Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “PERIODICITY”

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.

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

At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Geometric effects of global lateral heterogeneity on long-period surface wave propagation

The present investigation has the objective to document examples of anomalous long-period surface wave amplitude behavior and to provide a preliminary appraisal of the effects of global lateral heterogeneity on surface wave propagation from a ray theory perspective. Attention is given to remarkable long-period surface wave anomalies described in literature, an equidistance azimuthal plot centered on the Iranian source region, Rayleigh wave and Love wave spectra for the 256-s period arrivals for the Tabas earthquake, constrained moment tensor and fault model inversion solutions ofr Iranian earthquakes, aspects of surface wave ray tracing, and a table of Rayleigh wave amplitude anomalies for Iranian earthquakes. Surface wave ray-tracing calculations for models of global phase velocity variations proposed by Nakanishi and Anderson (1984) are found to show that large-amplitude anomalies will be observed for Love and Rayleigh waves with periods of 100-250 s.

Lay, T.↗
NASA NTRS1986

A 6 centimeter radio survey of short-period active binary stars

Thirteen binaries with periods in the range of 0.2-2.0 days have been observed at 6 cm wavelength with the VLA. Eight out of these thirteen systems were detected, of which seven are RS Canum Venaticorum systems and one is an Algol system, with observed fluxes in the range of 0.3-5.0 mJy. The individual characteristics of the detected sources are briefly discussed. As a group, relative to active binaries of longer orbital periods, the short-period active binaries have a slightly lower mean radio luminosity. There is also a clear correlation of high radio luminosity with high X-ray luminosity evident in these short-period systems, although a functional dependence cannot be determined from noncontemporary data.

Drake, S. A.↗
NASA NTRS1986

An exact solution for orbit view-periods from a station on a tri-axial ellipsoidal planet

This paper presents the concise exact solution for predicting view-periods to be observed from a masked or unmasked tracking station on a tri-axial ellipsoidal surface. The new exact approach expresses the azimuth and elevation angles of a spacecraft in terms of the station-centered geodetic topocentric coordinates in an elegantly concise manner. A simple and efficient algorithm is developed to avoid costly repetitive computations in searching for neighborhoods near the rise and set times of each satellite orbit for each station. Only one search for each orbit is necessary for each station. Sample results indicate that the use of an assumed spherical earth instead of an 'actual' tri-axial ellipsoidal earth could introduce an error up to a few minutes in a view-period prediction for circular orbits of low or medium altitude. For an elliptical orbit of high eccentricity and long period, the maximum error could be even larger. The analytic treatment and the efficient algorithm are designed for geocentric orbits, but they should be applicable to interplanetary trajectories by an appropriate coordinates transformation at each view-period calculation. This analysis can be accomplished only by not using the classical orbital elements.

Tang, C. C. H.↗
NASA NTRS1986

The rotation period of Uranus

On 24 January 1986 the spacecraft Voyager 2 transversed the innermost magnetosphere of the planet Uranus, coming as close as 4.2 Uranus radii to the planet. It is pointed out that the magnetic field data provide a direct measure of the rotation period of the planet's interior, where the field is generated. Two period determinations are reported. A combination of the obtained values provides a weighted mean value of P = 17.24 + or - 0.01 h. It is concluded that the 17.24-h rotation period has important consequences for studies of atmospheric dynamics and the internal structure and composition of Uranus. Thus, inferences regarding the internal structure can be drawn from the relationship between the observed planetary oblateness, rotation period, and gravitational moment.

Desch, M. D.↗
NASA NTRS1986

Quasi-periodic pulsations in solar hard X-ray and microwave flares

For more than a decade, various studies have pointed out that hard X-ray and microwave time profiles of some solar flares show quasi-periodic fluctuations or pulsations. Nevertheless, it was not until recently that a flare displaying large amplitude quasi-periodic pulsations in X-rays and microwaves was observed with good spectral coverage and with a sufficient time resolution. The event occurred on June 7, 1980, at approximately 0312 UT, and exhibits seven intense pulses with a quasi-periodicity of approximately 8 seconds in microwaves, hard X-rays, and gamma-ray lines. On May 12, 1983, at approximately 0253 UT, another good example of this type of flare was observed both in hard X-rays and in microwaves. Temporal and spectral characteristics of this flare are compared with the event of June 7, 1980. In order to further explore these observational results and theoretical scenarios, a study of nine additional quasi-periodic events were incorporated with the results from the two flares described. Analysis of these events are briefly summarized.

