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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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At least 739 records · Page 41
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

State-Constrained Optimal Control Problems of Impulsive Differential Equations

The present paper studies an optimal control problem governed by measure driven differential systems and in presence of state constraints. The first result shows that using the graph completion of the measure, the optimal solutions can be obtained by solving a reparametrized control problem of absolutely continuous trajectories but with time-dependent state-constraints. The second result shows that it is possible to characterize the epigraph of the reparametrized value function by a Hamilton-Jacobi equation without assuming any controllability assumption.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2019

Diagnostic accuracy of acoustic radiation force impulse elastography (ARFI) in comparison to other non-invasive modalities in staging of liver fibrosis in chronic HCV patients: single-center experience

Purpose: To evaluate the reliability of ARFI elastography for liver fibrosis staging and compare it to other non-invasive assessment of hepatic fibrosis (FIB-4 and APRI) in chronic HCV (CHC) patients. Methods: A single-center, prospective study included 2103 CHC patients. Liver stiffness (LS) was evaluated by TE and ARFI elastography. FIB-4 and APRI were calculated. The area under the receiver-operating characteristic curve (AUROCs) was used to assess the diagnostic performance of ARFI elastography for staging of liver fibrosis using TE as a reference standard. Results: The best cut off values of ARFI elastography for diagnosis of ≥ F2, ≥ F3and F4 were 1.36 m/s, 1.45 m/s, and 1.7 m/s with AUROCs of 0.89, 0.94 and 0.95, respectively. ARFI elastography cut offs are lower in patients with normal ALT level compared to those with ALT level (1.1–< 3 ULN) and those with ALT level ≥ 3ULN (1.35 m/s vs 1.39 m/s vs 1.54 for F ≥ 2, 1.44 m/s vs 1.58 m/s vs 1.6 m/s for F3, 1.69 m/s, 1.84 m/s, 1.86 m/s for F4). FIB-4 (0.82–0.86) and APRI (0.78–0.82) yielded lower AUC in prediction of significant fibrosis and cirrhosis than ARFI elastography (0.89–0.95). Conclusion: ARFI elastography is a reliable method for non-invasive staging of liver fibrosis in CHC patients when compared to TE with a good diagnostic performance comparable to FIB-4 and APRI scores for the prediction of significant fibrosis and cirrhosis.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2018

On Stability of Linear Systems with Impulsive Action at the Matrix

We discuss properties of stability and asymptotic stability of solutions to linear systems of differential equations with generalized actions in matrices of systems. We obtain sufficient conditions that guarantee the stability and asymptotic stability of solutions to these system. A distinctive feature of systems considered is the fact that the right-hand sides of systems contain the ill-posed operation of multiplication of discontinuous functions by generalized functions.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2019

Options and uncertainties in planetary defense: Impulse-dependent response and the physical properties of asteroids

In this paper we discuss that though rare, asteroid impacts are inevitable, and with the current state of technology, kinetic impactors are the preferred but not the complete solution. If the time to impact is short, or the threatening body too large, nuclear deflection serves as a final option. This work is part of an integrated study by National Aeronautics and Space Administration (NASA) and the National Nuclear Security Administration (NNSA) to better determine the relative efficacy of these complimentary approaches. In particular, we examine the important material properties that affect each approach, to improve critical characterization efforts, and reduce uncertainty in the limits of the impactor technology. Impact speeds for kinetic impactors on Near-Earth Object (NEO) intercept trajectories commonly range from 5 to 20 km/s, resulting in significant crater ejecta and a momentum enhancement above that carried by the impactor. This enhancement depends substantially on the strength and porosity of the asteroid, as well as the impact speed. Here simulations from different codes are presented, along with constraints from experimental measurements. The uncertainties due to ignorance of the strength and porosity of the impact point are significant in determining the limits of impactor sufficiency. The nuclear approach is considered within the context of current capabilities, posing no need to test, as extant and well-understood devices are sufficient for the largest known Potentially Hazardous Objects (PHOs). Results of x-ray sources with realistic spectra as well as blackbody spectra are given, along with some assessment on composition dependence.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Performance evaluation of a Terry GS-2 steam impulse turbine with air-water mixtures

Terry steam turbines are widely used in various industries because of their robust design. Within the nuclear power generation industry, they are used in the Reactor Core Isolation Cooling System to remove decay heat during reactor isolation events. During the Fukushima Daiichi nuclear power station disaster in Japan in 2011, the Reactor Core Isolation Cooling System and associated Terry turbine operated for over 70 hours in Unit 2; this runtime is well beyond the expected operating duration. Theories suggest the turbine was subjected to a two-phase inlet flow, which could degrade the turbine performance. In this work, an experimental test rig was constructed to test a full-scale Terry model GS-2 steam turbine under two-phase air/water flows. Steady-state efficiency and torque performance maps of the turbine were developed over a range of turbine inlet pressures (1.38–4.83 bar or 20–70 psia), air mass fractions (0.05–1.0) and rotational speeds up to 4000 RPM. Furthermore, turbine performance followed expected trends with torque varying linearly and efficiency varying quadratically with rotational speed. In addition, high-speed images of the two-phase flow entering the turbine were also analyzed to understand how changes in inlet pressure and air mass fraction affect the flow regime and homogenization. The present tests with air–water two-phase mixtures are an important step towards providing an understanding of the full-scale Terry turbine’s behavior and performance curves under two-phase conditions. The results of this work will be combined with air/water and steam/water data gathered using a small-scale Terry ZS-1 steam turbine in order to understand the scaling relationship between large and small size Terry turbines and fluid pairs. The combined data set will enable further development of analytical models over a wide range of conditions and may be used to provide technical justification for expanded use of the Terry turbines in nuclear power plant safety systems and other systems.

