Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “surface mass variations”

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 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

Relating polarization phase difference of SAR signals to scene properties

This paper examines the statistical behavior of the phase difference Delta-phi between the HH-polarized and VV-polarized backscattered signals recorded by an L-band SAR over an agricultural test site in Illinois. Polarization-phase difference distributions were generated for about 200 agricultural fields for which ground information had been acquired in conjunction with the SAR mission. For the overwhelming majority of cases, the Delta-phi distribution is symmetric and has a single major lobe centered at the mean value of the distribution Delta-phi. Whereas the mean Delta-phi was found to be close to zero degrees for bare soil, cut vegetation, alfalfa, soybeans, and clover, a different pattern was observed for the corn fields; the mean Delta-phi increased with increasing incidence angle Theta = 35 deg. The explanation proposed for this variation is that the corn canopy, most of whose mass is contained in its vertical stalks, acts like a uniaxial crystal characterized by different velocities of propagation for waves with horizontal and vertical polarization. Thus, it is hypothesized that the observed backscatter is contributed by a combination of propagation delay, forward scatter by the soil surface, and specular bistatic reflection by the stalks. Model calculations based on this assumption were found to be in general agreement with the phase observations.

Ulaby, Fawwaz T.↗
NASA NTRS1987

Groundwater sapping channels: Summary of effects of experiments with varied stratigraphy

Experiments in the recirculating flume sapping box have modeled valley formation by groundwater sapping processes in a number of settings. The effects of the following parameters on sapping channel morphology were examined: surface slope; stratigraphic variations in permeability cohesion and dip; and structure of joints and dikes. These kinds of modeling experiments are particularly good for: testing concepts; developing a suite of distinctive morphologies and morphometries indicative of sapping; helping to relate process to morphology; and providing data necessary to assess the relative importance of runoff, sapping, and mass wasting processes on channel development. The observations from the flume systems can be used to help interpret features observed in terrestrial and Martian settings where sapping processes are thought to have played an important role in the development of valley networks.

Kochel, R. Craig↗
NASA NTRS1977

Lunar gravity - A mass point model

A point-mass representation of a quasi-global lunar gravity field is developed by processing Apollo 15 and 16 subsatellite and Lunar Orbiter 5 Doppler tracking data. The model is generated by reducing the long-period variations in the mean orbital elements. The model consists of 117 point masses distributed over the region of + or - 30 deg in latitude about the lunar equator and resolves all the previously known 'mascons' on the near side as broad positive-gravity regions. The near-side acceleration map evaluated at 100 km above the lunar surface shows good agreement with line-of-sight acceleration results. The lunar far-side gravity map shows strong broad positive-gravity regions for the highland areas. However, all the major ringed basins are resolved as localized negative anomalies in contrast with the near-side basins. This model does not indicate any evidence for the existence of any mascon-type feature on the lunar far-side.

Ananda, M. P.↗
NASA NTRS1990

Investigation of biogeophysical feedback on the African climate using a two-dimensional model

A numerical scheme is specifically designed to develop a time-dependent climate model to ensure the conservation of mass, momentum, energy, and water vapor, in order to study the biogeophysical feedback for the climate of Africa. A vegetation layer is incorporated in the present two-dimensional climate model. Using the coupled climate-vegetation model, two tests were performed involving the removal and expansion of the Sahara Desert. Results show that variations in the surface conditions produce a significant feedback to the climate system. It is noted that the simulation responses to the temperature and zonal wind in the case of an expanded desert agree with the climatological data for African dry years. Perturbed simulations have also been performed by changing the albedo only, without allowing the variation in the vegetation layer. It is shown that the variation in latent heat release is significant and is related to changes in the vegetation cover. As a result, precipitation and cloud cover are reduced.

Xue, Yongkang↗
NASA NTRS1992

Quasi-periodic climate change on Mars

The paper examines evidence that the Martian climate undergoes quasi-periodic variations, including the polar layered terrain, differences between the residual polar caps, and the current net southward flow of H2O. The driving functions for these variations are oscillations in the elements of the Martian orbit coupled with precession of the Martian spin axis. These 'astronomic variations' control the distribution of the insolation, which in turn influences the partition of volatiles between atmospheric and surface reservoirs. The major effects anticipated at low obliquity are growth of the polar caps, substantial decrease in surface pressure, cessation of duststorms, release of CO2 from the regolith, and poleward migration of H2O ground ice. At high obliquity, the mass of the perennial polar caps decreases and permanent CO2 frost disappears, CO2 desorbs from the regolith at high latitudes, the surface pressure may increase to several times its current value, and the atmospheric dust load increases.

