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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.

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NASA NTRS2002

Coastal Research Imaging Spectrometer

The Coastal Research Imaging Spectrometer (CRIS) is an airborne remote-sensing system designed specifically for research on the physical, chemical, and biological characteristics of coastal waters. The CRIS includes a visible-light hyperspectral imaging subsystem for measuring the color of water, which contains information on the biota, sediment, and nutrient contents of the water. The CRIS also includes an infrared imaging subsystem, which provides information on the temperature of the water. The combination of measurements enables investigation of biological effects of both natural and artificial flows of water from land into the ocean, including diffuse and point-source flows that may contain biological and/or chemical pollutants. Temperature is an important element of such measurements because temperature contrasts can often be used to distinguish among flows from different sources: for example, a sewage outflow could manifest itself in spectral images as a local high-temperature anomaly.anomaly. Both the visible and infrared subsystems scan in "pushbroom" mode: that is, an aircraft carrying the system moves along a ground track, the system is aimed downward, and image data are acquired in acrosstrack linear arrays of pixels. Both subsystems operate at a frame rate of 30 Hz. The infrared and visible-light optics are adjusted so that both subsystems are aimed at the same moving swath, which has across-track angular width of 15. Data from the infrared and visible imaging subsystems are stored in the same file along with aircraft-position data acquired by a Global Positioning System receiver. The combination of the three sets of data is used to construct infrared and hyperspectral maps of scanned areas shown.

Source record↗
DOE OSTI2022

The MeerKAT Galaxy Cluster Legacy Survey I. Survey Overview and Highlights

MeerKAT’s large number (64) of 13.5 m diameter antennas, spanning 8 km with a densely packed 1 km core, create a powerful instrument for wide-area surveys, with high sensitivity over a wide range of angular scales. The MeerKAT Galaxy Cluster Legacy Survey (MGCLS) is a programme of long-track MeerKAT L-band (900–1670 MHz) observations of 115 galaxy clusters, observed for ~6–10 h each in full polarisation. The first legacy product data release (DR1), made available with this paper, includes the MeerKAT visibilities, basic image cubes at ~8" resolution, and enhanced spectral and polarisation image cubes at ~8" and 15" resolutions. Typical sensitivities for the full-resolution MGCLS image products range from ~3–5 μJy beam –1 . The basic cubes are full-field and span 2° × 2°. The enhanced products consist of the inner 1.2° × 1.2° field of view, corrected for the primary beam. The survey is fully sensitive to structures up to ~10' scales, and the wide bandwidth allows spectral and Faraday rotation mapping. Relatively narrow frequency channels (209 kHz) are also used to provide H I mapping in windows of 0 < z < 0.09 and 0.19 < z < 0.48. Here, in this paper, we provide an overview of the survey and the DR1 products, including caveats for usage. We present some initial results from the survey, both for their intrinsic scientific value and to highlight the capabilities for further exploration with these data. These include a primary-beam-corrected compact source catalogue of ~626 000 sources for the full survey and an optical and infrared cross-matched catalogue for compact sources in the primary-beam-corrected areas of Abell 209 and Abell S295. We examine dust unbiased star-formation rates as a function of cluster-centric radius in Abell 209, extending out to 3.5 R 200 . We find no dependence of the star-formation rate on distance from the cluster centre, and we observe a small excess of the radio-to-100 μm flux ratio towards the centre of Abell 209 that may reflect a ram pressure enhancement in the denser environment. We detect diffuse cluster radio emission in 62 of the surveyed systems and present a catalogue of the 99 diffuse cluster emission structures, of which 56 are new. These include mini-halos, halos, relics, and other diffuse structures for which no suitable characterisation currently exists. We highlight some of the radio galaxies that challenge current paradigms, such as trident-shaped structures, jets that remain well collimated far beyond their bending radius, and filamentary features linked to radio galaxies that likely illuminate magnetic flux tubes in the intracluster medium. We also present early results from the H I analysis of four clusters, which show a wide variety of H I mass distributions that reflect both sensitivity and intrinsic cluster effects, and the serendipitous discovery of a group in the foreground of Abell 3365.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2004

