SEARCH · Search NASA

Results for “Decay”

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.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 325 records · Page 18

All sources NASA NTRS DOE OSTI

NASA NTRS1983

Decay of long-lived particles in the early universe

It is pointed out that radiative decay of massive fermions can distort the cosmic background radiation. The present investigation is concerned with a study of decay lifetimes in the range from 10 to 100,000 years. Attention is given to the physics involved in determining the effect of radiative decay of massive fermions on observed photon backgrounds. The case of particles which decoupled when the effective number of species in equilibrium was in the range from 50 to 100 is considered, and constraints on particle masses and lifetimes are placed on the basis of observed photon fluxes. This approach provides results with special applications to particles predicted by supersymmetry theories and to right-handed neutrinos. Implications for galaxy formation are also discussed.

Silk, J.↗

NASA NTRS1982

Correlation of flight effects on centerline velocity decay for cold-flow acoustically excited jets

Acoustic excitation can influence the large scale shear layer structure of jet flow, thereby causing the jet centerline velocity to decay more rapidly. this phenomena has numerous practical applications, such as the reduction of jet/flap impingement velocity in order to reduce flap structural loads for under-the-wing STOL aircraft concepts. Cold flow centerline velocity decay data obtained with acoustic excitation of the jet exhaust flow are correlated with and without flight effects. Initially, static data are correlated in terms of an acoustic parameter involving the threshold sound level needed to initiate acoustic excitation of the jet plume. It is shown that for a given flight speed, the same acoustic excitation parameter as that developed for the static condition is valid. Finally, an aerodynamic flight effects parameter is included to correlate the static and flight centerline velocity decay data with and without acoustic excitation.

Vonglahn, U. H.↗

NASA NTRS1985

Decay of a coherent scalar disturbance in a turbulent flow

The time evolution of an initially coherent, sinusoidal passive-scalar disturbance is considered when the wavelength q is less than the length scale of the surrounding isotropic turbulent flow. In 64 sup 3 direct numerical simulations a Gaussian prescription for the average scalar amplitude breaks down after a timescale associated with the wavenumber of the disturbance and there is a transition to a new characteristic decay. The Gaussian prescription is given by exp(-(1/2) q-squared w(t)), where a form for w(t), the Lagrangian mean square displacement of a single fluid particle, is proposed. After the transition the decay is given by exp(-t/tau), where tau is the new characteristic timescale. If q k(sub K), then 1/tau = 1/tau(sub D) + 1/tau(sub K), where k(sub K) is the Kolmogorov wavenumber, tau(sub D) is the diffusive timescale and tau(sub K) is the Kolmogorov timescale. An experiment originally proposed by de Gennesis considered in which the evolution of a coherent laser-induced pattern is read by a diffracting laser. The theory of this experiment involves the dispersion of particle pairs, but it is shown that in a certain limit it reduces to the single Fourier-mode problem and can be described in terms of single particle diffusion. The decay of a single mode after the transition in the simulation best describes the experiment.

Kerr, R. M.↗

NASA NTRS1985

Solar pressure and molecular decay in cometary atmospheres

The effects of solar pressure and molecular decay on number density in cometary atmospheres are rigorously separated and scale lengths for each are determined from an analysis of observed brightness profiles in the solar and antisolar directions. It is found that the pressure scale length of CN is approximately 160,000 km and that of C2 is approximately 110,000 km. The scale length for molecular decay, heretofore incorrectly inferred from the observational data, is approximately 3 times as long as the pressure scale lengths. It is difficult to determine adequately from observations that extend no more than about 100,000 km from the comet nucleus. The scale length for molecular decay by photodissociation or whatever cause is found to be about 350,000 km for C2 and 500,000 km for CN.

Beard, D. B.↗

NASA NTRS1985

An approximate model of vortex decay in the atmosphere

An approximate analysis of atmospheric effects on wake vortex motion and decay is presented. The effects of density stratification, turbulence, and Reynolds number are combined in a single model so that the relative importance of different parameters can be determined. Predicted wake motion is shown to be in good agreement with limited data from both ground facility and flight test measurements taken under low turbulence conditions. Wake decay was found to depend strongly on both density stratification and turbulence. For typical levels of turbulence, wake decay was found to result from the 'Crow instability' except under strongly stratified conditions.

Greene, G. C.↗

NASA NTRS1985

Velocity and temperature decay characteristics of inverted-profile jets

In order to design efficient, lightweight flap systems for future engine under-the-wing STOL aircraft, the velocity and temperature decay rate of the jet plume must be increased relative to that for single-stream nozzles in order to provide local flap loads and structural temperatures within acceptable limits. The jet plume decay rate of dual flow engines can be increased by resorting to inverted-profile velocity and temperature nozzle concepts. The peak axial decay characteristics of model-scale, two-stream inverted-profile nozzle flows are empirically correlated. Also discussed are the radial and spreading characteristics of inverted-profile nozzle flows.

