Search

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]

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]

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]

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]

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]

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]

Nonstationary characteristics of detached flow have been experimentally studied during interaction of the boundary layer with a shock wave that appears on a profiled bump in transonic flow. The experiments were performed with variable shock-wave intensity and position in a T-325 wind tunnel. The flow was studied using methods of schlieren imaging, measuring average pressure and its pulsations on the surface of a model, and determining velocity fields by particle image velocimetry. Analysis of the experimental data showed that the observed shock-wave oscillations and flow pulsations in the detachment zone were related to disturbances present in the oncoming boundary layer. [ABSTRACT FROM AUTHOR]

Background-oriented schlieren is a method of visualizing refractive disturbances by comparing digital images with and without a refractive disturbance distorting a background pattern. Traditionally, backgrounds consist of random distributions of high-contrast color transitions or speckle patterns. To image a refractive disturbance, a digital image correlation algorithm is used to identify the location and magnitude of apparent pixel shifts in the background pattern between the two images. Here, a novel method of using color gradient backgrounds is explored as an alternative that eliminates the need to perform a complex image correlation between the digital images. A simple image subtraction can be used instead to identify the location, magnitude, and direction of the image distortions. Gradient backgrounds are demonstrated to provide quantitative data only limited by the camera's pixel resolution, whereas speckle backgrounds limit resolution to the size of the random pattern features and image correlation window size. Quantitative measurement of density in a thermal boundary layer is presented. Two-dimensional gradient backgrounds using multiple colors are demonstrated to allow measurement of two-dimensional refractions. A computer screen is used as the background, which allows for rapid modification of the gradient to tune sensitivity for a particular application. [ABSTRACT FROM AUTHOR]

We examine the dynamics of a high-speed shock-induced flow near the open end of a shock tube using the particle image velocimetry (PIV) and the background oriented schlieren (BOS) methods along with two- and three-dimensional numerical simulations. In experiments, planar shock waves ( $$M=1.3$$ -1.6) are discharged from a rectangular ( $$24\,\hbox {mm} \times 48\,\hbox {mm}$$ ) low-pressure section of a shock tube open to the atmosphere. Due to the rectangular exit geometry, the resulting flow is highly three-dimensional and, thus, more complicated, compared to well-studied circular/axisymmetric geometries. The study focuses on the spatio-temporal flow structure up to 1 ms after the shock wave diffraction. PIV and BOS visualization techniques share the same post-processing principle, and the iterative multi-step cross-correlation algorithm applied in the PIV software is adapted here for the calculation of background pattern displacement on the BOS images. Particular attention is given to the resolution of flow regions where sharp gradients are present, such as a diffracted shock front or embedded shocks. Computational fluid dynamic simulations of the problem are also conducted to validate the experimental results and methods and to gain more insight into the three-dimensional flow dynamics. PIV and BOS images are found to be consistent with the corresponding numerical flow visualizations. [ABSTRACT FROM AUTHOR]

We present an improvement to the standard synthetic schlieren technique to obtain the temperature distribution of a fluid inside of a Hele-Shaw cell. We aim to use the total variation $$L^1$$ -norm optical flow method to treat experimental images and to obtain quantitative results of the development of thermal convection inside a cell, by detecting the gradients of the optical refractive index. We present a simple algorithm to set the optical flow parameters, which is based on the comparison between the optical flow output and the result obtained by digital PIV using the structural index metric. As an example of the application of the proposed method, we analyze laboratory experiments of thermal convection in porous media using a Hele-Shaw cell. We demonstrate that the application of the proposed method produces important improvements versus digital PIV, for the quantification of the gradients of the refractive index including the detection of small-scale convective structures. In comparison with correlation-based digital methods, we demonstrate the advantages of the proposed method, such as denoising and edge capture. These features allow us to obtain the temperature, for this experimental setting, with better image resolution than other techniques reported in the literature. [ABSTRACT FROM AUTHOR]

