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At least 649 records · Page 36

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NASA NTRS1985

Effects of upstream-biased third-order space correction terms on multidimensional Crowley advection schemes

The impact of upstream-biased corrections for third-order spatial truncation error on the stability and phase error of the two-dimensional Crowley combined advective scheme with the cross-space term included is analyzed, putting primary emphasis on phase error reduction. The various versions of the Crowley scheme are formally defined, and their stability and phase error characteristics are intercompared using a linear Fourier component analysis patterned after Fromm (1968, 1969). The performances of the schemes under prototype simulation conditions are tested using time-dependent numerical experiments which advect an initially cone-shaped passive scalar distribution in each of three steady nondivergent flows. One such flow is solid rotation, while the other two are diagonal uniform flow and a strongly deformational vortex.

Schlesinger, R. E.↗

NASA NTRS1985

Elements of a stochastic-dynamical theory of the long-term variability of the El Nino/Southern Oscillation

The rudiments of a stochastic-dynamical model for climatic systems with multiple equilibrium states are presented as a means for analyzing the long-term variability of the El Nino/Southern Oscillation (ENSO) events. It is shown that a combination of the unstable air-sea interaction, the seasonal cycle, and stochastic intraseasonal forcings must be considered in any model for ENSO. In particular, the instability in the air-sea interaction may be triggered by stochastic forcing. The possibility that stochastic events initiate the conditions leading to ENSO exacerbates the already difficult task of predicting ENSO patterns.

Lau, K.-M.↗

NASA NTRS1985

Energetic auroral and polar ion outflow at DE 1 altitudes Magnitude, composition, magnetic activity dependence, and long-term variations

Data acquired from the Dynamics Explorer I Energetic Ion Composition Spectrometer in the period from September 1981 to May 1984 are used to determine the magnitude of the terrestrial ion outflow in the 0.01-17 keV/el range. The data are also employed to investigate the mass composition and topology (local time and invariant latitude distributions) of the ion outflow, as well as the outflow's magnetic activity dependence and long-term variation. The relative importance of auroral versus polar cap upflowing ions as a source of energetic plasma for various parts of the magnetosphere is examined.

Yau, A. W.↗

NASA NTRS1985

High temperature, short term tensile strength of C6000/PMR-15 composites

Tensile tests were conducted on 0 unidirectionally reinforced Celion 6000 graphite fibers in PMR-15 polyimide matrix. Tensile strengths for coupons subjected to short and long term uniform temperatures were obtained. Thick coupons, heated on one side to produce significant transient through thickness temperature gradients, were tested and compared to the strength of specimens with uniform temperature distributions. All coupons were radiantly heated and reached maximum test temperatures within 15 sec. Tensile loads were applied to the coupons after 15 sec of elevated temperature exposure. Loading rates were selected so that specimen failures occurred within a maximum of 45 sec after reaching the test temperature. Results indicate that significant tensile strength remains beyond the material post cure temperature.

Digiovanni, P. R.↗

NASA NTRS1985

Joule-Thomson valves for long term service in space cryocoolers

Joule-Thomson valves for small cryocoolers have throttling passages on the order of 0.1 millimeter in diameter. Consequently, they can become plugged easily and stop the operation of the cooler. Plugging can be caused by solid particles, liquids or gases. Plugging is usually caused by the freezing of contaminant gases from the process stream. In small open loop coolers and in closed loop coolers where periodic maintenance is allowed, the problem is overcome by using careful assembly techniques, pure process gases, warm filters and cold adsorbers. A more thorough approach is required for closed loop cryocoolers which must operate unattended for long periods. This paper presents the results of an effort to solve this problem. The causes of plugging are examined, and various ways to eliminate plugging are discussed. Finally, the development of a J-T defroster is explained. It is concluded that a combination of preventive measures and a defroster will reduce the chance of cooler failure by plugging to such a degree that J-T coolers can be used for long term space missions.

