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At least 487 records · Page 27
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

The effect of decay of the amplitude of oscillation on random process models for QPO X-ray stars

Random process models provide a useful mathematical framework for analysis of the quasi-periodic oscillations (QPO) recently discovered in the X-ray emission from certain luminous Galactic X-ray stars. In this paper, consideration is given to the effects of the power spectrum of the decay of the amplitude of oscillation that is expected in some physical models for the QPO phenomena. The resulting changes in the power spectrum depend on the ratio of the decay time of the amplitude of oscillation to the lifetime of the shot envelope. For values of this ratio less than about 10, it is found that incoherent QPO peaks in the power spectrum are significantly reduced in height and total power and are broadened in width, making them more difficult to detect. Coherent terms in the power spectrum, if present, are reduced in height and total power, but their width is unaffected. The red noise component remains unchanged. The present results are applied to the beat-frequency modulated accretion model in order to set constraints on the physical properties of the boundary layer between the inner edge of the accretion disk and the neutron star magnetosphere.

Shibazaki, N.↗
NASA NTRS1987

ELF oscillations associated with electron beam injections from the Space Shuttle

ELF oscillations (f less than 500 Hz) were observed during the electron beam emissions of the space experiments with particle accelerators (SEPAC) flown on the Spacelab 1 Shuttle mission. The beams had energies up to 5 keV and currents up to 300 mA, and the oscillations were present in the data from a Langmuir probe, a floating probe, an electron energy analyzer, and a photometer. The power density and frequency of the fundamental increased with the Shuttle charge-up potential. The emission level observed during the beam sequences increased with the charge-up potential of the Orbiter, which largely depended on the wake structure. It is most likely that the ELF oscillations are expressions of fluctuations in the return current and the Shuttle potential and that these fluctuations are caused by processes involving charge imbalances in the near environment of the Shuttle, possibly in a comoving plasma cloud.

Cai, D.↗
NASA NTRS1988

Long-pulse generation with a stable-relaxation-oscillation Nd:YLF laser

A simple technique for long-pulse (0.2-2 microsec) generation with neodymium lasers has been demonstrated. Under the proper conditions, a normal-mode oscillator, operated in a single transverse mode, produces well-defined relaxation oscillations from which a single subpulse can be isolated for amplification. The characteristic subpulse temporal profile is ideal for saturated amplification without pulse shortening. Data are presented for a Nd:YLF system consisting of an oscillator followed by a 64-nm-long amplifier. Pulse energies in excess of 100 mJ were achieved with a small-signal gain of 630.

Harisson, J.↗
NASA NTRS1987

The impact damped harmonic oscillator in free decay

The impact-damped oscillator in free decay is studied by using time history solutions. A large range of oscillator amplitude is covered. The amount of damping is correlated with the behavior of the impacting mass. There are three behavior regimes: (1) a low amplitude range with less than one impact per cycle and very low damping, (2) a useful middle amplitude range with a finite number of impacts per cycle, and (3) a high amplitude range with an infinite number of impacts per cycle and progressively decreasing damping. For light damping the impact damping in the middle range is: (1) proportional to impactor mass, (2) additive to proportional damping, (3) a unique function of vibration amplitude, (4) proportional to 1-epsilon, where epsilon is the coefficient of restitution, and (5) very roughly inversely proportional to amplitude. The system exhibits jump phenomena and period doublings. An impactor with 2 percent of the oscillator's mass can produce a loss factor near 0.1.

Brown, G. V.↗
NASA NTRS1988

Resonant geomagnetic field oscillations and Birkeland currents in the morning sector

The relationship between large-scale Birkeland currents and resonant oscillations of the geomagnetic field is investigated on the basis of Viking magnetic-field, electric-field, and particle measurements and AMPTE-CEE and IMP-8 magnetic-field measurements obtained during April 23-25, 1986. The data are presented in extensive graphs and characterized in detail. The 3-16-mHz 5-60-nT magnetic-field oscillations detected by Viking are also seen by AMPTE-CEE below L = 8.8 and extend from the lower L shells to the boundary between regions 1 and 2 of the Birkeland-current system, and this boundary is shown to be closely related to the interface between the low-latitude boundary layer and the central plasma sheet. It is suggested that the magnetospheric oscillations may be driven by several different solar-wind features, possibly depending on the direction of the IMF.

Potemra, T. A.↗
NASA NTRS1988

On the dynamics of intraseasonal oscillations and ENSO

This paper investigates some basic properties of low-frequency phenomena in the tropical atmosphere and the coupled ocean-atmosphere system, with an aim of seeking a unified dynamical framework for studies of the mechanisms of intraseasonal oscillations and the El Nino Southern Oscillation (ENSO). The role of specific processes leading to intraseasonal oscillations and to ENSO, both separately and collectively, are considered, using a simple shallow-water system. The results demonstrated the effect of including a time-dependent moist atmosphere in studies of the coupled atmosphere-ocean system. It is suggested that the interaction of several atmospheric processes and oceanic processes through evaporation-SST feedback and wind stress at the atmosphere-ocean interface may be important for the onset and evolution of an ENSO event.

