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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 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Kilometer-sized waves in electron density in the Venusian nightside ionosphere

As periapsis of the Pioneer Venus Orbiter (PVO) descended into the lower nightside ionosphere of Venus in the Fall of 1992, wave-like ionospheric density strucutures began to appear on some of the volt-ampere characteristics of the Orbiter Electron Temperature Probe. The number of such events is insufficient to fully define their morphology but enough to provide an indication of the wave amplitudes, scale sizes, occurrence altitudes, and local time variation. The density variations were quasi-sinudoidal, with wavelengths of the order of 1 km along the nearly horizontal trajectory near periapsis. Nearly all of the wave events were encountered within an altitude band lying between 140 and 160 km, a region containing the steep negative N(sub e) gradient just above the ionospheric peak. The waves generally did not fill the occurrence band but were seen primarily as isolated events on curves taken intermittently as PVO crossed through the band. Peak-to-trough amplitudes (delta N/N) were in the range of 5% to 50%. The latitudinal extent of the waves could not be resolved because volt-ampere curves were obtained only intermittent, however, their occurrence on both inbound and outbound passages through the wave band suggests that the waves sometimes exist in layers that extend over at least 15 deg of latitude. The generation mechanism for these waves is unknown, but we suspect that it involves the steep density gradient that separates the main nightside ionosphere from the tenuous, and probably rapidly flowing plasma above.

Brace, Larry H.↗
NASA NTRS1993

Kilometer-Sized Waves in Electron Density in the Venusian Nightside Ionosphere

As periapsis of the Pioneer Venus Orbiter (PVO) descended into the lower nightside ionosphere of Venus in the Fall of 1992, wave-like ionospheric density structures began to appear on some of the volt-ampere characteristics of the Orbiter Electron Temperature Probe. The number of such events is insufficient to fully define their morphology but enough to provide an indication of the wave amplitudes, scale sizes, occurrence altitudes, and local time variation. The density variations were quasi-sinusoidal, with wavelengths of the order of 1 km along the nearly horizontal trajectory near periapsis. Nearly all of the wave events were encountered within an altitude band lying between 140 and 160 km, a region containing the steep negative N(sub e) gradient just above the ionospheric peak. The waves generally did not fill the occurrence band but were seen primarily as isolated events on curves taken intermittently as PVO crossed through the band. Peak-to-trough amplitudes ((Delta)N/N) were in the range of 5% to 50%. The waves exhibited little local time variation within the available viewing period (01-04 hrs), with perhaps a tendency for the waves to rise to slightly higher altitudes toward dawn. The latitudinal extent of the waves could not be resolved because volt-ampere curves were obtained only intermittently, however, their occurrence on both inbound and outbound passages through the wave band suggests that the waves sometimes exist in layers that extend over at least 15 deg of latitude. The generation mechanism for these waves is unknown, but we suspect that it involves the steep density gradient that separates the main nightside ionosphere from the tenuous, and probably rapidly flowing plasma above.

Brace, Larry H.↗
NASA NTRS1981

Electromagnetic-wave excitation in a large laboratory beam-plasma system

The mechanism by which unstable electrostatic waves of a beam-plasma system are converted into observed electromagnetic waves is of current interest in space physics and in tokamak fusion research. The process involved in the conversion of electrostatic to electromagnetic waves at the critical layer is well understood. However, the radiation from uniform plasmas cannot be explained on the basis of this process. In connection with certain difficulties, it has not yet been possible to establish the involved emission processes by means of experimental observations. In the considered investigation these difficulties are overcome by employing a large laboratory plasma in a parameter range suitable for detailed diagnostics. A finite-diameter electron beam is injected into a uniform quiescent afterglow plasma of dimensions large compared with electromagnetic wavelengths. The considered generation mechanism concerning the electromagnetic waves is conclusively confirmed by observing the temporal evolution of an instability

Whelan, D. A.↗
NASA NTRS2003

Modeling of Unsteady Three-dimensional Flows in Multistage Machines

Despite many years of development, the accurate and reliable prediction of unsteady aerodynamic forces acting on turbomachinery blades remains less than satisfactory, especially when viewed next to the great success investigators have had in predicting steady flows. Hall and Silkowski (1997) have proposed that one of the main reasons for the discrepancy between theory and experiment and/or industrial experience is that many of the current unsteady aerodynamic theories model a single blade row in an infinitely long duct, ignoring potentially important multistage effects. However, unsteady flows are made up of acoustic, vortical, and entropic waves. These waves provide a mechanism for the rotors and stators of multistage machines to communicate with one another. In other words, wave behavior makes unsteady flows fundamentally a multistage (and three-dimensional) phenomenon. In this research program, we have has as goals (1) the development of computationally efficient computer models of the unsteady aerodynamic response of blade rows embedded in a multistage machine (these models will ultimately be capable of analyzing three-dimensional viscous transonic flows), and (2) the use of these computer codes to study a number of important multistage phenomena.

