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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]

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]

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]

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]

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]

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]

Highlights: [•] Schlieren technique was developed for the experimental study of free convection. [•] The analysis of image data allowed the determination of boundary layer thickness. [•] The research tool enables the validation of criterial relationships. [Copyright &y& Elsevier]

In a Mach 3.8 wind tunnel, both instantaneous and time-averaged flow structures of different scales around a blunt double-cone with or without supersonic film cooling were visualized via nano-tracer planar laser scattering (NPLS), which has a high spatiotemporal resolution. Three experimental cases with different injection mass flux rates were carried out. Many typical flow structures were clearly shown, such as shock waves, expansion fans, shear layers, mixing layers, and turbulent boundary layers. The analysis of two NPLS images with an interval of 5 μs revealed the temporal evolution characteristics of flow structures. With matched pressures, the laminar length of the mixing layer was longer than that in the case with a larger mass flux rate, but the full covered region was shorter. Structures like K-H (Kelvin-Helmholtz) vortices were clearly seen in both flows. Without injection, the flow was similar to the supersonic flow over a backwardfacing step, and the structures were relatively simpler, and there was a longer laminar region. Large scale structures such as hairpin vortices were visualized. In addition, the results were compared in part with the schlieren images captured by others under similar conditions [ABSTRACT FROM AUTHOR]

Abstract: We compare the results of three quantitative schlieren techniques applied to the measurement and visualization of a two-dimensional laminar free-convection boundary layer. The techniques applied are Schardin''s “calibrated” schlieren technique, in which a weak lens in the field-of-view provides a calibration of light deflection angle to facilitate quantitative measurements, “rainbow schlieren”, in which the magnitude of schlieren deflection is coded by hue in the image, and “background-oriented schlieren” (BOS), in which quantitative schlieren-like results are had from measuring the distortion of a background pattern using digital-image-correlation software. In each case computers and software are applied to process the data, thus streamlining and modernizing the quantitative application of schlieren optics. (BOS, in particular, is only possible with digital-image-correlation software.) Very good results are had with the lens-calibrated standard schlieren method in the flow tested here. BOS likewise produces good results and requires less expensive apparatus than the other methods, but lacks the simplification of parallel light that they feature. Rainbow schlieren suffers some unique drawbacks, including the production of the required rainbow cutoff filter, and provides little significant benefit over the calibrated schlieren technique. [Copyright &y& Elsevier]

There is an important need for simple methods to visualize and measure whole-field airflow patterns in the HVAC field. The background-oriented schlieren (BOS) method is presented here as an answer to this need—a simple and effective method for visualizing refractive HVAC flowfields in situ. The equipment required is simple, portable, inexpensive, and readily available, thus superseding some previous approaches, including lens-and-grid schlieren techniques that require large fixed installations. For BOS a custom random-dot background pattern is used, and imaging is done by a consumer-grade digital single-lens-reflex (SLR) camera. After covering the principles of BOS, examples are shown of visualized airjets and plumes from heating vents and registers, a space-heater, a teakettle, and a human thermal plume and cough. BOS images of candle plumes are also used to explore several different visualization approaches, equipment arrangements, and image color scales. While current results are purely-qualitative visualizations, quantitative temperature measurements can also be made in certain cases where the flow is approximately two dimensional. Finally, BOS lends itself well to certain HVAC chores, such as the diagnosis of commercial kitchen ventilation airflows. [ABSTRACT FROM AUTHOR]

Abstract: Schlieren and its variants such as differential interferometry and schlieren microscopy, have been used extensively for flow visualization where first derivative fields are captured. The derivative fields obtained from the schlieren like methods can be further processed to estimate the temperature field when the first derivative fields relate to those of temperature. Temperature construction from first derivative field is an ill-posed problem owing to the experimental noise and a few discrete points where measured temperatures may be available. A new approach has been proposed where the domain is discretized into a large number of triangular elements and least-squares based finite-element analysis is performed over the discretized domain. The domain and boundaries are identified manually based on prior knowledge. Temperature fields have been constructed for experimentally obtained first derivative fields from a Differential Interferometer (DI) for different cases. The performance of the new methodology is found to be good. [Copyright &y& Elsevier]