Vonglahn, U.↗

NASA NTRS1986

Velocity and temperature decay characteristics of inverted-profile jets

Von Glahn, U.↗

NASA NTRS1985

The cancellation of magnetic flux. II - In a decaying active region

H-alpha filtergrams and videomagnetograms are used to study an active region during its period of decay on August 3-8, 1984; the decay had been initiated by a fragmentation process in which very small knots of magnetic flux separated from larger concentration of flux. The disappearance of magnetic flux was always observed when the small fragments of flux encountered other small fragments or concentrations of flux of opposite polarity. Such 'cancellations' are shared by both polarities of magnetic field, and it is deduced that the disappearance of flux occurred either at or within 5 arcsec of the apparent dividing line between the opposite polarities. All of the 22 flares observed during the decay of this region were initiated around sites where magnetic flux was cancelling or was deduced to be cancelling during the flares. It is hypothesized that cancellation was one of the necessary conditions for flaring in this active region.

Martin, S. F.↗

NASA NTRS1988

Diffusional decay of striations

This paper investigates the amount of decay of growth striations in bulk crystals grown from melts and solutions, due to solid-state diffusion during the cooling of the crystal to room temperature. A model is developed based on the assumption of a sinusoidal variation in the solid composition at the interface of a semiinfinite ingot growing at constant velocity, and the damping behavior of this variation is followed as it moves away from the solid-liquid interface. Numerical results are presented showing that an increase in the diffusivity causes the striations to penetrate deeper into the solid and, consequently causes the distance for 99.9 percent decay to increase. Analytical results are also presented for the decay of striations in an isothermal ingot induced by a sinusoidal fluctuation in the rate of crystal growth.

Gray, R. T.↗

NASA NTRS1989

Limits to the radiative decays of neutrinos and axions from gamma-ray observations of SN 1987A

Gamma-ray observations obtained by the SMM gamma-ray spectrometer in the energy range 4.1-6.4 MeV are used to provide limits on the possible radiative decay of neutrinos and axions emitted by SN 1987A. For branching ratio values for the radiative decay modes of less than about 0.0001, the present limits are more stringent than those based upon the photon flux from decaying relic neutrinos. The data are also used to set an axion mass limit.

Kolb, Edward W.↗

NASA NTRS1989

Does mass accretion lead to field decay in neutron stars

The recent discovery of cyclotron lines from gamma-ray bursts indicates that the strong magnetic fields of isolated neutron stars might not decay. The possible inverse correlation between the strength of the magnetic field and the mass accreted by the neutron star suggests that mass accretion itself may lead to the decay of the magnetic field. The spin and magnetic field evolution of the neutron star was calculated under the hypothesis of the accretion-induced field decay. It is shown that the calculated results are consistent with the observations of binary and millisecond radio pulsars.

Shibazaki, N.↗

NASA NTRS1989

The decay of the spectrum of the gravitational potential and the topography for the earth

The spectrum of the earth's gravitational potential and topography, as represented by spherical harmonic expansions to degree 180, have been computed. Modeling the decay in the form of (A x l) exp-Beta, values of A and Beta for several degree (l) ranges were computed. For degree range 5-180, Beta was 2.54 for the potential and 2.16 for equivalent rock topography. The potential decay was somewhat slower than that implied by Kaula's rule. However, at high degree ranges, the Beta values were larger agreeing better with recent determinations from terrestrial gravity data and geoid undulations implied by satellite altimetric data. The values imply that the potential decays faster at higher l values.

Rapp, R. H.↗

NASA NTRS1991

Stratospheric aerosol optical depth observed by the Stratospheric Aerosol and Gas Experiment. II - Decay of the El Chichon and Ruiz volcanic perturbations

The decay of the El Chichon perturbation to the optical depth of stratospheric aerosols at 1.02 micron, 0.525 micron, and 0.453 micron is calculated from the Stratospheric Aerosol and Gas Experiment II (SAGE II) data set for the period December 1984 to December 1988. It is found that the perturbed optical depths at middle and higher latitudes of both hemispheres exhibited an exponential decay superimposed by a seasonal oscillation with a maximum and a minimum occurring in local winter and local summer, respectively. Microphysical processes and variation of the tropopause height alone cannot explain this seasonal change of optical depth. The magnitudes of the exponential component at higher latitudes were, in general, larger than those at lower latitudes. For optical depths in tropical regions, the seasonal oscillations were small and were disturbed by the eruption of Nevado del Ruiz on November 13, 1985. The increase in the ratio of optical depth at 0.525 micron to that at 1.02 micron from about 2.0 at the beginning of 1985 to about 3.5 at the end of 1988 indicates the average size of aerosol particles in the stratosphere is diminishing since the eruption of El Chichon. The 1/e folding time for El Chichon decay derived from the SAGE II data set is in reasonably good agreement with those derived by other methods.

