Showing papers on "Schlieren published in 1977"

Optical observation of pre-breakdown and breakdown phenomena in transformer oil

Abstract: High-speed time-resolved Schlieren photographs are taken between impulse voltage application and breakdown in transformer oil using an image convertor camera at various applied voltages and framing speeds. Needle-to-plate electrodes are used and the oil is under vacuum. The pre-breakdown disturbance accompanied by shock waves emanates from the needle electrode for both polarities of the needle point. The disturbance from the negative point propagates step by step like a lightning discharge in air, while that from the positive point moves radially and reaches the plate electrode in one step. The pre-breakdown disturbance seems to be a gaseous phase which is generated in the liquid.

High spatial resolution schlieren photography.

Abstract: Some geometrical optical considerations for attaining spatial resolution of about 100 microM in a photographic schlieren experiment are presented. The topics of astigmatism, coma, and spherical aberration are considered briefly. Special emphasis is directed to diffraction effects in a single-mirror schlieren instrument. A method for estimating the size of limiting apertures in the knife-edge plane to allow a particular spatial resolution, compromised vs photographic sensitivity, is reported. A sample application for photographing the schlieren effects in a small spark discharge is included.

Optical studies of prebreakdown events in liquid dielectrics

Abstract: A combination of time-resolved Schlieren and scattered-light optical techniques has been used to examine the prebreakdown events in dielectric liquids. When a negative high-voltage pulse is applied to the point of a nonuniform gap, a disturbance, associated with discrete shock waves, propagates outwards from the point. For the opposite polarity, the disturbance moves at supersonic velocity, and is characteristically different in appearance and self luminosity. In both cases, the disturbance is identified as a region of closely packed microbubbles which ultimately fragments into isolated bubbles if breakdown does not intervene. The events recorded are explained in terms of cavitation nucleated by transient cavity collapse, and the polarity differences are attributed to field emission. The mechanism is further supported by experimental findings using prestressing techniques.

High-speed schlieren studies of electrical breakdown in liquid hydrocarbons

Abstract: High-speed schlieren studies have been carried out on the electrical breakdown of liquid hydrocarbons using a pulsed ruby laser. The development and propagation of charge carrier trajectories were recorded photographically on prebreakdown, breakdown, and postbreakdown events occurring in very pure, oxygen-free samples of hexane, benzene, toluene, and isooctane using a parallel plate electrode geometry. The initiation of breakdown was characterized by the growth of narrow streamers the creation of which was attributed to field injected electrons at local asperities of the cathode surface. Once the streamers reached the anode, large currents were found to flow through the gap leading to formation of a plasma column. Subsequent emissions occurring at the anode were attributed to field reversal. Details of the schlieren pictures have been interpreted in terms of classical electromagnetic theory.

Investigations into the structure of jet diffusion flames using time-resolved optical measuring techniques

Abstract: This paper describes an investigation into the turbulent structure of small jet diffusion flames, using a variety of measuring techniques, each being capable of time resolution. These include laser Doppler anemometry, quantitative Schlieren, cross-beam correlation techniques and thermocouple probes. Spectra are reported. The Schlieren spectra display both high and low frequency structures. In the flames the high frequency peaks, which are associated with vortex shedding, are found to be significantly different from those observed in the equivalent cold jets. Both the velocity and Schlieren spectra indicate that heat release has considerable influence upon the dissipation of the higher wave number turbulence fluctuation. The Schlieren spectra are dominated by a low frequency phenomenon, which appears to be of a quasi-laminar nature. This structure is found to be a well defined oscillation in the laminar flames; although undergoing a spectral broadening, it persists in the higher velocity turbulent flames investigated. Comparison of these spectra with far field noise spectra shows significant differences, although again indicating a dominance of low frequency structure.

Instabilities in detonation waves near the limits of propagation.

Abstract: Experimental work is described on the propagation of marginal detonation waves in propane-oxygen and hydrogen-oxygen systems diluted with oxygen, nitrogen and argon. The detonation tube was of rectangular section, with internal dimensions 2.3*1.0 cm, and overall length of 30 m. A continuous monitor of the wavefront velocity was achieved using a 3 cm wavelength microwave Doppler system. A cyclic wave phenomenon which is characterized by a normal and a slow velocity regime, is observed to be present near the limits of detonability in all except the hydrogen-oxygen-argon system. The relative durations of these regimes varies from one system to another and appears to be governed by the structural regularity of the detonation front. Schlieren records show that in the low-velocity regime the shock front and reaction zone are dissociated, with a separation varying between 1 and 10 cm, and that a weak transverse wave effect appears to be present at the lead shock which is non-planar.