Schlieren visualisations are used to investigate the influence of cylindrical surface curvature on both the time-invariant and oscillatory behaviour of the shock structures within underexpanded jets impinging on to a surface. At moderate standoff distances $$\left( \frac{h}{d} < 5\right) $$ , the impingement surface curvature affects the temporal behaviour of the flow significantly, but with only slight variations in the time-invariant structure. A convex surface curvature constrains flapping oscillations to a single plane, the normal of which is parallel to the cylinder axis; the oscillation frequency is largely unaffected. For the standoff distances and nozzle pressure ratios studied, concave surface curvature suppresses the formation of impingement tones. A mechanism is proposed whereby entrainment of the recirculated wall jet flow alters the shear layer receptivity to acoustic disturbances, breaking the acoustic feedback loop that drives the jet oscillations. [ABSTRACT FROM AUTHOR]

Disruption of thin (0.3-0.7 mm) horizontal layers of liquid (ethanol) was experimentally investigated during local heating with the substrate side at a heater diameter of 1.6 mm. The influence of the liquid layer thickness and the heating intensity on rupture dynamics was investigated. Visualization and inspection of the liquid layer were realized using the schlieren and shadow methods. General steps of the disruption are determined and the average formation rate of the dry spot is measured, which is 0.2-4 mm/s and increases with increasing of both the layer thickness and the heat flux. It is shown that one of the general factors that affect the rupture dynamics of the liquid layer and the formation of the dry spot in the area of local heating apart from the thermocapillary effect is evaporation. [ABSTRACT FROM AUTHOR]

The transient processes associated with the interaction of a Bunsen flame and nanosecond pulsed discharges (NPD) are explored experimentally with two optical methods. A nanosecond-gated schlieren system is employed to image the shockwave propagation and the hydrodynamic response of the flame to NPD while the time-resolved optical emission spectroscopy measurements are carried out to determine active species and temperature in the plasma region created by the discharges. Therefore, the unsteady process of the interaction of the flame with the discharges is recorded in real-time by the combined measurements. Numbers of experimental evidences for understanding the dynamics of non-equilibrium plasma produced by NPD and performing further numerical simulation are offered. [ABSTRACT FROM AUTHOR]

The spatial distribution of gas temperature in air plasma jet of dc glow microdischarge has been determined. The temperature field was measured by a thermocouple probe and compared to schlieren images. The jet can be separated in the radial direction into three characteristic regions with clearly pronounced boundaries. The central region represents a narrow hot zone corresponding to the visible plasma plume, in which the gas temperature varies from 50 to 200°C depending on the air flow rate and distance from the anode. This zone is surrounded by a warm “coat” of ~1-cm diameter and a temperature within 30-50°C. The outer region represented ambient air at room temperature. The zone of temperatures above 50°C did not extend to a distance above 3 cm from the output nozzle of the discharge cell. [ABSTRACT FROM AUTHOR]

Electrophysical, hydro-gas-dynamic, and thermal characteristics of a discharge arising between liquid electrodes are considered. Spatial visualization of flow patterns in the gas discharge region is performed by using the schlieren technique. [ABSTRACT FROM AUTHOR]

Weak spherical blast waves in static air and their breakup of ethanol and Jet A-1 kerosene droplets were investigated. The blast waves were created by laser-induced air breakdowns at ambient temperature and pressure. In the first part of this study, they were visualized with schlieren imaging, and their trajectories were tracked with high temporal resolution. The laser pulse energy was varied to create blast waves of different strengths. Their initial energies were determined by the application of a numerical and a semi-empirical blast wave model. In the second part, monodisperse ethanol and kerosene droplet chains were injected. Their interaction with the blast waves was visualized by the application of shadowgraph imaging. The perpendicular distance of the breakdown origin toward the droplet chains was varied to study the effect on the fuel droplets as a function of the distance. Droplets within a few millimeters around the breakdown origin were disintegrated into two to three secondary droplets. The blast-induced flow velocities on the post-shock side and the corresponding Weber numbers were calculated from the data of a non-dimensional numerical simulation, and a close look was taken at the breakup process of the droplets. The analysis showed that the aerodynamic force of the blast-induced flow was sufficient to deform the droplets into disk-like shapes, but diminished too fast to accomplish breakup. Due to the release of strain energy, the deformed droplets relaxed, stretched into filaments and finally disintegrated by capillary pinching. [ABSTRACT FROM AUTHOR]