Kieffer, Hugh H.↗
NASA NTRSDate not supplied

Uncertainty Quantification of Inducer Natural Frequency using Conditional Assessment of Modeling and Modal Testing of Simpler Systems

The low pressure fuel pump inducer of the new Space Launch System RS25 core stage engine operates in a highly complex environment that substantially affects its modal characteristics. Some of the more important effects are fluid-added mass resulting from operation within a light liquid (Hydrogen), and the magnification of this effect due to tight tip clearance. Since higher order cavitation has been identified as a significant harmonic driver, knowledge of the natural frequency of potentially excitable modes is critical for safe operation, but this frequency cannot be measured during the severe operational environment. A comprehensive testing and analysis program has therefore been performed over the last four years to identify the nominal value and uncertainty of the frequency by modeling and testing four simpler structures which share some of the characteristics of the operational inducer. This testing was used to assess and adjust modeling techniques and excellent correlation was achieved. Identification of the uncertainty in the inducer frequency itself was still problematic, however. This difficulty led to an investigation of Bayesian uncertainty quantification techniques, and to the application of the relatively simple technique of Multi-Variate Normal conditional distributions to calculate the inducer natural frequency uncertainty. Assumptions on prior distributions of uncertainty of the fluid-added mass and tip clearance effect are initially applied to models of each of the simple structures and the inducer itself, and these uncertainties are propagated to generate natural frequencies using design of experiments. Simple response surfaces are then created from this data in order to calculate a Covariance Matrix relating all of these natural frequencies. Finally, the results from modal test of the simple structures are considered to be observations and used to calculate the conditional variance of the desired inducer frequencies. As this method is less rigorous than more complicated Bayesian methods reported in the literature, a conservative factor is applied to the result, but the resulting uncertainty is still significantly less than originally estimated and will greatly assist certification of the inducer for use in the engine.

Uncertainty Quantification↗
NASA NTRSDate not supplied

Uncertainty Quantification of Inducer Eigenvalues using Conditional Assessment of Models and Modal Test of Simpler Systems

The low pressure fuel pump inducer of the new Space Launch System RS25 core stage engine operates in a highly complex environment that substantially affects its modal characteristics. Some of the more important effects are fluid-added mass resulting from operation within a light liquid (Hydrogen), and the magnification of this effect due to tight tip clearance. Since higher order cavitation has been identified as a significant harmonic driver, knowledge of the natural frequency of potentially excitable modes is critical for safe operation, but this frequency cannot be measured during the severe operational environment. A comprehensive testing and analysis program has therefore been performed over the last four years to identify the nominal value and uncertainty of the frequency by modeling and testing four simpler structures which share some of the characteristics of the operational inducer. This testing was used to assess and adjust modeling techniques and excellent correlation was achieved. Identification of the uncertainty in the inducer frequency itself was still problematic, however. This difficulty led to an investigation of Bayesian uncertainty quantification techniques, and to the application of the relatively simple technique of Multi-Variate Normal conditional distributions to calculate the inducer natural frequency uncertainty. Assumptions on prior distributions of uncertainty of the fluid-added mass and tip clearance effect are initially applied to models of each of the simple structures and the inducer itself, and these uncertainties are propagated to generate natural frequencies using design of experiments. Simple response surfaces are then created from this data in order to calculate a Covariance Matrix relating all of these natural frequencies. Finally, the results from modal test of the simple structures are considered to be observations and used to calculate the conditional variance of the desired inducer frequencies. As this method is less rigorous than more complicated Bayesian methods reported in the literature, a conservative factor is applied to the result, but the resulting uncertainty is still significantly less than originally estimated and will greatly assist certification of the inducer for use in the engine.