Coastal Research Imaging Spectrometer

The Coastal Research Imaging Spectrometer (CRIS) is an airborne remote sensing system designed specifically for research on the physical, chemical, and biological characteristics of coastal waters. The CRIS includes a visible-light hyperspectral imaging subsystem for measuring the color of water, which contains information on the biota, sediment, and nutrient contents of the water. The CRIS also includes an infrared imaging subsystem, which provides information on the temperature of the water. The combination of measurements enables investigation of biological effects of both natural and artificial flows of water from land into the ocean, including diffuse and point-source flows that may contain biological and/or chemical pollutants. Temperature is an important element of such measurements because temperature contrasts can often be used to distinguish among flows from different sources: for example, a sewage outflow could manifest itself in spectral images as a local high-temperature anomaly. Both the visible and infrared subsystems scan in pushbroom mode: that is, an aircraft carrying the system moves along a ground track, the system is aimed downward, and image data are acquired in across-track linear arrays of pixels. Both subsystems operate at a frame rate of 30 Hz. The infrared and visible-light optics are adjusted so that both subsystems are aimed at the same moving swath, which has across-track angular width of 15 . Data from the infrared and visible imaging subsystems are stored in the same file along with aircraft- position data acquired by a Global Positioning System receiver. The combination of the three sets of data is used to construct infrared and hyperspectral maps of scanned areas (see figure). The visible subsystem is based on a grating spectrograph and a rapid-readout charge-coupled-device camera. Images of the swatch are acquired in 256 spectral bands at wavelengths from 400 to 800 nm. The infrared subsystem, which is sensitive in a single wavelength band of 8 to 10 m, is based on a focal-plane array of HgCdTe photodetectors that are cooled to an operating temperature of 77 K by use of a closed-Stirling-cycle mechanical cooler. The nonuniformities of the HgCdTe photodetector array are small enough that the raw pixel data from the infrared subsystem can be used to recognize temperature differences on the order of 1 C. By use of a built-in blackbody calibration source that can be switched into the field of view, one can obtain bias and gain offset terms for individual pixels, making it possible to offset the effects of nonuniformities sufficiently to enable the measurement of temperature differences as small as 0.1 C.

Lucey, Paul G.↗
NASA NTRS2021

Microstructural Observations on Advanced Multilayer Protective Coatings for Nuclear Thermal Propulsion

The present research discusses the microstructures of a new multilayered coating concept proposed to solve the “midrange corrosion” problem observed in NbC and ZrC-coated graphite (Gr) nuclear fuels during the NERVA/Rover programs. The concept envisions designing a compliant multilayered coatings architecture to accommodate the thermal expansion mismatch strains between the ZrC outer coating and the Gr/(U,Zr)C fuel matrix along with using a diffusion barrier to minimize carbon and hydrogen diffusion, respectively. Multilayered coatings were deposited on Gr substrates by chemical vapor deposition (CVD). Graphite disks, as well as the inner channels of a 19-hole hexagonal rod 152.4 mm long, were coated by CVD in proof-of concept process demonstration studies. Coated disk specimens were thermally-cycled between ambient and 1900 K for as long as 20 cycles with a 1h hold time at the temperature using both stepped and non-interrupted thermal cycling methods. Detailed cross-sectional microstructural analyses were conducted on the thermally-cycled coated disks. These results showed that the coatings were intact after 20 thermal cycles at 1900 K although the bonding of the overlay Mo coating appeared to be influenced by the oxygen content in the layer. Seven transverse sections were cut along the length from the coated hexagonal rod for microstructural studies. All the nineteen channels were observed to have been coated in all the sections. This paper reports the results of detailed microstructural analyses, compositional line scans and x-ray dot maps conducted on three coated channels from a mid-section of the rod. The measured values of carbon (C) were observed to be exceptionally high in all the specimens, which were attributed to hydrocarbon contamination of the specimen surfaces. It was observed that the oxygen (O) levels were high in all the coating layers even after approximately correcting for the observed high values of C. The source of O contamination of the CVD reactor system was attributed to either the presence of oxychloride impurities in the precursor salts or a pin hole leak in the coating system. It is concluded that high values of O result in poor bonding of the Mo overlay to the ZrC layer. These trials have established that it is possible to deposit multilayered coatings by CVD in 19-hole hexagonal rods 152.4 mm long similar to the NERVA fuel rod design although the quality of the coatings and the uniformity of their thicknesses require further improvement.

