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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS1973

The critical state - A trapped wave model of vortex breakdown.

A model of vortex breakdown is presented, and its predictions are compared with the experiments of Sarpkaya (1971). The model is centered about a theory of long, weakly nonlinear waves propagating on critical flows in tubes of variable cross section. Although the weakly nonlinear theory must be extended beyond its domain of formal validity, many of the experimentally observed features of vortex breakdown are reproduced by the model. The description of the time evolution of the flowfield that is presented requires numerical calculations that are not simple, but some important conclusions may be determined by easy computations. In particular, the axial position of a breakdown may be found from a very simple equation.

Randall, J. D.↗
NASA NTRS1975

Time dependent breakdown in silicon dioxide films

An investigation was conducted regarding the possible existence of a time-dependent breakdown mechanism in thermal oxides of the type used as gate oxide in MOS circuits. Questions of device fabrication are discussed along with details concerning breakdown measurements and the determination of C-V characteristics. A relatively large prebreakdown current observed in one of the cases is related to the time-dependent breakdown.

Svensson, C.↗
NASA NTRS1975

Inherent time delay for dielectric breakdown in shock-loaded x-cut quartz

Dielectric breakdown in shock-loaded x-cut quartz is examined under conditions of both impact loading and Q-switched laser-irradiation loading. It is observed that breakdown is characterized by an inherent time delay which depends on the magnitude of the electric field. For pulse duration less than about 30 ns, dielectric breakdown is not observed.

Graham, R. A.↗
NASA NTRS1976

Time-dependent MOS breakdown

A general model for time-dependent breakdown in metal-oxide-silicon (MOS) structures is developed and related to experimental measurements on samples deliberately contaminated with Na. A statistical method is used for measuring the breakdown probability as a function of log time and applied field. It is shown that three time regions of breakdown can be explained respectively in terms of silicon surface defects, ion emission from the metal interface, and lateral ion diffusion at the silicon interface.

Li, S. P.↗
NASA NTRS1976

Effective defect density for MOS breakdown - Dependence on oxide thickness

A new procedure for measuring effective defect density takes time-dependent primary dielectric breakdown in MOS devices into account. A time-varying breakdown probability is obtained by applying a constant field stress to a statistically sufficient number of MOS capacitors. Effective defects are assumed distributed randomly in two dimensions, hence indistinguishable. The effective defect density is found to vary with oxide thickness; tentative explanations are ventured to account for this (interfacial tensile stress, ion patching enhanced by local trapping, breakdown in the form of filamentary discharges with release of stored electrostatic energy).

Li, S. P.↗
NASA NTRS1976

Solutions of the Navier-Stokes equations for vortex breakdown

A numerical investigation of vortex breakdown has been undertaken in an attempt to understand its properties, and the mechanisms responsible for it. Solutions of the full steady axisymmetric Navier-Stokes equations for breakdown in an unconfined viscous vortex have been obtained for core Reynolds numbers up to 200, for a two-parameter family of assumed upstream velocity distributions. Diffusion and convection of vorticity away from the vortex core, and the strong coupling between the circumferential and axial velocity fields in highly-swirling flows, are shown to lead to stagnation and reversal of the axial flow near the axis. The various theories of vortex breakdown are considered in light of the present numerical solutions.

Grabowski, W. J.↗
NASA NTRS1977

Record dielectric breakdown automatically

Automatic Monitoring System for time-dependent dielectric breakdown tests ninety-nine metal-oxide semiconductor capacitors simultaneously. Each breakdown generates voltage spike registered on readouts and indicated by LED. Latching circuit prevents recording of possible subsequent breakdowns in same capacitor. In addition to research use, system could also be adapted for quality control.