The flow over a missile-shaped configuration is investigated by means of Schlieren visualization in short-duration facility producing free stream Mach numbers of 5.75 and 8. This visualization technique is demonstrated with a 41° full apex angle blunt cone missile-shaped body mounted with and without cavity. Experiments are carried out with air as the test gas to visualize the flow field. The experimental results show a strong intensity variation in the deflection of light in a flow field, due to the flow compressibility. Shock stand-off distance measured with the Schlieren method is in good agreement with theory and computational fluid dynamic study for both the configurations. Magnitude of the shock oscillation for a cavity model may be greater than the case of a model without cavity. The picture of visualization shows that there is an outgoing and incoming flow closer to the cavity. Cavity flow oscillation was found to subside to steady flow with a decrease in the free stream Mach number. [ABSTRACT FROM AUTHOR]

Abstract  The background-oriented schlieren (BOS) flow visualization method has the potential for large-scale flow imaging outside the laboratory by using natural backgrounds instead of the artificial patterns normally used indoors. The natural surroundings of an outdoor test site can sometimes be used as such a background, subject to criteria of fine scale, randomness and contrast that are developed here. Some natural backgrounds are more appropriate than others for a given application. Backgrounds used here to visualize both high- and low-speed schlieren disturbances include a sunlit cornfield and a backlit grove of trees. A range of image post-processing methods is considered for qualitative BOS. It is found that high sensitivity and a broad measuring range are in conflict here, much as they are in traditional schlieren instruments. Applications of natural-BOS include explosive characterization, firearms and artillery testing, chemical and natural-gas leak detection, and related phenomena. [ABSTRACT FROM AUTHOR]

Turbulent flame acceleration process has been studied in a two-dimensional valveless pulsed detonation combustor (PDC) using Schlieren flow visualization, ionization probes, and companion high-frequency pressure instrumentation. Experiments were conducted using near stoichiometric propane-air mixtures in a 76.2 mm × 76.2 mm, 1.68 m long modular detonation tube with the windowed combustor under multicyclic conditions within the PDC operating in a valveless mode. The effect of the presence of the obstacle was investigated at various stages of the flame acceleration process in aerodynamically confined environment. Amplification of the leading shock waves through the interaction of the obstacle enhances the flame acceleration process owing to the reduction of post-shock induction time and the hydrodynamic instabilities at the baroclinic interface. The internal configuration of the combustor including the obstacle plays a major role of the flame acceleration due to the shock amplification pattern. Sequences of schlieren images also show the flow drifting and blowdown processes confirming that the gasdynamic valves are successfully operative in the valveless PDC. [ABSTRACT FROM AUTHOR]

Experiments to demonstrate the use of the background-oriented schlieren (BOS) technique in hypersonic impulse facilities are reported. BOS uses a simple optical set-up consisting of a structured background pattern, an electronic camera with a high shutter speed and a high intensity light source. The visualization technique is demonstrated in a small reflected shock tunnel with a Mach 4 conical nozzle, nozzle supply pressure of 2.2 MPa and nozzle supply enthalpy of 1.8 MJ/kg. A 20° sharp circular cone and a model of the MUSES-C re-entry body were tested. Images captured were processed using PIV-style image analysis to visualize variations in the density field. The shock angle on the cone measured from the BOS images agreed with theoretical calculations to within 0.5°. Shock standoff distances could be measured from the BOS image for the re-entry body. Preliminary experiments are also reported in higher enthalpy facilities where flow luminosity can interfere with imaging of the background pattern. [ABSTRACT FROM AUTHOR]

The article discusses the application of optical imaging techniques to hypersonic facilities. Examples of experimental measurements are also provided. Traditional Schlieren and shadowgraph techniques remain as inexpensive and easy-to-use flow visualization techniques. Planar laser induced fluorescence has been used to visualize complex flows and to measure quantities as temperature and velocity.

Groundwater seeps are known to occur in Eckernförde Bay, Baltic Sea. Their discharge rate and dispersion were investigated with a new schlieren technique application, which is able to visualize heterogeneous water parcels with density anomalies down to Δ σt = 0.049 on the scale of millimeters. With the use of an inverted funnel, discharged fluids can be captured and the outflow velocity can be determined. Overall, 46 stations could be categorized by three different cases: active vent sites, seep-influenced sites, and non-seep sites. New seep locations were discovered, even at shallow near-shore sites, lacking prominent sediment depression, which indicate submarine springs. The detection of numerous seeps was possible and the groundwater-influenced area was defined to be approximately 6.3 km2. Flow rates of between 0.05 and 0.71 l m−2 min−1 were measured. A single focused fluid plume, which was not disturbed by the funnel was recorded and revealed a flux of 59.6 ± 20 ml cm−2 min−1 and it was calculated that this single focused plume would be strong enough to produce a flow rate through the funnel of 1.32 ± 0.44 l m−2 min−1. The effect of different seep-meter funnel sizes is discussed. [ABSTRACT FROM AUTHOR]

