Search

The paper introduces a multi-scale processing method for quantitative study and visualization of convective heat transfer using diffractive optical element based background-oriented schlieren technique. The method relies on robust estimation of phase encoded in the fringe pattern using windowed Fourier transform and subsequent multi-scale characterization of the obtained phase using continuous wavelet transform. As the phase is directly mapped to the refractive index fluctuations caused by the temperature gradients, the multi-scale inspection provides interesting insights about the underlying heat flow phenomenon. The performance of the proposed method is demonstrated for quantitative flow visualization. [ABSTRACT FROM AUTHOR]

Gas to liquid (GTL) fuel has gained attention recently because of its clean combustion behavior. Experimental studies have been performed to investigate fundamental combustion characteristics such as laminar burning speed and flame structure of GTL/air/diluent premixed flames. In the present study, the GTL fuel was designated by Syntroleum S-8, supplied by US Air Force Research Laboratory (AFRL), which was synthesized from natural gas using the Fisher–Tropsch (F–T) process. A mixture of 32% iso -octane, 25% n-decane and 43% n-dodecane by volume was used as a surrogate for GTL fuel. In this work, two diluent concentrations of 5% and 10% were used. The diluent is a blend of 86% N 2 and 14% CO 2 having the same specific heat as the burned gases. Experiments were conducted using a spherical vessel for laminar burning speeds measurement and a cylindrical vessel to investigate the flame structures. The cylindrical vessel was set up in a Z-shape Schlieren system coupled with a high-speed CMOS camera that was used to capture evolutionary behavior of flames at up to 40,000 frames per second. A multi-shell thermodynamic model was used to calculate the laminar burning speed for the smooth and low stretch flames. During the flame expansion, measured pressure rise as a function of time was the input into the thermodynamic model. Power law correlations for laminar burning speeds of GTL/air/diluent premixed flames over a wide range of temperatures (from 490 K to 610 K), pressures (from 0.5 atm to 3.2 atm), equivalence ratios (from 0.7 to 1.2), and two different diluent concentrations of 5% and 10% have been reported. Experimental burning speed results were compared with simulation values calculated by the solution of one dimensional steady premixed flame code from CANTERA using Ranzi’s chemical kinetics mechanisms. Results of simulations are close to the measured values. [ABSTRACT FROM AUTHOR]

Abstract In order to understand the combustion characteristic clearly in the scramjet combustor, the effect of equivalence ratio and injector position on flow structure and flame development in a hydrogen fueled scramjet combustor was investigated experimentally in the present paper, various measurements included schlieren, shadow, differential interferometry, flame emission and OH*-PLIF were made during the experiments in an attempt to better understand the combustion flow field. The results were obtained under the inflow condition of Ma2.0, total temperature and total pressure were 950 K and 0.82 MPa, respectively. The combustion induced backpressure had not spread into the isolator when the equivalence ratio was 0.1, the flow structure was stable and the flame was located in the cavity shear layer in a line shape. But the backpressure of the cases when the equivalence ratio was 0.3 had spread into the isolator, the flow structure was unstable and the flame existed both in the cavity and in the core flow in a shredded paper shape. The hydrogen injected from the upstream injector burnt more intensively than that injected from the downstream injector under the same equivalence ratio condition. The above mentioned non-intrusive measuring methods had their own limitations, but the synchronous measurement method could well show a more comprehensive coupling information of the flow structure and combustion characteristics. [ABSTRACT FROM AUTHOR]

The oscillation characteristics of the shock train in an isolator have been investigated in a direct-connect wind tunnel at Mach 2.7. High-speed schlieren imaging and high-resonance frequency pressure measurements were used to capture the flow features during the shock train movement. The oscillation features without the effects of incident shocks were analyzed first. As the shock train moved upstream, the low-frequency part of the oscillation was found to develop. The analysis was then extended to complex situations with incident shocks. It was revealed that the shock wave-boundary layer interactions considerably influence the shock train behavior. The interactions were classified into three patterns: (I) single interaction, (II) multi-interactions on the same side, and (III) multi-interactions on different sides. Experimental results indicated that the oscillation could be affected in temporal scale by pattern II and enhanced in spatial scale by pattern III. The data also showed that the pressure rise induced upstream propagates to the exit, causing phase offsets in the wall pressure histories and making the pressure distributions diverge from their stable state. This phenomenon suggested a possible physical mechanism for the oscillation during shock train movement, which was verified by additional tests with large backpressure rising rate. It was found that there exists a critical frequency which is related to the pressure ramping rate during the oscillation. If the dominant frequency of the backpressure varies beyond this critical frequency, the pressure distribution could be forced into a steady state before the oscillation was induced. Otherwise the oscillation could not be suppressed. [ABSTRACT FROM AUTHOR]