Nuclear thermal propulsion, multilayer coatings, m↗
NASA NTRS2006

Radar Probing of Planetary Regoliths: An Example from the Northern Rim of Imbrium Basin

Imaging radar measurements at long wavelengths (e.g., >30 cm) allow deep (up to tens of meters) probing of the physical structure and dielectric properties of planetary regoliths. We illustrate a potential application for a Mars orbital synthetic aperture radar (SAR) using new Earth-based 70-cm wavelength radar data for the Moon. The terrae on the northern margin of Mare Imbrium, the Montes Jura region, have diffuse radar backscatter echoes that are 2-4 times weaker at 3.8-cm, 70-cm, and 7.5-m wavelengths than most other lunar nearside terrae. Possible geologic explanations are (1) a pyroclastic deposit associated with sinuous rilles in this region, (2) buried mare basalt or a zone of mixed highland/basaltic debris (cryptomaria), or (3) layers of ejecta associated with the Iridum and Plato impacts that have fewer meter-sized rocks than typical highlands regolith. While radar data at 3.8-cm to 7.5-m wavelengths suggest significant differences between the Montes Jura region and typical highlands, the surface geochemistry and rock abundance inferred from Clementine UV-VIS data and eclipse thermal images are consistent with other lunar terrae. There is no evidence for enhanced iron abundance, expected for basaltic pyroclastic deposits, near the source vents of the sinuous rilles radial to Plato. The regions of low 70-cm radar return are consistent with overlapping concentric ''haloes'' about Iridum and Plato and do not occur referentially in topographically low areas, as is observed for radar-mapped cryptomaria. Thus we suggest that the extensive radar-dark area associated with the Montes Jura region is due to overlapping, rock-poor ejecta deposits from Iridum and Plato craters. Comparison of the radial extent of low-radar-return crater haloes with a model for ejecta thickness shows that these rock-poor layers are detected by 70-cm radar where they are on the order of 10 m and thicker. A SAR in orbit about Mars could use similar deep probing to reveal the nature of crater - and basin-related deposits.

Iridum crater↗
NASA NTRS2005

Lunar and Planetary Science XXXVI, Part 20

The topics include: 1) Virtual Reality Technology as a Tool to Enhance Collaboration Between Space Exploration and Public Outreach: The Case Using the Mars Exploration Rover Images; 2) Atmospheric Electron-induced X-Ray Spectrometer (AEXS) Instrument Development; 3) Impact of Low Thermal Conductivity Layers on the Bulk Conductivity of a Martian Crustal Column; 4) Impacting Classroom Teachers Through Long-Term Professional Development; 5) Oxygen, Ca, and Ti Isotopic Compositions of Hibonite-bearing Inclusions; 6) Phenomenological Excitation Functions of Xe Isotopes with Protons on Nuclei of Cs, La and Ce; 7) Double-Diffusive Convection and Other Modes of Salinity-modulated Heat and Material Transport in Europa s Ocean; 8) Slope Morphologies of the Hellas Mensae Constructs, Eastern Hellas Planitia, Mars; 9) Development of Polygonal Thermal Contraction Patterns in a South Polar Trough, Mars 3 Years of Observations; 10) Martian Relevance of Dehydration and Rehydration in the Mg-Sulfate System; 11) Formation of Martian Volcanic Provinces by Lower Mantle Flushing? 12) Can Glasses Help Us to Unravel the Origin of Barred Olivine Chondrules? 13) Loki Patera: A Magma Sea Story; 14) Compositions of Partly Altered Olivine and Replacement Serpentine in the CM2 Chondrite QUE93005; 15) Model of Light Scattering by Lunar Regolith at Moderate Phase Angles: New Results; 16) Radiation Resistance of a Silicone Polymer Grease Based Regolith Collector for the HERA Near-Earth Asteroid Sample Return Mission; 17) Analysis of the Tectonic Lineaments in the Ganiki Planitia (V14) Quadrangle, Venus; 18) Nanometer-sized Diamonds from AGB Stars; 19) Quantifying Exact Motions Along Lineaments on Europa; 20) Geometry of Thrust Faults Beneath Amenthes Rupes, Mars; 21) Mapping of the Physical Characteristics and Mineral Composition of a Superficial Layer of the Moon or Mars and Ultra-Violet Polarimetry from the Orbital Station; 22) Negative Searches for Evidence of Aqueous Alteration on Asteroid Surfaces; 23) What Processes Have Shaped Basalt Boulders on Earth and Mars? Studies of Feature Persistence Using Facet Mapping and Fractal Analysis; 24) The Popigai Fluidizites: Dense Water Inclusions in Lechatelierite; Evidence for Shock-generated Carbonate and Hydrous Silicate Melts; 25) Missing Xenon Problem and Climate of the Early Earth; and 26) More on Magnetic Spectra from Correlated Crustal Sources on Mars