Hall, Kenneth C.↗
NASA NTRS1985

A new model of resonance in the winter stratosphere

It is generally accepted that the planetary waves observed in the winter stratosphere are primarily a response to dynamical forcing from the troposphere. Nevertheless, the mechanism by which wave amplitudes sometimes become larger remains uncertain. It is possible that anomalously large waves in the stratosphere might simply be the result of anomalously large tropospheric forcing. However, it has also been suggested that they are a response to a stratospheric-tropospheric cavity being in a near-resonant configuration. It has been suggested that nonlinear self-tuning effects could play an important role in the behavior of such a cavity. Self-tuning may occur when a system starts to one side of resonance, such that the mean-state change induced by growing waves brings the system closer to resonance. A new model of the stratospheric cavity is introduced and is then used to re-examine the possibility of wave growth in the real atmosphere and in atmospheric models due to self-tuning effects. The new model is based on the picture of the winter-time stratosphere which has been revealed by the observations of Ertel's potential vorticity, Q. Isentropic maps of Q show two rather distinct regions, the first containing the circumpolar vortex, where gradients of Q are large and Rossby waves may propagate easily. Surrounding this is a second, low-latitude region where the gradients are generally weak and where, because the Eulerian-mean flow is comparable with their phase speed, Rossby waves must be continually breaking. As the waves are observed to grow the relative sizes of these two regions change in time. This leads to the interesting possibility that self-tuning, mainly due to irreversible changes in the size of the polar vortex, is taking place.

Haynes, P. H.↗
NASA NTRS1979

Turboprop interior noise studies

The modal theory of acoustoelasticity is applied to the determination of the sound levels caused by a prescribed external sound excitation which is transmitted through a cylindrical shell. A circumferential traveling pressure wave excitation is studied as representative of a propeller sound field. It is shown how other excitations such as point mechanical loading, plane wave and reverberation random may be synthesized by superposition of circumferential waves. Representative numerical results illustrate the importance of structural and acoustic frequency matching in the determination of interior sound levels and clarify the role of the cylindrical shell ring frequency. An exploratory study of a double wall geometry is conducted.

Dowell, E. H.↗
NASA NTRS1985

The 4-5 day mode oscillation in zonal winds of Indian middle atmosphere during MONEX-79

In the early studies based on time series of balloon observations, the existence of 4 to 5 day period waves and 10 to 20 day wind fluctuations were found in the tropical lower stratosphere, and they are identified theoretically as the mixed Rossby-gravity wave and the Kelvin wave, respectively. On the basis of these studies, it was established that the vertically propagating equatorial waves play an important role in producing the QBO (quasi-biennial oscillation) in the mean zonal wind through the mechanism of wave-zonal interaction. These studies are mainly concentrated over the equatorial Pacific and Atlantic Oceans. Similar prominent wave disturbances have been observed over the region east of the Indian Ocean during a quasi-biennial oscillation. Zonal winds in upper troposphere and lower stratosphere (10 to 20) km of the middle atmosphere over the Indian subcontinent may bear association with the activity of summer monsoon (June-September). Monsoon Experiment (MONEX-79) has provided upper air observations at Balasore (21 deg. 30 min.N; 85 deg. 56 min.E), during the peak of monsoon months July and August. A unique opportunity has, therefore, been provided to study the normal oscillations present in the zonal winds of lower middle atmosphere over India, which may have implication on large scale wave dynamics. This aspect is examined in the present study.

Reddy, R. S.↗
NASA NTRS2013

Acoustic Mechanical Feedthroughs

Electromagnetic motors can have problems when operating in extreme environments. In addition, if one needs to do mechanical work outside a structure, electrical feedthroughs are required to transport the electric power to drive the motor. In this paper, we present designs for driving rotary and linear motors by pumping stress waves across a structure or barrier. We accomplish this by designing a piezoelectric actuator on one side of the structure and a resonance structure that is matched to the piezoelectric resonance of the actuator on the other side. Typically, piezoelectric motors can be designed with high torques and lower speeds without the need for gears. One can also use other actuation materials such as electrostrictive, or magnetostrictive materials in a benign environment and transmit the power in acoustic form as a stress wave and actuate mechanisms that are external to the benign environment. This technology removes the need to perforate a structure and allows work to be done directly on the other side of a structure without the use of electrical feedthroughs, which can weaken the structure, pipe, or vessel. Acoustic energy is pumped as a stress wave at a set frequency or range of frequencies to produce rotary or linear motion in a structure. This method of transferring useful mechanical work across solid barriers by pumping acoustic energy through a resonant structure features the ability to transfer work (rotary or linear motion) across pressure or thermal barriers, or in a sterile environment, without generating contaminants. Reflectors in the wall of barriers can be designed to enhance the efficiency of the energy/power transmission. The method features the ability to produce a bi-directional driving mechanism using higher-mode resonances. There are a variety of applications where the presence of a motor is complicated by thermal or chemical environments that would be hostile to the motor components and reduce life and, in some instances, not be feasible. A variety of designs that have been designed, fabricated and tested will be presented

