Showing papers on "Schlieren published in 1988"

High‐speed photography of surface geometry effects in liquid/solid impact

Abstract: Recent theoretical studies of liquid/solid impact, in particular the geometric wave theory of Lesser and Field, have emphasized the importance of the detailed geometry in the contact area. In parallel with the theoretical work, we have developed a two‐dimensional technique using gels for impact and shock studies. A combination of high‐speed photography and schlieren optics allows the shocks in the liquid and solid, if it is transparent, to be visualized, as well as important processes, such as jetting, to be recorded. This paper describes the gel technique and gives results for a range of surface geometries for collision velocities of a few hundred meters per second. The relevance to damage initiation in liquid/solid impact problems, such as rain erosion, steam turbine blade erosion, and cavitation, are discussed.

Experimental investigation of the aerodynamic breakup of liquid drops

Abstract: An experimental investigation was made of the deformation and the mechanism of stripping-type breakup of liquid drops. Experiments were conducted in a 60 X 150 mm cross-sectional shock tube equipped with pulsed laser holographic interferometry. Water drops having diameters of 1030 and 4300 fim were examined for shock wave Mach numbers from 1.3 to 1.5 in atmospheric air. The Weber and Reynolds numbers under these conditions were in the range of 600 to 7600 and 1.38 to 10.4 x 10, respectively. In previous works, the experimental data of the stripping-type breakup were obtained by spark shadowgraphs and streak schlieren methods and high-speed movies. However, in previous works, due to the effect of light scattering through the micromist, the structure of disintegrating drops could not be visualized. The purpose of the present work is, by using holographic interferometry, to re-examine the classical problem of the stripping-type breakup of liquid drops. As a result, a four-stage mechanism of the stripping-type breakup of liquid drops was established.

Propagation of weak shocks through a random medium

Abstract: The propagation of weak shock waves (M_s = 1.007, 1.03 and 1.1) through a statistically uniform random medium has been investigated experimentally in a shock tube. The wave-from geometry, rise time and amplitude of initially plane shocks which have propagated through a random mixture of helium and refrigerant 12 are measured. The effect of shock propagation on the properties of the random medium is visualized with schlieren and shadow photography. The pressure histories of the distorted shock waves reflecting from a normal end wall are observed to be both peaked and rounded. In the rounded case the perturbed shock is found to be made up of a succession of weak, slightly curved fronts with a total effective rise time orders of magnitude greater than the classical Taylor thickness. The radius of curvature of the weakest shocks after propagating through the random medium is inferred from observations at two downstream stations to be about 7 times the integral scale of the gas inhomogeneities. It is concluded that the observed distortions of the wave fronts can best be explained in terms of random focusing and defocusing of the front by the inhomogeneities in the medium. A ray-tracing calculation has been used to interpret the experimental observations. It is found that geometrical considerations are sufficient to account for many of the effects observed on the shocks.

Structure of attached and lifted gas jet flames in hysteresis region

Abstract: The flame structure and hysteresis chacracteristics of lifting and reattaching propane jet flames over contoured nozzles were studied. High-speed schlieren photography was employed to investigate the flow field in the near-nozzle region of cold jets, attached flames, lifted flames, and their transitions. Fine-wire thermocouples were used in conjunction with a computer data acquisition system to record temperature field in the stabilization region. Gas sampling and analysis were employed to determine the mean concentrations of oxygen and carbon monoxide The mean velocities were determined with a total pressure tube. The flow in the flame stabilization region is observed to be laminar in attached flames and dominated by organized structures in lifted flames. The characteristics of those structures have been measured during liftoff and reattachment transitions and compared with cold jet data. The average dimensionless celerities and Strouhal numbers of the organized structures in the burning parts of the flames were measured as 0.94 and 2.8 during liftoff and 0.62 and 1.30 during reattachment, respectively. The schlieren photographs and radial profiles of temperature and concentrations establish that the flame zone is located away from the shear layer. Further, they show that the molecular diffusion controls the lifting process. The dynamics of the organized structures that result in a wider shear layer govern the reattachment process. This difference between the structures of the flame base in the two configurations accounts for the hysteresis phenomenon. The variations of liftoff and reattachment velocities with nozzle diameter, fuel molecular weight and addition of inert diluents to the jet fluid were measured to substantiate these observations.

