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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 829 records · Page 46
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Overview of recent observations and simulations from the American WAKE experimeNt (AWAKEN) field campaign

The American WAKE experimeNt (AWAKEN) is a large-scale field campaign focused on gathering new detailed observations of wind farm–atmosphere interactions for improved understanding of wind farm physics and overall performance. The field campaign, centered around five wind farms in northern Oklahoma, began in September 2022 and will run through at least October 2023. This paper summarizes some of the initial observations and simulations performed as part of the AWAKEN project.

17 WIND ENERGY↗
DOE OSTI2012

Structural and dynamical properties of liquid Cu[subscript 80]Si[subscript 20] alloy studied experimentally and by ab initio molecular dynamics simulations

The local structures and dynamical properties of the liquid Cu{sub 80}Si{sub 20} alloy have been studied by x-ray diffraction and ab initio molecular dynamics (MD) simulations. The pair-correlation functions and the structure factors derived from the three-dimensional coordinates of the MD configurations agree well with the experimental results. The local structure of the liquids is analyzed using Honeycutt-Andersen (HA) indices, Voronoi tessellation (VT), and an atomic cluster-alignment method. The HA indices analysis shows that the pentagonal bipyramid, a fragment of an icosahedron (ICO), plays a dominant role in the short-range order (SRO) of the Cu{sub 80}Si{sub 20} liquid. The HA indices corresponding to the pentagonal bipyramid increase dramatically with decreasing temperature. VT analysis indicates that, while the liquid does exhibit a strong icosahedral SRO, fcc-like SRO is also measurable. The results from VT analysis are further confirmed using the recently developed atomic cluster-alignment method. Finally, self-diffusion constants, as a function of temperature for both Cu and Si species, are calculated.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Simple concentration-dependent pair interaction model for large-scale simulations of Fe-Cr alloys

This work is motivated by the need for large-scale simulations to extract physical information on the iron-chromium system that is a binary model alloy for ferritic steels used or proposed in many nuclear applications. From first-principles calculations and the experimental critical temperature we build a new energetic rigid lattice model based on pair interactions with concentration and temperature dependence. Density functional theory calculations in both norm-conserving and projector augmented-wave approaches have been performed. A thorough comparison of these two different ab initio techniques leads to a robust parametrization of the Fe-Cr Hamiltonian. Mean-field approximations and Monte Carlo calculations are then used to account for temperature effects. The predictions of the model are in agreement with the most recent phase diagram at all temperatures and compositions. The solubility of Cr in Fe below 700 K remains in the range of about 6 to 12%. It reproduces the transition between the ordering and demixing tendency and the spinodal decomposition limits are also in agreement with the values given in the literature.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Capacitive charge storage at an electrified interface investigated via direct first-principles simulations [Direct Simulation of Capacitive Charging of Graphene and Implications for Supercapacitor Design]

Understanding the impact of interfacial electric fields on electronic structure is crucial to improving the performance of materials in applications based on charged interfaces. Supercapacitors store energy directly in the strong interfacial field between a solid electrode and a liquid electrolyte; however, the complex interplay between the two is often poorly understood, particularly for emerging low-dimensional electrode materials that possess unconventional electronic structure. Typical descriptions tend to neglect the specific electrode-electrolyte interaction, approximating the intrinsic “quantum capacitance” of the electrode in terms of a fixed electronic density of states. Instead, we introduce a more accurate first-principles approach for directly simulating charge storage in model capacitors using the effective screening medium method, which implicitly accounts for the presence of the interfacial electric field. Applying this approach to graphene supercapacitor electrodes, we find that results differ significantly from the predictions of fixed-band models, leading to improved consistency with experimentally reported capacitive behavior. The differences are traced to two key factors: the inhomogeneous distribution of stored charge due to poor electronic screening and interfacial contributions from the specific interaction with the electrolyte. Lastly, our results are used to revise the conventional definition of quantum capacitance and to provide general strategies for improving electrochemical charge storage, particularly in graphene and similar low-dimensional materials.

25 ENERGY STORAGE↗
DOE OSTI2016

Reinventing atomic magnetic simulations with spin-orbit coupling

We propose a powerful extension to the combined molecular and spin dynamics method that fully captures the coupling between the atomic and spin subsystems via spin-orbit interactions. Moreover, the foundation of this method lies in the inclusion of the local magnetic anisotropies that arise as a consequence of the lattice symmetry breaking due to phonons or crystallographic defects. By using canonical simulations of bcc iron with the system coupled to a phonon heat bath, we show that our extension enables the previously unachievable angular momentum exchange between the atomic and spin degrees of freedom.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2010

Quantum Simulation of Multiple-Exciton Generation in a Nanocrystal by a Single Photon

We have shown theoretically that efficient multiple-exciton generation (MEG) by a single photon can be observed in small nanocrystals. Our quantum simulations that include hundreds of thousands of exciton and multiexciton states demonstrate that the complex time-dependent dynamics of these states in a closed electronic system yields a saturated MEG effect on a picosecond time scale. Including phonon relaxation confirms that efficient MEG requires the exciton-biexciton coupling time to be faster than exciton relaxation time.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2017

Simulation–Extrapolation for Bias Correction with Exposure Uncertainty in Radiation Risk Analysis Utilizing Grouped Data

