SEARCH · Engineering Papers

Results for “gravity model”

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.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 595 records · Page 33

All sources NASA NTRS DOE OSTI

NASA NTRS1995

Lithospheric bending at subduction zones based on depth soundings and satellite gravity

A global study of trench flexure was performed by simultaneously modeling 117 bathymetric profiles (original depth soundings) and satellite-derived gravity profiles. A thin, elastic plate flexure model was fit to each bathymetry/gravity profile by minimization of the L(sub 1) norm. The six model parameters were regional depth, regional gravity, trench axis location, flexural wavelength, flexural amplitude, and lithospheric density. A regional tilt parameter was not required after correcting for age-related trend using a new high-resolution age map. Estimates of the density parameter confirm that most outer rises are uncompensated. We find that flexural wavelength is not an accurate estimate of plate thickness because of the high curvatures observed at a majority of trenches. As in previous studies, we find that the gravity data favor a longer-wavelength flexure than the bathymetry data. A joint topography-gravity modeling scheme and fit criteria are used to limit acceptable parameter values to models for which topography and gravity yield consistent results. Even after the elastic thicknesses are converted to mechanical thicknesses using the yield strength envelope model, residual scatter obscures the systematic increase of mechanical thickness with age; perhaps this reflects the combination of uncertainties inherent in estimating flexural wavelength, such as extreme inelastic bending and accumulated thermoelastic stress. The bending moment needed to support the trench and outer rise topography increases by a factor of 10 as lithospheric age increases from 20 to 150 Ma; this reflects the increase in saturation bending moment that the lithosphere can maintain. Using a stiff, dry-olivine rheology, we find that the lithosphere of the GDH1 thermal model (Stein and Stein, 1992) is too hot and thin to maintain the observed bending moments. Moreover, the regional depth seaward of the oldest trenches (approximately 150 Ma) exceeds the GDH1 model depths by about 400 m.

Levitt, Daniel A.↗

NASA NTRS2004

Satellite Geopotential Anomaly Constraints for the Crust of the Greenland-Iceland Region

Satellite magnetometer observations of the Greenland-Iceland region compare quite well with lower altitude data. The satellite magnetic data suggest magnetically enhanced crust was emplaced by the Iceland Plume. Crustal thicknesses, which may be more than 30 km for the Greenland-Scotland Ridge, were obtained from inversion of the compensating terrain gravity effects that were estimated by spectral correlation analysis of the free-air gravity anomalies and terrain gravity effects. Regional magnetic anomaly maxima overlie possible thickened crust from eastern Iceland to the Greenland Coast. The Iceland-Faroe Ridge may involve thinner crust than the Greenland-Iceland portion of the Greenland-Scotland Ridge. The gravity derived crustal model exceeds a 0.7 correlation with available seismic estimates. In thermally active areas our gravity Moho estimates are systematically deeper than the seismic estimates suggesting local density reductions of the underlying lower crust/upper mantle. In south central Greenland, on the other hand, the gravity Moho estimates are shallower than seismic estimates to suggest a local enhancement of the lower crust/upper mantle density. The dichotomous crust of the Greenland-Iceland and Iceland-Faroe Ridges suggests unequal crustal development by the Iceland Plume and the Mid-Atlantic Ridge, where more crustal material may have been contributed to the North Atlantic Plate than the Eurasian Plate. A new thermal modeling scheme based on Poisson's relation between point pole gravity and thermal potentials allows estimation of magnetic crustal thicknesses. Subsequent magnetic anomaly inversion for susceptibility contrasts infers crustal development of the Greenland-Scotland Ridge by temporally variable pulses in plume strength.