10.1119/1.5042245 Using schlieren optics as a tool to see the invisible, we describe a technique of visualizing traveling ultrasonic (28 kHz) sound waves in real time. Suitable for lecture demonstration purposes or as an instructional laboratory experiment, our setup can readily demonstrate the reflection of sound waves from surfaces, diffraction effects around objects, interference, and standing waves. Additionally, the incorporation of color filters provides information such as gradient directions and sound wave phase differences not obtainable with just a white light source. As an example, acoustic standing waves are analyzed. [ABSTRACT FROM AUTHOR]

In flow analysis, such as flow injection analysis, liquid lens is formed at the boundary between two adjacent liquid media which have different refractive indices. Light refraction at the liquid interface gives the so-called ‘schlieren signal’. Schlieren effect is both concentration-dependent and temperature-dependent. In this work, the schlieren signal from temperature difference was quantitatively investigated for application in enthalpimetric measurement. The schlieren phenomena was then exploited for chemical analysis. A thermal insulated single flow line manifold was constructed using deionized water at 23 °C as the carrier. Deionized water at various temperatures in the range of 5–85 °C was injected into the carrier flow. A correlation between the schlieren signal and sample temperature was observed. A heat exchanger unit (HEU), consisting of a small volume glass-reaction chamber with a surrounding water jacket, was constructed. The unit was thermally insulated in a double layer cylindrical PVC unit. For demonstrating the applicability of temperature-dependent schlieren effect in chemical analysis, the exothermic oxidation reaction between acid dichromate and ethanol or ascorbic acid was employed with heat transferring to the surrounding water layer. When an aliquot of water from the HEU is injected into the constant temperature flow line the observed schlieren signal was dependent on the analyte concentration. Linear calibration (r 2 > 0.99) were obtained covering the concentration range of ethanol and ascorbic acid as found in samples. The developed flow system provides good precision (RSD < 5%) with sample throughput of 4 sample h −1 . The system were applied to the determination of ethanol in Thai white spirit and ascorbic acid in vitamin C tablets, respectively. The quantitative results obtained from the schlieren method were in agreement with the comparative methods. [ABSTRACT FROM AUTHOR]

The Schlieren method has been used before to visualize weak shock waves radiated from the open ends of brass instruments, but no attempt has previously been undertaken, however, to measure the geometry of the radiated wavefronts using the Schlieren images. In this paper Schlieren visualization is used to estimate the geometry of the two-dimensional shock wavefronts radiated from the bell of a trumpet at different frequencies. It is observed that the geometry of the shocks does change with frequency, in the expected manner. The propagation speeds of these shocks are also calculated, and they too exhibit the anticipated behavior. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, flow visualization of the oblique shock waves of a single‐crystal silicon microscale supersonic wind tunnel is described. The microscale supersonic wind tunnel has a convergent–divergent section and a throat area of 1000 μm × 300 μm, and designed for realizing supersonic flow of Mach 2 in microscale domain. The schlieren system with integrated optical microscope was used for visualization of the oblique shock waves generated in the microscale domain. To investigate the effect of nonisentropic flow state on the inclination angle to the shock, a numerical integration of the modified Shapiro's differential equation was applied. [ABSTRACT FROM AUTHOR]

Focusing Schlieren technique is used to measure the span-wise length of the bifurcated normal shock in a rectangular duct. This is done by inclining the Schlieren setup by a small angle to the regular schlieren light path. The projection of the normal plane of the shock can be seen in the images, and can be used to deduce the spanwise length of the shock. The measurement is further verified by traversing the focusing schlieren setup to mark the physical distance between the ends of the normal part of the shock. This is the first reported work of measurement of spanwise length of a normal shock in a duct, and its temporal variation. [ABSTRACT FROM AUTHOR]

Highlights • Jet noise of a synthetic jet actuator is isolated from its diaphragm noise to assess aeroacoustic characteristics. • Audible whistling from the actuator occurs within a similar Strouhal number range as other pipe and orifice flow systems. • A threshold in the jet Reynolds number is established for the onset of flow-induced sound. • Good agreement between the acoustic spectra, velocity spectra and Schlieren visualisation of the synthetic jet. Abstract In this paper, the aeroacoustic characteristics of a circular orifice, synthetic jet actuator in quiescent conditions is investigated. Electromagnetic actuation, in the form of a shaker-driven actuator with latex diaphragm, proved to be desirable over piezoelectric actuation for this work due to the reduced diaphragm noise contribution to overall actuator self-noise, hence making it easier to identify jet-related noise. Acoustic and velocity data, collected from microphone measurements in an anechoic chamber and hotwire measurements respectively, were compared for correlation. Schlieren visualization was also used to show synthetic jet development near the orifice. Flow-induced sound in the form of an audible whistling was found to occur for a Strouhal number range of 0.24< St <0.50. A threshold in the jet Reynolds number of 600 < Rej<750 was established for the onset of whistling from the actuator for all drive voltages. Coherence between the acoustic spectra and velocity power spectra is shown, with evidence of a feedback mechanism consisting of vortices shed at a frequency that coincides with acoustic modes of the actuator. [ABSTRACT FROM AUTHOR]

