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At least 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Surface deformation and geoid anomalies over single and double-layered convective systems

Using a primitive variable formulation of the finite-element method, the differences in the surface observables, such as topography and geoid, produced by single- and double-layered thermal convection, were compared. Both constant and depth-dependent viscosities have been considered. For the same Rayleigh number, larger surface perturbations are produced by single-cell convection. For the same Nusselt number, the magnitudes of the surface observables are greater for double-layered convection. For the same surface heat-flux, surface topographies have similar magnitudes, but the relative amplitudes of geoid anomalies depend greatly on the style of viscosity stratification. This difference in the geoid between the two systems increases with greater surface heat-flow, regardless of viscosity structure.

Koch, M.↗
NASA NTRS2003

Estimates of the Spectral Aerosol Single Sea Scattering Albedo and Aerosol Radiative Effects during SAFARI 2000

Using measurements of the spectral solar radiative flux and optical depth for 2 days (24 August and 6 September 2000) during the SAFARI 2000 intensive field experiment and a detailed radiative transfer model, we estimate the spectral single scattering albedo of the aerosol layer. The single scattering albedo is similar on the 2 days even though the optical depth for the aerosol layer was quite different. The aerosol single scattering albedo was between 0.85 and 0.90 at 350 nm, decreasing to 0.6 in the near infrared. The magnitude and decrease with wavelength of the single scattering albedo are consistent with the absorption properties of small black carbon particles. We estimate the uncertainty in the single scattering albedo due to the uncertainty in the measured fractional absorption and optical depths. The uncertainty in the single scattering albedo is significantly less on the high-optical-depth day (6 September) than on the low-optical-depth day (24 August). On the high-optical-depth day, the uncertainty in the single scattering albedo is 0.02 in the midvisible whereas on the low-optical-depth day the uncertainty is 0.08 in the midvisible. On both days, the uncertainty becomes larger in the near infrared. We compute the radiative effect of the aerosol by comparing calculations with and without the aerosol. The effect at the top of the atmosphere (TOA) is to cool the atmosphere by 13 W/sq m on 24 August and 17 W/sq m on 6 September. The effect on the downward flux at the surface is a reduction of 57 W/sq m on 24 August and 200 W/sq m on 6 September. The aerosol effect on the downward flux at the surface is in good agreement with the results reported from the Indian Ocean Experiment (INDOEX).

Bergstrom, Robert W.↗
NASA NTRS2000

Structure, Hydrodynamics, and Phase Transition of Freely Suspended Liquid Crystals

Smectic liquid crystals are phases of rod shaped molecules organized into one dimensionally (1D) periodic arrays of layers, each layer being between one and two molecular lengths thick. In the least ordered smectic phases, the smectics A and C, each layer is a two dimensional (2D) liquid. Additionally there are a variety of more ordered smectic phases having hexatic short range translational order or 2D crystalline quasi long range translational order within the layers. The inherent fluid-layer structure and low vapor pressure of smectic liquid crystals enable the long term stabilization of freely suspended, single component, layered fluid films as thin as 30A, a single molecular layer. The layering forces the films to be an integral number of smectic layers thick, quantizing their thickness in layer units and forcing a film of a particular number of layers to be physically homogeneous with respect to its layer structure over its entire area. Optical reflectivity enables the precise determination of the number of layers. These ultrathin freely suspended liquid crystal films are structures of fundamental interest in condensed matter and fluid physics. They are the thinnest known stable condensed phase fluid structures and have the largest surface-to-volume ratio of any stable fluid preparation, making them ideal for the study of the effects of reduced dimensionality on phase behavior and on fluctuation and interface phenomena. Their low vapor pressure and quantized thickness enable the effective use of microgravity to extend the study of basic capillary phenomena to ultrathin fluid films. Freely suspended films have been a wellspring of new liquid crystal physics. They have been used to provide unique experimental conditions for the study of condensed phase transitions in two dimensions. They are the only system in which the hexatic has been unambiguously identified as a phase of matter, and the only physical system in which fluctuations of a 2D XY system and Kosterlitz Thouless phase transition has been observed and 2D XY quasi long range order verified. Smectic films have enabled the precise determination of smectic layer electron density and positional fluctuation profile and have been used to show that the interlayer interactions in anti-ferroelectric tilted smectics do not extend significantly beyond nearest neighbors. The interactions which are operative in liquid crystals are generally weak in comparison to those in crystalline phases, leading to the facile manipulation of the order in liquid crystals by external agents such as applied fields and surfaces. Effects arising from weak ordering are significantly enhanced in ultrathin free films and filaments wherein the intermolecular coupling is effectively reduced by loss of neighbors. Over the past four years this research, which we now detail, has produced a host of exciting new discoveries and unexpected results, maintaining the position of the study of freely suspended liquid crystal structures as one of most exciting and fruitful areas of complex fluid physics. In addition, several potentially interesting microgravity free film experiments have been identified.

