SEARCH · Engineering Papers

Results for “important processes”

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.

SourceResearch connectionFrom yearThrough year

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

At least 1,045 records · Page 58

All sources NASA NTRS DOE OSTI

NASA NTRS1987

Growth and characterization of cubic SiC single-crystal films on Si

Morphological and electrical characterization results are presented for cubic SiC films grown by chemical vapor deposition on single-crystal Si substrates. The films, up to 40 microns thick, were characterized by optical microscopy, (SEM), (TEM), electron channeling, surface profilometry, and Hall measurements. A variety of morphological features observed on the SiC films are described. Electrical measurements showed a decrease in the electron mobility with increasing electron carrier concentration, similar to that observed in Si. Room-temperature electron mobilities up to 520 sq cm/V-s (at an electron carrier concentration of 5 x 10 to the 16th/cu cm) were measured. Finally, a number of parameters believed to be important in the growth process were investigated, and some discussion is given of their possible effects on the film characteristics.

Powell, J. Anthony↗

NASA NTRS1987

Enhanced beam coupling modulation using the polarization properties of photorefractive GaAs

Observation is reported of a rotation in the polarization of the two photorefractive recording beams in GaAs for a configuration with the internally generated space-charge field along the line 110 crystallographic orientation. This rotation is a result of simultaneous constructive and destructive beam coupling in each beam for the optical electric field components along the two electrooptically induced principal dielectric axes of the crystal. By turning one of the beams on and off, the intensity of the other beam after the crystal and a polarization analyzer can be modulated by as much as 500 percent. This result is of particular importance for optical information processing applications.

Partovi, Afshin↗

NASA NTRS1987

Principles for problem aggregation and assignment in medium scale multiprocessors

One of the most important issues in parallel processing is the mapping of workload to processors. This paper considers a large class of problems having a high degree of potential fine grained parallelism, and execution requirements that are either not predictable, or are too costly to predict. The main issues in mapping such a problem onto medium scale multiprocessors are those of aggregation and assignment. We study a method of parameterized aggregation that makes few assumptions about the workload. The mapping of aggregate units of work onto processors is uniform, and exploits locality of workload intensity to balance the unknown workload. In general, a finer aggregate granularity leads to a better balance at the price of increased communication/synchronization costs; the aggregation parameters can be adjusted to find a reasonable granularity. The effectiveness of this scheme is demonstrated on three model problems: an adaptive one-dimensional fluid dynamics problem with message passing, a sparse triangular linear system solver on both a shared memory and a message-passing machine, and a two-dimensional time-driven battlefield simulation employing message passing. Using the model problems, the tradeoffs are studied between balanced workload and the communication/synchronization costs. Finally, an analytical model is used to explain why the method balances workload and minimizes the variance in system behavior.

Nicol, David M.↗

NASA NTRS1988

Reducing adhesion and agglomeration within a cloud of combustible particles

The study of combustible particle clouds inside flame tubes is of fundamental scientific interest as well as a practical concern. Only the suspended concentration is important to the combustion process, so that assurances must be provided that a minimum of particles adheres to the tube wall. This paper demonstrates experimentally the ability to minimize adhesion and agglomeration of acoustically-mixed lycopodium particles within a 5-cm diameter lexan flame tube. The area density of particles (ADP) adhering to the wall of bare lexan tubes was measured at greater than 100 particles/sq mm. The nature of adhesion was found to be clearly electrostatic, with the ADP level aggravated by increased mixing time, vigor, and the concentration of particles. Increases in the conductivity of the air and the tube wall did not affect ADP levels substantially. However, the observed adhesion was reduced to less than 10 p/sq mm when the air was ionized by use of an alpha emitter mounted on the inner walls of the flame tube.

Ross, Howard D.↗

NASA NTRS1987

Medical effects of iodine disinfection products in spacecraft water

Various iodination products (IDPs), including iodinated and iodine-induced new compounds, will be present in the iodine-disinfected water that is expected to be used by crews on the NASA Space Station and on long duration missions. The metabolic intermediaries created by such a process may be more important to crew health than the parent IDPs, and reclamation and recycling may be expected to produce additional products. These medical effects may be expressed in crews as hypersensitivity, allergic, acute toxic, and chronic toxic reactions, as well as modifications of immune system response.

Sauer, Richard L.↗

NASA NTRS1988

Applications of tethered satellites to some problems of terrestrial physics

An evaluation is made of the applicability of tethered satellites to geomagnetic field and low altitude magnetospheric/ionospheric plasma investigations. Geomagnetic field observations from tethered satellites with gradiometers promise higher field measurement accuracy and the separation of current effects; plasma observations are important in understanding such processes as the deposition of magnetospheric energy into the upper atmosphere, and the influence of magnetospheric current distribution on this process. The unusual requirements that emerge for attitude knowledge and dynamic isolation in the satellite's instrumentation are discussed.

