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At least 1,171 records · Page 65
All sourcesNASA NTRSDOE OSTI
NASA NTRS2019

Observing System Experiments for Evaluating the Impact of Satellite Sea Surface Salinity on Seasonal Predictions from the GMAO S2S System

We assess the impact of satellite sea surface salinity (SSS) observations on dynamical ENSO (El Nino-Southern Oscillation) forecasts. Assimilation of SSS improves the mixed layer depth (MLD) and modulates the Kelvin waves associated with ENSO. In column 2, the initialization differences between experiments that assimilate SSS minus those withholding SSS assimilation are presented. Column 3 shows examples of forecasts generated for the different phases of ENSO assimilating the different satellite SSS. In general, for all phases of ENSO, SSS assimilation improves forecasts. The far right column compares ensemble means for assimilation of individual and combined SMOS, Aquarius, SMAP (Soil Moisture and Ocean Salinity, NASA Aquarius, Soil Moisture Active Passive) SSS forecasts. Finally, the latest forecasts are presented comparing assimilation versus no- assimilation of satellite SSS for single forecasts over the last year.

Hackert, E.↗
DOE OSTI2018

Seasonal Evolution of the Subglacial Hydrologic System Modified by Supraglacial Lake Drainage in Western Greenland

The impact of summer surface melt on Greenland Ice Sheet dynamics is modulated by the state of the subglacial hydrologic system. Studies of ice motion indicate that efficiency of the subglacial system increases over the melt season, decreasing the sensitivity of ice motion to surface melt inputs. However, the behavior of the subglacial hydrologic system is complex and some characteristics are still poorly constrained. Here we investigate the coevolution of subglacial hydrology and ice motion in the Pâkitsoq region of western Greenland during the 2011 melt season. We analyze measurements from 11 Global Positioning System stations, from which we derive ice velocity, longitudinal strain rates, and basal uplift, alongside observations of surface ablation and supraglacial lake drainages. We observe ice acceleration after the onset of local surface melting, followed by gradual ice deceleration, consistent with increasing subglacial efficiency. In the study area, supraglacial lake drainages cooccur with a change in regional strain rate patterns and ice deceleration, suggesting that lake drainages contribute to rapid subglacial reorganization. At lower ice surface elevations (below ~900 m above sea level), ice motion is correlated with both total basal uplift and its rate of change, while at higher elevations (~900–1,100 m above sea level), ice motion correlated only with the basal uplift rate. This pattern suggests that continued cavity growth or subglacial sediment dynamics may be important in the apparent increase in subglacial drainage efficiency at higher elevations in the ablation zone. Furthermore, our results further suggest that transient subglacial behavior is important in the seasonal evolution of ice motion.

58 GEOSCIENCES↗
NASA NTRS1984

Numerical simulation of the tip vortex off a low-aspect-ratio wing at transonic speed

The viscous transonic flow around a low aspect ratio wing was computed by an implicit, three dimensional, thin-layer Navier-Stokes solver. The grid around the geometry of interest is obtained numerically as a solution to a Dirichlet problem for the cube. A low aspect ratio wing with large sweep, twist, taper, and camber is the chosen geometry. The topology chosen to wrap the mesh around the wing with good tip resolution is a C-O type mesh. The flow around the wing was computed for a free stream Mach number of 0.82 at an angle of attack of 5 deg. At this Mach number, an oblique shock forms on the upper surface of the wing, and a tip vortex and three dimensional flow separation off the wind surface are observed. Particle path lines indicate that the three dimensional flow separation on the wing surface is part of the roots of the tip vortex formation. The lifting of the tip vortex before the wing trailing edge is observed by following the trajectory of particles release around the wing tip.

Mansour, N. N.↗
NASA NTRS1985

The shuttle glow: A program to study the ram-induced phenomena

In January 1984, a proposal was submitted to NASA Headquarters entitled The Shuttle Glow: A Program to Determine the Physics of the Ram induced Phenomena. This proposal included the following elements in a shuttlebased experiment: (1) The use of a special flat generating surface 1 x 3 m on which the glow can be produced and observed. This surface will be maneuvered to vary the orientation of ram flow and of projected component of geomagnetic field. (2) Remotely mounted optical instruments to view the glowing layer on the plate. By scanning, the variation of radiance as a function of wavelength and standoff distance from the plate will be observed looking parallel to the plate. (3) The preferred location of the generating plate and in situ diagnostics is on the end of the Remote Manipulator System (RMS) arm.

