Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “Sheet”

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 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS2014

Future Antarctic Bed Topography and Its Implications for Ice Sheet Dynamics

The Antarctic bedrock is evolving as the solid Earth responds to the past and ongoing evolution of the ice sheet. A recently improved ice loading history suggests that the Antarctic Ice Sheet (AIS) has generally been losing its mass since the Last Glacial Maximum. In a sustained warming climate, the AIS is predicted to retreat at a greater pace, primarily via melting beneath the ice shelves.We employ the glacial isostatic adjustment (GIA) capability of the Ice Sheet System Model (ISSM) to combine these past and future ice loadings and provide the new solid Earth computations for the AIS.We find that past loading is relatively less important than future loading for the evolution of the future bed topography. Our computations predict that the West Antarctic Ice Sheet (WAIS) may uplift by a few meters and a few tens of meters at years AD 2100 and 2500, respectively, and that the East Antarctic Ice Sheet is likely to remain unchanged or subside minimally except around the Amery Ice Shelf. The Amundsen Sea Sector in particular is predicted to rise at the greatest rate; one hundred years of ice evolution in this region, for example, predicts that the coastline of Pine Island Bay will approach roughly 45mmyr−1 in viscoelastic vertical motion. Of particular importance, we systematically demonstrate that the effect of a pervasive and large GIA uplift in the WAIS is generally associated with the flattening of reverse bed slope, reduction of local sea depth, and thus the extension of grounding line (GL) towards the continental shelf. Using the 3-D higher-order ice flow capability of ISSM, such a migration of GL is shown to inhibit the ice flow. This negative feedback between the ice sheet and the solid Earth may promote stability in marine portions of the ice sheet in the future.

ice dynamics↗
NASA NTRSDate not supplied

The Impact of Ice Sheet Surface Representation on Surface Mass Balance in the Goddard Earth Observing System

Surface conditions and their impacts on surface mass balance (SMB) play important roles in ice sheet dynamics and ice sheet interactions with the overlying atmosphere and surrounding ocean. The NASA Global Modeling and Assimilation Office’s (GMAO) Goddard Earth Observing System (GEOS) – an ecosystem of models and reanalyses – represent ice sheet and glacier SMB components, including prognostic surface albedo evolution, fractional snow cover, and snowpack hydrology and meltwater retention. We find the successful representation of ice sheet surface mass balance in MERRA-2, and similar systems, is partly due to compensating biases in surface energy budget. In the atmospheric system, subsequent changes to cloud microphysics and in the longwave radiative transfer model are found to have reduced biases in surface net longwave radiation fluxes while exacerbating surface net shortwave radiation biases, producing erroneously high near-surface temperatures and surface melt in summer months. These issues can be exacerbated by poor experiment initialization and the use of two-moment cloud microphysics within the GEOS ocean-atmosphere coupled system. Here, we document the spatial and temporal extent and causes of these biases in the ice sheet surface energy budget across GEOS systems and implement a range of model improvements to mitigate these issues. We examine the combined impact on these changes on regional energy budget and SMB using both free running and replay experiments (to simulate the impact in reanalyses). Overall, these improvements increase modeled ice sheet surface realism and provide a strong basis for more accurate ice sheet SMB in GMAO’s future reanalyses and forecasting systems.

Lauren C Andrews↗
NASA NTRS2012

Dynamic Inland Propagation of Thinning Due to Ice Loss at the Margins of the Greenland Ice Sheet

Mass-balance analysis of the Greenland ice sheet based on surface elevation changes observed by the European Remote-sensing Satellite (ERS) (1992-2002) and Ice, Cloud and land Elevation Satellite (ICESat) (2003-07) indicates that the strongly increased mass loss at lower elevations (<2000 m) of the ice sheet, as observed during 2003-07, appears to induce interior ice thinning at higher elevations. In this paper, we perform a perturbation experiment with a three-dimensional anisotropic ice-flow model (AIF model) to investigate this upstream propagation. Observed thinning rates in the regions below 2000m elevation are used as perturbation inputs. The model runs with perturbation for 10 years show that the extensive mass loss at the ice-sheet margins does in fact cause interior thinning on short timescales (i.e. decadal). The modeled pattern of thinning over the ice sheet agrees with the observations, which implies that the strong mass loss since the early 2000s at low elevations has had a dynamic impact on the entire ice sheet. The modeling results also suggest that even if the large mass loss at the margins stopped, the interior ice sheet would continue thinning for 300 years and would take thousands of years for full dynamic recovery.

