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At least 847 records · Page 47
All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

Flame stabilization in DME spray flames under engine-relevant conditions characterized by OH* chemiluminescence and formaldehyde laser-induced fluorescence

The transient and quasi-steady flame structures of Dimethyl Ether (DME) fuel sprays, produced by a single-hole injector (Spray D), were investigated using Planar Laser-Induced Fluorescence (PLIF) and chemiluminescence imaging in a constant-volume chamber under Engine Combustion Network (ECN) Spray A conditions (900 K ambient temperature, 60 bar ambient pressure, 1500 bar injection pressure, and 22.8 kg/m 3 ambient density). Low-temperature chemical reaction zones were visualized using formaldehyde (CH 2 O) PLIF with 355 nm excitation, while high-temperature flame regions were captured via chemiluminescence imaging of excited-state hydroxyl radicals (OH*). Both transient and quasi-steady flame structures clearly show the transition from CH 2 O to OH*, highlighting the progression from low- to high-temperature combustion, while the position of the flame is displaced for DME compared to reference hydrocarbon n-dodecane. Homogeneous reactor calculations with detailed chemistry and using adiabatic mixing for initial temperature show that CH 2 O peaks are significantly higher for DME at the same equivalence ratio, with a higher heat-release during the cool-flame regime with respect to the fuel heating value. Thus, the cool-flame dynamic as a precursor to high-temperature combustion and flame stabilization exhibit distinct behavior for DME relative to conventional hydrocarbons, and these phenomena are effectively resolved through the soot-free nature of DME and the high-speed, time-resolved diagnostics.

CH2O laser-induced fluorescence↗
NASA NTRS1987

An improved main reflector design for compact range applications

Compact ranges offer many advantages over other types of ranges, and as a result much effort is being directed toward the improvement of their performance. The use of concave edge contours and blended rolled edge terminations to reduce the unwanted energy diffracted into the target zone from the termination of the main reflector is discussed. The proposed shaping of the edge contour minimizes the diffracted fields by virtue of reducing the spread factor; whereas, the blended rolled edge terminations reduce the diffracted fields by creating a smooth transition in the reflected field. Two design examples are treated in order to illustrate these concepts.

Pistorius, Carl W. I.↗
NASA NTRS2000

Crack Turning in Integrally Stiffened Aircraft Structures

Current emphasis in the aircraft industry toward reducing manufacturing cost has created a renewed interest in integrally stiffened structures. Crack turning has been identified as an approach to improve the damage tolerance and fail-safety of this class of structures. A desired behavior is for skin cracks to turn before reaching a stiffener, instead of growing straight through. A crack in a pressurized fuselage encounters high T-stress as it nears the stiffener--a condition favorable to crack turning. Also, the tear resistance of aluminum alloys typically varies with crack orientation, a form of anisotropy that can influence the crack path. The present work addresses these issues with a study of crack turning in two-dimensions, including the effects of both T-stress and fracture anisotropy. Both effects are shown to have relation to the process zone size, an interaction that is central to this study. Following an introduction to the problem, the T-stress effect is studied for a slightly curved semi-infinite crack with a cohesive process zone, yielding a closed form expression for the future crack path in an infinite medium. For a given initial crack tip curvature and tensile T-stress, the crack path instability is found to increase with process zone size. Fracture orthotropy is treated using a simple function to interpolate between the two principal fracture resistance values in two-dimensions. An extension to three-dimensions interpolates between the six principal values of fracture resistance. Also discussed is the transition between mode I and mode II fracture in metals. For isotropic materials, there is evidence that the crack seeks out a direction of either local symmetry (pure mode I) or local asymmetry (pure mode II) growth. For orthotropic materials the favored states are not pure modal, and have mode mixity that is a function of crack orientation.

Pettit, Richard Glen↗
NASA NTRS2003

Volatiles Inventory to the Inner Planets Due to Small Bodies Migration

The concurrent processes of endogeneous and exogeneous origin are assumed to be responsible for the volatile reserves in the terrestrial planets. Volatiles inventory through collisions is rooted in orbital dynamics of small bodies including near-Earth objects (NEOs), short and long-period comets, and trans-Neptunian objects (TNOs), the latter probably supplying a large amount of Jupiter crossing objects (JCOs). Our model testifies that even a relatively small portion (approx. 0.001) of JCOs which transit to orbits with aphelia inside Jupiter's orbit (Q<4.7 AU) and reside such orbits during more than 1 Myr may contribute significantly in collisions with the terrestrial planets. The total mass of volatiles delivered to the Earth from the feeding zone of the giant planets could be greater than the mass of the Earth's oceans.

