resolution is limited by scattering of the photolyzing light by the SiO 2 nodules of the porous matrices, but thermal consolidation of the matrices occurs without loss of pattern resolution. Consistent with a random distribution of pores throughout these silica matrices, the change in pattern dimension on consolidation can be calculated from the void volume of the matrix. Various optical structures created by these photodeposition techniques and their optical performance are described. [ABSTRACT FROM AUTHOR]

Deals with a study which presented the influence of biogeochemical processes on the inherent optical properties (IOP) distribution in the vicinity of the bottom on horizontal scales of tens of meters, vertical scales of tens of centimeters and temporal scales of minutes in terms of net effect on IOP. Materials and methods; Information on horizontal gradients in optical properties across coral-sand boundaries; Number of observations for which each property was higher above the reef or sand.

The initiation of ethylene combustion (at φ=1 and φ=1.5) was studied over a wide range of temperatures (∼800-1620 K) in shock waves. Ignition delay data showed the presence of two or three different chemically controlling regimes in the combustion. At low temperatures (∼900-1000 K) the time development of a combustion bubble was tracked via Schlieren techniques and was observed to grow slowly. Unburnt gas between the bubble and end wall that was compressed and heated by the latter exploded and gave rise to a strong combustion wave that traveled back through the partially burning gas. Emission measurements showed the presence of CH*, OH*, C 2 *, and a continuum emission attributed to CO 2 * flame bands. C 2 * was found to be only important in richer mixtures. CH* was formed only microseconds ahead of the other diatomics. [ABSTRACT FROM AUTHOR]

Thick and thin freestanding films for the sample showing the subphases, AF and SmC γ * were investigated. The thick film shows optically uniaxiality due to the helix along the smectic layer normal. However, Schlieren textures emerge in the thin film where the helical pitch is less than one pitch. It indicates that the subphases observed have biaxiality. Meanwhile, the decrease of the transmission in the bright brush in Schlieren textures compared with that in the SmC* phase support the distorted Ising model. [ABSTRACT FROM AUTHOR]

The purpose of this study is to investigate the relation between ignition systems (or energy) and flame kernel development. In this research, three different kinds of ignition systems and several different kinds of spark plug electrodes are designed and manufactured. The flame propagation velocity in a constant volume combustion chamber is measured by both a laser deflection method using an He-Ne laser and a Schlieren method using a high speed digital camera. In order to control the ignition energy, the dwell time is varied by a spark time controller. The results indicate that, when the ignition energy is increased by extending the dwell time, flame kernel growth accelerates. As the electrode gap width is increased, the breakdown energy is also increased, which stimulates the initial kernel development. The materials, diameter and shape of the electrode affect the discharged energy, the energy transfer efficiency and heat losses and, thus, these also affect the flame kernel development. The average difference in measurement of the flame velocity by the laser deflection method and Schlieren method is only 1.73 per cent. The laser deflection method is found to be preferable because it is more effective and employs simpler means for the analysis of flame kernel development. [ABSTRACT FROM AUTHOR]

A model of the human eye with a gradient index crystalline lens is presented. The crystalline lens shape at different accommodation levels is described by a single function which is a combination of hyperbolic cosine functions and hyperbolic tangent functions. Using the experimental data published in the literature a model of the variations of the external lens shape was created. Formulae for the lens shape parameters and gradient index distribution for different accommodation levels are given. [ABSTRACT FROM AUTHOR]

In the present work, prebreakdown phenomena in dielectric liquids which have various molecular structure, especially those with chlorine or fluorine atoms and some double bonds included in their molecular formula, are observed in detail for positive and negative point polarities, under the application of an impulse voltage (1.1/225 µs) to the point electrode. As test liquids, cyclic hydrocarbon liquids such as cyclohexane, benzene, chlorocyclohexane, monochlorobenzene, hexafluorobenzene, and cyclohexene are used. Results obtained in these liquids are compared with those obtained in straight-chain hydrocarbon liquids. As a result, the propagation of negative streamers, especially in dielectric liquids with chlorine or fluorine atoms in their molecular formula, is promoted, whereas the propagation of positive streamers in dielectric liquids with some double bonds is promoted. The former is related to a large electron affinity, the latter is due to π electrons, for both cyclic and straight-chain hydrocarbon liquids. © 2002 Scripta Technica, Electr Eng Jpn, 139(1): 1–8, 2002; DOI 10.1002/eej.1140 [ABSTRACT FROM AUTHOR]

We present equations for analysing the primary spherical aberration of the crystalline lens, both as a whole and of its contributing parts. We apply these equations to two examples of gradient index lenses. The spherical aberration of one lens is well outside that expected for real lenses, but that of the other is within this range. We assess the sensitivity of the aberration to changes in surface shape and gradient index distribution. The process is applicable to other models of the gradient index of the lens. [ABSTRACT FROM AUTHOR]