Clark, Noel A.↗
NASA NTRS2004

Structure and Dynamics of Freely Suspended Liquid Crystals

Smectic liquid crystals are phases of rod shaped molecules organized into one dimensionally (1 D) periodic arrays of layers, each layer being between one and two molecular lengths thick. In the least ordered smectic phases, the smectics A and C, each layer is a two dimensional (2D) liquid. Additionally there are a variety of more ordered smectic phases having hexatic short range translational order or 2D crystalline or quasi long range translational order within the layers. The inherent fluid-layer structure and low vapor pressure of smectic liquid crystals enables the long term stabilization of freely suspended, single component, layered fluid films as thin as 30A, a single molecular layer. The layering forces the films to be an integral number of smectic layers thick, quantizing their thickness in layer units and forcing a film of a particular number of layers to be physically homogeneous with respect to its layer structure over its entire area. Optical reflectivity enables the precise determination of the number of layers. These ultrathin freely suspended liquid crystal films are structures of fundamental interest in condensed matter and fluid physics. They are the thinnest known stable fluid structures and have the largest surface-to-volume ratio of any stable fluid preparation, making them ideal for the study of the effects of reduced dimensionality on phase behavior and on fluctuation and interface phenomena. Their low vapor pressure and quantized thickness enable the effective use of microgravity to extend the study of basic capillary phenomena to ultrathin fluid films. Freely suspended films have been a wellspring of new LC physics. They have been used to provide unique experimental conditions for the study of condensed phase transitions in two dimensions. They are the only system in which the hexatic has been unambiguously identified as a phase of matter, and the only physical system in which fluctuations of a 2D XY system and Kosterlitz Thouless phase transition has been observed and 2D XY quasi long range order verified. Smectic films have enabled the precise determination of smectic layer electron density and positional fluctuation profiles and have been used to show that the interlayer interactions in antiferroelectric tilted smectics do not extend significantly beyond nearest neighbors. Freely suspended films played a pivotal role in the recent discovery of macroscopic chiral-polar ordering in fluids of achiral molecules. The interactions which are operative in liquid crystals are generally weak in comparison to those in crystalline phases, leading to the facile manipulation of the order in liquid crystals by external agents such as applied fields and surfaces. Effects arising from weak ordering are significantly enhanced in ultrathin free films and filaments, in which the intermolecular coupling is effectively further reduced by loss of neighbors. Over the past four years this research, which we now detail, has produced a host of exciting new discoveries and unexpected results, maintaining the study of freely suspended liquid crystal structures as one of most exciting and fruitful areas of complex fluid physics. In addition, a class of experiments on the behavior of 1D interfaces in 2D films have been pursued with results that point to potentially quite interesting effects in microgravity.

Clark, Noel A.↗
NASA NTRS1972

Attenuation of stress waves in single and multi-layered structures

Analytical and experimental studies were made of the attenuation of the stress waves during passage through single and multilayer structures. The investigation included studies on elastic and plastic stress wave propagation in the composites and those on shock mitigating material characteristics such as dynamic stress-strain relations and energy absorbing properties. The results of the studies are applied to methods for reducing the stresses imposed on a spacecraft during planetary or ocean landings.

Yang, J. C. S.↗
NASA NTRS1990

Transition from moving to stationary double layers in a single-ended Q machine

Large-amplitude (less than about 100 percent) relaxation oscillations in the plasma potential are known to be generated when the cold endplate of a single-ended Q machine is biased positively. These oscillations are associated with double layers that form near the hot plate (plasma source) and travel toward the endplate at about the ion-acoustic velocity. At the endplate they dissolve and then form again near the hot plate, the entire process repeating itself in a regular manner. By admitting a sufficient amount of neutral gas into the system, the moving double layers were slowed down and eventually stopped. The production of stationary double layers requires an ion source on the high-potential side of the double layers. These ions are provided by ionization of the neutral gas by electrons that are accelerated through the double layer. The dependence of the critical neutral gas pressure required for stationary double-layer formation on endplate voltage, magnetic field strength, and neutral atom mass has been examined. These results are discussed in terms of a simple model of ion production and loss, including ion losses across the magnetic field.

