Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “layered structure”

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.

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

At least 595 records · Page 33
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Microwave Radiometer Observations of Snowpack Properties and Comparison of U.S. Japanese Results

Microwave data collected by field experiments over Vermont and Hokkaido and Nimbus-7 SMMR over North Dakota and Hokkaido were studied. The measured 37 GHz brightness temperatures show considerable effect of volume scattering by snow grains. The 37 GHz brightness for a new snowpack with average grain radius of 0.25 mm is generally about 40 K higher than the naturally compacted pack with average grain radius of 0.4 mm. The scattering effect is much less distinct for the 6.6 GHz. However, the layering effect is much stronger at the longer wavelength. For 10.7 and 18 GHz, the effect of layering and scattering vary due to different combinations of internal snow grain distribution and layering structures. Over the Hokkaido test site, the SMMR data are too coarse for the snow field. A better spatial resolution is required to study these snow fields.

Chang, A. T. C.↗
NASA NTRS1984

Usefulness of multifrequency MST radar measurements, part 2.6B

Scattering of radio waves from atmospheric refractive-index irregularities induced by turbulence was invoked almost four decades ago to explain the characteristics of signals received on VHF/UHF ionospheric and tropospheric forward-scatter links. Due to the bistatic geometry of these links a slender, horizontally extended, common volume or cell is formed in space. The principal contribution to scattering arises from refractive-index fluctuations in this volume at the Bragg wave number K approx. sub B = K approx. sub i -k approx. sub s vectors. It has been surmised that the use of more than one frequency in probing the middle-atmosphere regions should help resolve several issues pertaining to the scattering mechanism. These issues are briefly re-examined in this note. The implications of the radar equation are discussed. The problems arising due to layered structure of turbulence and the choice of frequencies most suitable for multifrequency measurements are considered.

Rastogi, P. K.↗
NASA NTRS1984

On the dynamics of stratocumulus formation and dissipation

Results obtained with a mixed layer model are used to study the dynamics of stratocumulus formation and dissipation in subtropical marine stratocumulus cloud regimes. The model used allows entrainment to be driven by shear as well as buoyancy, and includes a very crude parameterization of the partial blackness of thin cloud layers. Model results show that for some values of the large-scale divergence there are three equilibrium mixed layer structures, two of which are stable. One of the stable equilibria is cloudy, deep, and buoyancy-driven, while the other is clear, shallow, and shear-driven. It is found that as a result of hysteresis effects a transient increase in the large-scale divergence can produce a long-lasting break in the clouds.

Randall, D. A.↗
NASA NTRS1985

Onboard near-optimal climb-dash energy management

This paper studies optimal and near-optimal trajectories of high-performance aircraft in symmetric flight. Onboard, real-time, near-optimal guidance is considered for the climb-dash mission, using some of the boundary-layer structure and hierarchical ideas from singular perturbations. In the case of symmetric flight, this resembles neighborhood-optimal guidance using energy-to-go as the running variable. However, extension to three-dimensional flight is proposed, using families of nominal paths with heading-to-go as the additional running variable. Some computational results are presented for the symmetric case.

Weston, A.↗
NASA NTRS1986

Analytical and numerical studies of the thermocapillary flow in a uniformly floating zone

The microgravity environment of an orbiting vehicle permits crystal growth experiments in the presence of greatly reduced buoyant convection in the liquid melt. Crystals grown in ground-based laboratories do not achieve their potential properties because of dopant variations caused by flow in the melt. The floating zone crystal growing system is widely used to produce crystals of silicon and other materials. However, in this system the temperature gradient on the free sidewall surface of the melt is the source of a thermocapillary flow which does not disappear in the low-gravity environment. The idea of using a uniform rotation of the floating zone system to confine the thermocapillary flow to the melt sidewall leaving the interior of the melt passive is examined. A cylinder of fluid with an axial temperature gradient imposed on the cylindrical sidewall is considered. A half zone and the linearized, axisymmetric flow in the absence of crystal growth is examined. Rotation is found to confine the linear thermocapillary flow. A simplified model is extended to a full zone and both linear and nonlinear thermocapillary flows are studied theoretically. Analytical and numerical methods are used for the linear flows and numerical methods for the nonlinear flows. It was found that the linear flows in the full zone have more complicated and thicker boundary layer structures than in the half zone, and that these flows are also confined by the rotation. However, for the simplified model considered and for realistic values for silicon, the thermocapillary flow is not linear. The fully nonlinear flow is strong and unsteady (a weak oscillation is present) and it penetrates the interior. Some non-rotating flow results are also presented. Since silicon as a large value of thermal conductivity, one would expect the temperature fields to be determined by conduction alone. This is true for the linear and weakly nonlinear flows, but for the stronger nonlinear flow the results show that temperature advection is also important. Uniform rotation may still be a means of confining the flow and the results obtained define the procedure to be used to examine this hypothesis.