vonFrese, R. R.↗

NASA NTRS1976

Shield volcanism and lithospheric structure beneath the Tharsis plateau, Mars

The heights of four great shield volcanoes, when interpreted as reflecting the local hydrostatic head on a common source of upwelling magma, provide significant constraints on models of lithospheric structure beneath the Tharsis plateau. If Bouguer gravity anomalies are modeled in terms of a variable thickness crust, and a two-component (crust/mantle) earth-like structure is assumed for the Martian lithosphere, the derived model lithosphere beneath the Tharsis plateau has the following properties: (1) the upper low-density 'crustal' component is thickened beneath the Tharsis plateau; (2) the lower high-density 'mantle' component is thinned beneath the Tharsis plateau; and (3) there is a net gradient on the base of the Martian lithosphere directed downward away from beneath the summit of the Tharsis plateau. A long history of magmatic intrusion is hypothesized to have been the cause of the updoming of the Tharsis plateau and the maintenance of the plateau in a state of only partial compensation.

Blasius, K. R.↗

NASA NTRS1991

Regolith-structure modeling

Early lunar missions have provided a basic understanding of the physical and strength properties of lunar regolith, which have been shown to differ from those of dry terrestrial granular soils. Lunar regolith is predominantly a fine sand of which nearly 40 percent can be characterized as silt with a particle size smaller than 100 micrometers. The top 10 to 20 cm of the regolith can be characterized as being in a loose to medium-loose state. The density of the regolith, however, rapidly increases below a depth of 20 cm. The highly irregular and angular shapes of the regolith particles tend to interlock and create relatively strong mechanical bonds that give the particulate mass substantial cohesive properties and smaller amounts of tensile strength properties. In addition, the friction angle of lunar regolith at medium to high densities is quite high and often exceeds 55 degrees. These known properties of lunar regolith have been matched in a terrestrially-manufactured analog known as Minnesota Lunar Simulant. A variety of experiments were conducted using this simulant to both verify existing information and generate new information on the physical and constitutive properties of lunar regolith. These experiments include maximum and minimum density determinations, specific mass of solids, grain-size distribution, conventional triaxial compression and extension, isotropic compression, one-dimensional compression, direct shear, and direct tension. Direct shear experiments were conducted under atmospheric and vacuum conditions. Results of the physical and strength experiments compare closely to results obtained from lunar missions. Results of simulant strength experiments performed in vacuum indicated no observable difference from results obtained in air. A test bed currently under study is one involving a regolith shield covering a first-generation human habitat module. It is understood that regolith in depths ranging from 3 to 5 meters is required for radiation shielding for habitation and workspace. The habitat module is treated as a rigid cylindrical tube with a smooth exterior. By making the cylinder rigid, a complex interaction problem is reduced to a situation where we can consider the support regolith and the shielding regolith as behaving independently of the structural properties of the cylindrical structure. Medium-dense lunar simulant was placed around a scaled model of the habitat module to provide a radiation shield. This embankment-type shield was constructed in relatively thin but fine layers by compacting, by mechanical vibratory means, layer upon layer of simulant placed adjacent to the horizontally-aligned cylinder. The slope angles were constructed at 55 degrees. The model described above was studied in a geotechnical centrifuge, which allows for the scaling of model dimensions to prototype dimensions by increasing the acceleration of gravity on the model. The deformation response can be scaled up to prototype dimensions to provide an assessment of the deformation patterns of the lunar structure. The actual process of local and/or global growth of instabilities or skip planes can also be observed.

Ko, Hon-Yim↗

NASA NTRS1993

Ishtar deformed belts: Evidence for deformation from below?

The mountain belts of Ishtar Terra are unique on Venus. Models for their formation include mantle upwelling, mantle downwelling, and horizontal convergence. The present forms of these models are too simple to predict surface strain, topography, or gravity. More detailed models will require specific constraints as imposed by geologic relations. In order to develop specific constraints for geodynamic models, we examine the geology of Ishtar Terra as viewed in Magellan SAR imagery in an attempt to interpret regional surface strain patterns. In this paper, we present geologic and structural relations that leads us to postulate that Ishtar deformed belts result from shear forces within the mantle acting on the lithosphere, and not by horizontal forces from colliding plates. We propose that the surface strains result from differential strain and displacement of domains within the upper mantle, and that further analysis of Ishtar deformation may allow us to identify individual domains within the mantle, and to constrain displacement trajectories between domains.

