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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 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

Phase linearity of the 914H coupled-cavity traveling wave tube

Tests of phase deviation from linearity were made on two 914H coupled-cavity traveling wave tubes (TWT). One tube had a voltage standing wave ratio (VSWR) of 2.4 and the other 1.4. The data showed that phase deviation is primarily a function of the amplitude and shape of the output VSWR. It was predicted that the low-VSWR tube would give a better system performance than the tube with a high VSWR. This prediction was confirmed by the Advanced Communications Technology Satellite (ACTS) system tests performed at the NASA Lewis Research Center. A possible improvement in the construction and stability of coupled-cavity TWT's is discussed.

Kavanagh, Frank E.↗
NASA NTRS1977

Potential damage to DC superconducting magnets due to the high frequency electromagnetic waves

Experimental data are presented in support of the hypothesis that a dc superconducting magnet coil does not behave strictly as an inductor, but as a complicated electrodynamic device capable of supporting electromagnetic waves. Travel times of nanosecond pulses and evidence of sinusoidal standing waves were observed on a prototype four-layer solenoidal coil at room temperature. Ringing observed during switching transients appears as a sequence of multiple reflected square pulses whose durations are related to the layer lengths. With sinusoidal excitation of the coil, the voltage amplitude between a pair of points on the coil exhibits maxima at those frequencies such that the distance between these points is an odd multiple of half wavelength in free space. Evidence indicates that any disturbance, such as that resulting from switching or sudden fault, initiates multiple reflections between layers, thus raising the possibility for sufficiently high voltages to cause breakdown.

Gabriel, G. J.↗
NASA NTRS1989

Acoustic waves in gases with strong pressure gradients

The effect of strong pressure gradients on the acoustic modes (standing waves) of a rectangular cavity is investigated analytically. When the cavity response is represented by a sum of modes, each mode is found to have two resonant frequencies. The lower frequency is near the Viaesaela-Brundt frequency, which characterizes the buoyant effect, and the higher frequency is above the ordinary acoustic resonance frequency. This finding shows that the propagation velocity of the acoustic waves is increased due to the pressure gradient effect.

Zorumski, William E.↗
NASA NTRS1989

Middle atmosphere thermal structure during MAP/WINE

Middle atmosphere temperatures were measured during the MAP/WINE campaign by various ground-based techniques, by rocket instruments, and by satellites. Respective data were analyzed for atmospheric thermal mean state as well as for long and short period variations. A brief survey of the results is given. Monthly mean temperatures agree well with the new CIRA model. Long period (planetary) waves frequently exhibit peculiar vertical amplitude and phase structures, resembling those of standing waves. Short period oscillations tend to begin breaking well below the stratosphere.

Offermann, D.↗
NASA NTRS1988

Multipoint measurements of upstream waves

Two-wave MHD populations are seen at collisionless shocks: precursor waves standing in the shock ramp which form an integral part of the shock and upstream waves which are usually attempting to propagate upstream but are carried back toward the shock by the solar wind flow. Both types of waves are observed at interplanetary shocks and planetary bow shocks. The difficulty in studying interplanetary shocks is that the shock normal is hard to determine accurately but multiple spacecraft measurements are of some assistance in this regard. Two types of multispacecraft studies have been used, closely spaced ones such as with ISEE-1 and -2 and more distantly separated ones such as with ISEE and UKS. These studies suggest that the paradigm proposed here for the evolution of large amplitude or 'fully developed' turbulence needs some revision.

Russell, C. T.↗
NASA NTRS1991

The location of the source of high-frequency solar acoustic oscillations

Recently Libbrecht and Jefferies et al. have reported regular peaks in the solar oscillation power spectrum extending well above 5.3 mHz, the maximum frequency of trapped acoustic modes. Kumar et al. argued that these peaks are primarily due to the interference of traveling waves which are excited due to acoustic emission from turbulent convection. In contrast with the standing wave P-mode frequencies below 5.3 mHz, the positions of the high-frequency interference peaks (HIPs) are dependent on the location of the source of the acoustic oscillations. In the present work, Kumar et al.'s argument is strengthened, and more importantly, use is made of the above dependence to determine the acoustic source strength as a function of depth. It is found that the acoustic source profile, and thus the convective velocity, is peaked about 200 km deeper than what is expected from standard mixing length theory.

Kumar, Pawan↗
NASA NTRS1983

Theory of hydromagnetic waves in the magnetosphere

A survey of theoretical and experimental research on the origin and characteristics of low-frequency hydromagnetic (HM) waves in the magnetosphere is presented, with a focus on advances in theory made in the last ten years. Basic wave theory and a collisionless plasma theory are applied to the magnetosphere as a HM system. Continuous energy sources are considered, such as the Kelvin-Helmholtz instability, the ring-current plasma, and drift instabilities. Other topics discussed include the theory of inhomogeneous HM waves, signal behavior in atmosphere and ionosphere, Alfven waves and ionosphere-magnetosphere coupling, Pi2 signals, damping, pulsating aurora, heavy-ion scattering, and standing waves in high-speed flows (like the wake phenomena caused on Jupiter by the passing of Io, observed by Voyager 1).