Source record↗
NASA NTRS2000

A Hot-Electron Far-Infrared Direct Detector

A new approach is proposed to improve the sensitivity of direct-detection bolometers at millimeter, submillimeter and far-infrared wavelengths. The idea is to adjust a speed of the thermal relaxation of hot-electrons in a nanometer size normal metal or super-conductive transition edge bolometer by controlling the elastic electron mean free path. If the bolometer contacts are made of a superconductor with high critical temperature (Nb, Pb etc.) then the thermal diffusion into the contacts is absent because of the Andreev's reflection and the electron-phonon relaxation is the only mechanism for heat removal. The relaxation rate should behave as T(sup 4)l at subkelvin temperatures (l is the electron elastic mean free path) and can be reduced by factor of 10-100 by decreasing l. Then an antenna- or waveguide-coupled bolometer with a time constant about 10(exp -3) to 10(exp -5) s at T approximately equals 0.1-0.3 K will exhibit photon-noise limited performance in millimeter and submillimeter range. The choice of the bolometer material is a tradeoff between a low electron heat capacity and fabrication. A state-of-the-art bolometer currently offers NEP = 10(exp -17) W(Square root of (Hz)) at 100 mK along with a approximately equals 2 msec time constant. The bolometer we propose will have a figure-of-merit, NEP(square root (r)), which is 10(exp 3) times smaller. This will allow for a tremendous increase in speed which will have a significant impact for observational mapping applications. Alternatively, the bolometer could operate at higher temperature with still superior sensitivity. This device can significantly increase a science return and reduce the cost for future observational missions. This research was performed by the Center for Space Microelectronics Technology, Jet Propulsion Laboratory, California Institute of Technology, and was sponsored by NASA, Office of Space Science.

Karasik, B. S.↗
NASA NTRS1990

Unbound molecular complexes in M33

The HI content of the nearby spiral galaxy M33 (d approx. 0.8 Mpc) has been the subject of many detailed studies (most recently Deul and van der Hulst 1987), but similar data on the molecular gas component has been lacking. This galaxy is currently undergoing vigorous high-mass star formation, as evidenced by the many OB associations and HII regions, and so is expected to possess at least some molecular gas. Interferometric studies have detected molecular clouds similar to Galactic Giant Molecular Clouds (GMCs) (Boulanger et al. 1988; Wilson et al. 1988). Researchers have recently mapped the nuclear region of M33 in the CO J=1-0 line with a 12 m telescope (half power beam width 55 inches approx. 210 pc) out to a radius of 3.5 minutes in order to trace the detailed distribution of the molecular gas. The resulting map reveals six large complexes with diameters of 200 to 400 pc. Interferometer observations of these regions have resolved them into individual molecular clouds similar to Galactic GMCs (Wilson et al. 1988). The complexes shown are much larger than individual Galactic GMCs (Sanders, Scoville, and Solomon 1985), but are somewhat smaller on average than the large associations seen in the grand-design spiral galaxy M51 (Rand and Kulkarni 1989). If we extend the velocity-diameter relation observed for Galactic GMCs to these larger structures, the predicted velocity widths are a factor of 1.5 to 3 times greater than the observed full-width half-maximum velocities (13 to 29 km (s-1). The peak brightness temperatures are approx. 0.2 K, at least a factor of 10 lower than what is observed for Galactic GMCs. If these large structures are made up of objects similar to Galactic GMCs, the area filling factor of the small clouds is roughly 10 percent. These results suggest that these complexes are not very large GMCs, but rather are associations of many individual GMCs or GMCs embedded in a diffuse gas component.