Fowlis, W. W.↗
NASA NTRS1985

High transconductance InGaAs/AlGaAs pseudomorphic modulation-doped field-effect transistors

Pseudomorphic In0.15Ga0.85As/A10.15Ga0.85As modulation-doped field effect transistors (MODFET's) exhibiting extremely good dc characteristics have been successfully fabricated. dc transconductance in these strained-layer structures of 270 mS/mm was measured for 1-micron gate, normally-on devices at 300 K. Maximum drain current levels are 290 mA/mm, with excellent pinch-off and saturation characteristics. The transconductance increased to 360 mS/mm at 77 K while no persistent photoconductivity or drain collapse was observed. Preliminary microwave results indicate a 300-K current gain cutoff frequency of about 20 GHz. These results are equivalent to the best GaAs/AlGaAs MODFET results and are due in part to the improved transport properties and carrier confinement in the InGaAs quantum well.

Ketterson, A.↗
NASA NTRS1986

A new method for fabrication of diamond-dust blocking filters

Thermal embedding of diamond dust onto a polyethylene-coated Al plate has been used to make a blocking filter for FIR applications. The Al plate is sandwiched between two Mylar 'blankets' and the air between the layers is removed by means of a small vacuum pump. After the polyethylene is heated and softened, the diamond dust is applied to the polyethylene coating using a brush. The optimum diamond dust grain sizes corresponding to polyethylene layer thicknesses of 9-12 microns are given in a table, and the application of the blocking filter to spectrometric measurements in the FIR is described. An exploded view diagram of the layered structure of the blocking filter is provided.

Collard, H. R.↗
NASA NTRS1986

Daytime lidar measurements of tidal winds in the mesospheric sodium layer at Urbana, Illinois

For more than 15 years lidar systems have been used to study the chemistry and dynamics of the mesospheric sodium layer. Because the layer is an excellent tracer of atmospheric wave motions, sodium lidar has proven to be particularly useful for studying the influence of gravity waves and tides on mesospheric dynamics. These waves, which originate in the troposphere and stratosphere, propagate through the mesosphere and dissipate their energy near the mesopause making important contributions to the momentum and turbulence budget in this region of the atmosphere. Recently, the sodium lidar was modified for daytime operation so that wave phenomena and chemical effects could be monitored throughout the complete diurnal cycle. The results of continuous 24 hour lidar observations of the sodium layer structure are presented alond with measurement of the semidiurnal tidal winds.

Kwon, K. H.↗
NASA NTRS1986

Generation of attenuation corrected images from lidar data

The interpretation of data generated by aerosol backscatter lidars is often facilitated by presentation of RHI and PPI images. These pictures are especially useful in studies of atmospheric boundary layer structure where convective elements, stratifications and aerosol laden plumes can be easily delineated. Procedures used at the University of Wisconsin to generate lidar images on a color enhanced raster scan display are described.

Eloranta, E. W.↗
NASA NTRS1987

Some aspects of double layer formation in a plasma constrained by a magnetic mirror

The shift from wave-generated anomalous resistivity toward the more large-scale effects of magnetic confinement of current carrying plasmas was inspired by the more extensive data on auroral particle distribution functions that were made available, data that may often seem consistent with a dissipation-free acceleration of auroral electrons over an extended altitude range. Efforts to interpret these data have brought new vigor to the concept that a smooth and static electric field can be self-consistently generated by suitable pitch angle anisotropies among the high altitude particle populations, different for electrons and ions, and that such an electric field is both necessary and sufficient to maintain the plasma in a quasi-neutral steady state. Certain aspects of this concept are reviewed and criticized, both from a general theoretical standpoint and from the standpoint of what is known about the magnetospheric environment. It is argued that this concept has flaws and that the actual physical problem is considerably more complicated, requiring a more complex electric field, possibly including double layer structures.

Lennartsson, W.↗
NASA NTRS1986

Modelling short channel mosfets for use in VLSI

In an investigation of metal oxide semiconductor field effect transistor (MOFSET) devices, a one-dimensional mathematical model of device dynamics was prepared, from which an accurate and computationally efficient drain current expression could be derived for subsequent parameter extraction. While a critical review revealed weaknesses in existing 1-D models (Pao-Sah, Pierret-Shields, Brews, and Van de Wiele), this new model in contrast was found to allow all the charge distributions to be continuous, to retain the inversion layer structure, and to include the contribution of current from the pinched-off part of the device. The model allows the source and drain to operate in different regimes. Numerical algorithms used for the evaluation of surface potentials in the various models are presented.

Klafter, Alex↗
NASA NTRS1987

Computer modelling of aluminum-gallium arsenide/gallium arsenide multilayer photovoltaics

The modeled cascade cells offer an alternative to conventional series cascade designs that require a monolithic intercell ohmic contact. Selective electrodes provide a simple means of fabricating three-terminal devices, which can be configured in complementary pairs to circumvent the attendant losses and fabrication complexities of intercell ohmic contacts. Moreover, selective electrodes allow incorporation of additional layers in the upper subcell which can improve spectral response and increase radiation tolerance. Realistic simulations of such cells operating under one-sun AMO conditions show that the seven-layer structure is optimum from the standpoint of beginning-of-life efficiency and radiation tolerance. Projected efficiencies exceed 26 percent. Under higher concentration factors, it should be possible to achieve efficiencies beyond 30 percent. However, to simulate operation at high concentration will require a model for resistive losses. Overall, these devices appear to be a promising contender for future space applications.