Lau, K.-M.↗
NASA NTRS1988

Observations of the 30-60 day oscillation in zonal mean atmospheric angular momentum and high cloud cover

Zonally averaged momentum and high-cloud data were analyzed for the periods of April-October 1979 and November 1982-October 1983 periods. The analysis yielded belt correlation fields implying a symmetry and propagation of momentum out of the tropics in the momentum data and an asymmetrical oscillation across the equatorial zone in the high-cloud data. The 30-60 day oscillation was found to be the dominant periodicity in tropical and subtropical belts in both the momentum and cloud fields, with variation of time spectra evident between 1979 and 1983. The lag correlations between high cloud belts and momentum belts showed strong correlations with the 30-60 day oscillation present. Anomalously high/low zonal mean high cloudiness in the tropics was accompanied by anomalously high/low zonal mean momentum in the tropics, with the latter anomalies subsequently propagating into midlatitudes.

Risbey, James S.↗
NASA NTRS1989

Reflection Oscillators Containing Series-Resonant Crystals

Crystal-controlled transistor reflection oscillator easily tunable and stable, consumes little power, and costs less than other types of oscillators operating at same frequencies. Made possible by design concept that includes operation of transistor well beyond 3-dB frequency of current-versus-frequency curve. Concept takes advantage of newly available crystals resonating at frequencies up to about 1 GHz. Useful in determining approximate frequency of oscillation and major elements determining frequency.

Kleinberg, Leonard E.↗
NASA NTRS1987

Mars global atmospheric oscillations: Transients and dust storm relations

It is showed that periods of enhanced quasitidal pressure oscillations occur during the season preceding the global dust storm season during every Martian year. Periods of these oscillations often differ slightly from the diurnal and semidiurnal, and the oscillations may correspond to atmospheric normal modes excited by solar heating. They may also contribute to the initiation of some global dust storms.

Tillman, James E.↗
NASA NTRS1989

Numerical analysis of flow through oscillating cascade sections

The design of turbomachinery blades requires the prevention of flutter for all operating conditions. However, flow field predictions used for aeroelastic analysis are not well understood for all flow regimes. The present research focuses on numerical solutions of the Euler and Navier-Stokes equations using an ADI procedure to model two-dimensional, transonic flow through oscillating cascades. The model prescribes harmonic pitching motions for the blade sections for both zero and nonzero interblade phase angles. The code introduces the use of a deforming grid technique for convenient specification of the periodic boundary conditions. Approximate nonreflecting boundary conditions were coded for the inlet and exit boundary conditions. Sample unsteady solutions were performed for an oscillating cascade and compared to experimental data. Also, test cases were run for a flat plate cascade to compare with the unsteady, small-perturbation, subsonic analyis. The predictions for oscillating cascades with nonzero interblade phase angle cases, which were near a resonant condition, differ from the experiment and theory. The zero degree interblade phase angle cases, which were near a resonant condition, differ from the experiment and theory. Studies on reflecting versus nonreflecting inlet and exit boundary conditions show that the treatment of the boundary can have a significant effect on the first harmonic, unsteady pressure distributions for certain flow conditions.

Huff, Dennis L.↗
NASA NTRS1989

Low frequency oscillations in total ozone measurements

Low frequency oscillations with periods of approximately one to two months are found in eight years of global grids of total ozone data from the Total Ozone Mapping Spectrometer (TOMS) satellite instrument. The low frequency oscillations corroborate earlier analyses based on four years of data. In addition, both annual and seasonal one-point correlation maps based on the 8-year TOMS data are presented. The results clearly show a standing dipole in ozone perturbations, oscillating with 35 to 50 day periods over the equatorial Indian Ocean-west Pacific region. This contrasts with the eastward moving dipole reported in other data sets. The standing ozone dipole appears to be a dynamical feature associated with vertical atmospheric motions. Consistent with prior analyses based on lower stratospheric temperature fields, large-scale standing patterns are also found in the extratropics of both hemispheres, correlated with ozone fluctuations over the equatorial west Pacific. In the Northern Hemisphere, a standing pattern is observed extending from the tropical Indian Ocean to the north Pacific, across North America, and down to the equatorial Atlantic Ocean region. This feature is most pronounced in the NH summer.

Gao, X. H.↗
NASA NTRS1989

Solar g-mode oscillations: Comparison of SMM-ACRIM and ground-based observations

Progress was made in access to data and in developing programs for its analysis. The difficulties in completing the work in the planned time can be traced to several factors. The correction of the Stanford oscillation using gridded intensity data was not successful. It was concluded that due to poor continuity of the 1985 and 1986 data due to clouds, that a joint analysis with the ACRIM data (best solar oscillation data to date) on the summer 1987 observations should be performed. The 1988 Stanford oscillation data are being examined and the cross comparison of the ACRIM spectrum with the Standford spectrum for 1987 in the g-mode regime will shortly begin.