A telecentric optical system is applied to the background-oriented schlieren (BOS) technique to improve accuracy, overcoming the drawbacks of conventional diverging light observation. This paper describes the optical arrangement and formula for telecentric BOS measurement and presents measurement results obtained by the colored-grid background-oriented schlieren technique to confirm the theoretical prediction. The application of the new approach for a large-scale supersonic wind tunnel test is reported. [ABSTRACT FROM AUTHOR]

We introduce a new approach to capturing refraction in transparent media, which we call light field background oriented Schlieren photography. By optically coding the locations and directions of light rays emerging from a light field probe, we can capture changes of the refractive index field between the probe and a camera or an observer. Our prototype capture setup consists of inexpensive off-the-shelf hardware, including inkjet-printed transparencies, lenslet arrays, and a conventional camera. By carefully encoding the color and intensity variations of 4D light field probes, we show how to code both spatial and angular information of refractive phenomena. Such coding schemes are demonstrated to allow for a new, single image approach to reconstructing transparent surfaces, such as thin solids or surfaces of fluids. The captured visual information is used to reconstruct refractive surface normals and a sparse set of control points independently from a single photograph. [ABSTRACT FROM AUTHOR]

Results of gasdynamic and spectroscopic investigations of optical discharge in the subsupersonic flow of a homogeneous fuel-air (CH + air) mixtures are presented. The combustion was initiated and maintained by optical discharge created using a CO laser. The laser radiation propagated across the flow and was focused by a lens on the axis of the supersonic jet ( M = 2). Emission-spectroscopy techniques provided data on the composition of radiating combustion products and the intensity of components emitted in the region of optical discharge. Patterns of Toepler's visualization of the flow structure in the schlieren scheme are presented. The images were monitored by a high-speed video camera operating at an exposure time of 1.5 μs and a frame frequency of 1000 s. [ABSTRACT FROM AUTHOR]

Three variants of schlieren techniques are employed to investigate the development of second-mode instability waves in the hypersonic boundary layer of a slender cone in a reflected shock tunnel. First, a previously proposed technique using high frame rate (i.e., at least as high as the dominant instability frequency) schlieren visualization with a continuous light source is shown to provide repeatable measurements of the instability propagation speed and frequency. A modified version of the technique is then introduced whereby a pulsed light source allows the use of a higher-resolution camera with a lower frame rate: this provides significant benefits in terms of spatial resolution and total recording time. A detailed picture of the surface-normal intensity distribution for individual wave packets is obtained, and the images provide comprehensive insight into the unsteady flow structures within the boundary layer. Finally, two-point schlieren deflectometry is implemented and shown to be capable of providing second-mode growth information in the challenging shock tunnel environment. [ABSTRACT FROM AUTHOR]

In this study, we successfully utilize the microscale schlieren method to visualize the microscale mass transport near the vapor-liquid interface during boiling of 2-propanol/water mixtures in a square capillary. Because the variation in the refractive index with composition is much greater than that with temperature, the microscale schlieren method proves to be a powerful tool for investigating the solutocapillary convection without the interference of thermocapillarity. When the difference between the equilibrium vapor and liquid mole fractions is large, we observe high concentration gradients near the vapor-liquid interface due to both mass diffusion and the solutocapillary effects. Although the solutocapillary convection is decidedly affected by the eruptive nature of the boiling process, the near-bubble mass transport still plays a vital role in boiling heat transfer. In a square capillary of d = 900 μm, mass diffusion dominates and the depletion of 2-propanol near the vapor-liquid interface increases. This leads to an increase in the local bubble point causing the deterioration of heat transfer for 2-propanol/water mixtures. However, in the smaller square capillary of d = 500 μm, the solutocapillary effect becomes more important. The induced convection near the contact line helps to augment the boiling heat transfer at x = 0.015, despite the fact that mass diffusion tends to cause a higher concentration gradient normal to the bubble front during the boiling process. Herein, we prove that the microscale schlieren method is able to provide valuable insight into the leverage between different mechanisms in heat transfer during the vaporization process of 2-propanol/water mixtures in a square capillary. [ABSTRACT FROM AUTHOR]

