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At least 433 records · Page 24

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DOE OSTI2023

Emergence of liquid following laser melting of gold thin films

X-ray structural science is undergoing a revolution driven by the emergence of X-ray Free-electron Laser (XFEL) facilities. The structures of crystalline solids can now be studied on the picosecond time scale relevant to phonons, atomic vibrations which travel at acoustic velocities. In the work presented here, X-ray diffuse scattering is employed to characterize the time dependence of the liquid phase emerging from femtosecond laser-induced melting of polycrystalline gold thin films using an XFEL. In a previous analysis of Bragg peak profiles, we showed the supersonic disappearance of the solid phase and presented a model of pumped hot electrons carrying energy from the gold surface to scatter at internal grain boundaries. This generates melt fronts propagating relatively slowly into the crystal grains. By conversion of diffuse scattering to a partial X-ray pair distribution function, we demonstrate that it has the characteristic shape obtained by Fourier transformation of the measured F ( Q ). The diffuse signal fraction increases with a characteristic rise-time of 13 ps, roughly independent of the incident pump fluence and consequent final liquid fraction. This suggests the role of further melt-front nucleation processes beyond grain boundaries.

36 MATERIALS SCIENCE↗

NASA NTRS1975

Interactions of satellite-speed helium atoms with satellite-surfaces. 1: Spatial distributions of reflected helium atoms

Interactions of satellite-speed helium atoms with practical satellite surfaces were investigated experimentally, and spatial distributions of satellite-speed helium beams scattered from four different engineering surfaces were measured. The 7000 m/sec helium beams were produced using an arc-heated supersonic molecular beam source. The test surfaces included cleaned 6061-T6 aluminum plate, anodized aluminum foil, white paint, and quartz surfaces. Both in-plane (in the plane containing the incident beam and the surface normal) and out-of-plane spatial distributions of reflected helium atoms were measured for six different incidence angles (0, 15, 30, 45, 60, and 75 deg from the surface normal). It was found that a large fraction of the incident helium atoms were scattered back in the vicinity of the incoming beam, particularly in the case of glancing incidence angles. This unexpected scattering feature results perhaps from the gross roughness of these test surfaces. This prominent backscattering could yield drag coefficients which are higher than for surfaces with either forward-lobed or diffusive (cosine) scattering patterns.

Liu, S. M.↗

NASA NTRS1988

Influence of land surface roughness on atmospheric circulation and precipitation - A sensitivity study with a general circulation model

The influence of land surface roughness on the large scale atmospheric circulation and rainfall is examined by comparing three sets of simulations made with a general circulation model in which the land surface roughness length, z(0), was reduced from 45 cm to 0.02 cm. It is found that the reduced surface roughness produced a two-fold increase in the boundary layer wind speed, a two-fold decrease in the magnitude of the surface stress, and almost no change in the surface evaporation and surface sensible heat flux. It is suggested that the height of the earth's vegetation cover has a large influence on the boundary layer water vapor transport convergence and the rainfall distribution.

Sud, Y. C.↗

NASA NTRS1991

A hotspot model for leaf canopies

The hotspot effect, which provides important information about canopy structure, is modeled using general principles of environmental physics as driven by parameters of interest in remote sensing, such as leaf size, leaf shape, leaf area index, and leaf angle distribution. Specific examples are derived for canopies of horizontal leaves. The hotspot effect is implemented within the framework of the model developed by Suits (1972) for a canopy of leaves to illustrate what might occur in an agricultural crop. Because the hotspot effect arises from very basic geometrical principles and is scale-free, it occurs similarly in woodlands, forests, crops, rough soil surfaces, and clouds. The scaling principles advanced are also significant factors in the production of image spatial and angular variance and covariance which can be used to assess land cover structure through remote sensing.