Hansen, V. L.↗

NASA NTRS2000

A Study of Bubble and Slug Gas-Liquid Flow in a Microgravity Environment

The influence of gravity on the two-phase flow dynamics is obvious.As the gravity level is reduced,there is a new balance between inertial and interfacial forces, altering the behavior of the flow. In bubbly flow,the absence of drift velocity leads to spherical-shaped bubbles with a rectilinear trajectory.Slug flow is a succession of long bubbles and liquid slug carrying a few bubbles. There is no flow reversal in the thin liquid film as the long bubble and liquid slug pass over the film. Although the flow structure seems to be simpler than in normal gravity conditions,the models developed for the prediction of flow behavior in normal gravity and extended to reduced gravity flow are unable to predict the flow behavior correctly.An additional benefit of conducting studies in microgravity flows is that these studies aide the development of understanding for normal gravity flow behavior by removing the effects of buoyancy on the shape of the interface and density driven shear flows between the gas and the liquid phases. The proposal calls to study specifically the following: 1) The dynamics of isolated bubbles in microgravity liquid flows will be analyzed: Both the dynamics of spherical isolated bubbles and their dispersion by turbulence, their interaction with the pipe wall,the behavior of the bubbles in accelerated or decelerated flows,and the dynamics of isolated cylindrical bubbles, their deformation in accelerated/decelerated flows (in converging or diverging channels), and bubble/bubble interaction. Experiments will consist of the use of Particle Image Velocimetry (PIV) and Laser Doppler Velocimeters (LDV) to study single spherical bubble and single and two cylindrical bubble behavior with respect to their influence on the turbulence of the surrounding liquid and on the wall 2) The dynamics of bubbly and slug flow in microgravity will be analyzed especially for the role of the coalescence in the transition from bubbly to slug flow (effect of fluid properties and surfactant), to identify clusters that promote coalescence and transition the void fraction distribution in bubbly and slug flow,to measure the wall friction in bubbly flow. These experiments will consist of multiple bubbles type flows and will utilize hot wire and film anemometers to measure liquid velocity and wall shear stress respectively and double fiber optic probes to measure bubble size and velocity as a function of tube radius and axial location.

McQuillen, J.↗

NASA NTRS2013

Large-Scale Liquid Hydrogen Tank Rapid Chill and Fill Testing for the Advanced Shuttle Upper Stage Concept

Cryogenic upper stages in the Space Shuttle program were prohibited primarily due to a safety risk of a 'return to launch site' abort. An upper stage concept addressed this concern by proposing that the stage be launched empty and filled using shuttle external tank residuals after the atmospheric pressure could no longer sustain an explosion. However, only about 5 minutes was allowed for tank fill. Liquid hydrogen testing was conducted within a near-ambient environment using the multipurpose hydrogen test bed 638.5 ft3 (18m3) cylindrical tank with a spray bar mounted longitudinally inside. Although the tank was filled within 5 minutes, chilldown of the tank structure was incomplete, and excessive tank pressures occurred upon vent valve closure. Elevated tank wall temperatures below the liquid level were clearly characteristic of film boiling. The test results have substantial implications for on-orbit cryogen transfer since the formation of a vapor film would be much less inhibited due to the reduced gravity. However, the heavy tank walls could become an asset in normal gravity testing for on-orbit transfer, i.e., if film boiling in a nonflight weight tank can be inhibited in normal gravity, then analytical modeling anchored with the data could be applied to reduced gravity environments with increased confidence.