ice dynamics↗
NASA NTRS2019

initMIP-Antarctica: an Ice Sheet Model Initialization Experiment of ISMIP6

Ice sheet numerical modeling is an important tool to estimate the dynamic contribution of the Antarctic ice sheet to sea level rise over the coming centuries. The influence of initial conditions on ice sheet model simulations, however, is still unclear. To better understand this influence, an initial state intercomparison exercise (initMIP) has been developed to compare, evaluate, and improve initialization procedures and estimate their impact on century-scale simulations. initMIP is the first set of experiments of the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6), which is the primary Coupled Model Intercomparison Project Phase 6 (CMIP6) activity focusing on the Greenland and Antarctic ice sheets. Following initMIP-Greenland, initMIP-Antarctica has been designed to explore uncertainties associated with model initialization and spin-up and to evaluate the impact of changes in external forcings. Starting from the state of the Antarctic ice sheet at the end of the initialization procedure, three forward experiments are each run for 100 years: a control run, a run with a surface mass balance anomaly, and a run with a basal melting anomaly beneath floating ice. This study presents the results of initMIP-Antarctica from 25 simulations performed by 16 international modeling groups. The submitted results use different initial conditions and initialization methods, as well as ice flow model parameters and reference external forcings. We find a good agreement among model responses to the surface mass balance anomaly but large variations in responses to the basal melting anomaly. These variations can be attributed to differences in the extent of ice shelves and their upstream tributaries, the numerical treatment of grounding line, and the initial ocean conditions applied, suggesting that ongoing efforts to better represent ice shelves in continental-scale models should continue.

ice sheet model↗
NASA NTRS2024

PRODEM: An Annual Series of Summer DEMs (2019 through 2022) of the Marginal Areas of the Greenland Ice Sheet

Surface topography across the marginal zone of the Greenland Ice Sheet is constantly evolving in response to changing weather, season, climate, and ice dynamics. However, current digital elevation models (DEMs) for the ice sheet are usually based on data from a multi-year period, thus obscuring these changes over time. Here we present four 500 m resolution summer DEMs (PRODEMs) of the Greenland Ice Sheet marginal zone for 2019 through 2022. The PRODEMs cover the marginal zone from the ice edge to 50 km inland, hence capturing all Greenland outlet glaciers. Each PRODEM is based on data fusion of CryoSat-2 radar altimetry and ICESat-2 laser altimetry using regionally varying kriging of elevation anomalies relative to ArcticDEM. The PRODEMs are validated using leave-one-out cross-validation, and PRODEM19 is further validated against an external data set, showcasing their ability to correctly represent surface elevations within the associated spatially varying prediction uncertainties. We observe a general lowering of surface elevations during the 4-year PRODEM period, but the spatial pattern of change is highly complex and with annual changes superimposed. The PRODEMs enable detailed studies of the marginal ice sheet elevation changes. With their high spatio-temporal resolution, the PRODEMs will be of value to a wide range of researchers and users studying ice sheet dynamics and monitoring how the ice sheet responds to changing environmental conditions. PRODEMs from summer 2019 through 2022 are available at https://doi.org/10.22008/FK2/52WWHG (Winstrup, 2024), and we plan to annually update the product henceforth.

Digital Elevation Models↗
NASA NTRS1973

Drying apparatus for photographic sheet material

An elongated drying chamber is provided with transport means for carrying photographic sheet material edgewise with the sheets in end-to-end relationship past a plurality of tubes that issue drying air streams. The tubes are slotted a distance equal to substantially the full width of the sheet material for complete, gentle drying by sheets of air. A common plenum supplies the tubes with heated air; the air is directed from the tube slots at a pronounced angle to the sheet surface to provide for arraying the tubes close to the surface for maximum drying effect while minimizing the danger of mechanical interference between the edges of the sheets and the slots in the tubes. The driver for the transport is housed in an enclosure between the plenum and the drying chamber; an air return duct is provided along another side to complete insulation of the drying chamber from ambient conditions.

Epstein, P.↗
NASA NTRS1973

On the structure of the magnetotail current sheet.

A self-consistent tail current sheet model described by an exact analytic solution of the time-independent Vlasov-Maxwell equations is presented. The model has a 'slingshot' field configuration with field lines outside the plasma sheet slightly flared in the antisolar direction. It is pointed out that when the model parameters are adjusted to agree with the spatial variation along the tail the required thickness of the neutral sheet must be about 2.5 earth radii, instead of less than or equal to 1 earth radius, as indicated by observations. Furthermore, it is shown qualitatively that a considerable velocity shear must be present in the tail current sheet if the plasma sheet is indeed much thicker than the neutral sheet.