Uncertainty Quantification↗
NASA NTRS2001

Gravitational Signal of Mass Redistribution Due to Interannual Meteorological Oscillations in Atmosphere and Ocean

Interannual meteorological oscillations (ENSO, QBO, NAO, etc.) have demonstrable influences on Earth's rotation. Here we study their effects on global gravitational field, whose temporal variations are being studied using SLR (satellite laser ranging) data and in anticipation of the new space mission GRACE. The meteorological oscillation modes are identified using the EOF (empirical orthogonal function)/PC (principal component) decomposition of surface fields (in which we take care of issues associated with the area-weighting and non-zero mean). We examine two fields, one for the global surface pressure field for the atmosphere obtained from the NCEP reanalysis (for the past 40 years), one for the surface topography field for the ocean from the Topex/Poseidon (T/P) data (for the past 8 years). We use monthly maps, and remove the mean-monthly ("climatology") values from each grid point, hence focusing only on non-seasonal signals. The T/P data were first subject to a steric correction where the steric contribution to the ocean surface topography was removed according to output of the numerical POCM model. The respective atmospheric and oceanic contributions to the gravitational variation, in terms of harmonic Stokes coefficients, are then combined mode-by-mode. Since the T/P data already contain the oceanic response to overlying atmospheric pressure, no regards to the inverted-barometer behavior for the ocean need be considered. Results for the lowest-degree Stokes coefficients can then be compared with space geodetic observations including the Earth's rotation and the SLR data mentioned above, to identify the importance of each meteorological oscillations in gravitational variation signals.

Chao, B. F.↗
NASA NTRS1992

Full-scale high angle-of-attack tests of an F/A-18

This paper presents an overview of high angle-of-attack tests of a full-scale F/A-18 in the 80- by 120-Foot Wind Tunnel of the National Full-Scale Aerodynamic Complex at NASA Ames Research Center at Moffett Field, California. A production aircraft was tested over an angle-of-attack range of 18 to 50 deg and at wind speeds of up to 100 knots. These tests had three primary test objectives. Pneumatic and mechanical forebody flow control devices were tested at full-scale and shown to produce significant yawing moments for lateral control of the aircraft at high angles of attack. Mass flow requirements for the pneumatic system were found to scale with freestream density and speed rather than freestream dynamic pressure. Detailed measurements of the pressures buffeting the vertical tail were made and spatial variations in the buffeting frequency were found. The LEX fence was found to have a significant effect on the frequency distribution on the outboard surface of the vertical fin. In addition to the above measurements, an extensive set of data was acquired for the validation of computational fluid dynamics codes and for comparison with flight test and small-scale wind tunnel test results.

Meyn, Larry A.↗
NASA NTRS1983

Cosmic-ray record in solar system matter

The interaction of galactic cosmic rays (GCR) and solar cosmic rays (SCR) with bodies in the solar system is discussed, and what the record of that interaction reveals about the history of the solar system is considered. The influence of the energy, charge, and mass of the particles on the interaction is addressed, showing long-term average fluxes of solar protons, predicted production rates for heavy-nuclei tracks and various radionuclides as a function of depth in lunar rock, and integral fluxes of protons emitted by solar flares. The variation of the earth's magnetic field, the gardening of the lunar surface, and the source of meteorites and cosmic dust are studied using the cosmic ray record. The time variation of GCR, SCR, and VH and VVH nuclei is discussed for both the short and the long term.

Reedy, R. C.↗
NASA NTRS2013

Stratosphere-to-Troposphere Transport Revealed by Ground-based Lidar and Ozonesonde at a Midlatitude Site

This paper presents ozone structures measured by a ground-based ozone lidar and ozonesonde at Huntsville, Alabama, on 27-29 April 2010 originating from a stratosphere-to-troposphere transport event associated with a cutoff cyclone and tropopause fold. In this case, the tropopause reached 6 km and the stratospheric intrusion resulted in a 2-km thick elevated ozone layer with values between 70 and 85 ppbv descending from the 306-K to 298-K isentropic surface at a rate of 5 km day1. The potential temperature was provided by a collocated microwave profiling radiometer. We examine the corresponding meteorological fields and potential vorticity (PV) structures derived from the analysis data from the North American Mesoscale model. The 2-PVU (PV unit) surface, defined as the dynamic tropopause, is able to capture the variations of the ozone tropopause estimated from the ozonesonde and lidar measurements. The estimated ozone/PV ratio, from the measured ozone and model derived PV, for the mixing layer between the troposphere and stratosphere is approximately 41 ppbv/PVU with an uncertainty of approximately 33%. Within two days, the estimated mass of ozone irreversibly transported from the stratospheric into the troposphere is between 0.07 Tg (0.9 10(exp33) molecules) and 0.11 Tg (1.3 10(exp33) molecules) with an estimated uncertainty of 59%. Tropospheric ozone exhibited enormous variability due to the complicated mixing processes. Low ozone and large variability were observed in the mid-troposphere after the stratospheric intrusion due to the westerly advection including the transition from a cyclonic system to an anticyclonic system. This study using high temporal and vertical-resolution measurements suggests that, in this case, stratospheric air quickly lost its stratospheric characteristics once it is irreversibly mixed down into the troposphere.