Ar-circulated hydrogen engine replaces atmospheric nitrogen with Ar so as to increase engine efficiency and avoid NOx forming. Due to the differences in gas molecular weight of injection gas and ambient gas, two injection strategies were proposed for this engine. This study aims at measuring the characteristics of non-reactive H 2 and O 2 jets in Ar atmosphere by Schlieren method. The experimental results demonstrate that the increase of injection pressure or the decrease of ambient pressure leads to an increase of penetration. Jet penetration length of O 2 is slightly lower than H 2 by 0–6%. The dispersion angle of gas jet increases slightly with the increase of injection pressure and ambient pressure. Under all the experimental pressure boundary conditions, the dispersion angles of H 2 jets are between 31° and 36° and the dispersion angles of O 2 jets are between 22° and 28°. The entrainment rate of gas jet increases with the increase of injection pressure, ambient pressure and injection time. Jet entrainment rate of O 2 is 2.43–2.98 times that of H 2 jet because of the lower volume flow rate of O 2 jet. H 2 jet entrains more total argon volume for its higher penetration and dispersion angle. The actual engine prefers to use the hydrogen injection strategy. [ABSTRACT FROM AUTHOR]

We present a high-speed single-mirror double-pass coincident schlieren system and corresponding algorithms for the visualization of acoustic waves and recovery of their associated audio signals. Schlieren systems are extensively used to visualize strong shockwaves, such as those from supersonic motion or explosions. Recently, they have also been used to visualize lower amplitude non-linear acoustic phenomena, such as the weak shockwaves arising from impact events including hand claps, belt snaps, and towel cracks. Time-invariant sounds produced by loudspeakers have also been imaged, in one case leading to frequency analysis, although these have been limited to high-frequency signals at very high sound pressure levels. The research presented here shifts the focus from sound-field visualization towards audio signal recovery. A comprehensive exploration of several parameters for imaging sound sources, including frequency, wave form, and amplitude, is presented. In addition, we address for the first time the recovery of phase information, which would be essential for speech intelligibility, and the more general case of non-contact sound field reconstruction. Through image and signal processing, it is shown that audio signals can be recovered from high-speed schlieren video whose acoustic waves appear to be below the limit of visibility, and were previously deemed unrecoverable by virtue of their frequency and sound pressure level. This includes sounds at frequencies and loudnesses relevant for human hearing, producing the first ‘schlieren microphone’. [ABSTRACT FROM AUTHOR]

This paper presents a review and an analysis of the development of speckle photography from the first experiments performed at the National Physical Laboratory of Great Britain up to the present time. The possibility of diagnosing turbulent and three-dimensional flows by the speckle photography methods has been shown. The single-exposure scheme of speckle photography has been analyzed and the possibility of using multiaspect speckle photography in reconstructive tomography of complex three-dimensional flows has been shown. Methods based on speckle technologies such as Talbot interferometry, PIV and BOS techniques have been analyzed. Particular consideration is given to the application of new measuring technologies in diagnosing turbulent flows and reconstructive tomography of three-dimensional flows. [ABSTRACT FROM AUTHOR]

As in most of industrial cases, in Spark-Ignition (SI) engines, the expansion of premixed flames is strongly affected by turbulence flow fields. However, as the flame radius and curvature are non-negligible during the combustion development, global stretch effect can drastically change the turbulent flame propagation speed. In this paper, spherical turbulent premixed flames are studied inside a constant-volume vessel for a wide range of air-isooctane mixture and initial turbulence by using simultaneously Mie scattering tomography and 2-views Schlieren techniques. With the 2-views Schlieren diagnostic, the flame volume can be reconstructed and a more ‘real’ flame radius can be determined. The comparison between this radius, which is more usually determined by only 1-view Schlieren, and the one obtained by tomography was possible, and enabled to select non-conveyed flames with a global spherical shape. In the first part, the values of the correction factor for Schlieren images, first introduced by Bradley et al. in 2003, is discussed as function of the turbulent intensity and initial pressure. In the second part, the effect of turbulent intensity and initial pressure for isooctane/air mixtures on flame propagation speed evolution is evaluated as function of the stretch rate. [ABSTRACT FROM AUTHOR]

The authors propose a new method for evaluating the contact area of a surface discharge on water by analyzing the resistance of water solution. This method supplements optical observations of light emissions and the Schlieren method. The contact area was modeled as a disk electrode, and the theoretical resistance of water was calculated under the assumption of a mutual similarity between the current and electrostatic fields. A simulated electrostatic field was calculated using a common charge-simulation method. The experimental values of the resistance of water were obtained by dividing the measured voltage drop at water by the measured current. The contact area evaluation was performed by comparing the experimental and theoretical resistance values. The contact area increases with increasing the applied voltage and is practically independent of water depth. [ABSTRACT FROM AUTHOR]