Kan, J. R.↗
NASA NTRS1974

Shear-stress intensity factors for elastic sheets with cover plates

Shear stress intensity factors are calculated for three problems concerning inextensible cover plates either bonded to or embedded in an elastic sheet which is under uniaxial tension. The stress intensity factors are small when the ratio of sheet thickness to cover plate length is small and, as the ratio increase, rapidly approach their asymptotic values for infinite sheet thickness. In the problem of the embedded cover plate, the stress intensity factor also depends on the Poisson's ratio of the sheet material. The dependence on Poisson's ratio, however, is significant only when the ratio of sheet thickness to cover plate length is small. Some possible implications of the present results for debonding of reinforced sheets under cyclic loading are briefly discussed.

Fichter, W. B.↗
NASA NTRS1976

Multiple-satellite studies of magnetospheric substorms - Radial dynamics of the plasma sheet

The radial dynamics of the nighttime plasma sheet during substorms is examined. The spatial dependence of plasma sheet variations at different radial distances is studied on the basis of simultaneous recordings from two closely spaced satellites. The simultaneous measurements of the plasma sheet behavior earthward and tailward of r = 15 earth radii confirm substorm models which predict a thinning of the near-earth plasma sheet before the formation of an X-type neutral line, followed by a thickening on the earthward side and a further thinning on the tailward side. Temporal correlations between the plasma sheet variations and substorm development on the ground are studied by obtaining accurate timing of individual substorm expansion onsets. In particular, during multiple onset storms, the near-earth plasma sheet is found to experience a series of multiple expansions and contractions, which usually occur in a one-to-one relationship with ground Pi 2 bursts and are well correlated with auroral zone and low-altitude magnetic disturbances.

Pytte, T.↗
NASA NTRS1978

High beta plasma in the dynamic Jovian current sheet

The equatorial current sheet, which Pioneer 10 repeatedly encountered on its outbound pass through the Jovian magnetosphere, frequently was associated with intense fluxes of energetic protons. Simultaneous observations of the changes in the energetic proton flux and in the magnetic-field magnitude demonstrate that the current sheet is embedded in a high-beta plasma in which high-energy (above 60 keV) ions frequently are the dominant constituents. Large differences in the plasma temperature and the thickness of this plasma sheet between encounters only 10 hours apart indicate that the Jovian plasma sheet is very dynamic on a time scale of hours. Occasional observations of significant temporal variations in the magnetic field and particle populations during periods within the plasma sheet may represent in situ observations of Jovian magnetic disturbances. Comparison with previous observations suggests that low-energy (not more than 5 keV) plasma contributes less than 3% to the current-sheet energy density.

Walker, R. J.↗
NASA NTRS1980

Variability of plasma sheet dynamics

IMP 7 observations of plasma sheet structure and dynamics made during a 24-h period in which the satellite traversed the tail plasma sheet at a radial distance of 35 earth radii and remained within 2 earth radii of the expected position at the neutral sheet and in which at least five substorms occured on the ground are presented. High-time-resolution measurements of the magnetic field, plasma flow and greater than 50-keV protons obtained by the satellite and ground-based magnetic measurements are discussed for a 4-hr interval in which two small substorms occurred, a 6-hr geomagnetically quiet interval in which moderate variable plasma flows were observed, a very rapid, smooth neutral sheet crossing and earthward plasma flow during the expansion phase of a small substorm, a moderate-size substorm in which a prolonged interval of field-aligned tailward flow commenced at onset, a strong tailward and dawnward flow burst in an interval between substorms and a large substorm in which strong tailward flow was observed. Observations indicate the significant distortion of the tail field, possibly by plasma flow stresses, a neutral sheet structure occasionally resembling the hydromagnetic rotational discontinuity and a high level of magnetic turbulence during an earthward plasma flow which may contribute towards plasma sheet dissipation.

Coroniti, F. V.↗
NASA NTRS1980

The Jovian magnetotail and its current sheet

Analyses of Voyager magnetic field measurements have extended the understanding of the structural and temporal characteristics of Jupiter's magnetic tail. The magnitude of the magnetic field in the lobes of the tail is found to decrease with Jovicentric distance approximately as r to he-1.4, compared with the power law exponent of -1.7 found for the rate of decrease along the Pioneer 10 outbound trajectory. Voyager observations of magnetic field component variations with Jovicentric distance in the tail do not support the uniform radial plasma outflow model derived from Pioneer data. Voyager 2 has shown that the azimuthal current sheet which surrounds Jupiter in the inner and middle magnetosphere extends tailward (in the anti-Sun direction) to a distance of at least 100 R sub J. In the tail this current sheet consists of a plasma sheet and embedded neutral sheet. In the region of the tail where the sheet is observed, the variation of the magnetic field as a result of the sheet structure and its 10 hr periodic motion is the dominant variation seen.

