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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 505 records · Page 28
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NASA NTRS1991

Evidence of chaotic pattern in solar flux through a reproducible sequence of period-doubling-type bifurcations

A preliminary study of the limits to solar flux intensity prediction, and of whether the general lack of predictability in the solar flux arises from the nonlinear chaotic nature of the Sun's physical activity is presented. Statistical analysis of a chaotic signal can extract only its most gross features, and detailed physical models fail, since even the simplest equations of motion for a nonlinear system can exhibit chaotic behavior. A recent theory by Feigenbaum suggests that nonlinear systems that can be led into chaotic behavior through a sequence of period-doubling bifurcations will exhibit a universal behavior. As the control parameter is increased, the bifurcation points occur in such a way that a proper ratio of these will approach the universal Feigenbaum number. Experimental evidence supporting the applicability of the Feigenbaum scenario to solar flux data is sparse. However, given the hypothesis that the Sun's convection zones are similar to a Rayleigh-Bernard mechanism, we can learn a great deal from the remarkable agreement observed between the prediction by theory (period doubling - a universal route to chaos) and the amplitude decrease of the signal's regular subharmonics. It is shown that period-doubling-type bifurcation is a possible route to a chaotic pattern of solar flux that is distinguishable from the logarithm of its power spectral density. This conclusion is the first positive step toward a reformulation of solar flux by a nonlinear chaotic approach. The ultimate goal of this research is to be able to predict an estimate of the upper and lower bounds for solar flux within its predictable zones. Naturally, it is an important task to identify the time horizons beyond which predictability becomes incompatible with computability.

Ashrafi, S.↗
NASA NTRS1991

Long-term behaviour of starspots. III - Active longitudes on the long-period RS CVn star HK Lacertae

Thirteen years of photometry were used to study active longitudes on the surface of HK Lacertae. The decaying and forming phenomena of active areas in the years of 1980-1981, 1983-1984, and 1988-1989 are shown. New activity centers suddenly occur near the same orbital phase (0.6) and have lifetimes of several (greater than 6) years. The average rotational period of HK Lac is 24.2852 d; compared to the orbital period of 24.4284 d, this implies a 32 degree/year drift of the active areas on the stellar surface. Two possible interpretations for the period difference are discussed: pseudosynchronism and rigidity of magnetic patterns. The active longitudes on HK Lac are compared with similar phenomena found on the sun.

Olah, K.↗
NASA NTRS1992

Optimal and robust controllers for periodic and multirate systems

The problem of optimal rejection of bounded persistent disturbances is solved in the case of linear discrete-time periodic systems. The solution consists of solving an equivalent time-invariant standard l1 optimization problem subject to an additional constraint. This constraint assures the causality of the resulting periodic controller. By the duality theory, the problem is shown to be equivalent to a linear programming problem, which is no harder than the standard l1 problem. Also, it is shown that the method of solution presented applies exactly to the problem of disturbance rejection in the case of multirate sampled data systems. Finally, the results are applied to the problem of robust stabilization of periodic and multirate systems.

Dahleh, Munther A.↗
NASA NTRS1992

Does an orbiting star cause periodic modulation of X-rays from NGC6814?

An obvious candidate for the phenomenon underlying the periodicity in the X-ray emission from the Seyfert galaxy NGC6814 is the orbital motion of a star or low-mass compact object around the central black hole. It is shown here that the presence of an orbiting star could be easily verified by looking for the effects of Lense-Thirring precession of the orbital plane caused by the dragging of inertial frames around a rotating black hole. Precession-induced variations in the waveform and in the phase of the observed periodicity should have a period of between a month and a year. Such variations could account for the different waveforms present in the Ginga and Exosat data set from observations of NGC6814 and may be detectable in existing Ginga and future Rosat, OSSE/BRO, and Astro-D data.

Sikora, Marek↗
NASA NTRS1992

Periodic variability in the optical counterpart of X-ray Nova Muscae 1991

Analysis of over 1000 CCD images of the optical counterpart of the black hole candidate Nova Muscae 1991, taken 3-5 months after the initial outburst, reveal the existence of a luminosity hump of about 0.2 mag which recurs every 10.5 hr. This period presumably either is, or is closely related to, the orbital period, which is thus similar to that of the confirmed black hole system A0620 - 00. The light-curve shape, the lack of significant color variations, and marginal evidence for a change in the recurrence period suggest that the luminosity humps in Nova Muscae may be related to the superhump phenomenon in the SU UMa subclass of dwarf novae. Other implications of the analogy between soft X-ray transients and dwarf novae are discussed.

Bailyn, Charles D.↗
NASA NTRS1991

Periodic trim solutions with hp-version finite elements in time

Finite elements in time as an alternative strategy for rotorcraft trim problems are studied. The research treats linear flap and linearized flap-lag response both for quasi-trim and trim cases. The connection between Fourier series analysis and hp-finite elements for periodic a problem is also examined. It is proved that Fourier series is a special case of space-time finite elements in which one element is used with a strong displacement formulation. Comparisons are made with respect to accuracy among Fourier analysis, displacement methods, and mixed methods over a variety parameters. The hp trade-off is studied for the periodic trim problem to provide an optimum step size and order of polynomial for a given error criteria. It is found that finite elements in time can outperform Fourier analysis for periodic problems, and for some given error criteria. The mixed method provides better results than does the displacement method.