Kosugi, Takeo↗
NASA NTRS1986

Orbital period changes in massive X-ray binaries

Data from long-term pulse-timing observations of Cen X-3 are compiled and analyzed to determine the evolution of the orbital period, and the implications of the results for other massive X-ray binaries are discussed. Third-body effects, tidal effects, and mass-loss/transfer effects are considered as possible causes for the orbital-period decay observed in Cen X-3, and it is concluded that strong tidal coupling between the companion star and the orbital causes the period decay via orbital instability and/or mass loss. Applying these results to the Roche-lobe-overflow models of Savonije (1983), the evolutionary expansion rate of the Cen X-3 companion and the Roche-lobe shrinking rate are estimated as about 2 x 10 to the -7th/yr and about 1 x 10 to the -6th/yr, respectively, implying an X-ray lifetime of about 5000 yr. Estimates of the companion mass-change rate and period decay for Cen X-3 and four other objects are presented in a table.

Kelley, Richard L.↗
NASA NTRS1988

Period doubling in the outer heliosphere

From approximately July 28 to November 26, 1984, IMP 8 at 1 AU observed quasi-periodic interaction regions in the solar wind characterized by a peak every 13.4 days in the magnetic field strength, plasma density, and temperature, corresponding to an inertial period of 12.5 days. When the same solar wind reached Voyager 2 (which moved from 15.2 to 16.1 AU during the corresponding time interval, September 27, 1984, to January 27, 1985), the enhancements in the magnetic field strength and the plasma density and temperature recurred with a period of approximately 25 days. Thus the period of the large-scale fluctuations in B, N, and T doubled between 1 AU and 15.2 AU. The magnetic field strength increased linearly with the density and the temperature in the Voyager 2 data. The tails of the distributions of B, N, and T in the Voyager 2 data were approximately exponential.

Burlaga, L. F.↗
NASA NTRS1988

Discovery of a 4.2 hour X-ray period in GX 9+9

The low-mass X-ray binary GX 9+9 (4U 1728-16) was observed during September 13-19, 1977, and March 11-17, 1978, with the HEAO A-1 instrument in scanning mode. An algorithm for detecting periodicities in the resulting light curves has been applied; it is based on the Scargle periodogram and takes into account the non-Gaussian nature of the intrinsic source variability. A 4.19 + or - 0.02 hr periodic modulation is detected in the September 1977 data. The folded light curve shows sinusoidal modulation with an amplitude of 3.8 percent. This amplitude is comparable to the 4.6 percent residual intrinsic source variability at all phases of the light curve. The 4.19 hr period is interpreted as the binary period of the system. The lack of eclipses and the sinusoidal light curve imply that the binary system is viewed at a low inclination angle of less than about 63 deg. Light curves of this type may be relatively common among low-mass X-ray binaries, but are difficult to detect without the long observations and high sensitivity possible with HEAO A-1.

Hertz, Paul↗
NASA NTRS1988

Periodic modulation of the atmosphere of Alpha Orionis

Alpha Orionis (Betelgeuse; M2 Iab) was monitored with IUE since 1984. Discovery of a 420-day periodic modulation of the flux in the optical and ultraviolet continua, and in the Mg II h and k line emission cores suggests that periodic photospheric pulsations were present from 1984 to 1986. This behavior continues through 1987. However, the general flux level of the ultraviolet continuum and the Mg II lines is decreasing, and the amplitude of the variation may be reduced. These decreases may be the emerging signature of an additional longer period. The density sensitive C II diagnostic, 2325.4/2328.1, indicates the chromospheric densities range between log N(sub e) (cm-3) = 8.7 and 9.5, but periodicities are not evident.