Webster, William J., Jr.↗

NASA NTRS1989

Encapsulants And Corrosion In Photovoltaic Modules

Paper reports studies of effects of moisture on photovoltaic modules, presenting data useful in further quantitative studies of such phenomena. Measured data used in computer simulation of two-dimensional conduction to analyze ionic-conduction characteristics of PVB- and EVA-encapsulated modules. Notes encapsulant plays important role in electrochemical processes in photovoltaic module. Indicates how variations in design parameters affect levels of leakage currents in modules. Points out likely leakage-current paths in modules at various temperatures and humidities. Compares results of field and laboratory tests of same specimens and notes greater severity of outdoor environment.

Mon, Gordon R.↗

NASA NTRS1989

Response of a chemically reacting shear layer to streamwise vorticity

A series of Direct Numerical Simulations are performed of a temporally evolving shear layer subject to both harmonic (2D) and streamwise (3D) forcing. The interaction and coupling of these various 2D and 3D modes is shown to significantly alter the development of the flow. The scale of the 3D modes is quite important to the coupling process with greatly enhanced mixing and product formation resulting from 3D modes that are rapidly amplified by the spanwise vorticity. In general, the longer wavelength 3D modes are found to be highly efficient at increasing the momentum transport while the shorter wavelengths increase mass transport.

Claus, R. W.↗

NASA NTRS1989

Balloon-borne measurements of aerosol, condensation nuclei, and cloud particles in the stratosphere at McMurdo Station, Antarctica, during the spring of 1987

Measurements of the vertical profile of particles with condensation nuclei counters and eight channel aerosol detectors at McMurdo Station, Antarctica, in 1987 verified observations made in 1986 concerning the absence of upwelling in the polar vortex and the presence of a condensation nuclei layer in conjunction with the ozone hole region. New observations of a bimodal aerosol size distribution, consisting of a large-particle mode mixed in with the small-particle sulfate mode, at temperatures below -79 C are consistent with the presence of nitric acid-water particles at low concentrations. Higher concentrations of large particles were observed in association with nacreous clouds. An unusual particle layer which contained enhanced concentrations of both the small-particle (sulfate) mode and the large-particle (nitric acid) mode was detected at temperatures below -85 C, suggesting simultaneous nucleation and growth phenomena. The vortex condensation nuclei layer was observed to form at the same time as the ozone hole, indicating that formation of the layer is triggered by photochemical processes and may be important in controlling ozone depletion above 22 km.

Hofmann, D. J.↗

NASA NTRS1989

Laboratory and theoretical studies of baroclinic processes

An understanding is being developed for processes which may be important in the atmosphere, and the definition and analysis of baroclinic experiments utilizing the geophysical fluid flow cells (GFFC) apparatus in microgravity space flights. Included are studies using numerical codes, theoretical models, and terrestrial laboratory experiments. The numerical modeling is performed in three stages: calculation of steady axisymmetric flow, calculation of fastest-growing linear eigenmodes, and nonlinear effects (first, wave-mean flow interactions, then wave-wave interactions). The code can accommodate cylindrical, spherical, or channel geometry. It uses finite differences in the vertical and meridional directions, and is spectral in the azimuthal. The theoretical work was mostly in the area of effects of topography upon the baroclinic instability problem. The laboratory experiments are performed in a cylindrical annulus which has a temperture gradient imposed upon the lower surface and an approximately isothermal outer wall, with the upper and inner surfaces being nominally thermally insulating.

Miller, Timothy↗

NASA NTRS1989

Response of a chemically reacting layer to streamwise vorticity

A series of Direct Numerical Simulations of a temporally evolving shear layer subject to both harmonic (2D) and streamwise (3D) forcing were performed. The interaction and coupling of these various 2D and 3D modes is shown to significantly alter the development of the flow. The scale of the 3D modes is quite important to the coupling process with greatly enhanced mixing and product formation resulting from 3D modes that are rapidly amplified by the spanwise vorticity. In general, the longer wavelength 3D modes are found to be highly efficient at increasing the momentum transport while the shorter wavelengths increase mass transport.

Claus, Russell W.↗

NASA NTRS1989

A PBL-radiation model for application to regional numerical weather prediction

Often in the short-range limited-area numerical weather prediction (NWP) of extratropical weather systems the effects of planetary boundary layer (PBL) processes are considered secondarily important. However, it may not be the case for the regional NWP of mesoscale convective systems over the arid and semi-arid highlands of the southwestern and south-central United States in late spring and summer. Over these dry regions, the PBL can grow quite high up into the lower middle troposphere (600 mb) due to very effective solar heating and hence a vigorous air-land thermal interaction can occur. The interaction representing a major heat source for regional dynamical systems can not be ignored. A one-dimensional PBL-radiation model was developed. The model PBL consists of a constant-flux surface layer superposed with a well-mixed (Ekman) layer. The vertical eddy mixing coefficients for heat and momentum in the surface layer are determined according to the surface similarity theory, while their vertical profiles in the Ekman layer are specified with a cubic polynomial. Prognostic equations are used for predicting the height of the nonneutral PBL. The atmospheric radiation is parameterized to define the surface heat source/sink for the growth and decay of the PBL. A series of real-data numerical experiments has been carried out to obtain a physical understanding how the model performs under various atmospheric and surface conditions. This one-dimensional model will eventually be incorporated into a mesoscale prediction system. The ultimate goal of this research is to improve the NWP of mesoscale convective storms over land.