We present results of an experimental study of blast wave propagation and particle dispersion induced by a free-field detonation of spherical charges made of a 125 g C-4 explosive surrounded by inert or reactive particles. Visualization of the flow was performed with a high-frame-rate video camera. Background oriented Schlieren (BOS) methods were adapted to process the images that allowed the detection of the shock waves. BOS analysis also revealed that particles form agglomerates, which may generate precursor perturbations on the recorded pressure signals. While inert glass particles notably delay the shock, the combustion of aluminium particles can accelerate it, especially if they are small atomized or flaked particles. When a mixture of inert glass particles with reactive particles is dispersed, the agglomerates are formed by coalescence of both materials. [ABSTRACT FROM AUTHOR]

Experimental testing was conducted for a planar shock wave of incident Mach number $$M_\mathrm{s} = 1.33$$ propagating through one of three compound parabolic profiles of 130, 195 or 260 mm in length, all of which exhibit an 80 % reduction in area. Both high-resolution single shot and low-resolution video were used in a schlieren arrangement. Results showed three main types of flow scenarios for propagation through a gradual area reduction, and an optimal net increase of 12.7 % in shock Mach number was determined for the longest profile, which is within 5 % of theoretical predictions using Milton's modified Chester-Chisnell-Whitham relation. [ABSTRACT FROM AUTHOR]

The fluid-dynamic characterization by means of Schlieren high-speed imaging of the effluent region of a single electrode plasma jet is presented. The plasma source is powered by a high-voltage generator producing pulses with nanosecond rise time. Time evolution of fluctuations generated in a free flow regime and when the jet is impinging on substrates of different geometries (plain substrates, Petri dishes, etc.) and materials (metal, dielectric covered metal, polystyrene) has been investigated. Plasma ignition causes fluid-dynamic instabilities moving in the direction of the jet flow and correlated with the high-voltage pulses: for low pulse repetition frequency (PRF) (<125 Hz), the movement of the turbulent front between two voltage pulses can be tracked, whereas for higher PRF (1,000 Hz) the flow is completely characterized by turbulent eddies in the effluent region, without relevant changes between subsequent voltage pulses. When the jet is impinging on a substrate, turbulent fronts propagate over the surface starting from the gas impinging zone. [ABSTRACT FROM AUTHOR]

A modification of the constant correction factor in the known equations of the background-oriented Schlieren is presented in order to be applicable to the near-field. Near-Field background-oriented Schlieren has the advantage over standard background-oriented Schlieren of being able to obtain reliable density distributions for set-ups in which the background pattern is placed directly behind the investigated flow field. It is proven that the modified correction factor depends solely on the distance between the background pattern and the flow field and on the external shape of the investigated flow field itself. The proof of principle and the accuracy of the proposed technique are obtained by the simulation of a 2D density variation with the use of glass wedge prism. The measurement of the whole-field density information of a supersonic underexpanded free jet is presented as an example that confirms the theoretical predictions. [ABSTRACT FROM AUTHOR]

The behavior of an aluminum door hanging at the exit of an open shock tube at different angles, from 5 $$^\circ $$ to 85 $$^\circ $$, and thereby providing partially open space for the exiting flow, was investigated experimentally. Experiments were conducted with an incident shock wave Mach number of $$M_\mathrm{is}=1.1$$ impinging on the partially opened door. Both pressure measurements in the vicinity of the door, on its center and inside the shock tube, and schlieren visualization were undertaken for studying the door movement and its maximum opening angle relative to its initial position. It was found that for an initial opening angle smaller than 25 $$^\circ $$ the door opened completely while for larger angles its motion is marginal. In addition, for an initial door opening angle of about 10 $$^\circ $$ the lowest pressures were recorded inside the shock tube behind the evolving waves after exiting of the incident shock wave. The present experimental results may be useful to numerical studies of fluid-structure interactions, e.g., in designing safety valves in jet engines. Such a device is needed for preventing rupture in the case when a sudden overpressure pulse is generated inside the aircraft engine compartment. [ABSTRACT FROM AUTHOR]