Song, Bin↗
NASA NTRS1982

Radar scattering from a diffuse vegetation layer over a smooth surface

A simple model is presented for the oblique backscatter and bistatic scatter from a smooth surface overlain by a diffuse layer. Only single scattering in the diffuse layer is taken into account. The model analysis shows that the combination of volume scattering and oblique reflection at the surface may increase appreciably the waves scattering. The scattering strongly depends on the properties of the smooth surface. These results support some of the observations made with the Seasat spaceborne imaging radar over flooded regions with heavy vegetation cover.

Engheta, N.↗
NASA NTRS1992

Structure of a swirling turbulent mixing layer

A single-stream swirling turbulent mixing layer was produced in which the angular momentum instability acts over most of the layer for most of its development length. The instability is associated with a large increase in the level of the conventional Reynolds stresses over those in an unswirled mixing layer. This paper presents measurements of the three-dimensionality of the mean flow at one axial location well downstream of the origin. The output of a fixed hot-wire probe was ensemble-averaged to produce an approximation to the mean flow field that would be seen by a rotating observer. The main goals of this study were to establish the presence or absence of Taylor-Goertler vortices found in unstable laminar boundary layers. The variations in the mean velocities and Reynolds stresses are generally consistent with a bodily 'wrinkling' of the mixing layer, without a significant variation in the layer thickness. The wrinkling is most probably a result of circumferential variations in the initial boundary layer, which in turn originate at the swirl generator.

Wood, D. H.↗
NASA NTRS2016

Conceptual Design of a Single-Aisle Turboelectric Commercial Transport With Fuselage Boundary Layer Ingestion

A single-aisle commercial transport concept with a turboelectric propulsion system architecture was developed assuming entry into service in 2035 and compared to a similar technology conventional configuration. The turboelectric architecture consisted of two underwing turbofans with generators extracting power from the fan shaft and sending it to a rear fuselage, axisymmetric, boundary layer ingesting fan. Results indicate that the turbo- electric concept has an economic mission fuel burn reduction of 7%, and a design mission fuel burn reduction of 12% compared to the conventional configuration. An exploration of the design space was performed to better understand how the turboelectric architecture changes the design space, and system sensitivities were run to determine the sensitivity of thrust specific fuel consumption at top of climb and propulsion system weight to the motor power, fan pressure ratio, and electrical transmission efficiency of the aft boundary layer ingesting fan.

Welstead, Jason R.↗
NASA NTRS1983

Production of large-area single-crystal wafers of cubic SiC for semiconductor devices

A reproducible process is described for growing a thick single-crystal layer of cubic SiC on a single-crystal Si wafer by chemical vapor deposition. A buffer layer, grown in situ, is used between the cubic SiC and the Si substrate to minimize the effect of lattice mismatch. Layers of up to 34 microns thick and several sq cm in area have been grown. Wafers are obtained by chemically removing the Si substrates from the grown layers. Excellent electron channeling patterns produced by these wafers indicate very good crystal quality. Preliminary electrical measurements have yielded electron mobilities up to 380 sq cm/Vs.

Nishino, S.↗
NASA NTRS1985

Lidar observations of the El Chichon cloud in the stratosphere over Fukuoka

A volcanic cloud in the stratosphere, originating from the March to April 1982 eruptions of El Chichon, has been observed for about 2.5 years at Fukuoka (33.5 degrees N, 130.4 degrees E) with two wavelengths of Nd-YAG lidar, 1.06 and 0.53 microns. Time and height variabilities of the cloud are described, using the 1.06 microns data, and some results of the two-wavelength measurements are presented. A sudden enormous increase in the total aerosol backscattering from the stratosphere (backscattering coefficient for 1.06 microns integrated over 13.5 to 28.5 km range) was followed by a decrease from late spring to summer with large fluctuations. The cloud initially appeared stratified into two layers: the upper one with fine structure and sharp edges in the easterly wind region and the lower dumpy one in the westerly wind region. Most of the aerosols were contained in the upper layer. The two layers merged into a broad, single-peaked layer as the easterly prevailed in the whole region in fall, when the total aerosol backscattering began to increase. The layer then decreased its peak height as it broadened. The difference in shape of both layers and the increase of total backscattering from fall can be interpreted by the difference in velocity of material transport in the easterly and the westerly wind region.