Southwood, D. J.↗
NASA NTRS1973

Analysis of the backscatter spectrum in an ionospheric modification experiment

Predictions of the backscatter spectrum are compared, including effects of ionospheric inhomogeneity with experimental observations of incoherent backscatter from an artificially heated region. Calculations show that the strongest backscatter echo received is not, in fact, from the reflection level, but from a region some distance below (about 0.5 km for an experiment carried out at Arecibo), where the pump wave from a HF transmitter approximately 100 kW) is below the threshold for parametric amplification. By taking the standing wave pattern of the pump into account, asymmetry is explained of the up-shifted and down-shifted plasma lines in the backscatter spectrum, and the several peaks typically observed in the region of the spectrum near the HF transmitter frequency.

Kim, H.↗
NASA NTRS1978

Theoretical and experimental studies of the atmospheric sodium layer

Atmospheric atomic sodium was studied with a laser radar system. Photocount data were processed using a digital filter to obtain continuous estimates of the sodium concentration versus altitude. Wave-like structures in the sodium layer were observed, and there was evidence for the presence of a standing wave in the layer. The bottomside of the layer was observed to undulate with a period of about 2 1/2 hours, and the layer was observed to broaden through the night. A meteor ablation-cluster ion theory of sodium was developed. The theory shows good agreement with existing atmospheric observations as well as laboratory measurements of rate constants.

Richter, E. S.↗
NASA NTRS1989

Filamentation instability of magnetosonic waves in the solar wind environment

Intense magnetosonic waves, originally propagating at the right angle with the interplanetary magnetic field, can excite a purely growing mode along the interplanetary magnetic field together with two symmetric magnetosonic sidebands propagating obliquely across the magnetic field. This instability process leads to the filamentation of the magnetosonic pump waves. These two excited magnetosonic sideband modes propagate together perpendicularly across the magnetic field and, meanwhile, form a standing wave pattern along the magnetic field. The thresholds of this filamentation instability can be exceeded in the solar wind environment. It is predicted that the density fluctuations produced by the filamentation instability along the interplanetary magnetic field have wavelengths greater than, at least, a few earth radii. The polarization of the obliquely propagating magnetosonic waves excited by the filamentation instability is determined by the characteristics of the magnetosonic pump waves and the environmental plasmas.

Kuo, S. P.↗
NASA NTRS1975

Material suspension within an acoustically excited resonant chamber

A method is described for positioning an object within a chamber, which is especially useful in performing manufacturing operations under zero gravity conditions. Sound waves are applied within the chamber in different directions and at a frequency for each direction that establishes a standing wave pattern so that the object is automatically urged towards the intersections of the nodes, or locations of minimum pressure.

Wang, T. G.↗
NASA NTRS1985

ISEE 1 and 2 observation of the spatial structure of a compressional Pc5 wave

A compressional Pc5 was observed on an ISEE 1 and 2 outbound path on September 28, 1981 at L = 5.6-7.3 near the magnetic equator at about 10 hr local time during the recovery phase of a geomagnetic storm. The wave propagated westward with a large azimuthal wave number of about 30 and exhibited in-phase oscillations of plasma density and magnetic field magnitude. During this event, component-dependent variations in phase and amplitude were observed for the magnetic field oscillations. The radial and compressional components had a constant phase and their amplitude was finite. In contrast, the azimuthal component changed its phase by 180 deg and its amplitude became zero during the middle of the wave event. The observation as indicating the spatial structure of the Pc5 wave is interpreted. The polarization reversal is likely to be caused by a crossing of a node of a standing wave located 4 deg above the geomagnetic equator.

Takahashi, K.↗
NASA NTRS1989

Acoustic wave propagation in heterogeneous structures including experimental validation

A finite element model was developed to solve for the acoustic pressure and energy fields in a heterogeneous suppressor. The derivations from the governing equations assumed that the material properties could vary with position resulting in a heterogeneous variable property two-dimensional wave equation. This eliminated the necessity of finding the boundary conditions between different materials. For a two media region consisting of part air and part bulk absorber, a model was used to describe the bulk absorber properties in two directions. Complex metallic structures inside the air duct are simulated by simply changing element properties from air to the structural material in a pattern to describe the desired shapes. To verify the numerical theory, experiments were conducted without flow in a rectangular duct with a single folded cavity mounted above the duct and absorbing material mounted inside a cavity. Changes in a nearly plane wave sound field were measured on the wall opposite the absorbing cavity. Fairly good agreement was found in the standing wave pattern upstream of the absorber and in the decay of pressure level opposite the absorber, as a function of distance along the duct. The finite element model provides a convenient method for evaluating the acoustic properties of bulk absorbers.