Temperature and velocity fields due to surface tension driven flow

Abstract: Temperature and velocity fields near an air-bubble in silicon-oil under a heated horizontal wall were investigated. The studies were made with silicon oils of different viscosities so that a wide range of Marangoni numbers was encountered. Schlieren interferograms were taken to analyse the temperature field. For the axisymmetric problem the Abel integral equation was solved numerically by using a coefficient procedure. From the recorded temperature distributions isotherms, radial temperature lines and the temperature along the bubble surface were determined graphically. At low Prandtl- and high Marangoni-numbers an oscillatory instability was observed. The flow field was investigated with neutral-buoyant light scattering glass spheres by observation of the meridian plane of the bubble with a thin light-sheet. The convective mechanism of the flow was recorded photographically by taking pictures with various exposure times. Thus a qualitative and quantitative description of the flow was possible.

Flow Field Effects on Flame Kernel Formation in a Spark-Ignition Engine

Abstract: The effects of mixture turbulence and mean flow velocity on flame kernel formation in a spark-ignition engine were investigated using a disc-shaped optically accessible side chamber. Three-dimensional flow predictions and LDA measurements were performed to get a better understanding of the flow field in this combustion chamber. Thinwire electrodes were located at a peripheral and a central position introduced into the combustion chamber via the glass windows. High-speed schlieren films at 40 kHz were taken from two orthogonal directions to visualize in detail the formation and development of the flame kernel. The purpose was to gain a better understanding of the interaction of turbulence and flame during the first milliseconds after spark breakdown of a transistorized coil ignition system and a capacitor discharge ignition system.

Large air-gap discharge and Schlieren techniques

Abstract: In Part one, the formation and development of leader filaments in large air-gap discharges are studied by Schlieren techniques. The time evolution of the thermal diameter of the leader at a given point in space is studied for several gap spacings ranging from 1 to 16.7 m and for different wave forms and voltage levels. In Part two, the authors review the main phases of the positive leader development. Experimental results obtained by Schlieren methods are used to test previous models of leader behaviour. Information is obtained about reduced field values and neutral temperature values in the leader channel.

Electrophoresis imaging system

Abstract: A method and system for imaging unstained smaple material constituents on a gel electrophoresis unit. The system includes a gel electrophoresis unit, preferably having a pH gradient gel thereon, phase detection optics including a collimated light source, shearing interferometer, phase contrast microscope or schlieren microscope, an image digitizer, and data processing electronics, such as a computer, for determining the concentration of sample constituents at each position on the electrophoresis unit. The method relies on the difference in index of refraction between sample material and gel material to introduce optical path differences in light rays passing through the material. The modified light is subjected to shearing interferometry to locate gel-sample boundaries, phase imaging to directly image the sample, or a laser scanner for directly measuring the deviation in a light beam due to the sample.

Development of Transparent Cylinder Engines for Schlieren Observation

Abstract: A square piston engine and a side-view collimating (SVC) cylinder engine are developed for side-view schlieren visualization of in-cylinder processes. The square piston engine has a square cylinder with two flat quartz windows permitting optical access to the entire cylinder volume. Compression sealing is provided by three polyimide ring assemblies which maintain the windows clean during the actual observational. In contrast with the square piston engine, the SVC cylinder engine permits to take schlieren photographs in practical engine geometries. This is made possible by designing the SVC cylinder so that it does not disperse parallel light rays.

Time-gated resonance schlieren studies of analyte distribution in a graphite furnace atomiser

Abstract: A time-gated resonance schlieren system has been used to observe the distribution of analyte in the furnace during the course of atomisation in graphite furnace atomic absorption spectrometry. The system is a simple one lens schlieren configuration using a pulsed dye laser tunable near the resonance wavelength of the analyte as its source and a modified 35-mm camera for image recording. It is capable of producing a series of schlieren photographs from a single firing of the atomiser. Experimental results for the atomisation of sodium suggest the existence of sodium atom concentration gradients within the furnace. For a time during atomisation, sodium atom density is lower near the furnace wall than in the centre of the furnace. It is postulated that the observed gradients are due to a strong interaction of the atomic sodium with the graphite. The gradient arises as atomic sodium, generated within the furnace through a gas phase dissociation mechanism, is adsorbed on to the furnace wall.