Fujiwara, M.↗
NASA NTRS2009

Fabrication of an Antenna-Coupled Bolometer for Cosmic Microwave Background Polarimetry

We describe the development of a detector for precise measurements of the cosmic microwave background polarization. The detector employs a waveguide to couple light between a pair of Mo/Au superconducting transition edge sensors (TES) and a feedhorn. Incorporation of an on-chip ortho-mode transducer (OMT) results in high isolation. The OMT is micromachined and bonded to the microstrip and TES circuits in a low temperature wafer bonding process. The wafer bonding process incorporates a buried superconducting niobium layer with a single crystal silicon layer which serves as the leg isolated TES membrane and as the microstrip dielectric. We describe the micromachining and wafer bonding process and report measurement results of the microwave circuitry operating in the 29-43GHz band along with Johnson noise measurements of the TES membrane structures and development of Mo/Au TES operating under '00mK.

Denis, K.L.↗
NASA NTRS2012

Inner Plasma Structure of the Low-Latitude Reconnection Layer

We report a clear transition through a reconnection layer at the low-latitude magnetopause which shows a complete traversal across all reconnected field lines during northwestward interplanetary magnetic field (IMF) conditions. The associated plasma populations confirm details of the electron and ion mixing and the time history and acceleration through the current layer. This case has low magnetic shear with a strong guide field and the reconnection layer contains a single density depletion layer on the magnetosheath side which we suggest results from nearly field-aligned magnetosheath flows. Within the reconnection boundary layer, there are two plasma boundaries, close to the inferred separatrices on the magnetosphere and magnetosheath sides (Ssp and Ssh) and two boundaries associated with the Alfvén waves (or Rotational Discontinuities, RDsp and RDsh). The data are consistent with these being launched from the reconnection site and the plasma distributions are well ordered and suggestive of the time elapsed since reconnection of the field lines observed. In each sub-layer between the boundaries the plasma distribution is different and is centered around the current sheet, responsible for magnetosheath acceleration. We show evidence for a velocity dispersion effect in the electron anisotropy that is consistent with the time elapsed since reconnection. In addition, new evidence is presented for the occurrence of partial reflection of magnetosheath electrons at the magnetopause current layer.

magnetosheath↗
NASA NTRS1984

Near-field turbulence properties of single- and two-stream plane mixing layers

Detailed mean-flow and turbulence measurements have been made in the near-field of two plane mixing layers in air with a maximum velocity of 21 m/s. The experimental rig enabled mixing layers of velocity ratios 0 and 0.46 to be generated simultaneously. Cases with both tripped and untripped initial boundary layers were studied. In all cases, it was found that the two-stream layer developed to the self-preserving state in a distance much shorter than the single-stream layer which followed accepted criteria for the development distance. The asymptotic levels of the turbulence quantities in the two-stream layer, and the development of the single-stream layer showed agreement with existing data. The results suggest that the two-stream mixing layer should provide a better test case for the development of turbulence models and calculation methods than the single-stream mixing layer.

Mehta, R. D.↗
NASA NTRS1993

Optimal Synthesis of Hot Composite Laminates with Interphase Layers

A method for the optimal grading of a single interphase layer in metal matrix composite laminates for the minimization of residual stresses is described. The capability to simultaneously tailor some fabrication parameters is also incorporated. Applications for unidirectional, cross-ply and quasi-isotropic Graphite/Copper laminates are investigated to assess the potential of interphase layer in reducing matrix residual stresses in various laminate configurations. Simultaneous optimization of interphase and fabrication characteristics appears to be more effective in decreasing residual stresses. The results also indicate that the interphase layer is more effective in lowering residual stresses in unidirectional composites and selectively within individual plies of a laminate. Embedded interphase layers in all the plies did not produce a significant global reduction in residual stresses.

Rabzak, Christopher↗
NASA NTRS1968

Reaction rates of graphite with ozone measured by etch decoration

Etch-decoration technique of detecting vacancies in graphite has been used to determine the reaction rates of graphite with ozone in the directions parallel and perpendicular to the layer planes. It consists essentially of peeling single atom layers off graphite crystals without affecting the remainder of the crystal.

Hennig, G. R.↗
NASA NTRS1991

Ir/IrSi3/Si Schottky-Barrier Infrared Detector

Quantum efficiency increased. Proposed Schottky-barrier infrared detector has double metallic layer of Ir and IrSi3 instead of single metallic layer of Ir, IrSi, or IrSi3. Offers advantages of both relatively high infrared absorption in thin film of Ir and stability and reproducibility of layer of IrSi3 in contact with Si. Also serves as barrier to chemical reactions between Ir overlayer and Si substrate. Detectors used to form focal-plane array integrated with charge-coupled-device-addressing and image-processing circuitry.

Lin, True-Lon↗