Baumeister, Kenneth J.↗
NASA NTRS1989

Acoustic wave propagation in heterogeneous structures including experimental validation

A finite element model was developed to solve for the acoustic pressure and energy fields in a heterogeneous suppressor. The derivations from the governing equations assumed that the material properties could vary with position resulting in a heterogeneous variable property two-dimensional wave equation. This eliminated the necessity of finding the boundary conditions between different materials. For a two-media region consisting of part air and part bulk absorber, a model was used to describe the bulk absorber properties in two directions. Complex metallic structures inside the air duct are simulated by simply changing element properties from air to the structural material in a pattern to describe the desired shapes. To verify the numerical theory, experiments were conducted without flow in a rectangular duct with a single folded cavity mounted above the duct and absorbing material mounted inside a cavity. Changes in a nearly plane wave sound field were measured on the wall opposite the absorbing cavity. Fairly good agreement was found in the standing wave pattern upstream of the absorber and in the decay of pressure level opposite the absorber, as a function of distance along the duct. The finite element model provides a convenient method for evaluating the acoustic properties of bulk absorbers.

Baumeister, Kenneth J.↗
NASA NTRS1977

Significant initial results from the environmental measurements experiment on ATS-6

The Applications Technology Satellite (ATS-6), launched into synchronous orbit on 30 May 1974, carried a set of six particle detectors and a triaxial fluxgate magnetometer. The particle detectors were able to determine the ion and electron distribution functions from 1 to greater than 10 to the 8th power eV. It was found that the magnetic field is weaker and more tilted than predicted by models which neglect internal plasma and that there is a seasonal dependence to the magnitude and tilt. ATS-6 magnetic field measurements showed the effects of field-aligned currents associated with substorms, and large fluxes of field-aligned particles were observed with the particle detectors. Encounters with the plasmasphere revealed the existence of warm plasma with temperatures up to 30 eV. A variety of correlated waves in both the particles and fields were observed: pulsation continuous oscillations, seen predominantly in the plasmasphere bulge; ultralow frequency (ULF) standing waves; ring current proton ULF waves; and low frequency waves that modulate the energetic electrons. In additon, large scale waves on the energetic-ion-trapping boundary were observed, and the intensity of energetic electrons was modulated in association with the passage of sector boundaries of the interplanetary magnetic field.

Fritz, T. A.↗
NASA NTRS1986

The resolved layer of a collisionless, high beta, supercritical, quasi-perpendicular shock wave. II - Dissipative fluid electrodynamics

Using the results of Scudder et al. (1986) on the bow shock wave observed by ISEE satellites, a quantitative description is presented of the electrodynamics of ion and electron fluids, and phase-standing wave interaction which manifests itself as a supercritical MHD shock. The cross-shock electrical profile was determined in both the normal incidence frame and in the deHoffman-Teller frame by two different methods, and the results were compared with dc electric field measurements.

Scudder, J. D.↗
NASA NTRS2003

Volumetric Near-Field Microwave Plasma Generation

A periodic series of microwave-induced plasmoids is generated using the outgoing wave from a microwave horn and the reflected wave from a nearby on-axis concave reflector. The plasmoids are spaced at half-wavelength separations according to a standing-wave pattern. The plasmoids are enhanced by an effective focusing in the near field of the horn (Fresnel region) as a result of a diffractive narrowing. Optical imaging, electron density, and rotational temperature measurements characterize the near field plasma region. Volumetric microwave discharges may have application to combustion ignition in scramjet engines.

Exton, R. J.↗
NASA NTRS2019

Laboratory Scale Testing of Ignition Overpressure for Space Vehicle Launch Pad Environments

A scale model of a NASA representative space vehicle is used to develop a refined estimate of the transient pressure loads that are expected to form at the base of the vehicle in the event of a vapor cloud explosion. Flammable vapor clouds are known to form prior to engine startup due to the significant amount of unburned hydrogen that is ejected from the combustion chamber. In the event of a vapor cloud explosion, the vehicle and payload must be able to withstand the resulting overpressure waves. The study comprises an array of pressure sensors located along the base heat shield of the scale model space vehicle as well as the interior wall and throat plug plane of the solid rocket booster. A spark source generator is used to simulate the overpressure wave produced by a vapor cloud explosion while measurements are acquired with and without the effect of a mobile launcher. Time- resolved schlieren images of the simulated vapor cloud explosion reveal the path and impact of both the initial wave and several reflected waves on the various components at the base of the space vehicle. In some instances, the reflected waves superpose to create waves that are higher in amplitude than the initial overpressure wave. A time frequency analysis of the pressure waveforms measured inside the solid rocket booster reveal a ring down tone corresponding to a standing wave that is four times the length of the nozzle.

Tinney, Charles E.↗