Recent theoretical and numerical investigations predict that localized nonlinear effects in the overlapping region of an incoming and reflected internal wave beam can radiate higher-harmonic beams. We present the first set of experimental visualizations, obtained using the digital Schlieren method, that confirm the existence of radiated higher-harmonic beams. For arrangements in which the angle of propagation of the second harmonic exceeds the slope angle, radiated beams are visualized. When the propagation angle of the second harmonic deceeds the slope angle no radiated beams are detected, as the associated density gradient perturbations are too weak for the experimental method. The case of a critical slope is also reported. [ABSTRACT FROM AUTHOR]

The computer simulation study for color-schlieren is to provide a new method for validations between experiment and theory, which can get colorschlieren image from the computational flow field data. This paper gives a brief description about the theory of method, technical way, algorithm, key problem that has been solved, the computational results and the future development direction. [ABSTRACT FROM AUTHOR]

Reports the use of the electron beam fluorescence (EBF) and focused schlieren (FS) methods for the high-speed flow visualization and characterization of the National Aeronautics and Space Administration's High-speed Flow Generator. Viability of the EBF and FS techniques; Development of a high-sensitivity FS system.

This paper reports the results of measurements of the ‘‘visual thickness,’’ obtained from flow visualization experiments by the schlieren method, of initially plane interfaces between two gases under impulsive accelerations. It is found that when such interfaces are processed by just one incident shock wave of strength of order Ms=1.5, their thickness increases slowly and they require observation over extended times; their growth rates are found to slow down with time, in agreement with simple theoretical arguments. The observed growth rates of thin interfaces formed by plastic membranes have been found to be substantially smaller than that reported by previous investigators. Also, thick, diffusively smoothed interfaces initially grow much more slowly than the discontinuous ones do. In these experiments, it is found that wall vortices formed by shock wave/boundary-layer interaction at the interface grow much more rapidly than the shock-processed interfaces in the bulk of the fluid. These wall structures can reduce the apparent growth of interfaces by vorticity-induced strain and impair the observation of the relevant interface phenomena. [ABSTRACT FROM AUTHOR]

A practical approach for deriving the absolute density field based on the background-oriented schlieren method in a high-speed flowfield was implemented. The flowfield of interest was a two-dimensional compressible flowfield created by two supersonic streams to simulate a linear aerospike nozzle operated under a supersonic in-flight condition. The linear aerospike nozzle had a two-dimensional cell nozzle with a design Mach number of 3.5, followed by a spike nozzle. The external flow simulating the in-flight condition was 2.0. The wall density distribution used as the wall boundary condition for Poisson's equation to solve the density field was derived by a simplified isentropic assumption based on the measured wall pressure distribution, and its validity was evaluated by comparing with that predicted by numerical simulation. Unknown coefficients in Poisson's equation were determined by comparing the wall density distribution with that predicted by the model. By comparing the derived density field based on the background-oriented schlieren method to that predicted by the model and numerical simulation, the absolute density field was derived within an error of 10% on the wall distribution. This practical approach using a simplified isentropic assumption based on measured pressure distribution thus provided density distribution with sufficient accuracy. [ABSTRACT FROM AUTHOR]

Understanding fluid flow is a difficult problem and of increasing importance as computational fluid dynamics (CFD) produces an abundance of simulation data. Experimental flow analysis has employed techniques such as shadowgraph, interferometry, and schlieren imaging for centuries, which allow empirical observation of inhomogeneous flows. Shadowgraphs provide an intuitive way of looking at small changes in flow dynamics through caustic effects while schlieren cutoffs introduce an intensity gradation for observing large scale directional changes in the flow. Interferometry tracks changes in phase-shift resulting in bands appearing. The combination of these shading effects provides an informative global analysis of overall fluid flow. Computational solutions for these methods have proven too complex until recently due to the fundamental physical interaction of light refracting through the flow field. In this paper, we introduce a novel method to simulate the refraction of light to generate synthetic shadowgraph, schlieren and interferometry images of time-varying scalar fields derived from computational fluid dynamics data. Our method computes physically accurate schlieren and shadowgraph images at interactive rates by utilizing a combination of GPGPU programming, acceleration methods, and data-dependent probabilistic schlieren cutoffs. Applications of our method to multifield data and custom application-dependent color filter creation are explored. Results comparing this method to previous schlieren approximations are finally presented. [ABSTRACT FROM PUBLISHER]

Discusses the Full-Scale Schlieren System visualization of supersonic bullet and muzzle blast from firing a .30-06 rifle. Schlieren images of the phenomenon; Observation of shock wave reflections; Bullet impact upon a target.