Due to the radial refractive index gradients that sometimes develop in flow cells during the travelling of the samples through them, the absorbance records obtained in such conditions have a strange shape, particularly at low analyte levels. To correct this phenomenon, called Schlieren effect, a simple and reliable signal processing algorithm is described in correlation to the detector characteristics of noise. The algorithm is illustrated in nitrite and nitrate determinations with proflavin from more real samples. It has a solid theoretical support and involves the subtraction from the sample records of a smoothed signal, acquired by working with a blank solution. [ABSTRACT FROM AUTHOR]

A new type of spherically symmetric gradient index element is presented. These elements are able to transform an optical field from a point source with a predetermined radiation pattern into an arbitrary amplitude distribution in the target plane. The design method is illustrated with four examples of amplitude formers that produce irradiance distributions that are constant, Gaussian, triangular and ring-shaped. [ABSTRACT FROM AUTHOR]

The effects of off-axis illumination and finite dimension on the irradiance distribution in a hybrid system formed by a zone plate and a tapered gradient-index (GRIN) medium are investigated. Results are applied to a GRIN medium with a divergent linear taper function and a sinusoidal zone plate of amplitude to show the transverse shift and the diffractional effect. [ABSTRACT FROM AUTHOR]

A general method is presented for designing spherically symmetric gradient-index axicons, which produce a variety of different intensity patterns along the axis, and with a boundary index larger than or equal to the index of the surrounding medium. As the design method is based on geometrical optics, tools to analyse the axicon using wave optics are given. [ABSTRACT FROM AUTHOR]

Examines the vortex shedding and wave propagation in transonic two-dimensional cavity flows. Use of schlieren technique in visualization; Analysis of pressure fluctuation in the cavity; Comparison of experimental results in terms of frequencies.

Studies the chromatic properties of SiO[sub2]-BaO-TiO[sub2]-K[sub2]O series radial gradient-index (GRIN) material. Representation for basic quantities of a radial GRIN lens; Measurement of chromatic aberrations and dispersion properties of the GRIN medium; Lens design example using the GRIN material.

Presents a study which investigated several polymer systems to test polymer-to-polymer miscibility for fabrication of plastic gradient index (GRIN) optic fiber. Properties and applications of GRIN materials; Methods to fabricate polymer GRIN fibers; Discussion on the results of the study.

Light propagation through a tapered gradient-index medium with a zone plate located at the input of the medium for non-uniform and uniform illumination is studied. Evolution of the beam half-width of the zone plate diffraction orders and an analogy with the conventional lens imaging formula are presented. A particular case is analysed. [ABSTRACT FROM AUTHOR]

Focuses on a study which evaluated the performance of a single bi-plano lens that uses low-dispersive radial gradient-index (GRIN) material. Representation for basic quantities of a radial GRIN lens; Optical design and evaluation of lens system; Dispersion property of the radial GRIN material.

In this paper, the propagation characteristics of a creepage discharge on the surface of a solid insulator with a back electrode in perfluorocarbon liquid under an impulse voltage application are investigated. The propagation process is observed in detail by means of a high-speed schlieren optical technique, and simultaneously waveforms of a current and a charge are also measured. Consequently, the polarity effect of the streamer propagation and the positive streamer which propagates stepwise can be seen. The latter is concurrent with the sparse current pulse which corresponds to the charge step-variation. These results are compared with ones obtained in transformer oil. © 2000 Scripta Technica, Electr Eng Jpn, 131(4): 19–28, 2000 [ABSTRACT FROM AUTHOR]

Abstract. A schlieren method was used to generate time-resolved images of the tunneling ionization front produced when an ultrashort high-power laser pulse irradiates He gas. By superimposing sequential schlieren images, we obtained information about the laser propagation and found that the ionization front propagated farther with decreasing density of the target gas. Ray-tracing suggested that this density dependence is a result of the spatial distribution of the laser intensity. [ABSTRACT FROM AUTHOR]

Examines the use of large-field Schlieren visualization systems to determine aerodynamic flow fields. Development of retroreflective versions for wind tunnel and convection flow; Examination of flow density variations; Identification of flow fields through a motion camera version of schlieren.

The propagation of a shock wave formed during the breakdown in the discharge chamber is studied experimentally and theoretically. The contactless optical schlieren visualization (shadow) method is used for the flow diagnostics. The dependences of spatiotemporal characteristics of shock wave propagation from the discharge chamber channel on the discharge energy are obtained experimentally. The exit velocity of the shock wave from the discharge channel and the energy spent and imparted to the shock wave are estimated analytically. [ABSTRACT FROM AUTHOR]

We propose a new class of Y-junctions based on wavefront processing to facilitate phase matching during branching. This is achieved by grading the lateral index profile of the waveguides. Simulations show that, at 85% power transmission, when compared to an all step-profile truncated-structural Y-branch (TSYB) that we proposed earlier, it widens the full branching angle by 1.5°. ©1999 John Wiley & Sons, Inc. Microwave Opt Technol Lett 22: 126–129, 1999. [ABSTRACT FROM AUTHOR]

This paper presents the spherically symmetric gradient-index axicon, a new kind of axicon that can be designed, using geometrical optics and conservation of energy in the geometric limit, to produce a variety of different intensity patterns along the axis. The performance of one particular axicon design is analysed using the scalar wave model. [ABSTRACT FROM AUTHOR]

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 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 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]

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]