Marov, M. Y.↗
NASA NTRS2019

Imaging the Key Stages of Planet Formation

New images of young stars are revolutionizing our understanding of planet formation. ALMA detects large grains in planet‐forming disks with few AU scale resolution and scattered light imaging with extreme adaptive optics systems reveal small grains suspended on the disks’ flared surfaces. Tantalizing evidence for young exoplanets is emerging from line observations of CO and H‐alpha. In this white paper, we explore how even higher angular resolution can extend our understanding of the key stages of planet formation, to resolve accreting circumplanetary disks themselves, and to watch planets forming in situ for the nearest star‐forming regions. We focus on infrared observations which are sensitive to thermal emission in the terrestrial planet formation zone and allow access to molecular tracers in warm ro‐vibrational states. Successful planet formation theories will not only be able to explain the diverse features seen in disks, but will also be consistent with the rich exoplanet demographics from RV and transit surveys. While we are far from exhausting ground‐based techniques, the ultimate combination of high angular resolution and high infrared sensitivity can only be achieved through mid‐infrared space interferometry.

John Monnier↗
NASA NTRS2011

The Little Photometer That Could: Technical Challenges and Science Results from the Kepler Mission

The Kepler spacecraft launched on March 7, 2009, initiating NASA's first search for Earth-size planets orbiting Sun-like stars. Since launch, Kepler has announced the discovery of 17 exoplanets, including a system of six transiting a Sun-like star, Kepler-11, and the first confirmed rocky planet, Kepler-10b, with a radius of 1.4 that of Earth. Kepler is proving to be a cornucopia of discoveries: it has identified over 1200 candidate planets based on the first 120 days of observations, including 54 that are in or near the habitable zone of their stars, and 68 that are 1.2 Earth radii or smaller. An astounding 408 of these planetary candidates are found in 170 multiple systems, demonstrating the compactness and flatness of planetary systems composed of small planets. Never before has there been a photometer capable of reaching a precision near 20 ppm in 6.5 hours and capable of conducting nearly continuous and uninterrupted observations for months to years. In addition to exoplanets, Kepler is providing a wealth of astrophysics, and is revolutionizing the field of asteroseismology. Designing and building the Kepler photometer and the software systems that process and analyze the resulting data to make the discoveries presented a daunting set of challenges, including how to manage the large data volume. The challenges continue into flight operations, as the photometer is sensitive to its thermal environment, complicating the task of detecting 84 ppm drops in brightness corresponding to Earth-size planets transiting Sun-like stars.

Kepler Mission↗
NASA NTRS2012

Transiting Exoplanet Survey Satellite (TESS)

The Transiting Exoplanet Survey Satellite (TESS) will discover thousands of exoplanets in orbit around the brightest stars in the sky. In a two-year survey, TESS will monitor more than 500,000 stars for temporary drops in brightness caused by planetary transits. This first-ever spaceborne all-sky transit survey will identify planets ranging from Earth-sized to gas giants, around a wide range of stellar types and orbital distances. No ground-based survey can achieve this feat. A large fraction of TESS target stars will be 30-100 times brighter than those observed by Kepler satellite, and therefore TESS . planets will be far easier to characterize with follow-up observations. TESS will make it possible to study the masses, sizes, densities, orbits, and atmospheres of a large cohort of small planets, including a sample of rocky worlds in the habitable zones of their host stars. TESS will provide prime targets for observation with the James Webb Space Telescope (JWST), as well as other large ground-based and space-based telescopes of the future. TESS data will be released with minimal delay (no proprietary period), inviting immediate community-wide efforts to study the new planets. The TESS legacy will be a catalog of the very nearest and brightest main-sequence stars hosting transiting exoplanets, thus providing future observers with the most favorable targets for detailed investigations.

Ricker, G. R.↗
DOE OSTI2024

Phase transitions in HfO2 probed by first-principles computations

Ever since ferroelectricity was discovered in HfO2, the question of its origin remains controversial. Here, we probe this question using a combination of Landau theory of phase transitions and first-principles computations. In such an approach, the energy landscape associated with the phase transition between cubic and different experimentally demonstrated phases of HfO2 (tetragonal, monoclinic, orthorhombic Pbca, orthorhombic Pnma, and orthorhombic Pca21) is explored using density functional theory calculations. Computations revealed that stabilization of all but orthorhombic Pbca phase is driven by a single unstable zone-boundary antipolar mode X2−. When coupled with zone-center modes (Γ1+ and Γ3+), it stabilizes the tetragonal phase. Coupling with four additional modes (Γ5+, X3−, X5−, X5+) results in the monoclinic phase, which is the ground state of the material. If, however, Γ5+ mode is replaced with Γ4− mode, orthorhombic polar phase Pca21 is stabilized. The application of this framework to examine the effect of electric field on the ferroelectric phase of hafnia reveals that the field of 5 MV/cm is capable of stabilizing ferroelectric phase over the monoclinic one at 0 K.