A numerical study is performed on the supersonic flow over an open cavity at Mach number of 1.5. A newly developed visualization method is employed to visualize the complicated flow structures, which provide an insight into major flow physics. Four types of shock/compressive waves which existed in experimental schlieren are observed in numerical visualization results. Furthermore, other flow structures such as multi-scale vortices are also obtained in the numerical results. And a new type of shocklet which is beneath large vortices is found. The shocklet beneath the vortex originates from leading edge, then, is strengthened by successive interactions between feedback compressive waves and its attached vortex. Finally, it collides against the trailing surface and generates a large number of feedback compressive waves and intensive pressure fluctuations. It is suggested that the shocklets beneath vortex play an important role of cavity self-sustained oscillation. [ABSTRACT FROM AUTHOR]

The arc discharge plasma actuator (ADPA) has wide application prospects in high‐speed flow control because of its local heating effect and strong disturbance. In this paper, the influence of ambient pressure, which ranges from 3 to 20 kPa, on the performance of a two‐electrode ADPA is investigated by a schlieren system. The duration of the arc heated region, as well as its area, is extracted by image processing. As the ambient pressure increases, different flow field evolutions occur. The duration of the ADPA heated region increases with the ambient pressure. The maximum duration reaches 1185.3 µs at 20 kPa. The velocity of the discharge‐induced blast shock wave first decreases gradually and then remains at 345 m/s for all air pressures. The blast shock wave has a higher velocity at lower pressures when it is freshly produced. A maximum blast shock wave velocity of 582 m/s is observed at the pressure of 7 kPa. The arc heated region is not sensitive to ambient pressure, but the deposited energy from the arc increases when the pressure increases. [ABSTRACT FROM AUTHOR]

Experimental investigation of various heat transfer mechanisms associated with isolated nucleate pool boiling have been presented. Measurements have been made in a complete non-intrusive manner using rainbow schlieren deflectometry technique. Boiling experiments have been performed for two levels of superheat with the bulk fluid maintained under saturated conditions. The rainbow schlieren images have first been subjected to qualitative interpretation wherein various sub-processes associated with the boiling phenomenon, such as development of thermal boundary layer on the substrate surface, inception of single bubble, growth of the vapor bubble till it departs, and scavenging of the superheat layer following the bubble departure have been discussed. Contributions of individual sub-processes towards the overall heat transfer rates achieved for a given superheat level have been determined through quantitative analysis of the images. Schlieren observations revealed the effect of varying superheat levels on parameters such as bubble diameter and departure time. Detailed heat transfer analysis revealed the dominance of evaporative heating in contributing towards the overall heat transfer rates. On the other hand, the contribution of natural convection from the heated substrate was found to be relatively small. In quantitative terms, the evaporative heating was seen to have an individual contribution as high as ≈66% to the overall heat transfer and ≈88% to the growth of the vapor bubble in the case of superheat level of 7 °C. [ABSTRACT FROM AUTHOR]

With the increase in the diesel injection pressure in modern diesel engines, the velocity of the diesel spray increases accordingly, and shock waves are induced when the spray is supersonic. To better understand the effect of shock waves on the spray development, the evolution of the high-pressure diesel spray and shock wave under different diesel injection pressures and ambient pressure has been investigated using the Schlieren imaging method. The results show that the shock wave affects the development of the spray and the mixing effect between the spray and surrounding air. The shock wave promotes development of the spray tip penetration and enhances the gas entrainment effect. However, the shock wave has an inhibitory effect on the spray development in the radial direction. In addition, the shock wave angle determines the range of influence on the ambient air, which affects atomization of the spray. The shock wave angle increases with the decrease in the spray front Mach number. An empirical correlation of the shock wave angle and spray front Mach number was proposed in this paper. The calculated results are in good agreement with the experimental data. In addition, a diagram of the shock wave generation and classification in a diesel engine is proposed. [ABSTRACT FROM AUTHOR]

Background Oriented Schlieren method was used to obtain diffusion coefficient of glucose in hydrogel in the experiment with radial symmetry. [ABSTRACT FROM AUTHOR]

High-pressure diesel-fuel sprays have been shown to emit shock wave under certain conditions, while the advanced injection strategy used in internal-combustion engines involve multiple injections taking place within a short time frame. Experimental study of double injection has shown in two instances that the first fuel-spray cloud could be accelerated before the arrival of the second fuel spray. Herein, it is hypothesized that a shock wave emitted from an injection interacts with the fuel-spray cloud of a preceding injection and is responsible of a push-away mechanism on the first droplet cloud reported on in the literature. In this context, the shock waves emitted by fuel-spray jets with a commercial 5-hole diesel injector injecting into a pressure vessel were characterized with schlieren visualizations and dynamic pressure measurements taken with single- and double-injection strategies. The experimental results confirm the shock presence based on schlieren measurements. The measured shock conditions show a different shock topology from most shock-tube experiments as the expansion wave closely followed the shock front, resulting in a thin shocked region and a short duration of the droplet exposition to the post-shock gas conditions. Experimental measurements were then used as initial conditions in a 1D multiphase simulation model allowing simulation of the shock-wave interaction with the droplet cloud under engine-related conditions. The model was used to conduct a parametric study on the droplet-cloud characteristics and showed that, as the cloud density increased, the shock intensity and droplet-induced velocity decreased. Finally, the model was used to illustrate that the push-away interaction mechanism could be explained by the shock wave–fuel-droplet-cloud interaction. [ABSTRACT FROM AUTHOR]