Detection of Separation Bubbles by Infrared Images in Transonic Turbine Cascades

Abstract: In a test facility for straight cascades, equipped with profiles designed for a highly loaded gas turbine rotor of a high-pressure stage, experiments were conducted to clarify some effects of shock wave–boundary layer interactions. The specific aim was to determine both the position and strength of compression shocks originating from profile wake flows and the position and extent of separation bubbles. The latter are most often detected by visualization methods like surface oil flow patterns or Schlieren photographs, as well as by typical properties in wall pressure distribution curves. In addition, the infrared image technique, which has found many applications in a wide range of technical activities in the recent years, may also be used. Compared with other methods, this technique has distinct advantages in fluid mechanics applications. The whole model can be observed without disturbing the boundary layer by tappings, measuring materials, or probes. Some typical infrared images are presented and interpreted using results of pressure distribution measurements, hot-film measurements, and surface oil flow visualizations.

Imaging of Engine Flow and Combustion Processes

Abstract: The authors used the planar imaging of laser induced fluorescence from OH to mark the position of the propagating flame front in the engine. These results are compared to similar data obtained using a Mie scattering technique employed in an earlier study. In additional, they used laser induced fluorescence imaging of nitrogen dioxide to observed the intake valve flow in a flow stand and schlieren flow visualization to observe liquid fuel sprays, valve flow, and flame propagation. Both planar imaging of Mie scattering and schlieren photography were used to observe liquid fuel sprays and the qualitative aspects of fuel-air mixing during intake valve flow.

Pressure distribution measurements at the shock wave focus in water by schlieren photography (kidney/gall stone lithotripsy)

Abstract: A shock wave in water was generated by an electric spark in the first focus of a truncated ellipsoidal reflector. From schlieren pictures of the reflector second focus region, taken at the moment the shock wave has reached it, radial pressure profiles of the focused shock wave were evaluated. The accuracy of the method used was tested by ray tracing through a semi-spherical layer inhomogeneity and by inverse reconstruction of the refractive index. The method was found to be useful for a qualitative indication of the pressure distribution at the second focus but laborious and inaccurate for a quantitative evaluation of the pressure.

Studies of the creepage discharge on the surface of liquids

Abstract: Aspects of propagation of the creepage discharge on the surface of liquids are investigated by a high-speed Schlieren method, when an impulse voltage is applied to a point-to-plane gap arranged at the interface between the liquids and air. Distilled water, rain water, electrolytic aqueous solutions, and insulating oil, liquids with very different conductivity, were selected as the sample liquids. In the same instant of taking high-speed time-resolved Schlieren photographs, prebreakdown currents were measured using a light-emitting diode system. Through these experiments, a polarity effect in the shape and the size of the density changes, features and mechanism of propagation of the density changes, and their mean propagation velocity are discussed in conjunction with current measurements. >

Duct resonance and its effect on the performance of high-pressure ratio axisymmetric ejectors

Abstract: The effect of duct resonance on the performance of high-pressure ratio axisymmetric ejectors was investigated in a series of experiments involving analysis of the emitted noise and schlieren photography of the ejector duct flow. The thrust characteristics obtained with changing blowing pressure were found to contain irregularities, and these were found to be associated with certain tones of acoustic radiation which tuned to the transverse modes of the mixing duct. When varying the blowing pressure for a given ejector the discrete tone fundamental frequency showed a staging behavior, and the frequencies at the discontinuities separating the stages coincided with the calculated modes of the duct. For a given frequency discontinuity, the Helmholtz number (based on duct diameter), was found to be the same for all ejectors tested. For each family of ejectors having the same duct-length to diameter ratio, the blowing pressure at which certain discontinuities occurred varied linearly with duct to nozzle area ratio, and the thrust irregularities assumed similar linear relationships. Increasing the duct length greatly improved thrust augmentation in the relatively high-pressure range and shifted thrust irregularities and the corresponding frequency discontinuities to lower pressure ranges. The schlieren photographs showed that the primary jet spread, and its mixing with the entrained secondary stream, were evidently enhanced by duct resonance. The jet appeared to be surrounded by interspersed helical eddies and to be oscillating in a spinning mode.

Optical studies of electrokinetic processes in dielectric fluids

Abstract: The behavior of transformer oil and other fluids used for the cooling and insulation of power system equipment is significantly influenced by enforced motion. Spatially resolved optical emission which illustrates the changes brought about by increased velocity in a stressed oil duct is presented. Schlieren records taken under the same circumstances permit the examination of concomitant hydraulic instabilities and their dependence on electric field through EHD vorticity generation. Optical data to show the influence of dielectric interfaces in the stressed gap and the impact of upstream charge injection on prebreakdown light emission are also presented. The results indicate that the modification of flow patterns and formation of a stagnant area are important aspects in determining the events leading to liquid breakdown, and have indicated the importance of oil motion per se in electrokinetic effects in liquid-cooled power system equipment. >