During previous investigations on pseudo-shock systems, we have observed reproducible differences between measurement and simulations for the pressure distribution as well as for size and shape of the pseudo-shock system. A systematic analysis of the deviations leads to the conclusion that small gaps of $$\Delta z=O(10^{-4})$$ m between quartz glass side walls and metal contour of the test section are responsible for this mismatch. This paper describes a targeted experimental and numerical study of the bypass mass flow within these gaps and its interaction with the main flow. In detail, we analyze how the pressure distribution within the channel as well as the size, shape and oscillation of the pseudo-shock system are affected by the gap size. Numerical simulations are performed to display the flow inside the gaps and to reproduce and explain the experimental results. Numerical and experimental schlieren images of the pseudo-shock system are in good agreement and show that especially the structure of the primary shock is significantly altered by the presence of small gaps. Extensive unsteady flow simulations of the geometry with gaps reveal that the shear layer between subsonic gap flow and supersonic core flow is subject to a Kelvin-Helmholtz instability resulting in small pressure fluctuations. This leads to a shock oscillation with a frequency of $$f= O(10^5) \hbox {s}^{-1}$$ . The corresponding time scale $$\tau $$ (s) is 16 times higher than the characteristic time scale $$\tau _\delta =\delta /U_\infty $$ of the boundary layer given by the ratio of the boundary layer thickness $$\delta $$ directly ahead of the shock and the undisturbed free stream velocity $$U_\infty $$ . To assess the reliability of our numerical investigations, the paper includes a grid study as well as an extensive comparison of several RANS turbulence models and their impact on the predicted shape of pseudo-shock systems. [ABSTRACT FROM AUTHOR]

The use of a short-duration laser as a light source for Schlieren flow visualization is described. The coherent nature of this light source has been removed using the laser-induced fluorescence principle. The quality of the speckle-free pictures obtained competes with the one achieved using conventional illumination techniques. Flow features are frozen in space thanks to the short duration of the laser flashes. The applicability of the technique is demonstrated with the characterization of hypersonic conical boundary layers at Mach 11. Pairs of images have been obtained using successive laser flashes with a separation time down to 2 μs. Instantaneous convection velocities and spectral properties of the dominant boundary layer disturbances have been determined. These characteristics compare favorably with the values determined from synchronized time-resolved wall pressure measurements. [ABSTRACT FROM AUTHOR]

In thermal spraying technique, the changes in the in-flight particle velocities are considered to be only a function of the drag forces caused by the dominating flow regimes in the spray jet. Therefore, the correct understanding of the aerodynamic phenomena occurred at nozzle out let and at the substrate interface is an important task in the targeted improvement in the nozzle and air-cap design as well as in the spraying process in total. The presented work deals with the adapting of an innovative technique for the flow characterization called background-oriented Schlieren. The flow regimes in twin wire arc spraying (TWAS) and high velocity oxygen fuel (HVOF) were analyzed with this technique. The interfering of the atomization gas flow with the intersected wires causes in case of TWAS process a deformation of the jet shape. It leads also to areas with different aero dynamic forces. The configurations of the outlet air-caps in TWAS effect predominantly the outlet flow characteristics. The ratio between fuel and oxygen determine the dominating flow regimes in the HVOF spraying jet. Enhanced understanding of the aerodynamics at outlet and at the substrate interface could lead to a targeted improvement in thermal spraying processes. [ABSTRACT FROM AUTHOR]

This study describes an attempt of quantitative visualization of open-air explosions via the background-oriented schlieren method (BOS). The shock wave propagation curve and overpressure distribution were extracted from the obtained images and compared with the results of the numerical analysis. The potential of extracting the density distribution behind the shock front is also demonstrated. Two open-air explosions were conducted; one with a $$36$$ -kg emulsion explosive and the other with a $$7.89$$ -kg composition C4 explosive. A high-speed digital video camera was used with a frame rate of $$10{,}000\,\mathrm{Hz}$$ and a pixel size of $$800 \times 600$$ . A natural background, including trees and grass, was used for BOS measurements instead of the random dots used in a laboratory. The overpressure distribution given by the passing shock was estimated from the visualized images. The estimated overpressures agreed with the values recorded by pressure transducers in the test field. The background displacement caused by light diffraction inside the spherical shock waves was in good agreement, except at the shock front. The results shown here suggest that the BOS method for open-air experiments could provide increasingly better quantitative and conventional visualization results with increasing spatial resolution of high-speed cameras. [ABSTRACT FROM AUTHOR]