Jupp, David L. B.↗

NASA NTRS1995

Small craters on the meteoroid and space debris impact experiment

Examination of 9.34 m(exp 2) of thick aluminum plates from the Long Duration Exposure Facility (LDEF) using a 25X microscope revealed 4341 craters that were 0.1 mm in diameter or larger. The largest was 3 mm in diameter. Most were roughly hemispherical with lips that were raised above the original plate surface. The crater diameter measured was the diameter at the top of the raised lips. There was a large variation in the number density of craters around the three-axis gravity-gradient stabilized spacecraft. A model of the near-Earth meteoroid environment is presented which uses a meteoroid size distribution based on the crater size distribution on the space end of the LDEF. An argument is made that nearly all the craters on the space end must have been caused by meteoroids and that very few could have been caused by man-made orbital debris. However, no chemical analysis of impactor residue that will distinguish between meteoroids and man-made debris is yet available. A small area (0.0447 m(exp 2)) of one of the plates on the space end was scanned with a 200X microscope revealing 155 craters between 10 micron and 100 micron in diameter and 3 craters smaller than 10 micron. This data was used to extend the size distribution of meteoroids down to approximately 1 micron. New penetration equations developed by Alan Watts were used to relate crater dimensions to meteoroid size. The equations suggest that meteoroids must have a density near 2.5 g/cm(exp 3) to produce craters of the shape found on the LDEF. The near-Earth meteoroid model suggests that about 80 to 85 percent of the 100 micron to 1 mm diameter craters on the twelve peripheral rows of the LDEF were caused by meteoroids, leaving 15 to 20 percent to be caused by man-made orbital debris.

Humes, Donald H.↗

NASA NTRS2004

Southern Alaska Glaciers: Spatial and Temporal Variations in Ice Volume

Although temperate mountain glaciers comprise less than 1% of the glacier-covered area on Earth, they are important because they appear to be melting rapidly under present climatic conditions and, therefore, make significant contributions to rising sea level. In this study, we use ICESat observations made in the last 1.5 years of southern Alaska glaciers to estimate ice elevation profiles, ice surface slopes and roughness, and bi-annual and/or annual ice elevation changes. We report initial results from the near coastal region between Yakutat Bay and Cape Suckling that includes the Malaspina and Bering Glaciers. We show and interpret ice elevations changes across the lower reaches of the Bagley Ice Valley for the period between October 2003 and May 2004. In addition, we use off-nadir pointing observations to reference tracks over the Bering and Malaspina Glaciers in order to estimate annual ice elevation change. Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and Shuttle Radar Topography Mission (SRTM) derived DEMs are used to estimate across track regional slopes between ICESat data acquisitions. Although the distribution and quantity of ICESat elevation profiles with multiple, exact repeat data is currently limited in Alaska, individual ICESat data tracks, provide an accurate reference surface for comparison to other elevation data (e.g. ASTER and SRTM X- and C-band derived DEMs). Specifically we report the elevation change over the Malaspina Glacier's piedmont lobe between a DEM derived from SRTM C-band data acquired in Feb. 2000 and ICESat Laser #2b data from Feb.-March 2004. We also report use of ICESat elevation data to enhance ASTER derived absolute DEMs. Mountain glaciers generally have rougher surfaces and steeper regional slopes than the ice sheets for which the ICESat design was optimized. Therefore, rather than averaging ICESat observations over large regions or relying on crossovers, we are working with well-located ICESat footprint returns to estimate glacier ice elevations and surface characteristics. Additional information is included in the original extended abstract.

Sauber, J.↗

NASA NTRS1997

Interferometric SAR for observation of glacier motion and firn penetration

A digital elevation model (DEM) for a swath in west Greenland above Jakobshavn Isbrae derived from a number ERS-1 interferograms combined so as to reduce phase errors and other problems is presented. The DEM shows a wealth of kilometer-scale, dynamically supported topography, which arises from ice sheet flow over the rough bed. A correlation is shown between topography and interferometric phase due solely to ice sheet motion, which clearly shows the translations of scatterers in the surface up and downslope in the topography. Finally, the low correlation in interferograms of ice sheet dry snow zones motivates investigation on the depth-locus of backscattering. A scattering model is presented, including realistic firn grain size distributions and layering, which shows that layering helps to localize backscattering from dry firn to shallower depths than would otherwise be expected.