The effect of C 2 H 4 and C 2 H 6 addition on the ignition of a stoichiometric hydrogen:oxygen mixture was experimentally analyzed using a shock tube with a discharge cell. Ignition delay time was measured behind a reflected shock wave after a high-voltage nanosecond discharge and in its absence using mixtures with small percentages of hydrocarbons. The obtained results were compared with ignition delay times in a mixture without hydrocarbon addition. It was shown that C 2 H 4 and C 2 H 6 inhibit the hydrogen–oxygen reaction and that the inhibition effect is much more profound when the mixtures are ignited after a high-voltage nanosecond discharge. A numerical simulation of the discharge and ignition phases was used to show the main mechanisms that control the inhibition of hydrogen:oxygen ignition by hydrocarbons. The densities of atoms, radicals, excited particles and charged particles produced in the discharge plasma were calculated and used as input parameters for ignition modeling. Reasonable agreement was obtained between the calculated and measured plasma-assisted ignition delay times in the mixtures under consideration. The analysis of primary reaction pathways for hydrocarbon species was used to demonstrate the inhibition mechanisms of ignition in hydrogen:oxygen mixtures excited by non-equilibrium discharge plasma. [ABSTRACT FROM AUTHOR]

The effect of cowl angle on the restart characteristics of simple ramp type hypersonic inlets was experimentally investigated in shock tunnel equipped with schlieren imagery and static pressure measurement. The cowl shock strength is found to be a key factor that determines the inlet restart and makes the restart contraction ratios significantly deviate from the Kantrowitz criterion. Stronger cowl shock tends to degrade the inlet restart capability by causing larger separation bubble and higher pressure loss during the restarting process. In particular, a sensitive range of the cowl angles, within which the restart contraction ratio decreases rapidly, was identified. A design concept of multiple noncoalesced cowl shocks was thus proposed and proven to significantly improve the inlet restart capability. [ABSTRACT FROM AUTHOR]

The initial stage of a nanosecond discharge in gaps with a high electric field at a cathode is studied by laser methods (interferometric, shadow, schlieren methods). The studies are performed in air at atmospheric pressure. Prominence is given to studying the evolution (appearance and growth) of the plasma channels at an anode and to estimating their parameters. [ABSTRACT FROM AUTHOR]

A green, simple, accurate and highly sensitive sequential injection lab-at-valve procedure has been developed for the simultaneous determination of ascorbic acid (Asc) and rutin using 18-molybdo-2-phosphate Wells-Dawson heteropoly anion (18-MPA). The method is based on the dependence of the reaction rate between 18-MPA and reducing agents on the solution pH. Only Asc is capable of interacting with 18-MPA at pH 4.7, while at pH 7.4 the reaction with both Asc and rutin proceeds simultaneously. In order to improve the precision and sensitivity of the analysis, to minimize reagent consumption and to remove the Schlieren effect, the manifold for the sequential injection analysis was supplemented with external reaction chamber, and the reaction mixture was segmented. By the reduction of 18-MPA with reducing agents one- and two-electron heteropoly blues are formed. The fraction of one-electron heteropoly blue increases at low concentrations of the reducer. Measurement of the absorbance at a wavelength corresponding to the isobestic point allows strictly linear calibration graphs to be obtained. The calibration curves were linear in the concentration ranges of 0.3–24 mg L −1 and 0.2–14 mg L −1 with detection limits of 0.13 mg L −1 and 0.09 mg L −1 for rutin and Asc, respectively. The determination of rutin was possible in the presence of up to a 20-fold molar excess of Asc. The method was applied to the determination of Asc and rutin in ascorutin tablets with acceptable accuracy and precision (1–2%). [ABSTRACT FROM AUTHOR]

By the methods of schlieren visualization, the evolution of elements of the fine structure of transverse vortex loops formed in the circular vortex behind the edge of a disk rotating in a continuously stratified fluid is traced for the first time. An inhomogeneous distribution of the density of a table-salt solution in a basin was formed by the continuous-squeezing method. The development of periodic perturbations at the outer boundary of the circular vortex and their transformation at the vortex-loop vertex are traced. A slow change in the angular size of the structural elements in the supercritical-flow mode is noted. [ABSTRACT FROM AUTHOR]