Two underexpanded free jets at fully expanded Mach numbers $$M_\mathrm{j}$$ = 1.15 and 1.50 are studied. Schlieren visualizations as well as measurements of static pressure, Pitot pressure and velocity are performed. All these experimental techniques are associated to obtain an accurate picture of the jet flow development. In particular, expansion, compression and neutral zones have been identified in each shock cell. Particle lag is considered by integrating the equation of motion for particles in a fluid flow and it is found that the laser Doppler velocimetry is suitable for investigating shock-containing jets. Even downstream of the normal shock arising in the $$M_\mathrm{j}$$ = 1.50 jet, the measured gradual velocity decrease is shown to be relevant. [ABSTRACT FROM AUTHOR]

High-speed schlieren visualization at 20,000 fps was performed to investigate the transient penetration depth of helium gas jets into air at 8 different pressure ratios. The injection pressure investigated was between 1.5 and 40 bar. The pressure in the mixing chamber was varied between 1.0 and 4.0 bar. The different pressure ratios were chosen to examine subsonic and underexpanded jets. To investigate the density dependence of the process, cryo injections were performed at injection temperatures of 224, 198 and 173 K at different pressure ratios. Using the image processing technique for detecting the jet tip penetration distance, presented in this article, it can be shown that for a given injector the jet penetration behavior is the same for the same pressure ratio. A new self-similar model was developed, to describe the influence of the injection and chamber gas density on the jet penetration behavior more accurately. The model was verified with the experimental data sets presented in this paper and with literature data. [ABSTRACT FROM AUTHOR]

A new variant of the background oriented schlieren technique is presented. Using a laser to generate a speckle reference pattern, several shortcomings of the conventional technique can be overcome. The arrangement decouples the achievable sensitivity from the placement constraint on the reference screen, facilitating the design of compact, high-sensitivity configurations. A new dual-pass imaging mode is introduced which further improves system performance and permits focusing on the target scene. Examples are presented that confirm the theoretical predictions. [ABSTRACT FROM AUTHOR]

The process of the impulse outflow of a gas jet occurring due to the discharge inside the discharge chamber is studied experimentally. The flow was investigated by means of the schlieren-visualization and the Particle Image Velocimetry optical techniques. During the study, impulse jet parameter variations were obtained. A number of features were revealed of the impulse outflow from the discharge chamber, such as formation of a second vortex ring, reduction of the vortex ring circulation during the outflow, and formation of vortices in front of the main vortex ring. [ABSTRACT FROM AUTHOR]

A new method is proposed for polarization modulation of random electromagnetic beams. The changes in the spectral degree and the state of polarization of such after their passage through the gradient-index (GRIN) waveguide lens system are investigated while the parameters of the system are adjusted. It is demonstrated that passage through the GRIN waveguide lens system can lead to more complex changes in the beam's polarization than its free-space propagation. [ABSTRACT FROM AUTHOR]

A simple method of generating polygonal gas interfaces is proposed by using the soap film technique. Thin pins are used as angular vertexes to connect the adjacent sides of polygonal soap films in order to avoid the pressure singularities around the vertexes caused by the surface tension. As a demonstration, three polygonal interfaces (i.e., square, equilateral triangle and diamond) are created in the test section of a shock tube. Experiments are then carried out for a planar shock wave (Mach number about 1.2) interacting with air/SF polygonal interfaces. Numerical simulations are also performed to validate the proposed method of the interface formation. Wave systems and interface structures can be clearly identified in experimental schlieren images and agree well with the numerical results. It is also indicated that the presences of thin pins and fine chamfers only have limited effects on the interface evolution and can be ignored at the very early stage. Experimental and numerical results about the movement of the distorted interface, the width and height of the interface structures are further compared and good agreement is achieved. It is then concluded that the polygonal interface formed by the proposed method is applicable for the Richtmyer-Meshkov instability study. [ABSTRACT FROM AUTHOR]