Bates, E. T., Jr.↗
NASA NTRS1976

Dielectric breakdown induced by picosecond laser pulses

The damage thresholds of transparent optical materials were investigated. Single picosecond pulses at 1.06 microns, 0.53 microns and 0.35 microns were obtained from a mode locked Nd-YAG oscillator-amplifier-frequency multiplier system. The pulses were Gaussian in space and time and permitted the determination of breakdown thresholds with a reproducibility of 15%. It was shown that the breakdown thresholds are characteristic of the bulk material, which included nine alkali halides, five different laser host materials, KDP, quartz, sapphire and calcium fluoride. The extension of the damage data to the ultraviolet is significant, because some indication was obtained that two- and three-photon absorption processes begin to play a role in determining the threshold. Throughout the visible region of the spectrum the threshold is still an increasing function of frequency, indicating that avalanche ionization is the dominant factor in determining the breakdown threshold. This was confirmed by a detailed study of the damage morphology with a high resolution microscope just above the threshold. The influence of self focusing is discussed, and evidence for beam distortion below the power threshold for complete self focusing is presented, confirming the theory of Marburger.

Smith, W. L.↗
NASA NTRS1977

Disrupted states of vortex flow and vortex breakdown

Flow visualization studies and laser Doppler anemometer measurements on swirling water flows reveal six distinct types of very large amplitude disturbance modes of the vortex core. Three, 'axisymmetric' and spiral vortex breakdowns, and the 'double helix,' have been described by others. A definite order of evolution in parameter space (Reynolds number and circulations) occurs, and is described. Puzzling responses of the axisymmetric and spiral vortex breakdowns to imposed flow transients reported previously are confirmed here, and are traced to the shedding of starting and stopping vortices from swirl vanes. Conclusions bearing upon the validity of some theories of vortex breakdown are possible from the data.

Faler, J. H.↗
NASA NTRS1978

An experimental map of the internal structure of a vortex breakdown

The flow field of an 'axisymmetric' vortex breakdown has been mapped using a laser-Doppler anemometer. The interior of the recirculation zone is dominated by energetic, non-axisymmetric, low frequency periodic fluctuations. Spectra for a number of points inside this zone, as well as time-averaged swirl and axial velocity profiles both inside and outside the recirculation zone, have been obtained. The time-averaged streamlines in the interior show an unexpected two-celled structure attributed to the action of the fluctuations. Although the present experiment deals with one particular breakdown, flow-visualization studies indicate that the case examined is typical of the 'axisymmetric' form of breakdown over a range of flow conditions.

Faler, J. H.↗
NASA NTRS1979

A group-velocity criterion for breakdown of vortex flow: An application to measured inlet profiles

Vortex flows exhibiting breakdown in a slightly divergent duct were measured. The slowly varying vortex flow field downstream of the entrance and upstream of the breakdown region is obtained numerically by using the inviscid quasi-cylindrical approximation. In these calculations, the Faler and Lebovich's experimental data were used as the starting conditions at the entrance of the duct. The group velocity of wave propagation for the axisymmetric mode (n = 0) and the asymmetric modes (n = + or - 1 and n = + or - 2) are calculated for the entrance conditions. For the theoretically predicted slowly varying flow field downstream of the entrance, the wave characteristics of the n = 0 and n = + or - 1 modes are presented. It was concluded that the flows which subsequently undergo vortex breakdown are all predicted to be supercritical and stable to infinitesimal inviscid disturbances, including the axially symmetric as well as the nonsymmetric perturbations.

Tsai, C. Y.↗
NASA NTRS1979

Spectral characteristics of vortex breakdown flowfields

Laser-Doppler anemometer measurements upstream and in the wakes of vortex breakdowns of bubble and spiral types are described. Spectral analysis of the data indicates prominent oscillations in the wakes at less than 10 Hz. These oscillations correspond closely to theoretical predictions of the linearly most unstable normal modes of the time-averaged mean flow profiles. The oscillations are more energetic, and vortex core expansions are greater for flows with a bubble form of vortex breakdown, which is therefore regarded as the stronger form of breakdown.