Three visualization methods, Schlieren, Shadowgraph, and Mie-scattering, were applied to compare diesel and gasoline spray structures in a constant volume chamber. Fuels were injected into a high pressure/high temperature chamber under the same in-cylinder pressure and temperature conditions of low load in a GDCI (gasoline direct injection compression ignition) engine. Two injection pressures (40 MPa and 80 MPa), two ambient pressures (4.2 MPa and 1.7 MPa), and two ambient temperatures (908 K and 677 K) were use. The images from the different methods were overlapped to show liquid and vapor phases more clearly. Vapor developments of the two fuels were similar; however, different liquid developments were seen. At the same injection pressure and ambient temperature, gasoline liquid propagated more quickly and disappeared more rapidly than diesel liquid phase. At the low ambient temperature and pressure condition, gasoline and diesel sprays with higher injection pressures showed longer liquid lengths due to higher spray momentum. At the higher ambient temperature condition, the gasoline liquid length was shorter for the higher injection pressure. Higher volatility of gasoline is the main reason for this shorter liquid length under higher injection pressure and higher ambient temperature conditions. For a design of GDCI engine, it is necessary to understand the higher volatility of gasoline. [ABSTRACT FROM AUTHOR]

The aim of the present study is to validate an experimental method for the measurement of in-fluid enthalpy variation by using speckle-based background-oriented schlieren (SBOS). The method is applied to solving the millijoule thermal-energy transfer to fluid induced by an electric discharge in the air. An intensified camera is used to match the electric discharge exposure time constraint. Speckle is used as the reference pattern, as it allows easier access to high-density patterns for subcentimetric imaging. Geometric optic principles are used to extract the line-of-sight integrated index of refraction. The local density field is then computed using the Gladstone-Dale equation through filtered back projection. The method is first validated in close experimental conditions provided by the density measurement of a millimeter-dimension CO 2 gaseous laminar jet. An analysis of the jet provides useful insights into the noise sources and uncertainties in the method, which is then transposed to the discharge. The results of enthalpy measurements are discussed. [ABSTRACT FROM AUTHOR]

In this work, we employ a combination of high-speed imaging and schlieren imaging, as well as multiphysics modelling, to elucidate the effects of the interaction between the laser beam and the powder bed. The formation of denuded areas where the powder was removed during single line and island scans over several layers were imaged for the first time. The inclination of the laser plume was shifted from forwards to backwards by changing power and scan speed, resulting in different denudation regimes with implications to the heat, mass and momentum transfer of the process. As the build progressed, denudation became less severe than for a single powder layer, but the occurrence of sintered and fused powder agglomerates, which were affected by the plume, increased. Schlieren imaging enabled the visualisation of the Ar gas flow, which takes place in the atmosphere above the bed due to the plume, in addition to its interaction with affected particles. Numerical modelling was used to understand and quantify the observed flow behaviour, through the hydrodynamic treatment of the laser plume as a multi-component Ar-Fe plasma. These results promote the characterisation of fluid dynamic phenomena during the laser powder-bed fusion (LPBF) process, which constitutes a key factor in the prevention of defects in additively manufactured parts. [ABSTRACT FROM AUTHOR]

Using light alcohols in spark-ignition engines can improve energy security, engine performance and pollutant emissions. Methanol has gained popularity due to its ease in production compared to ethanol. Methanol could absorb water easily. In the present work, the adiabatic laminar burning velocity of methanol containing water is investigated both experimentally and numerically. Numerical simulations using CHEMKIN-PRO were undertaken to predict the burning velocities of six mixtures with different water volume fractions (up to 0.6) from the latest San Diego chemical-kinetic mechanism. The burning velocities of three mixtures with different water volume fractions (up to 0.4) were measured using a constant volume vessel and a Schlieren imaging system for a wide range of temperature (380–450 K), pressure (100–400 kPa) and equivalence ratio (0.7–1.4). Results showed a decrease in burning velocity with pressure and an increase with temperature. Water as a diluent led to reduction of the burning velocity. The chemical-kinetic mechanism over predicts the burning velocity. [ABSTRACT FROM AUTHOR]

An algorithm for compensating image distortions under the influence of the surface deformation of the background screen when using the shadow background method. The efficiency of the algorithm is confirmed. We experimentally determined the optimal marker to be used for searching on the image. We examined the capabilities of the algorithm in compensating for the shift and rotation of the surface of the background screen with an mean square deviation of not more than 0.43 pixels, determined by cross-correlation processing. [ABSTRACT FROM AUTHOR]

Background oriented schlieren method is applied in diagnostics of shock waves in air. The method can be used for visualization of shock waves that are generated after explosion or due to motion at ultrasonic speeds. Experimental data make it possible to observe propagation of a shock wave in space, estimate the asymmetry of energy liberation in explosion, and determine parameters of shock wave. [ABSTRACT FROM AUTHOR]