Dupree, A. K.↗
NASA NTRS1988

Astrophysical implications of periodicity

Two remarkable discoveries of the last decade have profound implications for astrophysics and for geophysics. These are the discovery by Alvarez et al., that certain mass extinctions are caused by the impact on the earth of a large asteroid or comet, and the discovery by Raup and Sepkoski that such extinctions are periodic, with a cycle time of 26 to 30 million years. The validity of both of these discoveries is assumed and the implications are examined. Most of the phenomena described depend not on periodicity, but just on the weaker assumption that the impacts on the earth take place primarily in showers. Proposed explanations for the periodicity include galactic oscillations, the Planet X model, and the possibility of Nemesis, a solar companion star. These hypotheses are critically examined. Results of the search for the solar companion are reported. The Deccan flood basalts of India have been proposed as the impact site for the Cretaceous impact, but this hypotheisis is in contradiction with the conclusion of Courtillot et al., that the magma flow began during a period of normal magnetic field. A possible resolution of this contradiction is proposed.

Muller, Richard A.↗
NASA NTRS1989

Oscillations in D-region absorption at periods of one to two months

One to two month oscillations in D-region absorption are found in seven years of daily f-min data from low latitude stations at Singapore (1N, 104E) and Rarotonga (21S, 160W). Coherency (cross-spectral) analyses reveal that solar flux variations account for much of the f-min variance at these periods. Over the range of periods from 10 to 200 days, statistically significant linear correlation is found between the f-min time series and contemporaneous 10.7 cm solar flux mearurements at periods of 16 to 19 days, the 26 to 29 day solar rotation band, and a broad band covering 43 to 80 day periods.

Stanford, J. L.↗
NASA NTRS1988

An estimate of equatorial wave energy flux at 9- to 90-day periods in the Central Pacific

Deep fluctuations in current along the equator in the Central Pacific are dominated by coherent structures which correspond closely to narrow-band propagating equatorial waves. Currents were measured roughly at 1500 and 3000 m depths at five moorings between 144 and 148 deg W from January 1981 to March 1983, as part of the Pacific Equatorial Ocean Dynamics program. In each frequency band resolved, a single complex empirical orthogonal function accounts for half to three quarters of the observed variance in either zonal or meridional current. Dispersion for equatorial first meridional Rossby and Rossby gravity waves is consistent with the observed vertical-zonal coherence structure. The observations indicate that energy flux is westward and downward in long first meridional mode Rossby waves at periods 45 days and longer, and eastward and downward in short first meridional mode Rossby waves and Rossby-gravity waves at periods 30 days and shorter. A local minimum in energy flux occurs at periods corresponding to a maximum in upper-ocean meridional current energy contributed by tropical instability waves. Total vertical flux across the 9- to 90-day period range is 2.5 kW/m.

Eriksen, Charles C.↗
NASA NTRS1989

The intrinsic luminosity and initial period of pulsars

The observed properties of pulsars are modeled on the assumption that they evolve by emitting magnetic dipole radiation and have exponentially decaying magnetic fields. The models are constructed by using Monte Carlo techniques to generate a Galactic sample of pulsars having particular distributions of initial periods, magnetic dipole moments, and positions. By choosing a relationship between the radio luminosity L(r), the period (P), and the P derivative, it is determined which pulsars of the generated sample could be detected by one of the major pulsar surveys. Detailed account is taken of known selection effects in these surveys. Models in which the size of the pulsar emission cone is dependent on the period are considered. The properties of the detectable pulsars are compared with the properties of the observed pulsars, and the initial distributions and luminosity relation of the model are adjusted until a satisfactory agreement is obtained. A crucial point is that the observed relation between L(r) and P, P derivative is affected by selection effects and does not apply to the actual pulsar population. It is found that the best fits are obtained with models in which the pulsars are injected with relatively long periods, about 0.5 sec.