Flachbart, R. H.↗

NASA NTRS1992

Experimental results and numerical modeling of solidification during aircraft high-g arcs

The question of how the Coriolis and gravity gradient forces during high-gravity maneuvers compare to that for common centrifuges is addressed. Microstructural examination of samples solidified during high-gravity arcs reveals no evidence of convection dampening. As a first approximation, the high-gravity arc is modeled as a centrifuge with a radius of 20,480 ft and angular speed of 0.318 rpm. Scaling analysis indicates that the Coriolis and gravity gradient expected on the aircraft high gravity arc are less than that for the centrifuges by a factor of 100. Detailed Navier-Stokes analysis of the fluid flow and thermal fields during solidification of aluminum and Cd-Te during KC-135 high gravity show that convective flows of about 1 mm/s are induced. The thermal field is only slightly modified by the convection. Coriolis and gravity gradient during solidification in KC-135 high-gravity arcs, even at accelerations that have been shown to produce significant convective flow dampening in the centrifuge systems, are found to have no significant influence on the melt thermal and flowfields.

Curreri, P. A.↗

NASA NTRS2014

The QBO in Two GISS Global Climate Models: 1. Generation of the QBO

The adjustment of parameterized gravity waves associated with model convection and finer vertical resolution has made possible the generation of the quasi-biennial oscillation (QBO) in two Goddard Institute for Space Studies (GISS) models, GISS Middle Atmosphere Global Climate Model III and a climate/middle atmosphere version of Model E2. Both extend from the surface to 0.002 hPa, with 2deg × 2.5deg resolution and 102 layers. Many realistic features of the QBO are simulated, including magnitude and variability of its period and amplitude. The period itself is affected by the magnitude of parameterized convective gravity wave momentum fluxes and interactive ozone (which also affects the QBO amplitude and variability), among other forcings. Although varying sea surface temperatures affect the parameterized momentum fluxes, neither aspect is responsible for the modeled variation in QBO period. Both the parameterized and resolved waves act to produce the respective easterly and westerly wind descent, although their effect is offset in altitude at each level. The modeled and observed QBO influences on tracers in the stratosphere, such as ozone, methane, and water vapor are also discussed. Due to the link between the gravity wave parameterization and the models' convection, and the dependence on the ozone field, the models may also be used to investigate how the QBO may vary with climate change.

middle atmosphere dynamics↗

NASA NTRS1993

Flexure and the role of inplane force around coronae on Venus

Large coronae on Venus, such as Artemis and Latona, are rimmed by conspicuous trenches and associated outer rises. Sandwell and Schubert have observed that these systems resemble terrestrial subduction zones in planform and have succeeded in fitting an elastic plate bending equation to the inferred flexural topography. However, the first zero crossing bending moments required are -2.5 x 10(exp 17) N for Artemis and -5.0 x 10(exp 16) N for Latona. Since these moments are similar in magnitude to those of subducting slabs on Earth, a rollback subduction mechanism was proposed to explain the flexure around the largest coronae, although a differential thermal subsidence model is sufficient to account for the topography around some coronae. The purpose is to investigate the effect of inplane force as a possible alternative to large applied moments in producing flexure at Artemis and Latona. The close correlation of gravity to topography on Venus implies the absence of a low viscosity zone and the strong coupling of the lithosphere to mantle convection. If coronae are the surface manifestations of mantle plumes, they may be the sites of active convective stress coupling. As the upwelling reaches the lithosphere, it spreads radially outward, inducing shear tractions on the base of the plate. In addition, the hot, expanding corona may load the surrounding plate horizonally. Both the basal shear stresses and radial loading can be treated as an equivalent compressive inplane force in the mechanical lithosphere, which contributes to the bending of the outlying plate. Using a model that relates inplane force to the measured gravity anomalies, a rough value of the inplane force at Artemis was calculated. Recent Pioneer Venus spherical harmonic gravity models indicate a geoid anomaly of about 75 m over Artemis, which corresponds to an estimated inplane force on the order of -1x10(exp 13) N/m. The gravity model is unable to resolve Latona, but an inplane force of similar dimensions is assumed. The maximum possible inplane force based on the expected rheology can be constrained by using the approximate 5 K/km thermal gradient inferred from the best fit 30 km elastic plate at Artemis and Latona. For a dry olivine flow law in the upper mantle, the compressional load limit of the 60 km thick mechanical lithosphere is -4 x 10(exp 13) N/m. This value is equivalent to a load of -8 x 10(exp 13) N/m on a 30 km thick elastic plate.