Winebrenner, Dale P.↗

NASA NTRS1979

The distribution of lobate debris aprons and similar flows on Mars

Planet-wide mapping of lobate debris aprons and other similar flows on Mars shows a strong concentration in two latitudinal bands roughly 25 deg wide and centered at 40 deg N and 45 deg S. This distribution supports the idea that these flows form when erosional debris is transported downslope and becomes mixed with ice deposited from the atmosphere, as these latitudes should receive high seasonal H2O frost deposition relative to the rest of the planet. Flows are found in the northern hemisphere band wherever old highland surfaces occur but are found in the southern hemisphere only near the two major impact basins, Argyre and Hellas. These areas are apparently characterized by mass wasting that is rapid relative to most of the southern hemisphere highlands. The rate of mass wasting may be related to the degree of consolidation of highland material.

Squyres, S. W.↗

NASA NTRS1991

Airborne SAR determination of relative ages of Walker Valley moraines, eastern Sierra Nevada

A regional study of the distribution and elevations of Pleistocene moraines in the Andes requires a method of determining relative age from space. One of our primary objectives is to establish the relative chronology of major climatic events responsible for glaciation in the Andes and other regions that are difficult to access on the ground and where suitable material for absolute age determination is lacking. The sensitivity of radar to surface roughness makes it possible to develop a remotely-based relative dating technique for landforms for which surface age and roughness can be correlated. We are developing such a technique with Airborne Synthetic Aperture Radar (AIRSAR) imagery of the eastern Sierra Nevada where independent evidence is available for the ages and physical characteristics of moraines. The Sierra Nevada moraines are similar in form and environmental setting to Andean moraines that we have targeted for study during the pending Shuttle Imaging Radar-C (SIR-C) mission. SAR imagery is used to differentiate the ages of five moraine sequences of Walker Valley in the eastern Sierra Nevada. Other aspects of this investigation are briefly discussed.

Fox, A.↗

NASA NTRS1993

Remote sensing of Earth terrain

Progress report on remote sensing of Earth terrain covering the period from Jan. to June 1993 is presented. Areas of research include: radiative transfer model for active and passive remote sensing of vegetation canopy; polarimetric thermal emission from rough ocean surfaces; polarimetric passive remote sensing of ocean wind vectors; polarimetric thermal emission from periodic water surfaces; layer model with tandom spheriodal scatterers for remote sensing of vegetation canopy; application of theoretical models to active and passive remote sensing of saline ice; radiative transfer theory for polarimetric remote sensing of pine forest; scattering of electromagnetic waves from a dense medium consisting of correlated mie scatterers with size distributions and applications to dry snow; variance of phase fluctuations of waves propagating through a random medium; polarimetric signatures of a canopy of dielectric cylinders based on first and second order vector radiative transfer theory; branching model for vegetation; polarimetric passive remote sensing of periodic surfaces; composite volume and surface scattering model; and radar image classification.

Kong, J. A.↗

DOE OSTI2020

Two-Phase Fluid Flow Properties of Rough Fractures With Heterogeneous Wettability: Analysis With Lattice Boltzmann Simulations