Wilson, Christine D.↗
DOE OSTI2023

Residual-based error correction for neural operator accelerated infinite-dimensional Bayesian inverse problems

We explore using neural operators, or neural network representations of nonlinear maps between function spaces, to accelerate infinite-dimensional Bayesian inverse problems (BIPs) with models governed by nonlinear parametric partial differential equations (PDEs). Neural operators have gained significant attention in recent years for their ability to approximate the parameter-to-solution maps defined by PDEs using as training data solutions of PDEs at a limited number of parameter samples. The computational cost of BIPs can be drastically reduced if the large number of PDE solves required for posterior characterization are replaced with evaluations of trained neural operators. However, reducing error in the resulting BIP solutions via reducing the approximation error of the neural operators in training can be challenging and unreliable. We provide an a priori error bound result that implies certain BIPs can be ill-conditioned to the approximation error of neural operators, thus leading to inaccessible accuracy requirements in training. To reliably deploy neural operators in BIPs, we consider a strategy for enhancing the performance of neural operators: correcting the prediction of a trained neural operator by solving a linear variational problem based on the PDE residual. We show that a trained neural operator with error correction can achieve a quadratic reduction of its approximation error, all while retaining substantial computational speedups of posterior sampling when models are governed by highly nonlinear PDEs. The strategy is applied to two numerical examples of BIPs based on a nonlinear reaction–diffusion problem and deformation of hyperelastic materials. We demonstrate that posterior representations of the two BIPs produced using trained neural operators are greatly and consistently enhanced by error correction.

97 MATHEMATICS AND COMPUTING↗
NASA NTRS2001

Far-Infrared Extragalactic Surveys: Past, Present, and Future

As much as one third of the luminosity of the local universe is emitted in the far infrared. In order to understand the history of energy release in the universe, it is crucial to characterize this rest-frame far-infrared contribution from the present back to the era of initial galaxy formation. Over the redshift range from 0 to 10, this energy is received in the 80 micrometers to 1 mm spectral region. In the 1980's the Infrared Astronomy Satellite (IRAS) all-sky survey provided the first comprehensive view of the far infrared emission from the local universe. The diffuse background measurements by Cosmic Background Explorer Satellite (COBE) have provided constraints on the integral contributions from the high redshift universe. In the past five years, submillimeter measurements made using the SCUBA instrument have revealed powerful high redshift sources. To develop a clear history of energy release in the universe, we need numbers and redshifts of representative populations of energetically important objects. The near future will bring the Space Infrared Telescope Facility Multiband Imaging Photometer (SIRTF)(MIPS) survey, which will cover about 100 square degrees at wavelengths out to 160 micrometers, providing a large sample of energetically important galaxies out to z of approx.3. In 2005, the Japanese IRIS survey will provide a 160 micrometers full sky survey, which will provide larger samples of the high z galaxy populations and will find intrinsically rare high luminosity objects. The SPIRE instrument on the FIRST facility will extend these surveys to longer wavelengths, providing a view of the universe at higher redshifts in three spectral bands. A concept for an all-sky submillimeter survey is under development, called the Survey of Infrared Cosmic Evolution (SIRCE). With a 2 m cryogenic telescope, it can map the entire sky to the confusion limit in the 100 to 500 micrometers range in six months. This survey will provide photometric redshifts, number counts, and will find the most luminous objects in the universe. In the next decade, the opening of the submillimeter, combined with the near infrared capability of NGST will provide us with a clear picture of energy release in the early universe.