Interferometry with schlieren microscopy.

Abstract: It is demonstrated theoretically that if the stop in a schlieren system is no larger than the central maximum of the empty field Airy disk at the stop location, the system will behave as an interferometer for objects much smaller than the empty field. Interferograms produced by such a system are shown.

Resonant optical schlieren system for plasma flow studies

Abstract: A dye laser, pumped by ruby second harmonic radiation at 3471 A, emitting ∼5‐mJ, 20‐nsec pulses with a bandwidth of 0.3 A, has been developed as a light source in a schlieren optical system which utilizes enhanced refractivity to examine plasma flow phenomena. When the dye laser is tuned adjacent to the BaII ion resonance line at 4554 A, the sensitivity of the system to density gradients in a barium plasma becomes 100–1000 times greater than can be achieved with nonresonant light sources which rely on the free electron contributions to plasma refractivity. Absorption effects also yield data on plasma flow patterns.

Visualization of ultrasonic waves in air

Abstract: It is not generally recognized that airborne ultrasonic waves can be visualized by means of schlieren. We show that waves in the upper kilohertz region generated by an electrostatic transducer can be visualized with a diffraction‐limited optical system.

TV projection display using an amorphous-se-type Ruticon light valve

Abstract: A display system is described which uses a new high sensitivity a(amorphous)-Se-type Ruticon and projects TV-like images from a conventional 9-in diagonal monitor onto a 40 × 40-in screen. The a-Se-type Ruticon requires 200-400-V dc bias, f1.4 lens onto the photoconductor side of the Ruticon. The system uses no mechanical shutters, and image erasure is accomplished with exposure from a flash tube. Readout is performed with a schlieren system using an f2.0 projector lens, and a 360-W tungsten halogen lamp and a high gain screen. The readout optical system is capable of producing a maximum of 8.8 fc and 22:1 contrast ratio. Actual output brightness and contrast ratio in real-time operation are limited by the performance of the light valve. The image-storage characteristic of the Ruticon introduces brightness nonuniformity to the projected image.

Population and profile display of transparent bodies in a transparent mass

Abstract: Population and profile data of transparent bodies contained within a transparent mass are obtained using a schlieren optical system in combination with a television camera and receiver. The transparent bodies, normally indistinguishable from the transparent mass by the unaided eye, are detected by the schlieren optical system and then magnified and contrasted on the television receiver. The invention is especially useful as a means for quality control in various chemical operations.

An electrostatically driven surface for flexible wall drag reduction studies

Abstract: This paper describes the theoretical analysis, experimental development and surface motion data from an electrostatically driven flexible wall capable of producing high frequency, large amplitude, short wavelength standing wave motions. The driven wall was developed for use in studying flexible wall-turbulent boundary layer interactions. The theoretical analysis considers the coupled electrochemical system using a continuum model. For the system under consideration the electrical field is uncoupled from the mechanical system leading to a simplified analysis. The structural problem is a non-linear (moderately) large amplitude problem and details of the analysis are presented. Surface motion measurements, made using a modified schlieren type optical system, are discussed and details of the surface motion under different excitations are presented. Fair agreement is obtained between experimental measurements and theoretical calculations.

A triangular path inverting interferometer

Abstract: A triangular path inverting interferometer is described with application to the study of thermal 'schlieren'. This is practically free of any vibration and coherence troubles, and possesses the unique feature that either differential or total shear may be obtained only with proper positioning of the object; once aligned, the optical components need not be disturbed further. This simple and stable set up may find applications in diverse fields of schlieren studies for which the common forms of two beam interferometers are more often employed.

Investigation Of Very Short Cavitation Shock Waves By Coherent Optical Methods

Abstract: During the collapse of cavitation bubbles in fluids very short and intense shock waves are radiated. Their study is of interest since they contain information about the final collapse phase which is very difficult to record directly. Furthermore, the shock pulses may contribute to the material damage associated with cavitation. Special optical methods taking advantage of the coherence and intensity of laser light have been developed to gain essential data about the shock waves like pulse width and maximum pressure. In one method, classical techniques of observing phase objects like Schlieren or dark-ground are modified in such a way that the accordingly filtered image of the travelling shock wave is scanned by a small pinhole of 5 pm diameter in front of a photomultiplier. In a second method the far-field diffraction pattern produced by the shock wave is recorded photographically. Since this pattern, which is the spatial power spectrum of the shock wave, is independent of the position of the wave, exposure times may be much longer than those required in direct observation of the wave due to its fast propagation. Results of both methods agree rather well and yield pulse widths of the order of lo nsec and pressure amplitudes of some 5o atm referred to 1 mm distance from the center of collapse.