Brown, C. David↗

NASA NTRS1984

Tethers and asteroids for artificial gravity assist in the solar system

Analytical models are defined for gravity-assist trajectory changes for spacecraft passing massive compact bodies. The models are applied in an examination of the benefits of lowering a tether to an asteroid during a flyby in order to gain a trajectory change equivalent to that from a massive body (planet). Direct flybys yield velocity gains while retrograde flybys shed velocity. The magnitude of the effects are a function of the proximity to the body during flyby. This inherently limits the gravity assist technique used around planets, which usually have atmospheres and can have intense radiation fields. If a spacecraft could extend a tether (such as to be tested on the Orbiter) to snag on an asteroid surface, the potential trajectory/velocity change of the spacecraft would be limited mainly by the tether strength. The encounter physics are treated as a soft collision. Possible applications of the asteroid tether technique are outer planet, Mars and main belt asteroid exploration missions.

Penzo, P. A.↗

NASA NTRS1984

Optical analysis of the star-tracker telescope for Gravity Probe

A ray tracing modeling of the star tracker telescope for Gravity Probe was used to predict the character of the output signal and its sensitivity to fabrication errors. In particular, the impact of the optical subsystem on the requirement of 1 milliarc second signal linearity over a + or - 50 milliarc second range was examined. Photomultiplier and solid state detector options were considered. Recommendations are made.

Zissa, D. E.↗

NASA NTRS1985

Venus gravity - A harmonic analysis and geophysical implications

An improved theoretical model of Venusian global gravity has been obtained by fitting a tenth degree spherical harmonic series to 78 orbital arcs of Doppler tracking data from the Pioneer Venus Orbiter. Maps of the free-air anomaly and its formal error are presented. Isostatic anomaly and 'Geoid' maps are also presented, and their geophysical implications are discussed in details. Comparison with equivalent resolution topographic models reveals a strong correlation between long wavelength gravity and the topography of Venus. Analysis of the second degree harmonics showed two aspects of the orientation of the inertial axes of Venus: (1) a significant (about three degrees) departure of the axis of greatest inertia from the rotational axis; and (2) a near alignment of the axis of least inertia with the location of the subterrestrial point at the time of the next inferior conjunction with earth (December 16, 2101). A series of contour maps of the Venusian free-air anomalies is provided.

Mottinger, N. A.↗

NASA NTRS1996

Modeling of Thermal Performance of Multiphase Nuclear Fuel Cell Under Variable Gravity Conditions

A unique numerical method has been developed to model the dynamic processes of bulk evaporation and condensation processes, associated with internal heat generation and natural convection under different gravity levels. The internal energy formulation, for the bulk liquid-vapor phase change problems in an encapsulated container, was employed. The equations, governing the conservation of mass, momentum and energy for both phases involved in phase change, were solved. The thermal performance of a multiphase uranium tetra-fluoride fuel element under zero gravity, micro-gravity and normal gravity conditions has been investigated. The modeling yielded results including the evolution of the bulk liquid-vapor phase change process, the evolution of the liquid-vapor interface, the formation and development of the liquid film covering the side wall surface, the temperature distribution and the convection flow field in the fuel element. The strong dependence of the thermal performance of such multiphase nuclear fuel cell on the gravity condition has been revealed. Under all three gravity conditions, 0-g, 10(exp -3)-g, and 1-g, the liquid film is formed and covers the entire side wall. The liquid film covering the side wall is more isothermalized at the wall surface, which can prevent the side wall from being over-heated. As the gravity increases, the liquid film is thinner, the temperature gradient is larger across the liquid film and smaller across the vapor phase. This investigation provides valuable information about the thermal performance of multi-phase nuclear fuel element for the potential space and ground applications.