Behannon, K. W.↗
NASA NTRS1981

Explorer 34 magnetic field measurements near the tail current sheet and auroral activity

Explorer 34 (Imp 4) 2.56 s magnetic data obtained during 131 traversals of the tail current sheet are presented, along with the simultaneous 2.5 min auroral electrojet indices AE and AL. The normal magnetic field satellite crossing times and positions are tabulated for these 131 crossings. Normal magnetic field is defined in the center of the sheet: it is the vector magnetic field at the time of field minimum during the crossing. It is remarkable that the only normal components too large in magnitude to be classified as fine structure occur near the time of onset of an AE event. Cases are discussed where the normal component, defined near the plasma sheet edges, has the opposite sign compared to the normal component defined at the sheet center. For quiet times, the current sheet may be only about 1000 km thick within a 3 earth-radii plasma sheet, and may carry some 10-15% of the total tail current.

Speiser, T. W.↗
NASA NTRS1981

Lower-hybrid-drift instability and its associated anomalous resistivity in the neutral sheet of earth's magnetotail

A one-dimensional non-Maxwellian model for the steady neutral sheet at the center of the earth's plasma sheet is constructed, and its instability properties are examined with respect to the lower-hybrid-drift wave. It is shown that in the limit of a thin neutral sheet, the distribution of modified Alper and Boltzmann distributions is a more powerful source of free energy than the usual drifting Maxwellian. It is also shown that most regions within the neutral sheet are unstable, and the frequency spectrum of the unstable waves is nearly the same as in the drifting Maxwellian case. The growth rate is found to be enhanced, and the saturation level of the fluctuating field and the anomalous resistivity calculated in the entire sheet are found to be 3 to 4 times larger than those in a Maxwellian sheet.

Pu, Z.-Y.↗
NASA NTRS1981

The Jovian magnetotail and its current sheet

A summary is given of the results of a further analysis of the magnetic field measurements in Jupiter's magnetic tail and in the region of the tail current sheet by both V1 and V2. Four studies are discussed. The first is a determination of the variation of the average magnetic field magnitude in the lobes of the magnetotail, that is, outside the plasma sheet, as a function of distance from the planet. The second is an examination of the variation with distance from Jupiter of the ratio of the azimuthal component of the magnetic field to the product of the radial distance and the radial component of the magnetic field. The third is a study of the detailed structure and orientation of the tail current sheet system, including both detailed illustrations of typical sheet crossings and the results of a minimum variance analysis of all V2 tail sheet traversals. The fourth is an investigation of various models which predict the position of the current sheet in the outer Jovian magnetosphere as a function of both time and location.

Behannon, K. W.↗
NASA NTRS1984

The plasma sheet boundary layer

A spatially distinct, temporally variable, transition region between the magnetotail lobes and the central plasma sheet designated the plasma sheet boundary layer has been identified from a survey of particle spectra and three-dimensional distributions as sampled by the ISEE 1 LEPEDEA. The instrumentation and data presentation are described, and the signatures of the magnetotail plasma regimes are presented and discussed for the central plasma sheet and lobe and the plasma sheet boundary layer. Comparisons of plasma parameters and distribution fucntions are made and the evolution of ion velocity distributions within the plasma sheet boundary layer is discussed. The spatial distribution of the plasma sheet boundary layer is considered and ion composition measurements are presented.

Eastman, T. E.↗
NASA NTRS1984

Potential Climatic Effects on the Greenland Ice Sheet

The Greenland Ice Sheet covers an area of 1,720,000 sq. km and contains approximately 2,600,000 cu km of ice. Most of the ice sheet receives an excess of snow accumulation over the amount of ice lost to wind, meltwater run-off or other ablative processes. The majority of mass loss occurs at the margin of the ice sheet as either surface melt, which flows into the sea or calving of icebergs from the tongues of outlet glaciers. Many estimates of these processes were published. An average of five published estimates is summarized. If these estimates are correct, then the Greenland Ice Sheet is in approximate equilibrium and contributes 490 cu km/a of fresh water to the North Atlantic and Arctic Oceans. Climate effects, ice sheet flow, and application of remote sensing to tracking of the ice sheet are discussed.

Bindschadler, R. A.↗
NASA NTRS1988

A twin-mirrored galvanometer laser light sheet generator

A galvanometer mirror-based laser light sheet system has been developed for use in the Basic Aerodynamics Research Tunnel at NASA Langley. This system generates and positions single or multiple light sheets over aeronautical research models being tested in the low speed tunnel. This report describes a twin mirrored galvanometer laser light sheet generator and shows typical light sheet arrangements in use. With this system, illumination of smoke entrained in the flow over a delta wing model reveals the vortical flow produced by the separation of the flow at the leading edge of the model. The light sheet system has proven to be very adaptable and easy to use in sizing and positioning light sheets in wind tunnel applications.

Rhodes, David B.↗