Physics↗
NASA NTRS1982

The effect of water vapor on fatigue crack Growth in 7475-t651 aluminum alloy plate

The effects of water vapor on fatigue crack growth in 7475-T651 aluminum alloy plate at frequencies of 1 Hz and 10 Hz were investigated. Twenty-five mm thick compact specimens were subjected to constant amplitude fatigue testing at a load ratio of 0.2. Fatigue crack growth rates were calculated from effective crack lengths determined using a compliance method. Tests were conducted in hard vacuum and at water vapor partial pressures ranging from 94 Pa to 3.8 kPa. Fatigue crack growth rates were frequency insensitive under all environment conditions tested. For constant stress intensity factor ranges crack growth rate transitions occurred at low and high water vapor pressures. Crack growth rates at intermediate pressures were relatively constant and showed reasonable agreement with published data for two Al-Cu-Mg alloys. The existence of two crack growth rate transitions suggests either a change in rate controlling kinetics or a change in corrosion fatigue mechanism as a function of water vapor pressure. Reduced residual deformation and transverse cracking specimens tested in water vapor versus vacuum may be evidence of embrittlement within the plastic zone due to environmental interaction.

Dicus, D. L.↗
NASA NTRS1984

Effect of water vapor on fatigue crack growth in 7475-T651 aluminum alloy plate

The effects of water vapor on fatigue crack growth in 7475-T651 aluminum alloy plate at frequencies of 1 Hz and 10 Hz were investigated. Twenty-five mm thick compact specimens were subjected to constant amplitude fatigue testing at a load ratio of 0.2. Fatigue crack growth rates were calculated from effective crack lengths determined using a compliance method. Tests were conducted in hard vacuum and at water vapor partial pressures ranging from 94 Pa to 3.8 kPa. Fatigue crack growth rates were frequency insensitive under all environment conditions tested. For constant stress intensity factor ranges crack growth rate transitions occurred at low and high water vapor pressures. Crack growth rates at intermediate pressures were relatively constant and showed reasonable agreement with published data for two Al-Cu-Mg alloys. The existence of two crack growth rate transitions suggests either a change in rate controlling kinetics or a change in corrosion fatigue mechanism as a function of water vapor pressure. Reduced residual deformation and transverse cracking specimens tested in water vapor versus vacuum may be evidence of embrittlement within the plastic zone due to environmental interaction.

Dicus, D. L.↗
NASA NTRS2001

Kepler Mission: Detecting Earth-sized Planets in Habitable Zones

The Kepler Mission, which is presently in Phase A, is being proposed for launch in 5 years for a 4-year mission to determine the frequency of Earth-sized or larger planets in habitable zones in our galaxy. Kepler will be placed in an Earth-trailing orbit to provide stable physical environments for the sensitive scientific instruments. The satellite is equipped with a photometric system with the precision of 10E-5, which should be sufficient for detecting the transits of Earth-sized or larger planets in front of dwarf stars similar to the Sun. Approximately 100,000 or more sun-like stars brighter than the 14th apparently magnitude will be monitored continuously for 4 years in a preselected region of the sky, which is about 100 square degrees in size. In addition, Kepler will have a participating scientist program that will enable research in intrinsic variable stars, interacting binaries including cataclysmic stars and X-ray binaries, and a large number of solar analogs in our galaxy. Several ten thousand additional stars may be investigated in the guest observer program open to the whole world.