Anderson, H. R.↗
NASA NTRS1974

Observations of moon-plasma interactions by orbital and surface experiments

Extensive magnetic field observations together with crucial plasma measurements by the Explorer 35 lunar orbiter and Apollo surface and orbital experiments have established the basic nature of the moon's interaction with the solar wind and interplanetary magnetic field. The effective absorption of the incident solar wind by the moon creates a plasma void or cavity behind the moon. The cavity-associated magnetic signature is characterized by an enhancement in magnetic field magnitude B within the cavity as compared with the mean level of B in the surrounding interplanetary plasma and dips or decreases in B near the cavity boundaries with the solar wind. Apollo particle and field measurements on the lunar surface have provided evidence of a regional interaction of the highly conducting solar wind with lunar remanent magnetic fields. Simultaneous plasma and magnetic field data, from the spectrometer and the lunar surface magnetometer at the Apollo 12 location, show the compression of the local remanent field by large solar wind and magnetosheath plasma dynamic pressures.

Schubert, G.↗
NASA NTRS1994

An in situ evaluation of TOPEX/Poseidon altimetric measurements versus meaurements made by moorings and inverted echo sounders for sea surface height

The classical method of observing the sea surface height has been to make shipboard measurements of the vertical - density profile, and then calculating the surface height relative to a deeper reference surface. Two methods (a moored vertical string of instruments and an inverted echo sounder) were subsequently developed to obtain longer time in situ measurements. The first of these can be thought of as an extension of the discrete bottle hydrocast while the second integrates acoustically over the water column. One purpose of this note is to compare the result when coincidental observations are made by these two methods. This was done at two sites in the western tropical Pacific. Two inverted echo sounders were deployed alongside two enhanced TOGA-COARE moorings to be used in an in situ evaluation of TOPEX/Poseidon altimetric measurements of sea surface height. The mooring and inverted echo sounder data reproduced one another, at low frequency, with a correlation of 0.93 and 0.95 and the altimeter correlated with each of the above values ranging from 0.84 to 0.94. It is concluded that the altimetric measurements are statistically equivalent to the in situ measurements in the area of study.

Source record↗
NASA NTRS1992

A comparison of airborne and ground-based radar observations with rain gages during the CaPE experiment

The vicinity of KSC, where the primary ground truth site of the Tropical Rainfall Measuring Mission (TRMM) program is located, was the focal point of the Convection and Precipitation/Electrification (CaPE) experiment in Jul. and Aug. 1991. In addition to several specialized radars, local coverage was provided by the C-band (5 cm) radar at Patrick AFB. Point measurements of rain rate were provided by tipping bucket rain gage networks. Besides these ground-based activities, airborne radar measurements with X- and Ka-band nadir-looking radars on board an aircraft were also recorded. A unique combination data set of airborne radar observations with ground-based observations was obtained in the summer convective rain regime of central Florida. We present a comparison of these data intending a preliminary validation. A convective rain event was observed simultaneously by all three instrument types on the evening of 27 Jul. 1991. The high resolution aircraft radar was flown over convective cells with tops exceeding 10 km and observed reflectivities of 40 to 50 dBZ at 4 to 5 km altitude, while the low resolution surface radar observed 35 to 55 dBZ echoes and a rain gage indicated maximum surface rain rates exceeding 100 mm/hr. The height profile of reflectivity measured with the airborne radar show an attenuation of 6.5 dB/km (two way) for X-band, corresponding to a rainfall rate of 95 mm/hr.

Satake, Makoto↗
NASA NTRS1992

A comparison of airborne and ground-based radar observations with rain gages during the CaPE experiment

The vicinity of KSC, where the primary ground truth site of the Tropical Rainfall Measuring Mission (TRMM) program is located, was the focal point of the Convection and Precipitation/Electrification (CaPE) experiment in July and Aug. 1991. In addition to several specialized radars, local coverage was provided by the C-band (5 cm) radar at Patrick AFB. Point measurements of rain rate were provided by tipping bucket rain gage networks. Besides these ground-based activities, airborne radar measurements with X- and Ka-band nadir-looking radars on board an aircraft were also recorded. A unique combination data set of airborne radar observations with ground-based observations was obtained in the summer convective rain regime of central Florida. We present a comparison of these data intending a preliminary validation. A convective rain event was observed simultaneously by all three instrument types on the evening of 27 July 1991. The high resolution aircraft radar was flown over convective cells with tops exceeding 10 km and observed reflectivities of 40 to 50 dBZ at 4 to 5 km altitude, while the low resolution surface radar observed 35 to 55 dBZ echoes and a rain gage indicated maximum surface rain rates exceeding 100 mm/hr. The height profile of reflectivity measured with the airborne radar show an attenuation of 6.5 dB/km (two way) for X-band, corresponding to a rainfall rate of 95 mm/hr.