Fractures are conduits for fluid flow in low-permeability geological formations. Multiphase flow properties of fractures are important in natural processes and in engineering applications such as the evaluation of the sealing capacity of caprocks and productivity of hydrocarbon-bearing tight rocks. Investigations of flow and transport through fractures typically focus on the effects of fracture geometric and mechanical factors such as aperture, roughness, and compressibility. The wettability of the fracture surfaces and its influence on microscale interfacial phenomena and macroscale effective transport properties are seldom studied. In this study, we investigated the effect of heterogeneous wetting properties on the displacement of water by supercritical CO2 through a series of lattice Boltzmann method simulations. The results show the evolution of the CO2 plume within a fracture is controlled by both the roughness of the aperture field and the wetting distribution. We combined these factors into a capillary pressure map that can be related to the macroscopic flow behavior of the fracture. We observed that heterogeneous wetting distributions promote the residual trapping of water where lower capillary pressures allowed for isolated water pockets in higher capillary pressure zones. Analysis of fracture unsteady relative permeability shows the effect of wetting on permeability evolution and provides support for the viscous-coupling relative permeability model. Finally, analysis of the steady-state relative permeability and saturation demonstrates a strong correlation between permeability and the standard deviation of the capillary pressure field. Thus, characterizing the distribution of wetting properties of fractures is crucial to understanding multiphase fracture flow and transport properties.

58 GEOSCIENCES↗

NASA NTRS1971

Martian cratering. II - Asteroid impact history.

This paper considers the extent to which Martian craters can be explained by considering asteroidal impact. Sections I, II, and III of this paper derive the diameter distribution of hypothetical asteroidal craters on Mars from recent Palomar-Leiden asteroid statistics and show that the observed Martian craters correspond to a bombardment by roughly 100 times the present number of Mars-crossing asteroids. Section IV discusses the early bombardment history of Mars, based on the capture theory of Opik and probable orbital parameters of early planetesimals. These results show that the visible craters and surface of Mars should not be identified with the initial, accreted surface. A backward extrapolation of the impact rates based on surviving Mars-crossing asteroids can account for the majority of Mars craters over an interval of several aeons, indicating that we see back in time no further than part-way into a period of intense bombardment. An early period of erosion and deposition is thus suggested. Section V presents a comparison with results and terminology of other authors.

Hartmann, W. K.↗

NASA NTRS1982

Comparison of sigma/deg/ obtained from the conventional definition with sigma/deg/ appearing in the radar equation for randomly rough surfaces

Results from the radar cross section of rough surfaces calculated from a conventional definition and from a direct solution of the radar equation are compared. The purpose is to define conditions under which the conventional solution produces the same cross-section as the radar equation. A randomly rough conduction surface is considered for line sources and corrugated surfaces. Two problems are analyzed: first, a plane wave is assumed to be incident on the surface and the observed scattered fields are computed, and second, the scattered fields received by an antenna colocated with the emitting antenna are used to compute the available power. The fields are assumed to scatter incoherently and the stationary points are assumed to be homogeneously distributed over the surface. The cross sections are found to be equal if the observer is far from the surface compared to the radii of the surface at the specular points.

Levine, D. M.↗

NASA NTRS1982

Radar scattering laws for the lunar surface

Bistatic radar observations of quasi-specular scatter from the lunar surface at 13 and 116 cm wavelengths have been expressed in terms of probability densities of the underlying surface slope distributions. These show best agreement with a Hagfors scattering function, although other forms having predominantly Gaussian or exponential characteristics also occur. Estimates of root mean square (rms) surface slope derived from these function range from 4 deg rms in maria to at least 8 deg rms in highlands. These are values appropriate to 25 m horizontal scales and represent averages over tens of square kilometers of surface area. The effective scattering roughness varies with wavelength, most strongly in maria, where its behavior is consistent with that expected from an immature, i.e., unsaturated cratered surface. There is very little variation in scattering roughness with wavelength in highlands area.