Hou, Lin-Jun↗
NASA NTRS1993

Thermoviscoplastic analysis of fibrous periodic composites using triangular subvolumes

The nonlinear viscoplastic behavior of fibrous periodic composites is analyzed by discretizing the unit cell into triangular subvolumes. A set of these subvolumes can be configured by the analyst to construct a representation for the unit cell of a periodic composite. In each step of the loading history, the total strain increment at any point is governed by an integral equation which applies to the entire composite. A Fourier series approximation allows the incremental stresses and strains to be determined within a unit cell of the periodic lattice. The nonlinearity arising from the viscoplastic behavior of the constituent materials comprising the composite is treated as fictitious body force in the governing integral equation. Specific numerical examples showing the stress distributions in the unit cell of a fibrous tungsten/copper metal matrix composite under viscoplastic loading conditions are given. The stress distribution resulting in the unit cell when the composite material is subjected to an overall transverse stress loading history perpendicular to the fibers is found to be highly heterogeneous, and typical homogenization techniques based on treating the stress and strain distributions within the constituent phases as homogeneous result in large errors under inelastic loading conditions.

Walker, Kevin P.↗
NASA NTRS1992

An RR Lyrae period shift in terms of the Fourier parameter Phi sub 31

The Fourier phase parameter Phi sub 31 has been determined for RRc stars in five globular clusters, NGC 6171, M5, M3, M53, and M15. The results indicate that the RRc stars in a given cluster show a sequence of Phi sub 31 increasing with period, and that the higher the cluster metallicity, the higher the sequence lies in a plot of Phi sub 31 with period. The Phi sub 31 values for the stars in NGC 6171 and M5 presented here are based on observations made with the University of Toronto 0.61 m telescope at Las Campanas, Chile, while those for M3, M53, and M15 are based on published data. A bootstrap procedure has been used to establish the uncertainties in the Fourier parameters. The physical significance of the relationship among Phi sub 31, period, and metallicity is not yet understood. It will need to be tested with hydrodynamic pulsation models computed with new opacities.

Clement, Christine M.↗
NASA NTRS1992

The 14.8-h orbital period of GX339-4

We present the results of photometric observations of the black hole candidate GX339-4, obtained while the system was in an 'off' state. We show that a 14.8-h modulation was present, and provide evidence for a similar periodicity in the 'high' state from a reanalysis of previously published photometry and spectroscopy. The presence of the same period in both states implies that it is likely to be the orbital period of the system. The spectroscopy analysis provides evidence for an apparent change in the systemic velocity of the system. The amplitude of the observed radial velocity variations, however, permits only crude limits to be placed on the mass of the compact object. Only absorption-line spectroscopy of the secondary in the 'off' state will provide a convincing mass determination.

Callanan, P. J.↗
NASA NTRS1993

Simulation of the early startup period of high-temperature heat pipes from the frozen state by a rarefied vapor self-diffusion model

The heat pipe startup process is described physically and is divided into five periods for convenience of analysis. The literature survey revealed that none of the previous attempts to simulate the heat pipe startup process numerically were successful, since the rarefied vapor flow in the heat pipe was not considered. Therefore, a rarefied vapor self-diffusion model is proposed, and the early startup periods, in which the rarefied vapor flow is dominant within the heat pipe, are first simulated numerically. The numerical results show that large vapor density gradients existed along the heat pipe length, and the vapor flow reaches supersonic velocities when the density is extremely low. The numerical results are compared with the experimental data of the early startup period with good agreement.

Cao, Y.↗
NASA NTRS1993

PG 0308 + 096 and PG 1026 + 002 - Two new short period binary stars resulting from common-envelope evolution

Ultraviolet spectroscopy, optical spectroscopy, and spectrophotometry have been used to study the excess UV stars PG 0308 + 096 and PG 1026 + 002. Both objects are short-period binary systems, each containing a DA white dwarf star and a dM star. Orbital periods of approximately 0.284 day for PG 0308 + 096, and aproximately 0.597 day for PG 1026, have been found by spectroscopic analysis of the H-alpha emission line. Ly-alpha and Balmer line profile fitting were used to estimate the mass of white dwarf stars; mass estimates for the dM stars are based on their spectral types. The orbital inclinations are derived from these masses, the periods, and amplitudes of the H-alpha radial velocity curves. The equivalent width of the H-alpha emission line, in each binary system, varies with the orbital phase in such a manner as to imply that it arises, in large part at least, from the hemisphere of the M star that faces the white dwarf star.