Wagner, Michael Broderick↗
NASA NTRS1986

An MHD simulation of the effects of the interplanetary magnetic field By component on the interaction of the solar wind with the earth's magnetosphere during southward interplanetary magnetic field

The interaction between the solar wind and the earth's magnetosphere has been studied by using a time-dependent three-dimensional MHD model in which the IMF pointed in several directions between dawnward and southward. When the IMF is dawnward, the dayside cusp and the tail lobes shift toward the morningside in the northern magnetosphere. The plasma sheet rotates toward the north on the dawnside of the tail and toward the south on the duskside. For an increasing southward IMF component, the plasma sheet becomes thinner and subsequently wavy because of patchy or localized tail reconnection. At the same time, the tail field-aligned currents have a filamentary layered structure. When projected onto the northern polar cap, the filamentary field-aligned currents are located in the same area as the region 1 currents, with a pattern similar to that associated with auroral surges. Magnetic reconnection also occurs on the dayside magnetopause for southward IMF.

Ogino, T.↗
NASA NTRS1988

Electrochromic Variable-Emissivity Surfaces

Temperature controlled by altering infrared radiative properties. Infrared emissivity of electrochromically active layer changed by applying voltage to insert or remove Li atoms electrochemically. Change reversible and continuously variable between specified limits of layered structure.

Rauh, R. David↗
NASA NTRS1987

A component decomposition model for evaluating atmospheric effects in remote sensing

A radiance value of a target pixel recorded by a remote sensor can be decomposed into three components: (1) attenuated target signature, (2) pure atmospheric radiation, and (3) the contribution made by the ground through the atmospheric scattering process. Given the meteorological and optical parameters of a layer-structured atmosphere, its transmittance and radiance distribution can be accurately calculated with a plane-parallel radiative transfer model. For a uniform surface, the ground contribution can be obtained by comparing radiances for an atmosphere over a black but nonemitting surface and the same atmosphere with an underlying ground of given albedo or temperature. For an inhomogeneous surface, the first two components remain the same as long as the surface is a plane. The third may be estimated using the locally averaged top-of-atmosphere radiance. An atmospheric point spread function is calculated by a Monte Carlo approach and is used for retrieving the ground signature through a deconvolution procedure.

Li, S.↗
NASA NTRS1988

Observations of marine stratocumulus clouds during FIRE

The First International Satellite Cloud Climatology Project Regional Experiment (FIRE) to study extensive fields of stratocumulus clouds off the coast of California is presented. Measurements on the regional and detailed local scales were taken, allowing for a wide interpretation of the mean, turbulent, microphysical, radiative, and chemical characteristics of stratocumulus. Multiple aircraft and ground-based remote-sensing systems were used to study the time evolution of the boundary layer structure over a three-week period, and probes from tethered balloons were used to measure turbulence and to collect cloud-microphysical and cloud-radiative data. The observations provide a base for studying the generation maintenance and dissipation of stratocumulus clouds, and could aid in developing numerical models and improved methods for retrieving cloud properties by satellite.

Albrecht, Bruce A.↗
NASA NTRS1987

A magnetohydrodynamic simulation of reconnection in the magnetotail during intervals with southward interplanetary magnetic field

Results are reported from a simulation of the interaction between the solar wind and the earth magnetosphere, using a time-dependent three-dimensional MHD model. The calculation was performed for several orientations of the IMF between dawnward pointing and southward. When the IMF has a dawnward component, the plasma sheet rotates northward on the dawnside of the tail and toward the south on the duskside. As the southward component becomes larger, the plasma sheet becomes thinner and develops a wavy cross section because of patchy or localized tail reconnection. The field-aligned currents (FACs) associated with this localized reconnection have a filamentary layered structure. When projected onto the polar cap the filamentary FACs are located in the same region as the tail region 1 currents. At lower latitudes strong region 2 sense currents that originate in the plasma sheet are found. FACs are found on field lines that map to the polar cap even for southward IMF. These currents have many of the properties of the observed polar-cusp currents. The polar-cusp FACs evolve from the polar-cap NB(z) FACs as the IMF is rotated from northward to southward.

Walker, R. J.↗
NASA NTRS1989

Photovoltaic Hydrogen Sensor

Photovoltaic device senses hydrogen developed to test degradation of diodes with platinum flash gates on backs. Sensing element is p/n junction rather than conventional Schottky barrier or metal oxide/silicon field-effect transistor. Hydrogen-indicating electrical signal modulated optically rather than electrically. Layered structure of hydrogen detector and principle of operation resemble silicon solar photovoltaic cell. Hydrogen detector responds to hydrogen in atmosphere within minutes and recovers quickly when hydrogen removed.

Daud, Taher↗