Moseley, Samuel H., Jr.↗
DOE OSTI2022

The Andromeda gamma-ray excess: background systematics of the millisecond pulsars and dark matter interpretations

ABSTRACT Since the discovery of an excess in gamma rays in the direction of M31, its cause has been unclear. Published interpretations focus on dark matter or stellar related origins. Studies of a similar excess in the Milky Way centre motivate a correlation of the spatial morphology of the signal with the distribution of stellar mass in M31. However, a robust determination of the best theory for the observed excess emission is challenging due to uncertainties in the astrophysical gamma-ray foreground model. We perform a spectro-morphological analysis of the M31 gamma-ray excess using state-of-the-art templates for the distribution of stellar mass in M31 and novel astrophysical foreground models for its sky region. We construct maps for the old stellar populations of M31 based on data from the PAndAS survey and carefully remove the foreground stars. We also produce improved astrophysical foreground models via novel image inpainting techniques based on machine learning methods. Our stellar maps, mimicking the location of a population of millisecond pulsars in the bulge of M31, reach a 5.4σ significance, making them as strongly favoured as the simple phenomenological models usually considered in the literature, e.g. disc-like templates. This detection is robust to generous variations of the astrophysical foreground model. Once the stellar templates are included in the astrophysical model, we show that the dark matter annihilation interpretation of the signal is unwarranted. We demonstrate that about one million unresolved millisecond pulsars naturally explain the observed gamma-ray luminosity per stellar mass, energy spectrum, and stellar bulge-to-disc flux ratio.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Conversions between gas-phase metallicities in MaNGA

ABSTRACT We present polynomial conversions between each of 11 different strong line gas-phase metallicity calibrations, each based on ∼1.1 million star-forming spaxels in the public Sloan Digital Sky Survey (SDSS) Data Release 15 (DR15) Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey. For this sample, which is ∼20 times larger than previous works, we present fifth-order polynomial fits for each of 110 possible calibration conversions, for both Small Magellanic Cloud-type and Milky Way-type dust corrections. The typical 2σ scatter around our polynomial fits is 0.1 dex; we present the range over which the metallicities are valid. Conversions between metallicities which rely on the same set of line ratios, or a heavily shared set of emission lines, have reduced scatter in their conversions relative to those conversions with little overlap in required emission lines. Calibration conversions with less consistent sets of emission lines also have increased galaxy-to-galaxy variability, and this variability can account for up to 35 per cent of the total scatter. We also compare our conversions to previous work with the single fibre SDSS DR7 spectra along with higher spatial resolution data from the TYPHOON Integral Field Spectroscopy survey, resulting in comparison samples with spatial resolutions from several kpc down to ∼100 pc. Our metallicity conversions, obtained with the large sample of MaNGA, are robust against the influence of diffuse ionized gas, redshift, effective radius, and spatial blurring, and are therefore consistent across both integrated spectra and the high-resolution integral field spectroscopy data.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1998

Numerical Simulation of Combustion and Extinction of a Solid Cylinder in Low-Speed Cross Flow

The combustion and extinction behavior of a diffusion flame around a solid fuel cylinder (PMMA) in low-speed forced flow in zero gravity was studied numerically using a quasi-steady gas phase model. This model includes two-dimensional continuity, full Navier Stokes' momentum, energy, and species equations with a one-step overall chemical reaction and second-order finite-rate Arrhenius kinetics. Surface radiation and Arrhenius pyrolysis kinetics are included on the solid fuel surface description and a parameter Phi, representing the percentage of gas-phase conductive heat flux going into the solid, is introduced into the interfacial energy balance boundary condition to complete the description for the quasi-steady gas-phase system. The model was solved numerically using a body-fitted coordinate transformation and the SIMPLE algorithm. The effects of varying freestream velocity and Phi were studied. These parameters have a significant effect on the flame structure and extinction limits. Two flame modes were identified: envelope flame and wake flame. Two kinds of flammability limits were found: quenching at low-flow speeds due to radiative loss and blow-off at high flow speeds due to insufficient gas residence time. A flammability map was constructed showing the existence of maximum Phi above which the solid is not flammable at any freestream velocity.

