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Young, J. A.

Publications and source records attributed to Young, J. A..

12 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2019

Far Field Plume Distribution and Divergence for NEXT: DART Mission

In support of the Double Asteroid Redirection Test (DART) mission, laboratory measurements were made on the NEXT ion engine, which will be used for the spacecraft's in-space propulsion [1]. This study revisits a small range of mission-specific 2.7A throttle levels to understand the effect of in-flight flow rate variability, investigate intermediate throttle conditions, and improve measurement methodology. This paper specifically examines the far-field plume divergence and backflow ion flux distribution of the NEXT, while a companion paper examines the charge state distributions.

Throttle Levels↗
NASA NTRS1999

Design Requirements for Amorphous Piezoelectric Polymers

An overview of the piezoelectric activity in amorphous piezoelectric polymers is presented. The criteria required to render a polymer piezoelectric are discussed. Although piezoelectricity is a coupling between mechanical and electrical properties, most research has concentrated on the electrical properties of potentially piezoelectric polymers. In this work, we present comparative mechanical data as a function of temperature and offer a summary of polarization and electromechanical properties for each of the polymers considered.

Ounaies, Z.↗
NASA NTRS1999

Molecular Modeling of the Poling of Piezoelectric Polyimides

The computational method described in this paper allows the calculation of the dielectric relaxation strength of an amorphous polymer based solely upon its chemical structure. The 4,4' oxydiphthalic anhydride (ODPA) dianhydride and bis-aminophenoxybenzene (APB) diamine based polyimides, (beta-CN) APB-ODPA and APB-ODPA were studied. Amorphous cells were constructed and then poled using molecular dynamics. Dielectric relaxation strengths of Delta(epsilon) = 17.8 for (beta-CN) APB-ODPA and Delta(epsilon) = 7.7 for APB-ODPA were predicted. These values are in excellent agreement with the experimental values. It was found that both the pendant nitrile dipole and the backbone anhydride residue dipole make significant contributions to the polyimides dielectric response. Specifically, it was shown that the difference in the magnitude of the dielectric relaxations is directly attributable to the nitrile dipole. The size of the relaxations indicate an absence of cooperative dipolar motions, The model was used to explain these results in terms of the average orientation of the nitrile and anhydride dipoles to within 51 deg. and 63 deg., respectively, of the applied electric field.

Young, J. A.↗
NASA NTRS1997

Dielectric Properties of Piezoelectric Polyimides

Molecular modeling and dielectric measurements are being used to identify mechanisms governing piezoelectric behavior in polyimides such as dipole orientation during poling, as well as degree of piezoelectricity achievable. Molecular modeling on polyimides containing pendant, polar nitrile (CN) groups has been completed to determine their remanent polarization. Experimental investigation of their dielectric properties evaluated as a function of temperature and frequency has substantiated numerical predictions. With this information in hand, we are then able to suggest changes in the molecular structures, which will then improve upon the piezoelectric response.

Ounaies, Z.↗
NASA NTRS1989

Cryogenic optical tests of a lightweight HIP beryllium mirror

Five interferometric tests were conducted at cryogenic temperatures on a lightweight, 50 cm diameter, hot isostatic pressed (HIP) beryllium mirror in the Ames Research Center (ARC) Cryogenic Optics Test Facility. The purpose of the tests was to determine the stability of the mirror's figure when cooled to cryogenic temperatures. Test temperatures ranged from room ambient to 8 K. One cycle to 8 K and five cycles to 80 K were performed. Optical and thermal test methods are described. Data is presented to show the amount of cryogenic distortion and hysteresis present in the mirror when measured with an earlier, Shack interferometer, and with a newly-acquired, phase-measuring interferometer.

Melugin, Ramsey K.↗
NASA NTRS1985

Cross-equatorial and boundary layer exchange: A FGGE review

The Global Weather Experiment (FGGE) provided unique data on the interesting phenomenon of cross-equatorial flow. Such motion is a key element of the seasonal cycle of the tropics, especially in monsoonal regions. The IIb obserations, IIIb assimilations, and implied dynamics of the flows are reviewed. Additional emphasis is given to the low level branches concentrated in the planetary boundary layer, including air sea interaction and vertical turbulence processes. The results of a recent MONEX Boundary Layer Workshop are summarized.

Young, J. A.↗
NASA NTRS1984

Mechanics of a strong subsynoptic gravity wave deduced from satellite and surface observations

The three-dimensional structure and dynamics of a strong, tropospheric gravity wave event continuously observed from both satellite and surface facilities reveal the subsynoptic character of this 'wave of depression', suggesting that the gravity wave originated in the upper troposphere near a jet streak, and, being quasi-hydrostatic, was relatively nondispersive and long lived. Theory is applied in order both to explain and estimate gravity wave properties throughout the troposphere. Further improvements in the descriptions presented may require the development of solitary wave theory for deep depression waves in shear flow.

Pecnick, M. J.↗
NASA NTRS1982

Utilization of satellite and surface observations in a gravity wave study

The possibility of detecting gravity waves in the upper troposphere by means of satellite imagery is examined. Attention is focused on the gravity wave event of March 27, 1975, for which the surface wave is analyzed by a pressure perturbation analysis based on barograph traces and a surface wind divergence analysis. IR observations were also made of the event with instrumentation on board the synchronous meteorological satellite (SMS-1). Strong wind gusts were reported at surface stations immediately following severe drops in the surface pressures, events which were found not to be connected with convective activity. The wave was concluded to extend throughout the troposphere with an axis tilted with height, indicating the wave was generated from an upper level energy source. It is suggested that the gravity wave was an adjustment to ageostrophic motions within the jet stream entering the Midwest. The finding is in concert with previous observations of jet streaks upstream from surface gravity waves.

Pecnick, M. J.↗
NASA NTRS1972

Time-spectral characteristics of large-scale cloud systems in the tropical Pacific.

Study of time series of areal cloud coverage over the central Pacific to determine the large-scale variations of tropical disturbance activity for a four-month period in 1967. The persistent and fluctuating components are found to be more prevalent in regions of high and low mean cloudiness, respectively, with major longitudinal variations in activity most evident in the Southern Hemisphere. Normalized power spectra show that the transient activity occurring in the equatorial zone is dominated by shorter periods of about four days, while longer periods are dominant away from the equator. Coherency magnitudes and phases between different locations indicate that propagating cloud systems are most identifiable at lower frequencies in the Southern Hemisphere, and at somewhat higher frequencies in the Northern Hemisphere. Activity with periods in excess of five days consists largely of both westward and eastward propagation on the planetary wave scale. The shorter periods appear to consist of both synoptic-scale and planetary-scale modes, with westward propagation dominant near the equator.

Sikdar, D. N.↗