Lester, J. M.↗

NASA NTRS1985

The detection of long-term changes in stratospheric ozone Scientific requirements and current results from satellite-based measurement systems

The ability to detect subtle trends in upper stratospheric ozone places strict requirements on satellite-based Solar Backscatter Ultraviolet (SBUV) sensors intended for this purpose. Simulation of the long-term change in backscattered radiance indicates trends in the range 1.0-1.6 percent per decade depending on wavelength for an ozone depletion scenario based on chlorofluorocarbon (CFC) chemistry. The maximum percentage change in the measured quantity, as would be sensed by the Nimbus-7 SBUV experiment and future similar instruments, is roughly a factor of two less than the maximum percentage change in ozone, the quantity of geophysical interest. Furthermore, interannual variations, whose magnitude can be estimated from the SBUV data set, could obscure a CFC-related trend in radiance when viewed over a period less than a decade. Even when such atmospheric noise and possible solar cycle variations are neglected, the detection of an ozone trend of the magnitude predicted for CFC-related chemistry requires a set of radiance measurements that are internally consistent to 1 percent or better over a time span of a decade.

Frederick, J. E.↗

NASA NTRS1985

On the observed properties and long-term structure and evolution of white dwarfs in cataclysmic variables

An attempt is made to explain the observed surface temperatures and luminosities of cataclysmic variable white dwarfs based on the cooling physics and time-averaged structure of the white dwarf in response to accretion. The evolutionary changes in core temperature and core luminosity as an accreting white dwarf evolves quasi-statically are compared with the cooling evolution of a nonaccreting, one solar mass, pure C-12 core. Then, the evolutionary behavior of the white dwarf envelope in mass and temperature, in response to long-term accretion, is used to predict temperatures and luminosities of white dwarfs during quiescent intervals between nova outbursts. It is shown that the observed luminosities and effective temperatures of the bare white dwarfs detected in some cataclysmic variables are the expected intrinsic values associated with classical nova thermonuclear outburst cycles. Two alternate interpretations of their observed surface temperatures and luminosities are assessed.

Sion, E. M.↗

NASA NTRS1984

Assessing photovoltaic module life from long-term environmental tests

An assessment is made of results obtained by an experimental program having as its aim the identification of critical temperature-humidity bias degradation mechanisms in solar cell modules intended to have an operating lifetime of the order of 20-30 years. These mechanisms are studied in the context of conditions encountered over the course of long term operation in various sites in the U.S. Accelerated tests for the assessment of product lifetime entail the development of a correlation between test and application conditions; this has been accomplished through an analytical procedure for the correlation of time-varying field exposures to constant-stress accelerated environments.

Otth, D. H.↗

NASA NTRS1986

A long-term study of SiO (v = 1, J = 2-1) maser emission from evolved stars and Orion A

General characteristics of eight late-type stars and Orion A in the SiO (v = 1, J = 2-1) maser line are reported based on observations made with the 20-m Onsala telescope between 1977 and 1984. Five stars show periodic maser flux variations that are well correlated with the infrared light curve, and the maximum SiO intensity and the dynamic range vary significantly between different periods and sources. Some long-term variations in the center velocity of the maser emission have been noted, with Omicron Cet possibly being a member of a binary system. Grand averages have been used to study the general velocity structure, and a global SiO (v = 1, J = 2-1) profile for Mira variables shows that the strongest emission is blueshifted with respect to the stellar velocity, while as a whole the emission is redshifted. The analysis places the SiO masers in a region between the pulsating stellar atmosphere and the expanding circumstellar envelope, where the velocity structure bears no relation to the expansion velocity of the shell.

Nyman, L.-A.↗

NASA NTRS1985

The use of a simplified isentropic model for short-term aviation forecasting

A simple, multiple-layer, isentropic potential vorticity forecast model is described for predicting short-term atmospheric changes between 12-hourly radiosonde observations. The model and the model initialization technique are described, and a case study with both convection and strong horizontal wind shear is discussed. The wind and stream function field forecasts and the forecast areas of convective instability are constrasted with observational results to assess the model's strengths and weaknesses. Excellent agreement is obtained between model predictions and observations.

Homan, J. H.↗

NASA NTRS1986

Semi-analytical theory for long-term rotational motion of uncontrolled satellites

Analytical solutions/integrals of the free rotational motion of uncontrolled satellites which contain spinning rotors are employed in conjunction with a suitable analytical/numerical averaging approach to predict long-term attitude motion. This semi-analytical method of averaging is basically similar to the one used in purely analytical 'general perturbation' theories. However, in view of the complexities involved in analytical treatment of the problem, its application is primarily numerical in character. An example is presented illustrating the use of the theory to predict the attitude motion of an axisymmetric satellite containing symmetrical constant-speed rotors under gravity-gradient torque. The use of the theory is considerably more efficient than straightforward numerical integration of the complete set of equations of motion.