Tien, J. S.↗
DOE OSTI2026

Taylor approximation variance reduction for approximation errors in PDE-constrained Bayesian inverse problems

In numerous applications, surrogate models are used as a replacement for accurate parameter-to-observable mappings when solving large-scale inverse problems governed by partial differential equations (PDEs). The surrogate model may be a computationally cheaper alternative to the accurate parameter-to-observable mappings and/or may ignore additional unknowns or sources of uncertainty. The Bayesian approximation error (BAE) approach provides a means to account for the induced uncertainties and approximation errors, i.e. the errors between the accurate parameter-to-observable mapping and the surrogate. The statistics of these errors are, however, in general unknown a priori, and are thus calculated using Monte Carlo sampling. Although the sampling is typically carried out offline, i.e. before considering the data, the process can still represent a computational bottleneck. In this work, we develop a scalable computational approach for reducing the costs associated with the sampling stage of the BAE approach. Specifically, we consider the Taylor expansion of the accurate and surrogate forward models with respect to the uncertain parameter fields either as a control variate for variance reduction or as a means to directly and efficiently approximate the mean and covariance of the approximation errors. We propose efficient methods for evaluating the expressions for the mean and covariance of the Taylor approximations based on linear(-ized) PDE solves. Furthermore, the proposed approach is independent of the dimension of the uncertain parameter, depending instead on the intrinsic dimension of the data, ensuring scalability to high-dimensional problems. The potential benefits of the proposed approach are demonstrated for two high-dimensional inverse problems governed by PDE examples, namely for the estimation of a distributed Robin boundary coefficient in a linear diffusion problem, and for a coefficient estimation problem governed by a nonlinear diffusion problem.

Bayesian approximation error↗
DOE OSTI2021

Investigation of Scale-Dependent Groundwater/Surface-water Exchange in Rivers by Gradient Self-Potential Logging: Numerical Modeling and Field Experiments

Exchanges of groundwater and surface-water are fundamental to a wide range of water-supply and water-quality management issues but challenging to map beyond the reach scale. Waterborne gradient self-potential (SP) measurements are directly sensitive to water flow through riverbed sediments and can be used to infer exchange locations, direction (gain versus loss), scale, and relative changes, but to date applications to river corridor hydrology are limited. Numerical modeling and field experiments were therefore performed herein, each emphasizing waterborne gradient SP logging for identifying and locating focused vertical groundwater discharge (surface-water gain) and recharge (surface-water loss) in a river. Two and three-dimensional numerical models were constructed to simulate the polarities, appearances, and peak amplitudes of streaming-potential and electric-field anomalies on a riverbed and in the surface-water that were attributable to steady-state vertical fluxes of groundwater through high-permeability conduits in the riverbed. Effects of varied hydraulic length-scale of exchange and surface-water depth were tested through numerical modeling. Modeling results aided in data acquisition and interpretation for three repeated field experiments performed along a 1.5–2.0 km reach of the Quashnet River in Cape Cod, Massachusetts, where focused, meter-scale groundwater discharges occur at discrete locations within otherwise ubiquitous and more diffuse groundwater upwelling conditions. Strong gradient SP anomalies were repeatedly measured in the Quashnet River at previously confirmed locations of focused groundwater discharge, showing the efficacy of waterborne gradient SP logging in identifying and characterizing groundwater/surface water exchange dynamics at multiple river network scales.

Engineering↗
DOE OSTI2020

Molecular dynamics simulations for glass transition temperature predictions of polyhydroxyalkanoate biopolymers