piezoelectric devices↗
NASA NTRS2019

Source of the Bursty Bulk Flow Diffuse Aurora: Electrostatic Cyclotron Harmonic and Whistler Waves in the Coupling of Bursty Bulk Flows to Auroral Precipitation

Electron cyclotron harmonic (ECH) and whistler chorus waves are recognized as the two mechanisms responsible for the resonant wave‐particle interactions necessary to precipitate plasma sheet electrons into the ionosphere, producing the diffuse Aurora. Previous work has demonstrated ECH waves dominate electron scattering at L shells >8, while whistler chorus dominates scattering at L shells L < 8. However, we find from Time History of Events and Macroscale (THEMIS) Interactions during Substorms observations of fast flows at L = 12 that oblique whistler chorus emissions play the dominant role in scattering electrons. Previous works have identified whistler‐mode waves within fast flows that are produced by an electron temperature anisotropy Te,⊥/Te,||> 1, consistent with electron betatron acceleration. Here, however, we find whistler chorus emissions throughout an interval of fast flows where Te,⊥/Te,||< 1. Parallel electron beams account for the enhanced parallel electron temperature and serve as the instability mechanism for the whistler chorus. The parallel electron beams and associated cigar‐shaped distributions are consistent with Fermi acceleration at dipolarizations in fast flows. We demonstrate that the scattering efficiency of the whistler chorus exceeds that of ECH waves, which THEMIS also detects during the fast flows. The obliquity of the whistler waves permits efficient scattering of lower‐energy electrons into the diffuse aurora. We conclude that Fermi acceleration of electrons provides one important free‐energy source for the wave‐particle interactions responsible for coupling plasma sheet electrons into the diffuse aurora during substorm conditions.

Wendel, D. E.↗
NASA NTRS1990

Acousto-ultrasonic nondestructive evaluation of materials using laser beam generation and detection

The acousto-ultrasonic method has proven to be a most interesting technique for nondestructive evaluation of the mechanical properties of a variety of materials. Use of the technique or a modification thereof, has led to correlation of the associated stress wave factor with mechanical properties of both metals and composite materials. The method is applied to the nondestructive evaluation of selected fiber reinforced structural composites. For the first time, conventional piezoelectric transducers were replaced with laser beam ultrasonic generators and detectors. This modification permitted true non-contact acousto-ultrasonic measurements to be made, which yielded new information about the basic mechanisms involved as well as proved the feasibility of making such non-contact measurements on terrestrial and space structures and heat engine components. A state-of-the-art laser based acousto-ultrasonic system, incorporating a compact pulsed laser and a fiber-optic heterodyne interferometer, was delivered to the NASA Lewis Research Center.

Huber, Robert D.↗
NASA NTRS1976

Simultaneous equatorial observations of 1- to 30-Hz waves and pitch angle distributions of ring current ions

Eighteen events of large-amplitude (0.4-6 gammas) waves which may be propagating in the ion cyclotron mode have een observed by Explorer 45. Comparison with simultaneously measured proton distributions has allowed the events to be divided into two categories. The first category consists of waves accompanied by enhanced ion fluxes apparently injected into the plasmasphere with anisotropic pitch-angle distributions. This simultaneity suggests that these waves may be generated by the observed ring-current ions. Waves in the second category were found near or outside the plasmapause and were not correlated with any identifiable changes in the observed proton distribution. The generation mechanism for these waves remains unknown.

