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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 649 records · Page 36

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NASA NTRS1980

High-frequency sound propagation in a spatially varying mean flow

An equation for acoustic ray paths in a spatially varying mean flow was examined to determine some of the characteristics of the flow gradient effects on sound propagation. In a potential flow, the acoustic rays are deflected in the direction of increasing mean flow, and the gradient of the mean flow speed is the dominant factor causing the ray deflection. In contrast, in a sheared mean flow, the vorticity is the dominant factor in deflection of the acoustic rays.

Cho, Y. C.↗

NASA NTRS1981

High-frequency sound propagation in a spatially varying mean flow

An equation for acoustic ray paths in a spatially varying mean flow has been examined to determine some of the characteristics of the flow gradient effects on sound propagation. In a potential flow the acoustic rays are deflected in the direction of increasing mean flow, and the gradient of the mean flow speed is the dominant factor causing the ray deflection. In contrast, in a sheared mean flow, the vorticity is the dominant factor in deflection of the acoustic rays.

Cho, Y. C.↗

NASA NTRS1981

High frequency scattering by a thin dielectric slab

The geometrical theory of diffraction (GTD) is applied to the problem of a source in the presence of a three-dimensional thin dielectric slab. It is assumed that the reflection and transmission properties of the slab are known, and the slab is not lossy and/or multilayered. Two-dimensional thin dielectric layer scattering is analyzed when diffraction emanates from a single point, the source is not near the slab, and random energy approaching tangentially due to transmission is negligible. Coefficients are obtained for the scattering and the GTD solution is extended to the three dimensional case after development of a ray fixed coordinate system. The results are applied to a half wavelength dipole mounted above a square polystyrene covered ground plane.

Burnside, W. D.↗

NASA NTRS1983

Lightweight, high-frequency transformers

The 25-kVA space transformer was developed under contract by Thermal Technology Laboratory, Buffalo, N. Y. The NASA Lewis transformer technology program attempted to develop the baseline technology. For the 25-kVA transformer the input voltage was chosen as 200 V, the output voltage as 1500 V, the input voltage waveform as square wave, the duty cycle as continuous, the frequency range (within certain constraints) as 10 to 40 kHz, the operating temperatures as 85 deg. and 130 C, the baseplate temperature as 50 C, the equivalent leakage inductance as less than 10 micro-h, the operating environment as space, and the life expectancy as 10 years. Such a transformer can also be used for aircraft, ship and terrestrial applications.

Schwarze, G. E.↗

NASA NTRS1984

Evaluation of single crystal LaB6 cathodes for use in a high frequency backward wave oscillator tube

The results of thermionic emission and evaporation studies of single crystal LaB6 cathodes are given. A comparison between the (100), (210) and (310) crystal planes shows the (310) and (210) planes to possess a work function approx 0.2 eV lower than (100). This translates into a significant increase in current density, J, at a specified temperature. Comparison with a state-of-the-art impregnated dispenser cathode shows that LaB6 (310) is a superior cathode in nearly all respects except operating temperature at j 10 A/sq cm. The 1600 K thermionic and room temperature retarding potential work functions for LaB6 (310) are 2.42 and 2.50 respectively.

Swanson, L. W.↗

NASA NTRS1982

A study of the high frequency limitations of series resonant converters

A transformer induced oscillation in series resonant (SR) converters is studied. It may occur in the discontinuous current mode. The source of the oscillation is an unexpected resonant circuit formed by normal resonance components in series with the magnetizing inductance of the output transformers. The methods for achieving cyclic stability are: to use a half bridge SR converter where q0.5. Q should be as close to 1.0 as possible. If 0.5q1.0, the instability will be avoided if psi2/3q-1/3. The second objective was to investigate a power field effect transistor (FET) version of the SR converter capable of operating at frequencies above 100 KHz, to study component stress and losses at various frequencies.

Stuart, T. A.↗

NASA NTRS1984

Observations of deep convection from an airborne high-frequency (92 and 183 GHz) passive microwave radiometer

Spencer et al. (1983) have reported that very low Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR) brightness temperatures at 37 GHz over land coincide with heavy thunderstorm rainfall, while Wilheit et al. (1982) used an aircraft-mounted radiometer operating at 92 and 183 GHz to observe convective precipitation associated with a tropical storm over the ocean. A scanning version of the instrument employed by Wilheit et al. is the Advanced Microwave Moisture Sounder (AMMS). The present paper has the objective to summarize the preliminary results of AMMS observations of convective raining clouds and to determine whether empirical relationships between rain rate and microwave brightness temperature, such as those developed for 37 GHz satellite data by Spencer et al., can be extended to higher microwave frequencies.

Hakkarinen, I. M.↗

NASA NTRS1984

Designing a 25-kilowatt high frequency series resonant

The feasibility of processing 25 kW of power with a single, transistorized, 20 kHz, series resonant converter stage has been demonstrated by the successful design, development, fabrication, and testing of such a device. It employs four Westinghouse D7ST transistors in a full-bridge configuration and operates from a 250-to-350-Vdc input bus. The unit has an overall worst-case efficiency of 93.5% at its full rated output of 1000 V and 25 A dc. A solid-state dc input circuit breaker and output-transient-current limiters are included in and integrated into the design. Circuit details of the converter are presented along with test data.