Kondo, Yoji↗
NASA NTRS2006

Molecular-dynamics Simulation-based Cohesive Zone Representation of Intergranular Fracture Processes in Aluminum

A traction-displacement relationship that may be embedded into a cohesive zone model for microscale problems of intergranular fracture is extracted from atomistic molecular-dynamics simulations. A molecular-dynamics model for crack propagation under steady-state conditions is developed to analyze intergranular fracture along a flat 99 [1 1 0] symmetric tilt grain boundary in aluminum. Under hydrostatic tensile load, the simulation reveals asymmetric crack propagation in the two opposite directions along the grain boundary. In one direction, the crack propagates in a brittle manner by cleavage with very little or no dislocation emission, and in the other direction, the propagation is ductile through the mechanism of deformation twinning. This behavior is consistent with the Rice criterion for cleavage vs. dislocation blunting transition at the crack tip. The preference for twinning to dislocation slip is in agreement with the predictions of the Tadmor and Hai criterion. A comparison with finite element calculations shows that while the stress field around the brittle crack tip follows the expected elastic solution for the given boundary conditions of the model, the stress field around the twinning crack tip has a strong plastic contribution. Through the definition of a Cohesive-Zone-Volume-Element an atomistic analog to a continuum cohesive zone model element - the results from the molecular-dynamics simulation are recast to obtain an average continuum traction-displacement relationship to represent cohesive zone interaction along a characteristic length of the grain boundary interface for the cases of ductile and brittle decohesion. Keywords: Crack-tip plasticity; Cohesive zone model; Grain boundary decohesion; Intergranular fracture; Molecular-dynamics simulation

Yamakov, Vesselin I.↗
DOE OSTI2025

Factors influencing water quality in surface water and alluvial groundwaters downgradient of a reclaimed surface coal mine in the Powder River Basin of southeastern Montana, USA

Abstract Coal mining and reclamation can have a profound influence on hydrogeologic systems, with clear consequences for groundwater quality, yet their long-term influence on downgradient water quality over time following reclamation is less well documented. Geochemical trends were evaluated in water quality downgradient of a fully reclaimed landscape at the former Big Sky Mine in the Rosebud Creek watershed (southeastern Montana, USA), over a 3-year period (2020–2022), including bond release in 2022. Within 6 km downgradient from the reclaimed area, sulfate concentrations decreased from approximately 3500 to 1800 mg l −1 within the Miller Coulee alluvial aquifer. Major ions, δ 34 S SO4 values, and residence time tracers suggest that the observed decreases in sulfate concentration result from a combination of dilution by mixed-age inflows and incomplete transit of the high salinity plume from the mine boundary. Both bedrock and alluvial aquifers of the Rosebud Creek corridor contained contributions of millennia-old regional groundwater, which may serve to mitigate mine-derived high salinity waters. Rosebud Creek, which traverses the outflow zone of Miller Coulee in the study area, exhibited high sulfate concentrations during low flows and consistent downgradient increases in sulfate concentration. The possibility of plume dynamics in Miller Coulee suggests that the greatest water quality impacts may not yet have reached Rosebud Creek.

Keeshin, Skye I. (ORCID:0009000986449872)↗
NASA NTRS1979

Effects of precursor heating on radiative and chemically reacting viscous flow around a Jovian entry body

The influence of change in the precursor region flow properties on the entire shock layer flow phenomena around a Jovian entry body was investigated. The flow in the shock layer was assumed to be steady, axisymmetric, and viscous. Both the chemical equilibrium and the nonequilibrium composition of the shock layer gas were considered. The effects of transitional range behavior were included in the analysis of high altitude entry conditions. Realistic thermophysical and radiation models were used, and results were obtained by employing the implicit finite difference technique in the shock layer and an iterative procedure for the entire shock layer precursor zone. Results obtained for a 45 degree angle hyperboloid blunt body entering Jupiter's atmosphere at zero angle of attack indicates that preheating the gas significantly increases the static pressure and temperature ahead of the shock for entry velocities exceeding 36 km/sec. The nonequilibrium radiative heating rate to the body is found to be significantly higher than the corresponding equilibrium heating. The precursor heating generally increases the radiative and convective heating of a body. That increase is slightly higher for the nonequilibrium conditions.

Tiwari, S. N.↗
NASA NTRS1993

Radar backscattering from snow facies of the Greenland ice sheet: Results from the AIRSAR 1991 campaign