Ding, Z.↗

NASA NTRS2013

Global Propagation of Gravity Waves Generated with the Whole Atmosphere Transfer Function Model

A brief review is presented of the Transfer Function Model (TFM) [e.g., Mayr et al., Space Science Reviews, 1990], which describes acoustic gravity waves (AGW) that propagate across the globe in a dissipative and static (no winds) background atmosphere with globally uniform temperature and density variations extending from the ground to 700 km. Unique among existing models, the TFM can be placed between the analytical approach on one end, and the rigorous numerical approach of general circulation models (GCM). The time consuming numerical integration of the conservation equations is restricted to compute the transfer function (TF) for a broad range of frequencies and spherical harmonics. Given TF, the atmospheric response for a chosen source configuration is then obtained in short order. Computationally efficient, the model is well suited to serve as experimental and educational tool for simulating propagating wave patterns across the globe. By design, the TFM is also semi-analytical and therefore well suited to explore the different wave modes that can be generated under different dynamical conditions.

Transfer Function Model↗

NASA NTRS2004

Architecting a Simulation Framework for Model Rehosting

The utility of vehicle math models extends beyond human-in-the-loop simulation. It is desirable to deploy a given model across a multitude of applications that target design, analysis, and research. However, the vehicle model alone represents an incomplete simulation. One must also replicate the environment models (e.g., atmosphere, gravity, terrain) to achieve identical vehicle behavior across all applications. Environment models are increasing in complexity and represent a substantial investment to re-engineer for a new application. A software component that can be rehosted in each application is one solution to the deployment problem. The component must encapsulate both the vehicle and environment models. The component must have a well-defined interface that abstracts the bulk of the logic to operate the models. This paper examines the characteristics of a rehostable modeling component from the perspective of a human-in-the-loop simulation framework. The Langley Standard Real-Time Simulation in C++ (LaSRS++) is used as an example. LaSRS++ was recently redesigned to transform its modeling package into a rehostable component.

Madden, Michael M.↗

NASA NTRS1992

Gravity field covariance analysis for the TOPEX/Poseidon mission

The TOPEX/Poseidon satellite oceanography mission will require very accurate orbit determination in order to fulfill its mission requirements of altimetrically mapping the ocean surface with approximately 10 centimeter accuracy. To meet such stringent orbit determination specifications will require very accurate tracking data and very accurate dynamical models of the satellite motion. The accuracy of the TOPEX/Poseidon orbit is expected to be driven by the accuracy of the earth's gravity field model. Expected orbit accuracy for several recent gravity models is presented. Both the capability of the models for modeling the motion of TOPEX/Poseidon and their global modeling characteristics are discussed. In addition, the gravity model improvement that can be expected by utilizing GPS tracking of TOPEX/Poseidon is evaluated. This evalution is based on a recent simulation of a gravity field recovery using 10 days of TOPEX/Poseidon GPS tracking.

Rosborough, G. W.↗

NASA NTRS2012

Multi-Scale Modeling of Liquid Phase Sintering Affected by Gravity: Preliminary Analysis

A multi-scale simulation concept taking into account impact of gravity on liquid phase sintering is described. The gravity influence can be included at both the micro- and macro-scales. At the micro-scale, the diffusion mass-transport is directionally modified in the framework of kinetic Monte-Carlo simulations to include the impact of gravity. The micro-scale simulations can provide the values of the constitutive parameters for macroscopic sintering simulations. At the macro-scale, we are attempting to embed a continuum model of sintering into a finite-element framework that includes the gravity forces and substrate friction. If successful, the finite elements analysis will enable predictions relevant to space-based processing, including size and shape and property predictions. Model experiments are underway to support the models via extraction of viscosity moduli versus composition, particle size, heating rate, temperature and time.

Olevsky, Eugene↗