Scherrer, Philip H.↗
NASA NTRS1988

Fluid oscillation in the Drop Tower

An interfluid meniscus oscillates within a cylindrical container when suddenly released from earth's gravity and taken into a microgravity environment. Oscillations damp out from energy dissipative mechanisms such as viscosity and interfacial friction. Damping out of the oscillations by the latter mechanism is affected by the nature of the interfacial junction between the fluid-fluid interface and the container wall. Perfluoromethylcyclohexane and isopropanol in glass were the materials used for the experiment. The wetting condition of the fluids against the wall changes at the critical wetting transition temperature. This change in wetting causes a change in the damping characteristics.

Kaukler, William F.↗
NASA NTRS1988

Oscillator strengths for Si II

New accurate results are presented for absorption oscillator strengths of Si II resonance lines. In these ab initio calculations, an extensive basis set of 67 electronic configuration are employed, including fine-structure states and relativistic effects in the Breit-Pauli approximation. For the strong Si II lines, good agreement is found with previous LS-coupling f-values of Dufton et al. (1983). Oscillator strengths of selected weaker lines disagree in some cases with empirically determined f-values by Shull, Snow, and York (1981) and by Van Buren (1986). The theoretical results are combined with curve-of-growth analyses of Copernicus and IUE data to yield a list of recommended Si II oscillator strengths consistent with the errors in both theoretical and observational analyses. These f-values have been used (Van Steenberg and Shull) for a Galactic survey of interstellar silicon abundances with the International Ultraviolet Explorer satellite.

Luo, Ding↗
NASA NTRS1988

Seasonal variation of the semiannual oscillation

Observations of zonally average temperature from the Nimbus-7 Stratospheric and Mesospheric Sounder in 1979-82 show the existence of a significant season variation in the equatorial semiannual oscillation. The first semiannual cycle, beginning in the Northern Hemisphere winter (December-May), is much stronger than the second (June-November) cycle. The balanced winds are calculated from satellite data, showing corresponding seasonality in the semiannual oscillation wind regimes. Strong coupling is observed during the first cycle between equatorial and North Polar temperature. A model of this coupling suggests that planetary Rossby wave momentum deposition in the northern winter is the cause of the seasonal variation in the easterly phase of the semiannual oscillation.

Delisi, Donald P.↗
NASA NTRS1989

Numerical analysis of flow through oscillating cascade sections

The design of turbomachinery blades requires the prevention of flutter for all operating conditions. However, flow field predictions used for aeroelastic analysis are not well understood for all flow regimes. The present research focuses on numerical solutions of the Euler and Navier-Stokes equations using an ADI procedure to model two-dimensional, transonic flow through oscillating cascades. The model prescribes harmonic pitching motions for the blade sections for both zero and nonzero interblade phase angles. The code introduces the use of a deforming grid technique for convenient specification of the perioidic boundary conditions. Approximate nonreflecting boundary conditons were coded for the inlet and exit boundary conditions. Sample unsteady solutions were performed for an oscillating cascade and compared to experimental data. Also, test cases were run for a flat plate cascade to compare with the unsteady, small-perturbation, subsonic analysis. The predictions for oscillating cascades with nonzero interblade phase angle cases, which were near a resonant condition, differ from the experiment and theory. The zero degree interblade phase angle cases, which were near a resonant condition, differ from the experiment and theory. Studies on reflecting versus nonreflecting inlet and exit boundary conditions show that the treatment of the boundary can have a significant effect on the first harmonic, unsteady pressure distribution for certain flow conditions.

Huff, Dennis L.↗
NASA NTRS1989

Oscillations in stellar atmospheres

Atmospheric excitation and propagation of oscillations are analyzed for typical pulsating stars. The linear, plane-parallel approach for the pulsating atmosphere gives a local description of the phenomenon. From the local analysis of oscillations, the minimum frequencies are obtained for radially propagating waves. The comparison of the minimum frequencies obtained for a variety of stellar types is in good agreement with the observed periods of the oscillations. The role of the atmosphere in the globar stellar pulsations is thus emphasized.

Costa, A.↗
NASA NTRS1988

Exploratory study of temperature oscillations related to transient operation of a Capillary Pumped Loop heat pipe

An analytical study has been conducted for better understanding of a peculiar transient behavior which was displayed in testing of a Capillary Pumped Loop (CPL) heat pipe system. During several test runs of this CPL system varying degrees of surface temperature oscillations occurred in the inlet line of the evaporators. Although several theories have been forwarded to explain this observed phenomenon, a satisfactory understanding of causes of these oscillations is still missing. The present investigation derives the conditions which lead to such oscillatory temperature behavior in evaporator inlet section of the mentioned CPL system. Stability characteristics of these temperature oscillations were investigated.

Kiper, A. M.↗