Satake, Makoto↗
DOE OSTI2015

Surface and subsurface cracks characteristics of single crystal SiC wafer in surface machining

Different machining processes were used in the single crystal SiC wafer machining. SEM was used to observe the surface morphology and a cross-sectional cleavages microscopy method was used for subsurface cracks detection. Surface and subsurface cracks characteristics of single crystal SiC wafer in abrasive machining were analysed. The results show that the surface and subsurface cracks system of single crystal SiC wafer in abrasive machining including radial crack, lateral crack and the median crack. In lapping process, material removal is dominated by brittle removal. Lots of chipping pits were found on the lapping surface. With the particle size becomes smaller, the surface roughness and subsurface crack depth decreases. When the particle size was changed to 1.5µm, the surface roughness Ra was reduced to 24.0nm and the maximum subsurface crack was 1.2µm. The efficiency of grinding is higher than lapping. Plastic removal can be achieved by changing the process parameters. Material removal was mostly in brittle fracture when grinding with 325# diamond wheel. Plow scratches and chipping pits were found on the ground surface. The surface roughness Ra was 17.7nm and maximum subsurface crack depth was 5.8 µm. When grinding with 8000# diamond wheel, the material removal was in plastic flow. Plastic scratches were found on the surface. A smooth surface of roughness Ra 2.5nm without any subsurface cracks was obtained. Atomic scale removal was possible in cluster magnetorheological finishing with diamond abrasive size of 0.5 µm. A super smooth surface eventually obtained with a roughness of Ra 0.4nm without any subsurface crack.

36 MATERIALS SCIENCE↗
DOE OSTI2020

High Heat Flux Testing of Castellated Graphite Plasma- Facing Components

As part of the recovery project of the National Spherical Tokamak Experiment–Upgrade (NSTX-U), the divertor plasma-facing components (PFCs) were redesigned to handle significantly higher heat fluxes and longer pulse lengths than NSTX. The design process resulted in a castellated, graphite PFC tile. To verify the thermal performance of this design, dedicated electron beam, high heat flux (HHF) testing was carried out on a de-optimized mock-up PFC target. These tests demonstrated that the tile design is itself robust to large, localized thermal gradients. No mechanical damage to the mock-up was observed during HHF testing, though the actual PFC tile mechanical tie-down was not tested. Rather, when the surface temperature exceeded the sublimation temperature of graphite, carbon blooms from the mock-up tile surface were observed. Finally, this resulted in 1 to 2 mm of surface material ablating from the mock-up after repeated, highly localized electron beam exposures.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Rationalizing energy level alignment by characterizing Lewis acid/base and ionic interactions at printable semiconductor/ionic liquid interfaces

Charge transfer and energy conversion processes at semiconductor/electrolyte interfaces are controlled by local electric field distributions, which can be especially challenging to measure. Furthermore, we leverage the low vapor pressure and vacuum compatibility of ionic liquid electrolytes to undertake a layer-by-layer, ultra-high vacuum deposition of a prototypical ionic liquid EMIM + (1-ethyl-3-methylimidazolium) and TFSI – (bis(trifluoromethylsulfonyl)-imide) on the surfaces of different electronic materials. We consider a case-by-case study between a standard metal (Au) and four printed electronic materials, where interfaces are characterized by a combination of X-ray and ultraviolet photoemission spectroscopies (XPS/UPS). For template-stripped gold surfaces, we observe through XPS a preferential orientation of the TFSI anion at the gold surface, enabling large electric fields (~10 8 eV m –1 ) within the first two monolayers detected by a large surface vacuum level shift (0.7 eV) in UPS.

14 SOLAR ENERGY↗
DOE OSTI2015

Suppression of surface charge accumulation on Al{sub 2}O{sub 3}-filled epoxy resin insulator under dc voltage by direct fluorination

Surface charge accumulation on insulators under high dc voltage is a major factor that may lead to the reduction of insulation levels in gas insulated devices. In this paper, disc insulators made of Al{sub 2}O{sub 3}-filled epoxy resin were surface fluorinated using a F{sub 2}/N{sub 2} mixture (12.5% F{sub 2}) at 50 °C and 0.1 MPa for different durations of 15 min, 30 min and 60 min. A dc voltage was applied to the insulator for 30 min and the charge density on its surface was measured by an electrostatic probe. The results revealed significant lower surface charge densities on the fluorinated insulators in comparison with the original one. Surface conductivity measurements indicated a higher surface conductivity by over three orders of magnitude after fluorination, which would allow the charges to transfer along the surface and thus may suppress their accumulation. Further, attenuated total reflection infrared analysis and surface morphology observations of the samples revealed that the introduction of fluoride groups altered the surface physicochemical properties. These structure changes, especially the physical defects reduced the depth of charge traps in the surface layer, which was verified by the measurement of energy distributions of the electron and hole traps based on the isothermal current theory. The results in this paper demonstrate that fluorination can be a promising and effective method to suppress surface charge accumulation on epoxy insulators in gas insulated devices.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Generation of subsurface voids and a nanocrystalline surface layer in femtosecond laser irradiation of a single-crystal Ag target