Identification of vortex motions in turbulent mixing of choked jets

Abstract: A specially adapted schlieren system is used to generate fluctuating signals which respond strongly to large scale coherent components of a turbulent mixing jet flow and which have a relatively reduced response to random disturbances. The schlieren signals also provide a direct indication of the presence of vortex-like structures in the turbulent mixing layers by virtue of the phase relationship of the schlieren signals to the pressure field. This system gives a clear resolution of the fluctuating periodic effects associated with vortex structures in the flow from a choked convergent nozzle. It has thus been possible to determine that vortex-like eddies are associated with the feedback screech mechanism, and also generate periodic disturbances due to their passage through the diamond shaped wave structure in the flow. The regular disturbances in the flow move at 0.77 of the fully expanded flow velocity. Phase spectral observations demonstrate clearly the vortex like structure of coherent disturbances in the flow by virtue of the quadrature phase relation between the schlieren and microphone signals. Movement of the sensing microphone in the pressure field external to the flow shows disturbance propagation at the acoustic velocity, and also shows that disturbances at Strouhal numbers above 0.7 emanating from the inner mixing zone can be identified by an additional time delay to reach the microphone and only influence the microphone when it is located downstream of the flow sensing schlieren system due to confinement of pressure disturbances within Mach cones of the flow.

A model experiment of wave propergation in acoustic microscope

Abstract: Using stroboscopic photoelastic method together with schlieren technique, simultaneous visualization of ultrasonic waves in water and glass has been made as a model experiment of wave phenomenon occurring in a scanning acoustic microscope. Evidence of reradiation into water from leaky surface acoustic waves excited by the incident converging ultrasonic pulses of 5.5 and 1.2 MHz is shown.

Evidence of turbulence in a current disrupted region in a plasma focus

Abstract: Immediately after the current disruption in a plasma focus discharge, emission of neutrons, soft x rays, ions, and electrons are observed for several tens of nanoseconds. Using a laser scattering diagnostic and a Schlieren technique, it is shown that the plasma in the disrupted region is in a superthermal level in this phase. In the Schlieren technique, the same laser pulse used for the scattering system was used for the light source. The experiment showed that only in the disrupted region did enhanced scattering occur. The measured duration of the superthermal level is ∼10 nsec. When a probe gas is mixed with the working gas, D2, spectral lines of highly charged ions of probe gas are obtained in the disrupted region.

Entrainment enhancement of a supersonic jet for advanced ejectors

Abstract: Improved ejector performance is required for heat-actuated heat pump applications. Laboratory experiments were conducted to investigate the use of a new rectangular supersonic nozzle configuration for mixing enhancement. Noncircular nozzles were previously found to increase substantially the jet entrainment in subsonic flow. Both flow visualization using schlieren photographs and pitot measurements in the far field indicate that the improvement in mixing for a supersonic jet is as good as that for a subsonic jet if not better. Mixing enhancement using a rectangular jet provides a technology basis for the development of the desired ejectors for heat pump applications. 9 refs., 8 figs.

The bubble, drop, and particle unit (BDPU)

Abstract: The Bubble, Drop, and Particle Unit (BDPU) is an instrument conceived to study the dynamics of transparent fluid mixtures in Spacelab’s microgravity environment. The scientific objectives of BDPU in its present configuration will essentially be the investigation of (1) bubbles, drops, and particles in liquids with nonuniform temperature and concentration fields, (2) formation and dynam-ics of the solidification fronts, and (3) interaction between intrusions and solidification fronts. The heart of the facility is the exchangeable test container which can, within some limits, be adapted to the investigator’s requirements. The diagnostics of the fluid under investigation are essentially opti-cal: direct view, slice view, Schlieren, interferometry, and thermography. Several devices are used for fluid management and production of bubbles and drops in liquid matrices. BDPU is a candidate for the IML-2 mission.

Liquid crystal phase modulator for large screen projection

Abstract: A liquid-crystal (LC) phase modulator was investigated as a possible substitute for the electrically deformable oil film used for image generation in some large-screen projectors. The LC causes a voltage-dependent phase shift of the incident light, which is converted into amplitude-modulated light by means of a schlieren optical system. To achieve great optical efficiency, the LC cell is designed to work without polarizers. When used with thin-film-transistor arrays as drivers, a grey-scale projection is possible. >

Bullseye color schlieren flow visualization

Abstract: In this variant of schlieren flow visualization a tricolored filter pattern made up of concentric rings is used instead of a knife edge at the cut-off plane. The method is particularly useful in combustion studies since it provides the range necessary to visualize flames while retaining the sensitivity necessary to detect fluid motion in the surrounding gasses. That utility is demonstrated in this paper in terms of basic principles and by application. Photographs presented include images of both premixed and diffusion flames in an unsteady swirling flow within a cylinder.