Taylor, W. W. L.↗
NASA NTRS2019

Highly Structured Slow Solar Wind Emerging From an Equatorial Coronal Hole

At solar minimum, the solar wind is observed at high solar latitudes as a predominantly fast (> 500 km/s), highly Alfvenic, rarefied stream of plasma originating deep within coronal holes, while near the ecliptic plane it is interspersed with a more variable slow (< 500 kms) wind. The precise origins of the slow wind streams are less certain, with theories and observations supporting sources from the tips of helmet streamers, interchange reconnection near coronal hole boundaries, and origins within coronal holes with highly diverging magnetic fields. The heating mechanism required to drive the solar wind is also an open question and candidate mechanisms include Alfven wave turbulence, heating by reconnection in nanoflares, ion cyclotron wave heating and acceleration by thermal gradients1. At 1 au, the wind is mixed and evolved and much of the diagnostic structure of these sources and processes has been lost. Here we present new measurements from Parker Solar Probe at 36 to 54 solar radii that show clear evidence of slow, Alfvenic solar wind emerging from a small equatorial coronal hole. The measured magnetic field exhibits patches of large, intermittent reversals associated with jets of plasma and enhanced Poynting flux and interspersed in a smoother and less turbulent flow with near-radial magnetic field. Furthermore, plasma wave measurements suggest electron and ion velocity-space micro-instabilities that have been identified with plasma heating and thermalization processes. Our measurements suggest an impulsive mechanism associated with solar wind energization and a heating role for micro-instabilities and provide strong evidence for low latitude coronal holes as a significant contribution to the source of the slow solar wind.

Solar physics↗
NASA NTRS1976

Acceleration of auroral electrons in parallel electric fields

Rocket observations of auroral electrons are compared with the predictions of a number of theoretical acceleration mechanisms that involve an electric field parallel to the earth's magnetic field. The theoretical models are discussed in terms of required plasma sources, the location of the acceleration region, and properties of necessary wave-particle scattering mechanisms. We have been unable to find any steady state scatter-free electric field configuration that predicts electron flux distributions in agreement with the observations. The addition of a fluctuating electric field or wave-particle scattering several thousand kilometers above the rocket can modify the theoretical flux distributions so that they agree with measurements. The presence of very narrow energy peaks in the flux contours implies a characteristic temperature of several tens of electron volts or less for the source of field-aligned auroral electrons and a temperature of several hundred electron volts or less for the relatively isotropic 'monoenergetic' auroral electrons. The temperature of the field-aligned electrons is more representative of the magnetosheath or possibly the ionosphere as a source region than of the plasma sheet.

Kaufmann, R. L.↗
NASA NTRS1982

Phenomenological and mechanics aspects of nondestructive evaluation and characterization by sound and ultrasound of material and fracture properties

Developments in fracture mechanics and elastic wave theory enhance the understanding of many physical phenomena in a mathematical context. Available literature in the material, and fracture characterization by NDT, and the related mathematical methods in mechanics that provide fundamental underlying principles for its interpretation and evaluation are reviewed. Information on the energy release mechanism of defects and the interaction of microstructures within the material is basic in the formulation of the mechanics problems that supply guidance for nondestructive evaluation (NDE).

Fu, L. S. W.↗
NASA NTRS1990

A comparison of the radial distribution of molecular gas and non-thermal radio continuum in spiral disks

The present study includes 65 spiral galaxies selected from the Five College Radio Astronomy Observatory (FCRAO) Extragalactic CO Survey for which the major axis distributions of CO emission and 1.49 GHz radio continuum emission are well determined. The radial distribution of the CO emission has been measured with the FCRAO at positions along the major axis that are spaced by one half power beam width (HPBW) (45 seconds). The radial profile of the 1.49 GHz radio continuum emission was constructed by determining the radio emission at the location of the CO measurements from the 1.49 GHz maps of Condon (1987). Large, greater than a factor of ten, radially decreasing gradients in the star formation efficiency are observed for a small percentage, approx. 10 percent, of the spirals in this sample. The majority of spirals, however, are associated with small gradients in the star formation efficiency that do not systematically increase or decrease with radius. That the star formation efficiency does not systematically decrease with radius tends to argue against a global dynamical mechanism, such as a spiral density wave, for being the dominant mechanism triggering disk star formation for the majority of spirals in this sample. The results tend to support the view that the star formation in spiral disks is dominated by a local process that depends more on the molecular cloud properties than the dynamical structure of a galaxy.

Devereux, Nicholas A.↗
NASA NTRS1986

Can stochastic, dissipative wave fields be treated as random walk generators

A suggestion by Meek et al. (1985) that the gravity wave field be viewed as stochastic, with significant nonlinearities, is applied to calculate diffusivities. The purpose here is to calculate the diffusivity for stochastic wave model and compare it with previous diffusivity estimates. The researchers do this for an idealized case in which the wind velocity changes but slowly, and for which saturation is the principal mechanism by which wave energy is lost. A related calculation was given in a very brief way (Weinstock, 1976), but the approximations were not fully justified, nor were the physical pre-suppositions clearly explained. The observations of Meek et al. (1985) have clarified the pre-suppositions for the researchers and provided a rationalization and improvement of the approximations employed.

Weinstock, J.↗