42 ENGINEERING↗
DOE OSTI2018

Description of the code ANVIL (ANisotropic Vhf Impulse Location)

In this paper, we have developed a general, three-dimensional method to locate sources of earth-directed radiation which takes into account the flight paths of reflected signals. While time-of-arrival algorithms exist for locating radio sources using line-of-sight propagation paths, radio sources originating above the surface of the earth will not necessarily emit strong power along direct paths to satellites. A combination of direct and ground-reflected pulses or only ground-reflected signals from such sources could be received by satellite-borne sensors. The work presented here applies to satellite detection of sub-ionospheric radio sources in a vacuum environment with ideal reflection off the earth's surface. Because satellites are not static receivers, their configuration with respect to a radio source is not always optimal. Therefore, a statistical study is performed using 1000 randomized configurations of 24 satellites in middle earth orbit. For each configuration, the radio source latitude and longitude is fixed, and its altitude is varied from 1 to 97 km. An analytic direct-path algorithm using the five satellites nearest the radio source provides an initial guess for the radio source latitude and longitude. We find that, using this approximation, the mean error in the calculated nadir location has a maximum value of 134 m for the 97-km source altitude. The coordinates of the initial guess are used to define the origin of a grid within which an all-points search refinement is performed. Finally, using this procedure, the overall maximum mean error in radio source position is found to be on the order of the computational grid size.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2023

Characterization of Aquifer Poroelastic Response to Impulse and Oscillatory Well Pressure Using Distributed Acoustic Sensing

Abstract The storage of fluids in the subsurface is critical for a broad spectrum of applications including managed aquifer recharge, storage of liquefied carbon dioxide and hydrogen, geothermal heat extraction and exploitation of hydrocarbon. It is surprising then, that there has been relatively little measurement of the vertical distribution of poroelastic storage in geologic formations as compared with permeability. We present experiments in which fluid was injected into an important regional aquifer and the depth‐dependent strain response measured using fiber optic distributed acoustic sensing. The formation expansion and contraction in response to fluid injection were several 100 nanostrain. Strain, and the implied storage distribution, was highly localized in specific strata and demonstrated complex, hydromechanical behavior. This new window into fluid‐geomechanical coupling undermines some typically use models and observations currently in practice, but provides potential for complete representation and prediction of fluid storage in the subsurface.

58 GEOSCIENCES↗
DOE OSTI2018

Impulsive UV-pump/X-ray probe study of vibrational dynamics in glycine

We report an ab-initio study of a pump-probe experiment on the amino-acid glycine. We consider an UV pump followed by an X-ray probe tuned to carbon K-edge and study the vibronic structure of the core transition. The simulated experiment is feasible using existing free electron laser or high harmonic generation sources and thanks to the localization of the core orbitals posseses chemical selectivity. The present theory applies to other experimental schemes, including the use of a THz probe, available with present soft X-ray free electron lasers and/or high harmonic generation sources.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

Drifting localization of ionization runaway: Unraveling the nature of anomalous transport in high power impulse magnetron sputtering

The plasma over a magnetron's erosion ''racetrack'' is not azimuthally uniform but concentrated in distinct dense ionization zones which move in the ExB direction with about 10% of the electron ExB/B{sup 2} drift velocity. The ionization zones are investigated with a gated camera working in concert with a streak camera for Al, Nb, Cu, and W targets in Ar or Kr background gas. It is found that each ionization zone has a high plasma density edge, which is the origin of a plasma-generating electron jet leaving the target zone. Each region of strong azimuthal plasma density gradient generates an azimuthal electric field, which promotes the escape of magnetized electrons and the formation of electron jets and plasma flares. The phenomena are proposed to be caused by an ionization instability where each dense plasma zone exhibits a high stopping power for drifting high energy electrons, thereby enhancing itself.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Extended two-temperature model for ultrafast thermal response of band gap materials upon impulsive optical excitation

Thermal modeling and numerical simulations have been performed to describe the ultrafast thermal response of band gap materials upon optical excitation. A model was established by extending the conventional two-temperature model that is adequate for metals, but not for semiconductors. It considers the time- and space-dependent density of electrons photoexcited to the conduction band and accordingly allows a more accurate description of the transient thermal equilibration between the hot electrons and lattice. Ultrafast thermal behaviors of bismuth, as a model system, were demonstrated using the extended two-temperature model with a view to elucidating the thermal effects of excitation laser pulse fluence, electron diffusivity, electron-hole recombination kinetics, and electron-phonon interactions, focusing on high-density excitation.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Observation of a periodic runaway in the reactive Ar/O{sub 2} high power impulse magnetron sputtering discharge

This paper reports the observation of a periodic runaway of plasma to a higher density for the reactive discharge of the target material (Ti) with moderate sputter yield. Variable emission of secondary electrons, for the alternating transition of the target from metal mode to oxide mode, is understood to be the main reason for the runaway occurring periodically. Increasing the pulsing frequency can bring the target back to a metal (or suboxide) mode, and eliminate the periodic transition of the target. Therefore, a pulsing frequency interval is defined for the reactive Ar/O{sub 2} discharge in order to sustain the plasma in a runaway-free mode without exceeding the maximum power that the magnetron can tolerate.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