A novel structured light-field focusing system is developed for flowfield measurements and visualization. The imaging technique provides true planar, two-dimensional, refractive measurements of flow structures. The innovative method is based on the light-field principle and multiple light sources to structure the two-dimensional planar focused imaging. The system is further unique in that it provides velocity fields from particles at extreme sampling rates (>15 kHz) driven by the forward scatter relative to traditional particle image velocimetry (PIV) systems. The arrangement and characteristics of the system are presented. The method has also been used to visualize an under-expanded free jet and impinging jet. The technique is demonstrated on various flows including spray imaging and velocimetry at high acquisition rates to characterize the performance of the diagnostic system. Additionally, the limits of the system have been demonstrated as a method for micro-scale visualizations and dense medium imaging. The study developed a formulation for using the optical diagnostic technique. The optical system parameters can be tailored to drive a shallow depth of field. Detail optimization of the diagnostic technique to achieve shallow range depth of field at a high signal to noise ratios is discussed. Finally, design considerations are proposed which will be pertinent in future, large-scale experiments. [ABSTRACT FROM AUTHOR]

This paper describes the measurement results for the shock wave propagation during an explosion of an explosive material with plastic filler. The detection is carried out with the help of high-speed analog and digital video cameras. The air shock wave is visualized by means of shadow photography in transmitted light in the zone of separation from expanding explosion products and using a background oriented schlieren method in the far zone. The optical measurements of the air shock wave propagation are used to estimate a maximum pressure in the compression zone at different sections and compared with the data obtained by the pressure gauges. The calculated pressure peaks are in good agreement with the measured ones. The background oriented schlieren method proves to be a useful tool easily introduced in conventional large-scale polygon experiments. [ABSTRACT FROM AUTHOR]

Time-resolved schlieren measurements were conducted on a supersonic rectangular multi-stream nozzle used for three-stream engines. The jet utilizes a single expansion ramp nozzle (SERN) configuration along with an bypass stream, operating at an ideally expanded bulk flow of M j , 1 = 1.6 and bypass stream of M j , 3 = 1.0 . Experiments were conducted with and without an aft deck to model airframe integration. Schlieren imaging was acquired up to 100 kHz in two different orientations. Proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) were applied to the schlieren data to identify flow structures. Comparisons were made between the two decomposition methods in the form of spatial correlations. [ABSTRACT FROM AUTHOR]

High gas injection pressure leads to the formation of under-expanded gas jet at nozzle exit. The shock wave structure near the nozzle affects gas jet injection and mixing characteristics, which affects the combustion and emission performance of the engine. Gas jet injection process and shock wave structure evolution law under different nozzle pressure ratio (NPR) and orifice diameter have been investigated with the application of Schlieren imaging and numerical simulation. Results show that long injection duration exists in gas injection process after the end of injection. It is attributed to the slow pressure reduction velocity in the nozzle. This phenomenon is one of the key factors which influence the high HC emission of gas fuel port injection natural gas engines. The change of pressure inside the nozzle also affects the shock wave structure (Mach disk) near the nozzle. It is found that the increase of Mach disk width leads to a large jet near-field angle which promotes the spatial distribution and turbulent mixing of gas jet. In addition to experimental results, numerical simulation is used to analyze the injection characteristics of gas injection system. Increasing gas nozzle flow area and shortening gas nozzle length are regarded as effective ways to improve the responsiveness of gas injection system and reduce gas injection duration. The range of Reynolds number at exit of the nozzle is 0.5–2.5 × 10 5 under various gas injection pressures and orifice diameters. This presents that gas jet is easily influenced by air turbulence in intake manifold. Strong air turbulence in intake manifold is needed in a gas fuel port injection natural gas engine in order to obtain homogeneous gas/air mixture. [ABSTRACT FROM AUTHOR]

The unsteadiness of ramp-induced shock wave/boundary layer interaction under the disturbance of an array of plasma jets was experimentally investigated using high-speed Schlieren imaging. The plasma jet was created by arc discharge in a confined small cavity, with a volume of 50 mm 3 . The boundary layer thickness at the position where the upstream array of the actuators locates is δ =  2.11 mm. Two jet orifices with a diameter of D  = 1 mm, 2 mm ( δ / D =  2.11, 1.05) are designed. The multichannel discharge circuit was used to extend the spanwise disturbance region of the jets. The size of separation zone was reduced when interacting with the plasma jet plume. Meawhile, an upstream motion of the separation shock and reattachment shock is observed during the recompression process caused by the obstruction and accumulation of the jets. [ABSTRACT FROM AUTHOR]

The far-field distribution of lasers is an important parameter for measuring beam quality. To overcome the insufficiencies of the CCD Camera dynamic range when measuring the far-field focal spot, the schlieren method was used to measure the focal spot far-field spatial distribution. Focal spot reconstruction was achieved after gray image matching, calculating the center, and image merging of the main lobe and side lobe. In this study, we used an algorithm based on the optimal arc to obtain the center for the side lobe images by fitting the circle center to improve the accuracy of the focal spot reconstruction. The results showed that the schlieren method of measuring the focal spot reconstruction algorithm could effectively, accurately and completely obtain the far-field focal spot. [ABSTRACT FROM AUTHOR]