Yue, G. K.↗

NASA NTRS1991

Cold equation of state in a strong magnetic field - Effects of inverse beta-decay

The influence of a high magnetic field (B is greater than 10 exp 12 G) on the degenerate matter equation of state appropriate to a neutron star is studied. The regime dominated by relativistic electrons up to the neutron drip density is highlighted. The equilibrium matter composition and equation of state, allowing for inverse beta-decay. Two different equilibrium models are determined: an ideal neutron-proton-electron (npe) gas and the more realistic model of Baym, Pethick, and Sutherland (1971) consisting of a Coulomb lattice of heavy nuclei embedded in an electron gas. For a sufficiently high field strength, the magnetic field has an appreciable effect, changing the adiabatic index of the matter and the nuclear transition densities. The influence of a strong field on some simple nonequilibrium processes, including beta-decay and inverse beta-decay (electron capture) is also considered. The effects produced by the magnetic field are mainly due to the changes in the transverse electron quantum orbits and the allowed electron phase space induced by the field.

Lai, Dong↗

NASA NTRS1992

Constraints to the decays of Dirac neutrinos from SN 1987A

The decay mode of a keV-mass Dirac neutrino is addressed with reference to the nineteen neutrino events associated with SN 1987A that were detected by the Kamiokande II and Irvine-Brookhaven-Michigan detectors. A complementary constraint is presented which is based upon a distinctive signal associated with the decay of wrong-helicity neutrinos that was not seen: high-energy (50 MeV and higher) neutrino events. The absence of such events excludes the decay of wrong-helicity neutrinos into proper-helicity neutrinos for a Dirac neutrino of mass between 1 and 300 keV. The constraint also rules out models of the 17-keV neutrino.

Dodelson, Scott↗

NASA NTRS1992

Gamma-ray bursts and neutron star field decay

Assuming a Galactic origin of gamma-ray bursts, we use pulsar data to calculate the spatial distribution of neutron stars and determine the sampling depths of current detectors. Based on these distance limits, we calculate the corresponding age distribution of Galactic neutron stars and apply an exponential field decay model to test whether the observed high incidence rate of cyclotron lines is consistent with suggested field decay time scales of order 10 exp 7 years. We find that the properties of the observed population of gamma-ray bursts are inconsistent with the idea that bursts originate at arbitrary times on neutron stars whose fields decay on time scales shorter than about 10 exp 9 years. Possible interpretations of this inconsistency are discussed.

Hartmann, Dieter↗

NASA NTRS1993

Constraints on vacuum decay from the microwave background

We consider the possible decay of a vacuum with nonzero energy density into radiation. This is one way to introduce a time-varying cosmological constant, which has been suggested as a means of resolving the cosmological constant problem. We concentrate on the model of Freese et al., in which the vacuum energy density is given as a fraction x/(1 - x) of the energy density of radiation. Using equations for the visible extragalactic background light and assuming that the vacuum decay energy is converted entirely into photons with a Planckian spectrum, we show that the decay process would be capable of contributing significantly to the intensity of the cosmic microwave background. Comparison with COBE observations leads to the constraint x is equal to or less than 0.001, which is stronger than the upper limit of 0.07 obtained previously by Freese et al. from considerations of primordial nucleosynthesis.

Overduin, J. M.↗

NASA NTRS1993

Constraints on decaying neutrinos from the far-ultraviolet extragalactic background light

We consider light neutrinos as dark matter candidates, concentrating on those proposed by Sciama, with rest energies near 30 eV and decay lifetimes of (2 +/- 1) x 10 exp 23 s. Using equations developed by Wesson et al. (1987) for the visible extragalactic background light, and assuming that the neutrinos are clustered in galactic halos similar to that around our own Galaxy, we show that their decay would be capable of significant contributions to the far-ultraviolet background. Observations in this region by Martin and Bowyer (1989) limit the possible mass of the neutrino halos to 2 x 10 exp 11 M(solar), which is less than the required galactic 'missing mass'. In order to form galactic halos of mass 5 x 10 exp 11 M(solar), such neutrinos would have to decay with lifetimes of at least 3 x 10 exp 23 s to be consistent with the observations. Sciama's proposal remains marginally viable, but more observational data will test it and similar neutrino models critically.

Overduin, J. M.↗