Kuang, Shi↗
NASA NTRS2003

Low-Degree Time-Variable Gravity and Geophysical/Climatic Connections

Any large mass transport in the Earth system produces changes in the gravity field. Low harmonic degree components of such variations have been observed by the satellite-laser-ranging (SLR) technique, particularly in J2, the Earth's dynamic oblateness. J2 has long been observed to undergo a slight decrease due primarily to the post-glacial rebound of the mantle -- until around 1998, when it switched quite suddenly to an increase trend which continued to 2001 before turning back, signifymg a large, anomalous mass redistribution whose J2 effect overshadows that of the post-glacial rebound over interannual timescales. Intriguing evidences have been found in the extratropical Pacific basins, especially related to the Pacific Decadal Oscillation, and in land hydrology. We will examine the latest results based on ocean altimetry, sea-surface temperature, and ocean and hydrology model outputs. Besides 52, the SLR-derived time series of the Earth's low-degree gravity components also show shorter wavelength zonal and other longitudinal signals. While the formal uncertainty of these terms is significantly higher, some of these series have significant signal that show correlation to various climatic signals. For example, the SLR-observed S2,l tesseral (and 53 zonal) coefficients compare favorably with those computed from the NCEP atmospheric pressure field. There is a significant correlation of the sectoral S2,2 with the Southern Oscillation Index signifying the influence of El NinoLa Nina, but preceding SO1 by about 1 year. Cases such as these demonstrate the utility of assessing the mass component of climate variations.

Chao, Benjamin F.↗
NASA NTRS1980

Cl-36 in polar ice, rainwater and seawater

Concentrations of the cosmogenic radioisotope Cl-36 in Antarctic ice, rain, and an upper limit of the seawater value are determined using van de Graaff accelerator high energy mass spectrometry. Cl-36 concentrations in Antarctic ice range between 2.5 to 8.7 x 10 to the 6th atoms Cl-36/kg, while those concentrations in samples collected at the Alan Hills ice field locations where meteorites have been brought to the surface by glacial flow and ablation are found to vary by more than a factor of three. This variation is attributed either to the effects of atmospheric mixing and scavenging or to radioactive decay in old ice. The Cl-36 concentration found in a present sample of rainwater is much lower than that reported in samples collected in the early 1960's, suggesting the occurrence of a decrease in the concentration of atmospheric Cl-36 derived from nuclear weapons tests over this time period.

Finkel, R. C.↗
NASA NTRS1983

Aircraft aerodynamic prediction method for V/STOL transition including flow separation

A numerical procedure was developed for the aerodynamic force and moment analysis of V/STOL aircraft operating in the transition regime between hover and conventional forward flight. The trajectories, cross sectional area variations, and mass entrainment rates of the jets are calculated by the Adler-Baron Jet-in-Crossflow Program. The inviscid effects of the interaction between the jets and airframe on the aerodynamic properties are determined by use of the MCAIR 3-D Subsonic properties are determined by use of the MCAIR 3-D Subsonic Potential Flow Program, a surface panel method. In addition, the MCAIR 3-D Geometry influence Coefficient Program is used to calculate a matrix of partial derivatives that represent the rate of change of the inviscid aerodynamic properties with respect to arbitrary changes in the effective wing shape.

Gilmer, B. R.↗
NASA NTRS1986

Distributions of (C-13)O emission in the disks of late-type spiral galaxies

In studies of molecular clouds in external galaxies, the distribution of (C-12)O emissivity as a function of radius in a galaxy has been employed to infer the surface density of molecular hydrogen with galactocentric radius. In principle, observations of more optically thin molecular species, such as (C-13)O, can be used as an alternative tracer of the mass of molecular hydrogen in a galaxy. Since the (C-13)O lines have a much lower opacity than the saturated, nearly thermalized (C-12)O lines, their intensity variations as a function of galactocentric radius are more readily interpreted in terms of gas density variations. The first strip maps of the (C-13)O distributions in six spiral galaxies are presented, and the (C-13)O and (C-12)O radial distributions are compared. Attention is given to the reliability of H2 masses and H2 distributions derived from (C-12)O data.