Polyhydroxyalkanoates (PHAs) represent an emerging class of biosynthetic and biodegradable polyesters that exhibit considerable potential to replace petroleum-based plastics towards a sustainable future. Despite the promise, general structure–property mappings within this class of polymers remain largely unexplored. An efficient exploration of this vast chemical space calls for the development and validation of predictive methods for accurate estimation of a diverse range of properties for PHA-based polymers. Towards this aim, we present and validate in this work the results of our molecular dynamics (MD) simulation based approach aimed at predicting glass transition temperatures (T g ) of PHA-based polymers. Since generally available and widely used polymer forcefields exhibit a relatively poor performance for T g predictions, we have developed a new forcefield by modifying the polymer consistent force field (PCFF) via refining a selected set of torsion potentials of the polymer backbone using accurate density functional theory (DFT) computations. After carefully assessing the dependence of critical simulation parameters, such as, polymer chain length, number of polymer chains, supercell size, and thermal quenching rate used in the simulation, the applicability and transferability of the modified PCFF (mPCFF) is demonstrated by directly comparing the computed T g predictions of various polymers with different chemistries, polymer side chain lengths and functional groups forming the polymer side chains against the respective experimentally measured values. Furthermore, the transport properties such as self-diffusion coefficient and viscosity are computationally determined and their well-known correlation with the target properties is demonstrated. Lastly, we have employed the developed approach to predict T g values for a number of yet-to-be-synthesized PHA-based polymers with a diverse set of functional groups in the polymer side chains. The results are further rationalized by correlating the predicted T g values with the inter-chain H-bond formation tendencies of the different side chain functional groups. This work represents an important first step towards computationally guided design of PHA-based functional polymers and opens up new directions for a systematic investigation of composition- and configuration-dependent structure–property relationships in more complex binary and ternary copolymer systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Stable Rate of Slip Along the Karakax Section of the Altyn Tagh Fault from Observation of Interglacial and Postglacial Offset Morphology and Surface Dating

Digital elevation maps obtained using TanDEM-X and Pleiades data combined with newly obtained surface age estimates using cosmogenic radionuclide (CRN) and optically simulated luminescence (OSL) methods are used to quantify the slip rate along the western section of the Altyn Tagh Fault in southern Xinjiang. The reconstruction of the conical shape of massive alluvial fans inferred to be from the Eemian (115±7 ka) from CRN dating shows consistent left-lateral offsets of 300±20 m, yielding a slip rate of 2.6±0.3 mm/year. Successive episodes of incision have left cut terraces inset in wide canyons, 10–25 m below the fans' surface. The incision was followed by the deposition of a broad terrace of early Holocene age, which is reincised by modern stream channels. Near the village of Shanxili, a 200 m wide valley is partially dammed by a shutter ridge displaced by the fault. A fill terrace deposited upstream from the ridge has an OSL age of 8.8±0.6 ka. The 23±2 m offset of the riser incising the terrace indicates a minimum postdepositional movement on the fault, yielding a Holocene rate of 2.6±0.5 mm/year, consistent with the 115-ka average slip rate. Furthermore, scarp degradation analysis using mass diffusion reveals a nonlinear relationship between fault displacement and degradation coefficient along the progressively exposed fault scarp, a pattern suggesting either seismic clustering or variable diffusion rate since the Eemian. Together with the Gozha Co-Longmu Co fault to the south, the Karakax section of the Altyn Tagh Fault contributes to the eastward movement of the western corner of Tibet.

58 GEOSCIENCES↗
NASA NTRS2020

Chandra and XMM-Newton Observations of A2256: Cold Fronts, Merger Shocks, and Constraint on the IC Emission

We present the results of deep Chandra and XMM-Newton observations of a complex merging galaxy cluster Abell 2256 (A2256) that hosts a spectacular radio relic (RR).The temperature and metallicity maps show clear evidence of a merger between the western subcluster (SC) and the primary cluster (PC). We detect five X-ray surface brightness edges. Three of them near the cluster center are cold fronts (CFs): CF1is associated with the infalling SC; CF2 is located in the east of the PC; and CF3 is to the west of the PC core. The other two edges at cluster outskirts are shock fronts(SFs): SF1 near the RR in the NW has Mach numbers derived from the temperature and the density jumps, respectively, of MT=1.62±0.12andMρ=1.23±0.06; SF2 in the SE has MT=1.54±0.05andMρ=1.16±0.13. In the region of the RR, there is no evidence for the correlation between X-ray and radio substructures, from which we estimate an upper limit for the inverse-Compton emission, and therefore set a lower limit on the magnetic field (∼450 kpc from PC center) of B>1.0μG for a single power-law electron spectrum or B>0.4μG for a broken power-law electron spectrum. We propose a merger scenario including a PC, an SC, and a group. Our merger scenario accounts for the X-ray edges, diffuse radio features, and galaxy kinematics, as well as projection effects.

Chong Ge↗