Cavitation plays an important role in fuel atomization mechanisms, but the physics of cavitation and its impact on spray formation and injector efficiency are not well documented yet. Experimental investigations are required to support the development and the validation of numerical models and the design of tomorrow's injectors, in the context of pollutant and fuel consumption reduction. The complexity of modern injectors and the extreme conditions of injection do not facilitate experimental investigations. In this paper, experiments are conducted in a simplified geometry. The model nozzle consists of a transparent 2D micro-channel supplied with a test oil (ISO 4113). Three different optical techniques are proposed to investigate the channel flow, with the pressure drop between upstream and downstream chambers as a parameter. A shadowgraph-like imaging technique allows the observation of cavitation inception and vapor cavities development throughout the channel. The technique also reveals the presence of density gradients (pressure or temperature) in the channel flow. However, this additional information is balanced by difficulties in image interpretation, which are discussed in the paper. In addition, a combination of Schlieren technique and interferometric imaging is used to measure the density fields inside the channel. The three techniques results are carefully analyzed and confronted. These results reveal a wealth of information on the flow, with pressure waves generated by bubble collapses, turbulence in the wake of vapor cavities and bubble survival in flow regions of high pressure. Our results also show that cavitation inception is located in the shear layers between the recirculation zones and the main flow, relatively far from the inlet corner, where the pressure is minimum in average. To explain this behavior, we propose a scenario of cavitation inception based on the occurrence and the growing of instabilities in the shear layers. [ABSTRACT FROM AUTHOR]

Laboratory experiments have been performed to investigate the interaction of internal waves with a pycnocline. An oscillating cylinder generated internal wave beams, which were observed using the synthetic schlieren technique. Internal waves incident on the pycnocline layer excited higher-frequency modes. In the absence of shear, a discrete spectrum of harmonic modes was generated due to nonlinear effects. These harmonic modes might play a role in the formation of internal solitary waves which have been observed in ocean pycnoclines. With shear, a continuous spectrum of excited modes was found. [ABSTRACT FROM AUTHOR]

The mechanism behind cavity-induced pressure oscillations in supersonic flows past a deep rectangular cavity is not well understood despite several investigations having been carried out. In particular, the process by which the pressure wave is generated and the path of the pressure wave propagating inside the cavity remains unclear. In the present study, the pressure waves around a deep rectangular cavity over which nitrogen gas flows at a Mach number of 1.7 are visualized using the schlieren method. The length of the cavity is 14.0 mm. The depths of the cavity are selected as 20.0 and 11.7 mm, corresponding to length-to-depth ratios of 0.70 and 1.2, respectively. The pressure waves propagating inside as well as outside the cavity have been successfully visualized using a high-speed camera, and the propagation pattern of these waves is found to be different from that previously predicted by numerical simulation and from those expected in previous oscillation models. In addition, the pressure oscillation near the trailing edge of the cavity is also measured using semiconductor-type pressure transducers simultaneously with the capture of the schlieren images. As a result, the relationship between the shear-layer motion, pressure-wave generation, and pressure oscillation at the trailing edge of the cavity is clarified experimentally. [ABSTRACT FROM AUTHOR]

The feasibility of using high-power light-emitting diodes (LED) as a light source for high frame rate schlieren imaging is investigated. Continuous sequences of high-intensity light pulses are achieved by overdriving the LED with current pulses up to a factor of ten beyond its specifications. In comparison to commonly used pulsed light sources such as gas discharge lamps and pulsed lasers, the pulsed LED has several attractive advantages: the pulse-to-pulse intensity variation is on the same order of magnitude as the detector (camera) noise permitting quantitative intensity measurements. The LED's narrow emission bandwidth reduces chromatic abberations, yet it is spectrally wide enough to prevent the appearance of speckle and diffraction effects in the images. Most importantly, the essentially lag-free light emission within tens of nanoseconds of the applied current pulse allows the LED to be operated at varying frequencies (i.e., asynchronously), which generally is not possible with neither lasers nor discharge lamps. The pulsed LED source, driven by a simple driver circuit, is demonstrated on two schlieren imaging setups. The first configuration visualizes the temporal evolution of shock structures and sound waves of an under-expanded jet that is impinging on a rigid surface at frame rates of 500 kHz to 1 MHz. In a second application, long sequences of several thousand high-resolution images are acquired on a free jet at a frame rate of 1 kHz. The low-intensity fluctuation and large sample number allow a reliable computation of two-point correlation data from the image sequences. [ABSTRACT FROM AUTHOR]

By means of the falling soap film tunnel and the Schlieren optical method, the Marangoni convection were observed directly in the immediate interfacial neighborhood during the desorption process of acetone from the falling soap film. Moreover, the hydraulic characteristics of the falling soap film tunnel, the acetone concentration, the surface tension of the soap liquid and the mass transfer has been investigated in details through the experimental or theoretical method. [ABSTRACT FROM AUTHOR]