The optical texture of the nematic phase, variously known as theschlieren, structure à noyuaxornucleated domain texture, was identified over a century ago as being an array of point singularities. When viewed between crossed polars, patterns of dark brushes radiate from each point nucleus. Thesignandstrengthof each nucleus can be uniquely determined from the changes in the orientation of these brushes when either the sample or the crossed polars are rotated, from two formulae given by Chadrasekhar in 1977. However, these were given with little exemplification and have been largely overlooked. Consequently, the majority of the discussions given in current literature are either incomplete and confusing or, in some cases, incorrect. Here, we provide a detailed explanation of the textures and their behaviour as viewed with the most commonly used experimental geometry (i.e. with a rotating sample and stationary polars). [ABSTRACT FROM AUTHOR]

A three-dimensional transient mathematical model of the “inductively coupled plasma – vacuum interface of the mass spectrometer” system was developed. The model takes into account spatial and temporal dynamics of hot gas flow (plasma emulation) and allows calculation of evolution of spatial distribution of pressure, velocity and temperature fields outside and within the interface of the mass-spectrometer. The results of modeling are verified using the experimental setup of high-speed optical and schlieren visualization of gas flows. [ABSTRACT FROM AUTHOR]

Time-resolved schlieren measurements were conducted on a supersonic rectangular multi-stream nozzle used for three-stream engines. The jet utilizes a single expansion ramp nozzle (SERN) configuration along with an bypass stream, operating at an ideally expanded bulk flow of M j , 1 = 1.6 and bypass stream of M j , 3 = 1.0 . Experiments were conducted with and without an aft deck to model airframe integration. Schlieren imaging was acquired up to 100 kHz in two different orientations. Proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) were applied to the schlieren data to identify flow structures. Comparisons were made between the two decomposition methods in the form of spatial correlations. [ABSTRACT FROM AUTHOR]

The Richtmyer-Meshkov instability of an unperturbed air/SF6 interface subjected to a diffracted shock is experimentally studied by high-speed schlieren photography under cylindrical circumstances. The cylindrically converging diffracted shock (CCDS) is produced by a cylindrically uniform shock diffracting around a rigid cylinder(s), and the unperturbed interface is created by a soap film technique. The effects of coupling of multiple rigid cylinders and diverse spacings from the cylinder to interface on a flow field are highlighted. Schlieren images indicate that the amplitude of disturbances on the CCDS increases compared with the local shock radius. After the CCDS impact, a bulge is derived from the interface due to the shock-shock interaction inside the interface, and the number of bulges depends upon the number of cylinders. As the number of cylinders increases, the bulge becomes less pronounced, which is ascribed to additional shock-shock interactions inside the volume. As the distance between the cylinder and interface increases, an air cavity is first observed before the formation of a bulge. The amplitude of perturbation on the interface is found to reduce before the central reflected shock arrival because of the Rayleigh-Taylor stabilization effect. Through equating the preinterface disturbance of the CCDS to the pre-shock perturbation of the perturbed interface, the initially linear growth rate is theoretically computed based on the impulsive model considering the Bell-Plesset effect. The theoretical results are found to deviate greatly from the experimental counterparts. Instead, taking the post-shock interface amplitude as an initial interface amplitude, the model works well. Therefore, the interface perturbations produced are notably smaller than the disturbances causing them. Moreover, the nonlinear behavior of perturbation growth is estimated by the model considering the Rayleigh-Taylor effect. [ABSTRACT FROM AUTHOR]

In this paper, prebreakdown streamers in transformer oil and transformer oil-based TiO2 nanofluid were observed by the schlieren method under positive lightning impulse voltage. Streamers in the nanofluid have numerous branches with much shorter length, whereas in the pure oil, they exhibit only certain filaments with longer length. The discrepancy in positive streamer pattern of both oils is further investigated under a range of applied voltages. The complexity of streamer patterns was quantitatively described using the fractal analysis method. Results indicate that the fractal dimension of streamer patterns shows different changing tendencies in both oils, and it keeps higher value in the nanofluid than that in pure oil during the whole propagation process, well corresponding with streamer propagating structures. Moreover, a new parameter, the ratio of fractal dimension to propagation length (D/L), is introduced to classify the complex streamer patterns for the first time. Three propagation zones in both nanofluid and pure oil are clearly categorized by the value of D/L, providing a quantitative way to distinguish the streamer patterns. [ABSTRACT FROM PUBLISHER]

This paper introduces a numerical method of reconstructing three-dimensional (3D) ultrasound fields from their two-dimensional back-projections to understand the intensity distribution of nonplanar waves. The horizontal planes for line-focused ultrasound fields were roughly elliptical, whereas those for point-focused ultrasound fields were nearly circular. Experimental and simulated results indicated that the ultrasound intensity at the central axis can be calibrated from the light intensity by 3D reconstruction of the ultrasound field. The difference between the measured ultrasound intensity for nonplanar waves in schlieren imaging and the simulation results were discussed. From this work, the 3D ultrasound field becomes possible to be reconstructed. [ABSTRACT FROM AUTHOR]