Chang, Chia-Bo↗

NASA NTRS1989

Production of organic compounds in plasmas - A comparison among electric sparks, laser-induced plasmas, and UV light

In order to ascertain the features of organic compound-production in planetary atmospheres under the effects of plasmas and shocks, various mixtures of N2, CH4, and H2 modeling the atmosphere of Titan were subjected to discrete sparks, laser-induced plasmas, and UV radiation. The experimental results obtained suggest that UV photolysis from the plasma is an important organic compound synthesis process, as confirmed by the photolysis of gas samples that were exposed to the light but not to the shock waves emitted by the sparks. The thermodynamic equilibrium theory is therefore incomplete in the absence of photolysis.

Scattergood, Thomas W.↗

NASA NTRS1989

Tropical forecasting - Predictability perspective

Results are presented of classical predictability studies and forecast experiments with observed initial conditions to show the nature of initial error growth and final error equilibration for the tropics and midlatitudes, separately. It is found that the theoretical upper limit of tropical circulation predictability is far less than for midlatitudes. The error growth for a complete general circulation model is compared to a dry version of the same model in which there is no prognostic equation for moisture, and diabatic heat sources are prescribed. It is found that the growth rate of synoptic-scale errors for the dry model is significantly smaller than for the moist model, suggesting that the interactions between dynamics and moist processes are among the important causes of atmospheric flow predictability degradation. Results are then presented of numerical experiments showing that correct specification of the slowly varying boundary condition of SST produces significant improvement in the prediction of time-averaged circulation and rainfall over the tropics.

Shukla, J.↗

NASA NTRS1990

Ground-based experiments on thermal and thermosolutal convection in inclined low-aspect-ratio enclosures

Thermal and thermosolutal convection driven by buoyancy forces are investigated experimentally in inclined low-aspect-ratio enclosures. Thermal and thermosolutal convection are important in crystal growth processes. The transport process in the fluid phase during the growth of a crystal has a profound influence on the structure and quality of the solid phase. The present work is concerned with natural convection in a shallow rectangular enclosure with imposed inclined temperature and/or concentration gradients. Various complex flow patterns are observed with different experimental conditions. A flow instability under certain conditions is reported. The flow instability is found to depend not only on the thermal and solutal Grashof numbers, the aspect ratios, but also the inclined angles.

Wang, L. W.↗

NASA NTRS1990

Magnetic reconnection driven by current repulsion

The evolution of an equilibrium consisting of two magnetic islands with oppositely directed currents embedded in a strong magnetic field is investigated, using numerical simulation methods. The rapid development of an ideal magnetohydrodynamic instability is observed, which first rotates and then expels the islands. The growth rate is on the order of the inverse of the Alfven transit time and is much higher than that for magnetic island coalescence. In the nonlinear stage, resistivity becomes important as the reconnection process ensues and dissipates the magnetic energy. The growth rate of the instability is a weak function of the plasma beta and other plasma parameters such as S, the magnetic Reynolds number. An energy principle analysis, based on eigenfunctions obtained from the simulation, confirms the existence of the instability.

Richard, R. L.↗

NASA NTRS1990

Buoyancy-driven melt segregation in the earth's moon. I - Numerical results

The densities of lunar mare magmas have been estimated at liquidus temperatures for pressures from 0 to 47 kbar (0.4 GPa; center of the moon) using a third-order Birch-Murnaghan equation and compositionally dependent parameters from Large and Carmichael (1987). Results on primary magmatic compositions represented by pristine volcanic glasses suggest that the density contrast between very-high-Ti melts and their liquidus olivines may approach zero at pressures of about 25 kbar (2.5 GPa). Since this is the pressure regime of the mantle source regions for these magmas, a compositional limit of eruptability for mare liquids may exist that is similar to the highest Ti melt yet observed among the lunar samples. Although the moon may have generated magmas having greater than 16.4 wt pct TiO2, those melts would probably not have reached the lunar surface due to their high densities, and may have even sunk deeper into the moon's interior as negatively buoyant diapirs. This process may have been important for assimilative interactions in the lunar mantle. The phenomenon of melt/solid density crossover may therefore occur not only in large terrestrial-type objects but also in small objects where, despite low pressures, the range of melt compositions is extreme.

Delano, J. W.↗

NASA NTRS1989

Dissociative recombination in interstellar clouds

Molecular ions play a significant role in the chemistry and evolution of interstellar molecular clouds, even though these regions are overwhelmingly neutral. The dissociative recombination (DR) process governs the abundances of many of these ions and of related neutral species. The gas-phase ion-molecule chemistry of the simplest species is summarized, with emphasis on those problems which are most sensitive to uncertain rates or product branching ratios of DR processes. Examples of the kinds of information needed about DR processes are presented. The importance of the H3(+) ion and prospects for its direct observation are discussed.

Black, John H.↗