Momentum transfer to target from laser-produced plasma

Abstract: The impact transferred from laser‐produced plasma to target was measured, and the formed plasma was studied by schlieren and shadow photographs. The cylindrical detonation wave theory was in general agreement with the plasma features in the early energy deposition phase but failed to predict the momentum transfer from plasma to target. The momentum transfer agreed with the theory of spherical blast wave which follows the cylindrical detonation wave.

Boiling sublayers along heated walls

Abstract: The development of two-phase boundary layers along heated walls has been investigated by High-speed cinematography and Schlieren techniques. Particular reference is done for mixing and D.N.B. problems in L.W.R. rod bundles. In two-phase boundary layers, evidenced by Schlieren techniques, a determinant role is played by a boiling boundary sublayer which develops along the heated wall.

FURTHER INVESTIGATIONS ON THE ABNORMAL PHENOMENA OBSERVED IN THE LIGHT TRANSMITTED BY α-LiIO 3 SINGLE CRYSTALS UNDER THE ACTION OF AN ELECTROSTATIC FIELD

Abstract: The abnormal phenomena in the transmitted light passing through α-LiIO 3 single crystals under the application of a DC field parallel to the c-axis have been further investigated by means of the direct projection method, the optical Fourier transfor-mation method and the Schlieren method. The change in the spacial frequency spectrum of a z-cut crystal plate with the increase of the applied field intensity was examined. It was observed that the diffraction efficiency of the transmitted light shows a linear dependence on the applied voltage, but is independent of the polariza-tion of the incident light. It was also found that the light polarization is not changed by the action of the DC field and the effect of absorption is negligible. The relaxa-tion process of the change in diffraction efficiency with time was recorded quantita-tively. A tentative explanation of these phenomena has been given.

Electrostatic Probe Measurements in Propagating Flames : Part 2, The Relation between Flame Propagation Velocity and Ion Current

Abstract: An experimental study has been made on electrostatic probe measurements in flames propagating through a stoichiometric propane-air mixture in a tube in the initial pressure range from 200 to 500 mm Hg. The ion current records obtained by an electrostatic probe were examined in detail and were compared with the flame behavior recorded on the schlieren photographs taken with a high-speed camera. The results show that the flame behavior inferred from the ion current records coincides well with that observed on schlieren photographs. The maximum ion current was found to increase with the increase of the gas velocity in the high ion density region near the propagating flame front. The phenomena near the turbulent flame front could be understood to some extent from the ion current records.

A schlieren study of the generation of different types of ultrasonic waves in thin plates

Abstract: Using Schlieren photography a study of the wave generation process in thin material has been performed providing an illustration as to how the waves originate and their dependence on material thickness and ultrasonic beam shape.

Optical Measurement of Laser-Driven Imploding Shock Wave

Abstract: In this laser-plasma experiments, technique of simultaneous optical measurement using Jamin interferometer, Schlieren method and shadowgraph was developed. The spatial and temporal resolutions of these methods were 5 µm and 30 psec, respectively. The existence of a shock wave propagating into the target was confirmed. The energies of shock waves in the target and in air were estimated by using the blast wave theory.

Holographic flow visualization at the Langley CF4 tunnel

Abstract: A holographic flow visualization system was used to obtain shadowgraph, schlieren, and interferograms of the flow field at Langley's hypersonic (Mach 6) CF4 tunnel. The dual hologram technique which was used makes it possible to vary focusing, knife-edge position, and the orientation and spacing of the interference fringes after a tunnel run. The experimental arrangement necessary to produce high quality interferograms is discussed. Typical shadowgraphs, schlieren pictures, and interferograms are presented.

Analysis Of Microscopic Imaging

Abstract: A simple method of image analysis suitable for arbitrary objects under partially coherent illumination is presented and applied to phase, schlieren, and interference microscopy. While approximate in the sense that diffraction is ignored, the method permits analysis using only algebra, resulting in a mathematical image in closed form.