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At least 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
NASA NTRS1976

An analog technique for the measurement of damping from transient decay signals

A tracking filter that includes an output that is logarithmically proportional to the amplitude of the filtered signal is used to measure the damping exponent of a transient decay. This method is analogous to the digital technique referred to as the moving-block or peak-plot method. The method is simple to use and quite accurate, avoids the numerical computations associated with similar digital techniques, but is constrained by the poor time-domain response characteristics of commercial tracking filters presently available.

Bousman, W. G.↗
NASA NTRS1975

Saturation spectrum of the parametric decay instability in the presence of an external magnetic field

The effects of an external magnetic field on the saturation spectrum of parametric decay instabilities is examined by means of a modification of the Kuo and Fejer (1972) analysis. The main effect of the magnetic field is shown to be a decrease of the wavenumber with increasing angle between the wave vector and the magnetic field, particularly when the pump frequency is near the second harmonic of the cyclotron frequency. The spread of the phase velocities as measured in the direction of the magnetic field is significantly greater in the saturation spectrum when the pump frequency is near the second harmonic of the cyclotron frequency, facilitating the acceleration of tail electrons to energies of a few electron volts. It is suggested that this effect accounts for the greatly enhanced airglow observed (Haslett and Megill, 1974) under these conditions.

Chen, H. C.↗
NASA NTRS1976

Generation of atomic H in a hydrogen matrix by tritium decay

Webeler's (1976) experimental results for the generation of atomic hydrogen in a hydrogen matrix by tritium decay are reexamined with a variant of Rosen's (1976) mathematical treatment. The analysis retains Rosen's equations for the number densities of trapped and mobile hydrogen atoms, but replaces his enthalpy equation with an equation for the directly measured temperature. Theoretical expressions are derived for the dependence of storage time, recombination time, and maximum density of trapped hydrogen atoms as a function of temperature for a given tritium concentration. A comparison of predictions for the maximum trapped atomic hydrogen number density as a function of storage time reveals that Rosen's estimate for the maximum number density of hydrogen atoms for the zero magnetic field case is a little more optimistic than the estimate obtained in the paper.

Zeleznik, F. J.↗
NASA NTRS1977

The morphological and statistical properties of solar X-ray events with long decay times

Photographic data from the S-054 X-ray telescope on Skylab has been used to study the morphological and statistical properties of a class of X-ray events with long decay times. The events were selected for analysis based on their Solrad 1-8 A flux signatures. These events are associated statistically with long-lived active regions, white-light coronal transients, and H alpha prominence activations. These X-ray events usually consist of an arcade of loops with a bright linear structure running along the arcade. The heights of the loops, their expansion velocities, their lifetimes, and morphological appearances all suggest that at least some of these events are the X-ray analog of the loop prominence systems observed in H alpha by Bruzek.

Kahler, S.↗
NASA NTRS1977

On the origin of biological chirality via natural beta-decay

An hypothesis to account for the chirality (handedness) of some biological molecules is given. Experimental evidence suggests that longitudinally polarized electrons having the chirality of terrestrial beta-decay electrons remove dextro-leucine from a racemic mixture. If, by a similar mechanism, the terrestrial environment provided more levo- than dextro-amino acids, that would account for the chirality now observed in organic molecules. An isotope of potassium has been proposed as the natural beta-emitter responsible for biomolecular chirality; however, Carbon 14 may be an even more plausible candidate. Ready availability of the carbon isotope in the terrestrial environment of 4.5 aeons ago, and the role of leucine in protein synthesis indicate that these two agents may have been chief factors in the evolution of biomolecular chirality. Suggestions for further research in this area are made.