Robson, R. R.↗

NASA NTRS1986

High frequency sampling of the 1984 spring bloom within the mid-Atlantic Bight: Synoptic shipboard, aircraft, and in situ perspectives of the SEEP-I experiment

Moorings of current meters, thermistors, transmissometers, and fluorometers on the mid-Atlantic shelf, south of Long Island, suggest a cumulative seaward export of perhaps 0.35 g C/sq m/day between the 80 and 120 m isobaths during February-April 1984. Such a horizontal loss of algal carbon over the lower third of the water column would be 23 to 78% of the March-April 1984 primary production. This physical carbon loss is similar to daily grazing losses from zooplankton of 32-40% of the algal fixation of carbon. Metabolic demands of the benthos could be met by just the estimated fecal pellet flux, without direct consumption of algal carbon, while bacterioplankton needs could be served by excretory release of dissolved organic matter during photosynthesis. Sediment traps tethered 10 m off the bottom at the 120 m isobath and 50 m above the 500 m isobath caught as much as 0.16 to 0.26 g C /sq m/day during March-April 1984, in reasonable agreement with the flux estimated from the other moored instruments.

Walsh, J. J.↗

NASA NTRS1986

On uniformly valid high-frequency far-field asymptotic solutions of the Helmholtz equation

An asymptotic, large wave number approximation for the Helmholtz equation is derived. The theory is an extension of the geometric acoustic theory, and provides corrections to that theory in the form of multiplicative functions which satisfy parabolic equations. A simple example is used both to illustrate failure of the geometric theory for large propagation distances, and to show the improvement obtained by use of the new theory.

Mcaninch, G. L.↗

NASA NTRS1986

High-frequency noise of In(y)Ga(1-y)As/Al(x)Ga(1-x)As MODFETs and comparison to GaAs/Al(x)Ga(1-x)As MODFETs

Noise parameter measurements for recently developed 1 micron gate In(y)Ga(1-y)As/Al(0.15)Ga(0.85)As MODFETs have been performed at 8 GHz art room and cryogenic temperatures. Owing to the relatively small C(gs/sq rt) g(m) ratio in these devices compared to identical conventional GaAs/AlGaAs MODFETs, both room- and cryogenic temperature noise figures have been reduced. In addition, the light sensitivity and drift in noise figure at cryogenic temperatures observed in conventional GaAs/AlGaAs MODFETs have been substantially reduced.

Morkoc, H.↗

NASA NTRS1987

Designing High-Frequency Inductors

Procedures for ferrite-core and Molypermalloy-core inductors detailed. Report presents design procedures for two types of highfrequency inductors: those with iron or ferrite cores and lumped airgaps and those with Molypermally powder cores and distributed airgaps.

Mclyman, C. W.↗

NASA NTRS1987

Resistojet control and power for high frequency ac buses

Resistojets are operational on many geosynchronous communication satellites which all use dc power buses. Multipropellant resistojets were selected for the Initial Operating Capability (IOC) Space Station which will supply 208 V, 20 kHz power. This paper discusses resistojet heater temperature controllers and passive power regulation methods for ac power systems. A simple passive power regulation method suitable for use with regulated sinusoidal or square wave power was designed and tested using the Space Station multipropellant resistojet. The breadboard delivered 20 kHz power to the resistojet heater. Cold start surge current limiting, a power efficiency of 95 percent, and power regulation of better than 2 percent were demonstrated with a two component, 500 W breadboard power controller having a mass of 0.6 kg.

Gruber, Robert P.↗

NASA NTRS1987

Resistojet control and power for high frequency ac buses

Gruber, Robert P.↗

NASA NTRS1988

A high frequency GaAlAs travelling wave electro-optic modulator at 0.82 micrometers

Experimental GaAlAs modulators operating at 0.82 micrometers using a Mach-Zehnder interferometer configuration were designed and fabricated. Coplanar 50 ohm travelling wave microwave electrodes were used to obtain a bandwidth length product of 11.95 GHz-cm. The design, fabrication and dc performance of the GaAlAs travelling wave modulator is presented.

Chorey, Christopher M.↗

NASA NTRS1988

Decomposition of hydrazine by high frequency glow electrical discharges

Originally, de Groot et al. examined electrical conductivity phenomena in terms of the thermodynamics of irreversible processes. They considered the system to be discontinuous and calculated only the differences between the opposing ends of the process. Thus these theories are phenomenological. In the present work, thermodynamics is developed as a continuous system based on hydrodynamics and the entropy production for a thin capillary. It is shown that electroconductivity can be interpreted without difficulty. For the electric double layer, the effects of electrolytes and surface conductivity and the gap width were calculated for the Gouy model.

Suzuki, M.↗

NASA NTRS1988

High frequency GaAlAs modulator and photodetector for phased array antenna applications

A waveguide Mach-Zehnder electro-optic modulator and an interdigitated photoconductive detector designed to operate at 820 nm, fabricated on different GaAlAs/GaAs heterostructure materials, are being investigated for use in optical interconnects in phased array antenna systems. Measured optical attenuation effects in the modulator are discussed and the observed modulation performance up to 1 GHz is presented. Measurements of detector frequency response are described and results presented.

Claspy, P. C.↗

NASA NTRS1987

Modal survey tests for high-frequency vibration analysis of the Space Shuttle Main Engine

Variations in the modal characteristics of different Space Shuttle Main Engines (SSMEs) are presented herein to study their structural dynamics vibrations, especially under fluid/structure interaction. The results of extensive modal survey tests and analysis relative to significant features in the modal characteristics of different powerheads and components are discussed.

Panossian, H. V.↗