Simpson, R. A.↗

NASA NTRS1996

Structure Identification Within a Transitioning Swept-Wing Boundary Layer

Extensive measurements are made in a transitioning swept-wing boundary layer using hot-film, hot-wire and cross-wire anemometry. The crossflow-dominated flow contains stationary vortices that breakdown near mid-chord. The most amplified vortex wavelength is forced by the use of artificial roughness elements near the leading edge. Two-component velocity and spanwise surface shear-stress correlation measurements are made at two constant chord locations, before and after transition. Streamwise surface shear stresses are also measured through the entire transition region. Correlation techniques are used to identify stationary structures in the laminar regime and coherent structures in the turbulent regime. Basic techniques include observation of the spatial correlations and the spatially distributed auto-spectra. The primary and secondary instability mechanisms are identified in the spectra in all measured fields. The primary mechanism is seen to grow, cause transition and produce large-scale turbulence. The secondary mechanism grows through the entire transition region and produces the small-scale turbulence. Advanced techniques use Linear Stochastic Estimation (LSE) and Proper Orthogonal Decomposition (POD) to identify the spatio-temporal evolutions of structures in the boundary layer. LSE is used to estimate the instantaneous velocity fields using temporal data from just two spatial locations and the spatial correlations. Reference locations are selected using maximum RMS values to provide the best available estimates. POD is used to objectively determine modes characteristic of the measured flow based on energy. The stationary vortices are identified in the first laminar modes of each velocity component and shear component. Experimental evidence suggests that neighboring vortices interact and produce large coherent structures with spanwise periodicity at double the stationary vortex wavelength. An objective transition region detection method is developed using streamwise spatial POD solutions which isolate the growth of the primary and secondary instability mechanisms in the first and second modes, respectively. Temporal evolutions of dominant POD modes in all measured fields are calculated. These scalar POD coefficients contain the integrated characteristics of the entire field, greatly reducing the amount of data to characterize the instantaneous field. These modes may then be used to train future flow control algorithms based on neural networks.

Chapman, Keith↗

NASA NTRS1997

Electric Arc and Electrochemical Surface Texturing Technologies

Surface texturing of conductive materials can readily be accomplished by means of a moving electric arc which produces a plasma from the environmental gases as well as from the vaporized substrate and arc electrode materials. As the arc is forced to move across the substrate surface, a condensate from the plasma re-deposits an extremely rough surface which is intimately mixed and attached to the substrate material. The arc textured surfaces produce greatly enhanced thermal emittance and hold potential for use as high temperature radiator surfaces in space, as well as in systems which use radiative heat dissipation such as computer assisted tomography (CAT) scan systems. Electrochemical texturing of titanium alloys can be accomplished by using sodium chloride solutions along with ultrasonic agitation to produce a random distribution of craters on the surface. The crater size and density can be controlled to produce surface craters appropriately sized for direct bone in-growth of orthopaedic implants. Electric arc texturing and electrochemical texturing techniques, surface properties and potential applications will be presented.

Banks, Bruce A.↗

NASA NTRS1985

Surface characteristics of Venus derived from Pioneer Venus altimetry, roughness, and reflectivity measurements

Pioneer Venus data sets for global topography, surface roughness, and reflectivity are analyzed, and the geological significance of some of the distinctive surface types is locally calibrated by examining high-resolution images from the Venera lander spacecraft and the Arecibo Observatory. Regional rock and bedrock surfaces are found to cover the majority of the planet. The relatively smooth rock and bedrock surfaces probably represent lava flows. Porous and unconsolidated fine materials cover less than about 27 percent of the surface. The distribution of soil surfaces indicate that they are the result of local weathering and small amounts of lateral transport. Several hypotheses for the origin of the high-dielectric materials which cover a small portion of the surface are discussed.

Head, J. W., III↗

NASA NTRS1989

The infrared-to-radio ratio within NGC 5236 (M83) and NGC 6946

A detailed comparison of the distribution of 60-micron IR and 20-cm radio continuum emission within NGC 5236 (M83) and NGC 6946 is presented. In the disks of both galaxies, a slow decrease of the 60-micron-to-20-cm ratio with increasing radius is observed. Values of this ratio within the central regions are enhanced by roughly a factor of 3 with respect to the outer disks, whereas the corresponding enhancement in radio surface brightness is at least a factor of 20. The 100-micron-to-20-cm ratio shows a similar but shallower gradient. To account for these observations, it is suggested that spiral galaxies are characterized by an IR disk with a shorter scale length than that of the radio continuum disk, the latter being smeared as a result of cosmic-ray propagation.

Bicay, M. D.↗