Noyes, H. P.↗
NASA NTRS1978

Post-volcanic stratospheric aerosol decay as measured by lidar

The paper summarizes and discusses results of lidar observations, at Hampton (Virginia), of the stratospheric aerosol vertical distribution for a period of 22 months (October 1974 to July 1976) after the volcanic eruption of the Volcan de Fuego in Guatemala. Data are presented in terms of lidar scattering ratio, vertically integrated aerosol backscattering, layer structure and location, and rawinsonde temperature profiles as a function of time. The results reveal a sudden increase in the stratospheric aerosol content after the volcanic eruption as well as its subsequent decline. There exists a high degree of correlation between the integrated aerosol backscattering and the tropopause height such that as one decreases the other increases and vice versa. Rapid decay of the stratospheric aerosol is found to occur over the late winter to early spring period.

Mccormick, M. P.↗
NASA NTRS1978

Experimental investigation of effects of jet decay rate on jet-induced pressures on a flat plate: Tabulated data

Tabular data are presented for an experimental study of the effects of jet decay rate on the jet-induced pressure distribution on a flat plate for a single jet issuing at right angle to the flat plate into a uniform crossflow. The data are presented in four sections: (1) presents the static nozzle calibration data; (2) lists the plate surface static pressure data and integrated loads; (3) lists the jet centerline trajectory data; and (4) lists the centerline dynamic pressure data.

Kuhlman, J. M.↗
NASA NTRS1979

Computational simulation of turbulent vortex merger and decay

The interaction and eventual merger of corotational vortices and the decay of a single vortex have been studied by employing zero-, one- and two-equation turbulent-flow models in order to gain a better understanding of the role of turbulence. An implicit finite-difference procedure is used to integrate the unsteady, two-dimensional equations in a cross-plane. The zero- and one-equation formulations utilize a mixing-length model, which incorporates the streamline curvature effect by prescribing a spatially-varying mixing-length. In the two-equation model, the turbulence kinetic energy equation and a modified rate of dissipation equation which includes a streamline curvature correction are solved. Computational results of different models applied to various flow-configurations are presented and compared with available experimental data whenever possible.

Raj, P.↗
NASA NTRS1979

Physical parameters in long-decay coronal enhancements

Four well-observed long-decay X-ray enhancements (LDEs) are examined which were associated with filament eruptions, white-light transients, and loop prominence systems. In each case the physical parameters of the X-ray-emitting plasma are determined, including the spatial distribution and temporal evolution of temperature and density. The results and recent analyses of other aspects of the four LDEs are compared with current models of loop prominence systems. It is concluded that only a magnetic-reconnection model, such as that proposed by Kopp and Pneuman (1976) is consistent with the observations.

Maccombie, W. J.↗
NASA NTRS1980

Oscillating two-stream and parametric decay instabilities in a weakly magnetized plasma

The paper considers the effects of a weak ambient magnetic field on the oscillating two-stream and parametric decay instabilities with emphasis on the dependence of the angular variation of the instability thresholds and growth rates on the magnetic field. A dispersion relation is derived in the limit in which the electron plasma frequency is much greater than the cyclotron frequency, and is solved for dipole and monochromatic pump spectra. The analysis shows that the presence of a magnetic field can substantially enhance thresholds and reduce growth rates for waves propagating at an oblique angle with respect to the ambient magnetic field, so that the magnetic field has a stabilizing influence on the off-parallel propagating modes.

Freund, H. P.↗
NASA NTRS1980

Skylab orbital lifetime prediction and decay analysis

A record (from preflight planning to earth impact) of skylab's orbital lifetime predictions, its actual decay and analysis are presented. Skylab provided a unique opportunity to develop, confirm and check out procedures and computer programs used for predicting lifetime and reentry. It provided verification of aerodynamic environment which was predicted for several aerodynamic configurations. It also provided data on the density model's reaction to rapidly changing solar flux over relatively short time periods. The effects of solar flux (and the uncertainty in the solar flux predictions) on orbital lifetime are discussed.

Dreher, P. E.↗
NASA NTRS1980

Broadness, decay, and correlation functions of isotropic homogeneous turbulence

General theoretical relationships consistent with experiments are obtained for isotropic homogeneous incompressible-fluid turbulence governed by the Navier-Stokes flow equation. A key quantity that structures the turbulence is the 'broadness' B of the probability measure over velocity fields. Both the decay law and the longitudinal correlation function for small r appear as simple functions of this quasiconstant parameter B.