Young, J. S.↗
NASA NTRS1988

Multi-property modeling of ocean basin carbon fluxes

The objectives of this project were to elucidate the causal mechanisms in some of the most important features of the global ocean/atomsphere carbon system. These included the interaction of physical and biological processes in the seasonal cycle of surface water pCo2, and links between productivity, surface chlorophyll, and the carbon cycle that would aid global modeling efforts. In addition, several other areas of critical scientific interest involving links between the marine biosphere and the global carbon cycle were successfully pursued; specifically, a possible relation between phytoplankton emitted DMS and climate, and a relation between the location of calcium carbonate burial in the ocean and metamorphic source fluxes of CO2 to the atmosphere. Six published papers covering the following topics are summarized: (1) Mass extinctions, atmospheric sulphur and climatic warming at the K/T boundary; (2) Sensitivity of climate and atmospheric CO2 to deep-ocean and shallow-ocean carbonate burial; (3) Controls on CO2 sources and sinks in the earthscale surface ocean; (4) pre-anthropogenic, earthscale patterns of delta pCO2 between ocean and atmosphere; (5) Effect on atmospheric CO2 from seasonal variations in the high latitude ocean; and (6) Limitations or relating ocean surface chlorophyll to productivity.

Volk, Tyler↗
NASA NTRS1994

Atomic and molecular gas in interstellar cirrus clouds

The dust, atomic gas, and molecular gas content of a sample of 26 of isolated, degree-sized infrared clouds are compared. Half of the clouds have an infrared excess indicating the presence of H2, and 14 contain compact CO-emitting regions. Complete, high angular resolution H I and CO maps of one cloud, G236+39, resolve the transition between atomic and molecular H, as well as the location of CO formation. Assuming the infrared emission traces the total column density, H2 is inferred to be much more widely distributed than the CO. The CO rotational levels are subthermally excited, and the (2 - 1)/(1 - 0) line ratios suggest a density n(H2) approximately 200/cm(exp 3) where CO was detected. A model of H2 formation on grain surfaces balanced by self-shielded photodissociation fits the variation of infrared brightness with H I column density. Assuming a temperature of 80 K, typical of diffuse H2 (Savage et al. 1977), the H2 chemistry requires an average density n(H + 2H2) approximately 50/cm(exp 3). For G236+39, if the distance is 100 pc, the H I and H2 masses are estimated to be 90 and 70 solar mass, respectively. High-resolution H I and infrared maps of a smaller cloud, G249.0+73.7, reveal no evidence for molecular gas, which is likely due to the low total column density through this cloud. These results suggest the H2 and H I content are comparable for some interstellar cirrus clouds with column densities N(H I) greater than 4 x 10(exp 20)/cm(exp 2), even where CO was not detected.

Reach, William T.↗
NASA NTRS1994

Infrared observations of Comet Austin (1990 V) by the COBE/Diffuse Infrared Background Experiment

Comet Austin was observed by the Cosmic Background Explorer (COBE)/Diffuse Infrared Background Experiment (DIRBE) with broadband photometry at 1-240 micrometers during the comet's close passage by Earth in 1990 May. A 6 deg long (6 x 10(exp 6) km) dust tail was found at 12 and 25 micrometers, with detailed structure due to variations in particle properties and mass-loss rate. The spectrum of the central 42 x 42 sq arcmin pixel was found to agree with that of a graybody of temperature 309 +/- 5 K and optical depth 7.3 +/- 10(exp -8). Comparison with IUE and ground-based obervations indicates that particles of radius greater than 20 micrometers predominate by surface area. A mass-loss rate of 510 (+510/-205) kg/s and a total tail mass of 7 +/- 2 x 10(exp 10) kg was found for a model dust tail composed of Mie spheres with a differential particle mass distribution dn/d log m approx. m(exp -0.63) and 2:1 silicate:amorphous carbon composition by mass.

Lisse, C. M.↗