Saffer, Rex A.↗
NASA NTRS1993

On quasiperiodic variations of pulsars' periods - An alternative to the planetary interpretation of PSR1257+12

The quasi-periodic timing residuals of PSR1257+12 are considered to be a result of two large-scale internal motions of the neutron star: core-crust relative motion and precession. The model requires that the superfluid core of a neutron star is not pinned to the solid crust. A very good fit to the observations is obtained for the set of parameters that corresponds to a nearly orthogonal rotator, and the period of the core-crust relative rotation that is nearly twice the period of precession.

Dolginov, A. Z.↗
NASA NTRS1993

Periodic variation in the geomagnetic activity - A study based on the Ap index

The monthly and daily samples of the Ap index for the interval from 1932 through 1982 were studied using the power spectrum technique. Results obtained for Bartel's period (about 27 days), the semiannual period, the dual-peak solar cycle distribution of geomagnetic storms, and certain other medium-scale periodicities are examined in detail. In addition, results on the cumulative occurrence number of storms per decade as a function of the Ap and Dst indices for the storm are presented.

De Gonzalez, Alicia L. C.↗
NASA NTRS1993

Periodicities in the IUE particle radiation data

The magnetospheric charged particle flux measured serendipitously by the IUE satellite during perigee passages from 1978 February to 1989 February has been searched for periodicities using Fourier transform and correlation techniques. The analysis uncovers persistent modulations of the particle flux with periods of 28-33 days and 15-17 days. The particle flux seen by IUE in the outer van Allen belt is modulated to a significant extent by the magnetosphere's interaction with high-speed solar wind streams from coronal holes, in agreement with analyses of fluxes at other altitudes and of geomagnetic disturbance indices. It is suggested that recurrent global solar-surface activity, such as the 25.8-day periodicity discussed by Bai and Sturrock (1991) and the solar 'hot spots' reported by Bai (1987, 1988, 1990), may play a role in modulating the particle flux near solar maximum.

Arquilla, Richard↗
NASA NTRS1993

A doubly periodic moment method solution for the analysis and design of an absorber covered wall

The periodic moment method (PMM) solution for scattering from a doubly periodic array of lossy dielectric bodies is developed in this paper. The purpose of this study is to design electromagnetic wedge and pyramidal absorber for low reflectivity so that one can improve the performance of anechoic chamber measurements. The spectral-domain formulation and the moment method volume polarization current approach are applied to obtain the expressions used to determine the scattering from a doubly periodic array of lossy dielectric bodies. Through these studies, some wedge and pyramidal absorber configurations have been designed, fabricated, and then tested in the OSU/ESL compact range measurement facility. In this paper, wedge absorber, commercial pyramidal absorber, and some low-frequency pyramidal absorber designs are presented. By considering the complexity of dealing with real-world material structures, good agreement between calculations and measurements has been obtained.

Yang, Chang-Fa↗
NASA NTRS1993

An investigation of the fuel-optimal periodic trajectories of a hypersonic vehicle

Periodic trajectories were found to minimize the range-averaged fuel consumption. For a realistic hypersonic aircraft modeled as a point mass over a nonrotating, spherical Earth, the periodic orbit yielded a 15 percent improvement in fuel consumption over static cruise. Moreover, vehicle dynamic loading was contained within a realistic survivability envelope of 8 g's. The resulting periodic orbit is composed of very distinct flight regimes (Keplerian arc, atmospheric glide and powered climb), which may offer mission advantages over the static path.

Dewell, Larry D.↗
NASA NTRS1993

Origins of Solar Systems Workshop: The Origin, Evolution, and Detectability of Short Period Comets

The origin of the short period comets (SPC) (periods less than 200 years), the dynamical formation of their present reservoir(s), the cause and rate of their transport to the inner planetary region where they can be detected, and the magnitude of selection effects in their discovery are important research questions directly coupled to the goals of understanding the origin and evolution of the Solar System. To address these questions in an intensive way, an interdisciplinary, five month long Workshop from Jan. to May 1993 at Southwest Research Institute (SwRI) in San Antonio was convened. The goal of this Workshop was to advance the state of understanding about the origins, dynamical evolution, and present location of short period comets and their reservoir(s).

Stern, S. Alan↗
NASA NTRS1993

The long-term dynamical behavior of short-period comets

The orbits of the known short-period comets under the influence of the Sun and all the planets except Mercury and Pluto are numerically integrated. The calculation was undertaken in order to determine the dynamical lifetimes for these objects as well as explaining the current orbital element distribution. It is found that a comet can move between Jupiter-family and Halley-family comets several times in its dynamical lifetime. The median lifetime of the known short-period comets from the time they are first injected into a short-period comet orbit to ultimate ejection is approximately 50,000 years. The very flat inclination distribution of Jupiter-family comets is observed to become more distended as it ages. The only possible explanation for the observed flat distribution is that the comets become extinct before their inclination distribution can change significantly. It is shown that the anomalous concentration of the argument of perihelion of Jupiter-family comets near 0 and 180 deg is a direct result of their aphelion distance being close to 5.2AU and the comet being recently perturbed onto a Jupiter-family orbit. Also the concentration of their aphelion near Jupiter's orbit is a result of the conservation of the Tisserand invariant during the capture process.

Levison, Harold F.↗