Laser sheet technique for visualizing a periodic rotor wake

Abstract: Introduction L sheets have been used to illuminate cross sections of steady flowfields for several years Intense sheets of light only a few millimeters in thickness can be produced by either expanding a single laser beam with a cylindrical lens or by rapidly sweeping the beam by means of a vibrating mirror or other optical device' Generally, smoke or other seed particles which scatter light as they pass through the laser sheet are introduced upstream of the region of interest Streamlines and vortical structures are identified respectively as streaks and swirl patterns in the illuminated region' Recent experiments conducted in the John J Harper 231 m x 274 m (7 x 9 ft) low-speed wind tunnel at the Georgia Institute of Technology employed a strobed laser sheet to visualize the periodic flowfield between a model rotor and an airframe in forward flight A nonintrusive method was developed to determine accurately tip vortex trajectories in a uniformly seeded flow Unlike Schlieren, shadowgraphic, or interferometric methods, the laser sheet technique is well-suited for visualizing the incompressible (low Mach number) flows common to rotorcraft Quantitative data valuable for use in the interpretation of vortex/airframe interaction measurements were gathered using this technique, as reported in Ref 5 Used in conjunction with laser velocimeter measurements, the laser sheet flow visualization approach provides a powerful means for documenting vortex jitter or other flow instabilities that can affect measurements The methods described are adaptable to many situations requiring visualization of a periodic flow structure

Freely expanding detonation products: Scaling of rate processes

Abstract: Using the Los Alamos reactive hydrodynamics code KIVA, calculations have been made to simulate the free expansion of cylinders of detonation products into a high vacuum. The emphasis of this paper is on the scaling of rate processes with cylinder size and initial conditions as a function of position in the expanding mass. The processes considered include diffusion, unimolecular decomposition, biomolecular radical reactions, and vibrational relaxation. The calculations also give time-dependent velocity fields; schlieren images; and profiles of density, pressure, and temperature. Many features of the calculations can be compared with experimental observations, including time-delayed schlieren and shadowgraph snapshots, time-dependent absorption spectra, and time-of-arrival profiles of molecular species. Some unexpected insights, such as the effect of the equation of state on the shape of the expanding plume and the effect of position on the rate of quenching, are discussed. These calculations are being used to interpret the available experimental data and to design future experiments. 6 refs., 13 figs.

Beam-deflection technique for the measurement of electron density in laser-produced plasmas

Abstract: Beam-deflection techniques have been shown to perform well for measurements in fluid flows1 and flames.2 Because of the growing interest in laser-produced plasmas we have investigated the capability of beam-deflection techniques for plasma measurements. While other techniques, including Interferometry and schlieren techniques,3 are well established for measuring electron density In laser-produced plasmas, we show that a beam- deflection technique is simple to implement and has advantages over current techniques.

Flatness Analysis Of Hard Disks

Abstract: The special flatness requirements of hard disks are discussed. Color schlieren and grazing incidence interferometry, the two methods currently dominating the market, are compared with the more recently developed moire deflectometry method. We show that since the color schlieren does not detect vital disk flatness information and since interferometry is too vibration sensitive and relatively expensive, moire deflectometry is a superior method for flatness analysis of hard disks.

不足膨張噴流の干渉に関する研究 : 第1報,2個の平行な噴流の干渉特性

Abstract: From the standpoint of engineering, and industry or space engineering, it is important to clarify the basic interference characteristics of underexpanded jets issuing from several pipes towards the atmosphere. In this paper, the interference characteristics of three parallel underexpanded jets, with intervals between each jet axis four times the pipe inner diameters, were studied. In particular, the flow visualizations of both mean and instantaneous flows with time were conducted by the schlieren and shadowgraph methods. Moreover, the length of pseudo-shock waves, the mean pressure and velocity distributions along the vertical axes, and the equivelocity diagrams within a vertical section to the jet aces were investigated in detail. As result, it was found that three parallel jet flows could be classified into 4 patterns for mean Mach numbers, ranging from 1 to 2, at the pipe exit.