Structural transformations in a shallow surface region of a bulk Ag (001) target irradiated by a femtosecond laser pulse are investigated in large-scale atomistic simulations and experiments. The simulations reveal a complex interplay of fast laser melting, rapid resolidification, and dynamic relaxation of laser-induced stresses that leads to the formation of a subsurface porous region covered by a nanocrystalline surface layer. The generation of the porous region is consistent with the experimental observation of surface “swelling” occurring at laser fluences below the spallation/ablation threshold and may be related to the incubation effect in multipulse laser ablation of metals. In this work, the nanocrystalline layer is produced by massive nucleation of crystallites triggered by a deep undercooling of the melted surface region experiencing fast quenching at a rate on the order of 10 11 K/s. The predicted surface structure features random crystallographic orientation of nanograins and a high density of stacking faults, twins, and nanoscale twinned structural elements with fivefold symmetry, which suggests high hardness and possible enhancement of catalytic activity of the surface.

incubation effect in laser ablation↗
DOE OSTI2024

Influence of Spatter on Porosity, Microstructure, and Corrosion of Additively Manufactured Stainless Steel Printed Using Different Island Size

Specimens of 316 L stainless steel were printed using laser powder bed fusion (LPBF), a popular metal additive manufacturing (AM) technique, with varying island sizes. Not many researchers have considered the impact of spatter while optimizing LPBF printing parameters. In this research, the influence of spatter was considered while also investigating the effect of varied island size on the microstructure, surface roughness, microhardness, and corrosion resistance of LPBF-316 L. No correlation was observed between surface roughness or microhardness and minor variations in island size. However, a correlation was drawn between varied island sizes and porosity in LPBF-316 L. The specimens associated with larger island sizes showed significantly enhanced corrosion resistance due to fewer manufacturing defects and reduced porosity, attributed to the minimal influence of the spatter. Based on analysis, the LPBF parameters were revised, which lead to superior corrosion resistance of LPBF-316 L, attributed to high density and reduced porosity.

36 MATERIALS SCIENCE↗
NASA NTRS1990

Summer 1988 TDWR microburst analysis

The Terminal Doppler Weather Radar (TDWR) testbed system was operated during the months of July to August 1988 in a live operational demonstration providing microburst (and related weather hazard) protection to the Stapleton International Airport in Denver, CO. During this time period, the performance of the detection system was carefully monitored in an effort to determine the reliability of the system. Initial performance analysis indicates that the microburst detection component of TDWR satisfies the basic performance goals of 90 percent probability of detection and 10 percent probability of false alarm. An in-depth study of the system performance, based on analysis of both dual-Doppler radar observations and surface mesonet measurements, is in progress to provide a detailed understanding of the observability of microbursts by the radar, the ability of the algorithms to detect microburst observed by the radar, and the timeliness and accuracy of the microburst alarms provided to operational users.

Merritt, Mark W.↗
NASA NTRS1995

Semiempirical Analysis of Surface Alloy Formation

The BFS method for alloys is applied to the study of surface alloy formation. This method was previously used to examine the experimental STM observation of surface alloying of Au on Ni(110) for low Au coverages by means of a numerical simulation. In this work, we extend the study to include other cases of surface alloying for immiscible as well as miscible metals. All binary combinations of Ni, Au, Cu, and Al are considered and the simulation results are compared to experiment when data is available. The driving mechanisms of surface alloy formation are then discussed in terms of the BFS method and the available results.

Bozzolo, Guillermo↗
NASA NTRS2013

Improved Understanding of the Modeled QBO Using MLS Observations and MERRA Reanalysis

The Quasi-Biennial Oscillation (QBO) dominates the variability of the tropical stratosphere on interannual time scales. The QBO has been shown to extend its influence into the chemical composition of this region through dynamical mechanisms. We have started our analysis using the realistic QBO internally generated by the Goddard Earth Observing System Version 5 (GEOS-5) general circulation model coupled to a comprehensive stratospheric and tropospheric chemical mechanism forced with observed sea surface temperatures over the past 33 years. We will show targeted comparisons with observations from NASAs Aura satellite Microwave Limb Sounder (MLS) and the Modern Era Retrospective-Analysis for Research and Applications (MERRA) reanalysis to provide insight into the simulation of the primary and secondary circulations associated with the QBO. Using frequency spectrum analysis and multiple linear regression we can illuminate the resulting circulations and deduce the strengths and weaknesses in their modeled representation. Inclusion of the QBO in our simulation improves the representation of the subtropical barriers and overall tropical variability. The QBO impact on tropical upwelling is important to quantify when calculating trends in sub-decadal scale datasets.

MERRA↗