Summary In observational epidemiological studies, the exposure that is received by an individual often cannot be precisely observed, resulting in measurement error, and a common approach to dealing with measurement error is regression calibration (RC). Use of RC, which requires assumptions about the distribution of unknown error-free (true) variables, leads to concern about the possibility of bias due to misspecification of that distribution. The simulation–extrapolation (SIMEX) method, in contrast, does not require a distributional assumption. However, analyses of large cohorts may be performed by using grouped or person-year data, and application of SIMEX to grouped data is not straightforward, particularly when there is a mixture of classical and Berkson measurement errors. We compared RC and SIMEX with grouped data analyses to assess robustness of the RC method to misspecification of the true dose distribution. We also applied SIMEX assuming mixtures of classical and Berkson errors and compared the results with those obtained by using RC for classical error only. SIMEX had less bias than RC and performed well regardless of the true dose distribution, whereas RC based on a misspecified true dose distribution showed greater bias than when based on the correctly specified true dose distribution.

Misumi, Munechika↗
DOE OSTI2013

Multiphysics Simulations: Challenges and Opportunities

We consider multiphysics applications from algorithmic and architectural perspectives, where ‘‘algorithmic’’ includes both mathematical analysis and computational complexity, and ‘‘architectural’’ includes both software and hardware environments. Many diverse multiphysics applications can be reduced, en route to their computational simulation, to a common algebraic coupling paradigm. Mathematical analysis of multiphysics coupling in this form is not always practical for realistic applications, but model problems representative of applications discussed herein can provide insight. A variety of software frameworks for multiphysics applications have been constructed and refined within disciplinary communities and executed on leading-edge computer systems. We examine several of these, expose some commonalities among them, and attempt to extrapolate best practices to future systems. From our study, we summarize challenges and forecast opportunities.

Keyes, David↗
DOE OSTI2022

Parallel simulated annealing with embedded machine learning and multifidelity models for reactor core design

This paper presents extensions to a penalty-free, parallel simulated annealing (SA) algorithm for multi-constrained combinatorial optimization with the aim of embedding multi-fidelity physics models into the annealing procedure. The method uses a low-fidelity, quickly executing model for rapid design space exploration and a high-fidelity model for detailed constraint resolution and on-the-fly bias correction. Machine learning models updated within the annealing procedure were used to bridge the gap between the multi-fidelity models, which led to accurate rapid exploration and efficient detailed constraint resolution. A software implementation of the new multi-fidelity optimization methods, called ML-PSA, was demonstrated on a continuous multi-fidelity optimization problem and a constrained combinatorial PWR lattice design problem. These problems demonstrate some of the features, parallel performance characteristics, and extensible nature of the multi-fidelity SA methods. This paper shows that the developed software and procedure are a general optimization tool that can be applied to a wide variety of scientific and engineering design optimization applications. (authors)

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2018

Risk Reduction of CO 2 Storage with Stochastic Simulations

The purpose of this project was to create an efficient and flexible tool based on principal component analysis (PCA) for a generation of differentiable realizations of porosity and permeability fields. Of particular interest were data which had significant connectivity between patches of the same rock type, which is called “binary”. The efficiency requirement needed a so called kernel PCA, and the binary images were found with the method called optimization based PCA (OPCA) for which interpretation is provided. The ability to honor given data at particular locations is also incorporated. The tool worked very well and efficiently for two dimensional (2-D) simulations. Furthermore, the tool was applied to a three-dimensional (3-D) dataset and determined that conditioning can be used to maintain the connectivity between vertical layers.

47 OTHER INSTRUMENTATION↗
DOE OSTI2023

Simulation of Chilled-water Injection at EGS Collab Testbed 2 using the GEOS simulation framework

This work seeks to model the chilled-water injection that occurred at EGS Collab Testbed 2 as part of experiment 3, using the open-source GEOS simulation framework. Modeling of this process, previously conducted using STOMP-GT, confirmed that thermal breakthrough was expected to occur during the chill water injection and may have been masked by strong Joule-Thompsons effects induced by large pressure drops between the fractures and the production wells. Building a GEOS numerical model of this process will allow to include additional physics. In fact, GEOS can model fully coupled thermo-poromechanical processes in fractured porous media and can be employed to model the process of artificially enhancing rock permeability by hydraulically fracturing the rocks. For example, the GEOS model could help understanding the dynamic nature of the EGS collab experiment 3 flow system. Since the limited time and resources do not allow for an extensive study, the focus is on building a baseline model of this process. As such, the main outcomes of this modelling efforts are listed below.

15 GEOTHERMAL ENERGY↗
DOE OSTI2024

Simulating Superconducting Devices in Novel Materials Using the G4CMP Simulation Toolkit

Understanding the kinematics of phonons and charge propagation in superconducting devices is crucial for conducting low-threshold dark matter searches and minimizing correlated errors in superconducting qubits. For nearly a decade, the Geant4 Condensed Matter Physics (G4CMP) package, originally developed for the Cryogenic Dark Matter Search (CDMS) experiment, has been limited to simulating charge and phonon transport in silicon and germanium materials. In this work, we have expanded the capabilities of G4CMP to include novel substrate materials such as sapphire (Al2O3), Gallium Arsenide (GaAs), Lithium Fluoride (LiF), Calcium Tungstate (CaWO4), and Calcium Fluoride (CaF2). We demonstrate the use of this toolkit in generating phonon transport properties for these materials and compare the results with experimentally determined values where available. Additionally, we explore the performance of superconducting devices utilizing these materials.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