Emmering, Robert T.↗
NASA NTRS1989

Oscillations in D-region absorption at periods of one to two months

One to two month oscillations in D-region absorption are found in seven years of daily f-min data from low latitude stations at Singapore (1 deg N, 104 deg E) and Rarotonga (21 deg S, 160 deg W). Coherency (cross-spectral) analyses reveal that solar flux variations account for much of the f-min variance at these periods. Over the range of periods from 10 to 200 days, statistically significant linear correlation is found between the f-min time series and contemporaneous 10.7 cm solar flux measurements at periods of 16-19 days, the 26-29 day solar rotation band, and a broad band covering 43-80 day periods.

Stanford, J. L.↗
NASA NTRS1990

Periodic variations in the activity of Comet P/Halley during the 1985/1986 apparition

A search for periodic variation in the production of gas and dust by Comet Halley has been performed using narrowband photometric measurements from four sites - Lowell Observatory, Mauna Kea Observatory, Perth Observatory, and Cerro Tololo Inter-American Observatory. The method of phase dispersion minimization was applied to observations made during 164 observing nights between September 1985 and June 1986. A clear-cut variation, with a period near 7.4 days, was present throughout the postperihelion window. Less conclusive evidence of a similar period has been found in the pre-perihelion data. No indication of a shorter period or of strong sporadic activity exists in the data. The observations require that Halley's nucleus returns to essentially the same orientation with respect to the sun approximately every 7.4 days except for longer-timescale seasonal evolution. This fact precludes certain proposed models of nuclear motion.

Schleicher, David G.↗
NASA NTRS1990

Cloud parameters derived from GOES during the 1987 marine stratocumulus FIRE Intensive Field Observation (IFO) period

The Geostationary Operational Environmental Satellite (GOES) is well suited for observations of the variations of clouds over many temporal and spatial scales. For this reason, GOES data taken during the Marine Stratocumulus Intensive Field Observations (IFO) (June 29 to July 19, 1987, Kloessel et al.) serve several purposes. One facet of the First ISCCP Regional Experiment (FIRE) is improvement of the understanding of cloud parameter retrievals from satellite-observed radiances. This involves comparisons of coincident satellite cloud parameters and high resolution data taken by various instruments on other platforms during the IFO periods. Another aspect of FIRE is the improvement of both large- and small-scale models of stratocumulus used in general circulation models (GCMs). This may involve, among other studies, linking the small-scale processes observed during the IFO to the variations in large-scale cloud fields observed with the satellites during the IFO and Extended Time Observation (ETO) periods. Preliminary results are presented of an analysis of GOES data covering most of the IFO period. The large scale cloud-field characteristics are derived, then related to a longer period of measurements. Finally, some point measurements taken from the surface are compared to regional scale cloud parameters derived from satellite radiances.

Young, David F.↗
NASA NTRS1991

Steady-state phase error for a phase-locked loop subjected to periodic Doppler inputs

The performance of a carrier phase locked loop (PLL) driven by a periodic Doppler input is studied. By expanding the Doppler input into a Fourier series and applying the linearized PLL approximations, it is easy to show that, for periodic frequency disturbances, the resulting steady state phase error is also periodic. Compared to the method of expanding frequency excursion into a power series, the Fourier expansion method can be used to predict the maximum phase error excursion for a periodic Doppler input. For systems with a large Doppler rate fluctuation, such as an optical transponder aboard an Earth orbiting spacecraft, the method can be applied to test whether a lower order tracking loop can provide satisfactory tracking and thereby save the effect of a higher order loop design.

Chen, C.-C.↗
NASA NTRS1990

Vibration transmission through periodic structures using a mobility power flow approach

The transmission of vibrational power (time averaged) through multiple coupled (periodic) structures is examined. The analysis is performed in the frequency domain and the coupling between the sub-elements of the periodic structure is expressed in terms of structural mobility functions for the junction points and between the junction points of the sub-elements. Equal length spans between stiffeners or supports of the periodic structure are considered. Through the use of the mobility power flow approach, the influence of sub-element and junction parameters, including damping at the joints, can be investigated. The results from the analysis can be in the form of either structural intensity or alternatively structural power content for each of the sub-elements. The examples discussed are for a thin, perfectly periodic beam with a finite number of spans with different types of stiffeners and/or supports between the spans. The excitation of the structure is by a point load located midway along the first span.

Cuschieri, J. M.↗