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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 505 records · Page 28
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Accelerating the Convergence of Replica Exchange Simulations Using Gibbs Sampling and Adaptive Temperature Sets

We recently introduced a novel replica-exchange scheme in which an individual replica can sample from states encountered by other replicas at any previous time by way of a global configuration database, enabling the fast propagation of relevant states through the whole ensemble of replicas. This mechanism depends on the knowledge of global thermodynamic functions which are measured during the simulation and not coupled to the heat bath temperatures driving the individual simulations. Therefore, this setup also allows for a continuous adaptation of the temperature set. In this paper, we will review the new scheme and demonstrate its capability. The method is particularly useful for the fast and reliable estimation of the microcanonical temperature T (U) or, equivalently, of the density of states g(U) over a wide range of energies.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Accelerating charge estimation in molecular dynamics simulations using physics-informed neural networks: corrosion applications

Molecular Dynamics (MD) simulations are used to understand the effects of corrosion on metallic materials in salt brine. Reactive force fields in classical MD enable accurate modeling of bond formation and breakage in the aqueous medium and at the metal-electrolyte interface, while also facilitating dynamic partial charge equilibration. However, MD simulations are computationally intensive and unsuitable for modeling the long time scales characteristic of corrosive phenomena. To address this, we develop reduced-order machine learning models that provide accurate and efficient predictions of charge density in corrosive environments. Specifically, we use Long Short-Term Memory (LSTM) networks to forecast charge density evolution based on atomic environments represented by Smooth Overlap of Atomic Positions (SOAP) descriptors. A physics-informed loss function enforces charge neutrality and electronegativity equivalence. The atomic charges predicted by the deep learning model trained on this work were obtained two orders of magnitude faster than those from molecular dynamics (MD) simulations, with an error of less than 3% compared to the MD-obtained charges, even in extrapolative scenarios, while adhering to physical constraints. This demonstrates the excellent accuracy, computational efficiency, and validity of the developed model. Lastly, even though developed for corrosion, these protocols are formulated in a phenomenon-agnostic manner, allowing application to various variable-charge interatomic potentials and related fields.

Atomistic models↗
DOE OSTI2016

Ion and electron dynamics generating the Hall current in the exhaust far downstream of the reconnection x-line

We have investigated the ion and electron dynamics generating the Hall current in the reconnection exhaust far downstream of the x-line where the exhaust width is much larger than the ion gyro-radius. A large-scale particle-in-cell simulation shows that most ions are accelerated through the Speiser-type motion in the current sheet formed at the center of the exhaust. The transition layers formed at the exhaust boundary are not identified as slow mode shocks. (The layers satisfy mostly the Rankine-Hugoniot conditions for a slow mode shock, but the energy conversion hardly occurs there.) We find that the ion drift velocity is modified around the layer due to a finite Larmor radius effect. As a result, the ions are accumulated in the downstream side of the layer, so that collimated ion jets are generated. The electrons experience two steps of acceleration in the exhaust. The first is a parallel acceleration due to the out-of-plane electric field E{sub y} which has a parallel component in most area of the exhaust. The second is a perpendicular acceleration due to E{sub y} at the center of the current sheet and the motion is converted to the parallel direction. Because of the second acceleration, the electron outflow velocity becomes almost uniform over the exhaust. The difference in the outflow profile between the ions and electrons results in the Hall current in large area of the exhaust. The present study demonstrates the importance of the kinetic treatments for collisionless magnetic reconnection even far downstream from the x-line.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2025

Accelerating Innovative Energy Solutions Using Combustion Simulations

Combustion-based transportation, electricity generation, and industrial heating in manufacturing constitute the three largest sectors of energy demand. Some of the recent technology development in these sectors are: switching to low-carbon fuels for the transportation sector, increasing energy efficiency in the power sector, and capturing carbon emissions from conventional power generators. Several teams at the National Renewable Energy Laboratory have been actively advancing research in these areas by leveraging computational modeling of combustion processes across the heavy-duty land based transportation, aviation, and power generation sectors. This article summarizes some of these efforts, demonstrating the potential of advanced computational techniques to generate technological solutions that will transform the global energy system.

08 HYDROGEN↗
DOE OSTI2018

An Advanced Simulation Toolkit for Photon Band Gap Accelerators

Photon band gap (PBG) structures have been investigated for accelerator and microwave tube applications over the past decade, and these investigations have concentrated on two distinct concepts. In one approach, a traditional accelerator slow-wave structure is created with a sequence of cavities of a new type that each contains a specially designed lattice of rods that endow it with desirable properties. Another more recent application of the same concept is to use a photonic crystal to confine the synchronous mode at the boundary of a dielectric wall accelerator (DWA), but let other modes excited by the beam escape. A related concept is the photonic crystal accelerator.

43 PARTICLE ACCELERATORS↗
DOE OSTI2019

Advancing Understanding of Fission Gas Behavior in Nuclear Fuel through Leadership Class Computing

This report describes the outcome of the pilot SciDAC project jointly funded by the DOE Office of Nuclear Energy and the DOE Office of Science, Advanced Scientific Computing Research to advance understanding of fission gas behavior in nuclear fuel. The computational approach involved utilizing both a bottom up and top down multiscale materials modeling to evaluate the processes of xenon fission gas diffusion, xenon clustering and precipitation and the mechanisms of fission gas resolution by energetic fission fragments in uranium dioxide nuclear fuel. The computer science and applied mathematics research activities were focused on optimizing the uranium – oxygen – xenon interatomic potential implementation in the LAMMPS molecular dynamics code for large-scale simulations of fission gas resolution, in addition to improvements in visualization of accelerated molecular dynamics simulations and performance optimization of the Xolotl continuum cluster dynamics with a new grouping scheme to reduce the computational cost of modeling very large fission gas bubbles containing more than 10,000 xenon atoms. The most impactful result from this project stems from a systematic series of atomistic molecular dynamics to evaluate fission gas resolution from xenon bubbles that, combined with the fission product yields and consideration of electron-phonon coupling, informed a first principles model that predicts the magnitude of fission gas resolution in good agreement with empirical estimates.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2022

Parallel Simulation of Beam Dynamics in Particle Accelerators [Slides]

LW3D was used to explore the expected CSR from an undulator-based diagnostic using a first-principles approach. It was also used for comparison with an analytic Green-function approach. LW3D was extended to model transport systems involving dipoles and quadrupoles. It was used to explore the radiation produced in a 4-dipole bunch compressor. Radiation ahead of the electron bunch at the beamline exit was shown to be associated with wavefronts produced in upstream dipoles overtaking the bunch.

43 PARTICLE ACCELERATORS↗