Lahr, B. S.↗

NASA NTRS1986

Detection of stratospheric HNO3 and NO2 response to short-term solar ultraviolet variability

It is pointed out that variations in the solar ultraviolet irradiance with a period equal to or approximately one-half of the rotation period of the sun are currently observed by satellite monitoring. Information regarding the response of stratospheric species to solar ultraviolet variability is indispensable for an understanding of the photochemical behavior of the middle atmosphere. Relations are considered between LIMS (limb infrared monitor of the stratosphere) measurements of HNO3 and NO2, and the SBUV (solar backscatter ultraviolet) measurements of short-term variations in 205-nm radiation. It is found that the response of HNO3 is much stronger than, but in the opposite sense to the ozone response, while the NO2 response is in the opposite sense to the HNO3 response. Model calculations predict large variations in HNO3 over the 11-yr solar cycle.

Keating, G. M.↗

NASA NTRS1987

Long-Term Tests of 38 Ball-Bearing Greases

Perfluoroalkylpolyether lubricants performed best. Final report in series presents results of program of long-term testing of ballbearing greases in vacuum, oxidizing, and otherwise hostile environments. Earlier reports in same series described in "Tests of 38 Ball-Bearing Greases," (MFS-25624) and "Further Tests of 38 Ball-Bearing Greases." (MFS-27043)

Mcmurtrey, E.↗

NASA NTRS1987

Testing Long-Term Exposure To Vacuum

Facility for studying long-term effects of vacuum on materials described in report. Vacuum storage system maintains specimens at constant temperature and high vacuum. Enables specimens to be removed easily and safely for analysis. Nitrogen storage system maintains specimens at 77 degree F in pure, dry nitrogen gas under gauge pressure of 2 to 4 psi. Enables comparison of control specimens with specimens exposed to vacuum. Instrumentation-and-control system measures and records operating parameters and generates control signals and alarm indications as necessary.

Clark, D. L.↗

NASA NTRS1987

Computing Long-Term Orbital Motions

Drifts and lifetimes predicted. Long-term Orbit Predictor (LOP) is trajectory-propagation computer program used as analysis tool for studies of lifetimes of orbiting spacecraft. Used for any planetary orbiting missions. LOP written in FORTRAN 77.

Kwok, J. H.↗

NASA NTRS1986

Investigation of long term storage effects on aerospace nickel-cadmium cell performance

A study on evaluation of the long term storage effects on aerospace nickel-cadmium cells currently being performed at NASA/Goddard Space Flight Center (GSFC) is described. A number of cells of 6 Ah and 12 Ah capacities which were stored in shorted condition for 8 to 9 years at the GSFC were selected for this study. These cells will undergo electrical acceptance testing the the GSFC, and life cycling at the NASA Battery Test Facility at the Naval Weapons Facility at the Naval Weapons Support Center (NWSC) in Crane, Indiana; in addition, some cells from the study will undergo destructive analyses.

Yi, T. Y.↗

NASA NTRS1986

Evaluation of a nonlinear parameter extraction mathematical model including the term C(subm(sub delta e squared))

Shuttle flight test data were used to determine values for the short-period parameters. The best identified, as judged by its estimated standard deviation, was the elevon effectiveness parameter C (sub m (sub sigma e squared)). However, the scatter about the preflight prediction of C (sub m (sub sigma e squared)) was large. Other investigators have suggested that adding nonlinear terms to the mathematical model used to identify C (sub m (sub sigma e)) could reduce the scatter. The results of this investigation show that C (sub m (sub sigma e squared)) is the only identifiable nonlinear parameter applicable and that the changes in C (sub m (sub sigma e)) values when C (sub m (sub sigma e squared)) is included are in the order of ten percent for the data estimated.

Suit, W. T.↗

NASA NTRS1986

Long-term amplitude and phase stability of the 400-kW 2.115-GHz transmitter

Results of recent measurements of the long-term phase, amplitude and group delay stability of the 400-kW S-band (2.115-GHz) transmitter are reported. Various control parameters which are responsible for many of the observed instabilities are identified. Further tests to identify the parameters responsible for the remaining instabilities are suggested.

Hoppe, D. J.↗