Garg, A. K.↗
NASA NTRS1982

Improved Model for MOS Breakdown

With an improved model, accelerated high-field tests can be used to predict gate-oxide breakdown in metal-oxide-semi-conductor (MOS) structures. Principal mechanism in MOS breakdown is mobile-ion emission from metal/oxide interface, which occurs during application of positive gate-bias field. Breakdown is related to clustering of emitted ions at localized defect sites in oxide/silicon interface. Using new model to interpret data, tests that normally take several weeks at low fields to accumulate sufficient statistics can be completed in only a few hours at high fields.

Li, S. P.↗
NASA NTRS1980

Characteristics of edge breakdowns on Teflon samples

The characteristics of electrical discharges induced on silverbacked Teflon samples irradiated by a monoenergetic electron beam have been studied under controlled laboratory conditions. Measurements of breakdown threshold voltages indicate a marked anisotropy in the electrical breakdown properties of Teflon: differences of up to 10 kV in breakdown threshold voltage are observed depending on the sample orientation. The material anisotropy can be utilized in spacecraft construction to reduce the magnitude of discharge currents.

Yadlowsky, E. J.↗
NASA NTRS1985

Viscous flows in rotating pipes with internal cavities and related vortex breakdowns

The problem of predicting vortex breakdown, which is characterized by a drastic change in core size and trajectory, is still basically unsolved. In the present paper, viscous flow in a long rotating pipe is analyzed, as a model for the type of breakdown observed in wake vortices. The full Navier-Stokes equations are shown to simplify to a set of unlinked partial differential equations. These equations allow for two separate solutions, for both full pipe flows (the classic rotating Hagen-Poiseuille flow) and cavity flows, in which a central area, with no net mass flux or viscous dissipation, is produced. Conditions permitting switching from one type of flow to the other, with no change in the mass flux or velocities at the pipe surface, are found. It is then argued that these conditions indicate when vortex breakdown will occur.

Weihs, D.↗
NASA NTRS1985

Vortex breakdown simulation

A vortex breakdown was simulated by the vortex filament method, and detailed figures are presented based on the results. Deformations of the vortex filaments showed clear and large swelling at a particular axial station which implied the presence of a recirculation bubble at that station. The tendency for two breakdowns to occur experimentally was confirmed by the simulation, and the jet flow inside the bubble was well simulated. The particle paths spiralled with expansion, and the streamlines took spiral forms at the breakdown with expansion.

Nakamura, Y.↗
NASA NTRS1985

Water permeation and dielectric breakdown. Water permeability in Pub Tedlar. Pub/Tedlar as a function of temperature and humidity

Moisture transport and dielectric breakdown of polyvinyl butyral (PVB), Tedlar, and PVB/Tedlar composites were addressed. Data for the temperature range between 20 and 80 C showed that the moisture flux through the composite is governed by the slower material; and that the composite permeability is intermediate to those of the component material, as predicted by theory. Data for Tedlar at 71 C, showing the dependence of moisture flux on relative humidity, was also presented. Dielectric breakdown data were less precise and less conclusive. The generally applied theoretical model does not match the experimental data. The PVB/Tedlar composite exhibited greater voltage breakdown resistance than either component. Testing of EVA and EVA/Tedlar composites is underway.

Orehotsky, J.↗
NASA NTRS1985

System technology analysis of aeroassisted orbital transfer vehicles: Moderate lift/drag (0.75-1.5). Volume 3: Cost estimates and work breakdown structure/dictionary, phase 1 and 2

Technology payoffs of representative ground based (Phase 1) and space based (Phase 2) mid lift/drag ratio aeroassisted orbit transfer vehicles (AOTV) were assessed and prioritized. A narrative summary of the cost estimates and work breakdown structure/dictionary for both study phases is presented. Costs were estimated using the Grumman Space Programs Algorithm for Cost Estimating (SPACE) computer program and results are given for four AOTV configurations. The work breakdown structure follows the standard of the joint government/industry Space Systems Cost Analysis Group (SSCAG). A table is provided which shows cost estimates for each work breakdown structure element.

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