Rosen, G.↗
NASA NTRS1982

Beta decay and the origins of biological chirality - Experimental results

Preliminary experimental results are presented of an investigation of the possible role of preferential radiolysis by electrons emitted in the beta decay of radionuclides, a parity-nonconserving process, in the universal causation of the optical activity of biological compounds. Experiments were designed to measure the asymmetry in the production of triplet positronium upon the bombardment of an amino acid powder target by a collimated beam of positrons as positron helicity or target chirality is reversed. No asymmetry down to a level of 0.0007 is found in experiments on the D and L forms of cystine and tryptophan, indicating an asymmetry in positronium formation cross section of less than 0.01, while an asymmetry of 0.0031 is found for leucine, corresponding to a formation cross section asymmetry of about 0.04

Gidley, D. W.↗
NASA NTRS1983

Dynamic alignment and selective decay in MHD

Under some circumstances, incompressible magnetohydrodynamic turbulence will evolve toward a state in which the velocity fields and magnetic fields are aligned or anti-aligned. We propose a mechanism for this effect and illustrate with numerical computations. Under some other circumstances, the energy appears to decay selectively toward a minimum energy state in which the kinetic energy has disappeared. It has not been possible so far to identify a boundary in the phase space which divides the two regimes.

Matthaeus, W. H.↗
NASA NTRS1982

Particle physics meets cosmology - The search for decaying neutrinos

The fundamental physical implications of the possible detection of massive neutrinos are discussed, with an emphasis on the Grand Unified Theories (GUTs) of matter. The Newtonian and general-relativistic pictures of the fundamental forces are compared, and the reduction of electromagnetic and weak forces to one force in the GUTs is explained. The cosmological consequences of the curved-spacetime gravitation concept are considered. Quarks, leptons, and neutrinos are characterized in a general treatment of elementary quantum mechanics. The universe is described in terms of quantized fields, the noninteractive 'particle' fields and the force fields, and cosmology becomes the study of the interaction of gravitation with the other fields, of the 'freezing out' of successive fields with the expansion and cooling of the universe. While the visible universe is the result of the clustering of the quark and electron fields, the distribution of the large number of quanta in neutrino field, like the mass of the neutrino, are unknown. Cosmological models which attribute anomalies in the observed motions of galaxies and stars to clusters or shells of massive neutrinos are shown to be consistent with a small but nonzero neutrino mass and a universe near the open/closed transition point, but direct detection of the presence of massive neutrinos by the UV emission of their decay is required to verify these hypotheses.

Henry, R. C.↗
NASA NTRS1983

Transient decay times and mean values of unsteady oscillations in transonic flow

Kerlick and Nixon (1981) point out that if the time-marching solution is stopped before the transient is complete and the steady state is reached, then the incorrect conclusion may be reached that a change in the mean lift has occurred in response to the oscillating motion of the airfoil when in fact no such change has occurred. For a narrow Mach number range, however, the time for the transient to decay and for a steady state to be reached is extraordinarily long. What is more, for a very narrow range of Mach numbers, a nonzero mean value of lift can occur for an airfoil of symmetrical profile oscillations about a zero angle of attack. The reason why this nonzero average lift occurs only over a narrow range of Mach numbers has so far not been obtained.

Dowell, E. H.↗
NASA NTRS1983

Protons from the decay of solar flare neutrons

Fluxes of energetic protons in interplanetary space are observed which are interpreted as the decay products of neutrons generated in a solar flare on 1982 June 3 at 11:42 UT. Because of the particular geometry of this event the spectrum of neutrons escaping from the sun can be constructed with great accuracy in the kinetic energy range 10-100 MeV. The resulting spectrum places stringent constraints on the free parameters used in previously published calculations of neutron production in solar flares. An estimate is made of the diffusion mean free path of charged particles in the interplanetary medium in a new way.

Evenson, P.↗