In June 1991, the NASA/JPL airborne SAR (AIRSAR) acquired C- (lambda = 5.6cm), L- (lambda = 24cm), and P- (lambda = 68m) band polarimetric SAR data over the Greenland ice sheet. These data are processed using version 3.55 of the AIRSAR processor which provides radiometrically and polarimetrically calibrated images. The internal calibration of the AIRSAR data is cross-checked using the radar response from corner reflectors deployed prior to flight in one of the scenes. In addition, a quantitative assessment of the noise power level at various frequencies and polarizations is made in all the scenes. Synoptic SAR data corresponding to a swath width of about 12 by 50 km in length (compared to the standard 12 x 12 km size of high-resolution scenes) are also processed and calibrated to study transitions in radar backscatter as a function of snow facies at selected frequencies and polarizations. The snow facies on the Greenland ice sheet are traditionally categorized based on differences in melting regime during the summer months. The interior of Greenland corresponds to the dry snow zone where terrain elevation is the highest and no snow melt occurs. The lowest elevation boundary of the dry snow zone is known traditionally as the dry snow line. Beneath it is the percolation zone where melting occurs in the summer and water percolates through the snow freezing at depth to form massive ice lenses and ice pipes. At the downslope margin of this zone is the wet snow line. Below it, the wet snow zone corresponds to the lowest elevations where snow remains at the end of the summer. Ablation produces enough meltwater to create areas of snow saturated with water, together with ponds and lakes. The lowest altitude zone of ablation sees enough summer melt to remove all traces of seasonal snow accumulation, such that the surface comprises bare glacier ice.

Rignot, Eric↗
NASA NTRS2005

Eclipsing Binaries from the Kepler Mission

The Kepler Mission is a photometric space mission that will continuously observe a single 100 sq deg field of view (FOV) of greater than 100,000 stars in the Cygnus-Lyra region for 4 or more years with a precision of 14 ppm (R=12). The primary goal of the mission is to detect Earth-size planets in the habitable zone of solar-like stars. In the process, many eclipsing binaries (EB) will also be detected. Prior to launch, the stellar characteristics will have been detennined for all the stars in the FOV with R<16. As part of the verification process, stars with transits ~<5% will need to have follow-up radial velocity observations performed to determine the component masses and thereby separate transits caused by stellar companions from those caused by planets. The result will be a rich database on EBs. The community will have access to the archive for uses such as for EB modeling of the high-precision light curves. A guest observer program is also planned for objects not already on the target list.

Koch, David↗
NASA NTRS2016

Human Mars Landing Site and Impacts on Mars Surface Operations

This paper describes NASA's initial steps for identifying and evaluating candidate Exploration Zones (EZs) and Regions of Interests (ROIs) for the first human crews that will explore the surface of Mars. NASA's current effort to define the exploration of this planet by human crews, known as the Evolvable Mars Campaign (EMC), provides the context in which these EZs and ROIs are being considered. The EMC spans all aspects of a human Mars mission including launch from Earth, transit to and from Mars, and operations on the surface of Mars. An EZ is a collection of ROIs located within approximately 100 kilometers of a centralized landing site. ROIs are areas relevant for scientific investigation and/or development/maturation of capabilities and resources necessary for a sustainable human presence. The EZ also contains one or more landing sites and a habitation site that will be used by multiple human crews during missions to explore and utilize the ROIs within the EZ. With the EMC as a conceptual basis, the EZ model has been refined to a point where specific site selection criteria for scientific exploration and in situ resource utilization can be defined. In 2015 these criteria were distributed to the planetary sciences community and the in situ resource utilization and civil engineering communities as part of a call for EZ proposals. The resulting "First Landing Site/Exploration Zone Workshop for Human Missions to the Surface of Mars" was held in October 2015 during which 47 proposals for EZs and ROIs were presented and discussed. Proposed locations spanned all longitudes and all allowable latitudes (+/- 50 degrees). Proposed justification for selecting one of these EZs also spanned a significant portion of the scientific and resource criteria provided to the community. Several important findings resulted from this Workshop including: (a) a strong consensus that, at a scale of 100 km (radius), multiple places on Mars exist that have both sufficient scientific interest to sustain multiple crews of exploring astronauts, AND potential resource deposits for ISRU indicating the current EZ definition is viable and should be retained for now, (b) new data types (needed for more definitive analysis of EZs) argued strongly for a new orbiter mission, and possibly one or more surface missions, to obtain these data, (c) a general consensus that this Workshop was an excellent start to identifying a place where future human missions to Mars can productively explore this planet and learn to live and work there for the long term. Building on these findings, HEOMD and SMD are: (a) refining the EZ selection criteria and overall selection process to improve on lessons learned from the first EZ workshop, (b) using these proposed locations to develop "reference EZs" for assessment purposes (primarily engineering assessments), (c) gathering data and conducting analyses to better understanding the different potential sources for water, including the ease of extraction and purification, and (d) assessing trends in additional data that are needed to better characterize EZs proposed at the workshop and how these data needs impact the design and operation of future robotic Mars missions.

Hoffman, Stephen J.↗