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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 613 records · Page 34
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Efficient estimation of the modified Gromov–Hausdorff distance between unweighted graphs

Abstract Gromov–Hausdorff distances measure shape difference between the objects representable as compact metric spaces, e.g. point clouds, manifolds, or graphs. Computing any Gromov–Hausdorff distance is equivalent to solving an NP-hard optimization problem, deeming the notion impractical for applications. In this paper we propose a polynomial algorithm for estimating the so-called modified Gromov–Hausdorff (mGH) distance, a relaxation of the standard Gromov–Hausdorff (GH) distance with similar topological properties. We implement the algorithm for the case of compact metric spaces induced by unweighted graphs as part of Python library , and demonstrate its performance on real-world and synthetic networks. The algorithm finds the mGH distances exactly on most graphs with the scale-free property. We use the computed mGH distances to successfully detect outliers in real-world social and computer networks.

Oles, Vladyslav (ORCID:0000000188727463)↗
DOE OSTI2019

Extended virtual element method for the Laplace problem with singularities and discontinuities

In this paper, we propose the extended virtual element method (X-VEM) to treat singularities and crack discontinuities that arise in the Laplace problem. The virtual element method (VEM) is a stabilized Galerkin formulation on arbitrary polytopal meshes, wherein the basis functions are implicit (virtual)—they are not known explicitly nor do they need to be computed within the problem domain. In this work, suitable projection operators are used to decompose the bilinear form on each element into two parts: a consistent term that reproduces the first-order polynomial space and a correction term that ensures stability. A similar approach is pursued in the X-VEM with a few notable extensions. To capture singularities and discontinuities in the discrete space, we augment the standard virtual element space with an additional contribution that consists of the product of virtual nodal basis (partition-of-unity) functions with enrichment functions. For discontinuities, basis functions are discontinuous across the crack and for singularities a weakly singular enrichment function that satisfies the Laplace equation is chosen. For the Laplace problem with a singularity, we devise an extended projector that maps functions that lie in the extended virtual element space onto linear polynomials and the enrichment function, whereas for the discontinuous problem, the consistent element stiffness matrix has a block-structure that is readily computed. An adaptive homogeneous numerical integration method is used to accurately and efficiently (no element-partitioning is required) compute integrals with integrands that are weakly singular. Once the element projection matrix is computed, the same steps as in the standard VEM are followed to compute the element stabilization matrix. Numerical experiments are performed on quadrilateral and polygonal (convex and nonconvex elements) meshes for the problem of an $\textit{L}$-shaped domain with a corner singularity and the problem of a cracked membrane under mode III loading, and results are presented that affirm the sound accuracy and demonstrate the optimal rates of convergence in the $\textit{L}^2$ norm and energy of the proposed method.

42 ENGINEERING↗
DOE OSTI2021

An adjoint method for determining the sensitivity of island size to magnetic field variations

An adjoint method to calculate the gradient of island width in stellarators is presented and applied to a set of magnetic field configurations. The underlying method for calculation of the island width is that of Cary & Hanson ( Phys. Fluids B, vol. 3, issue 4, 1991, pp. 1006–1014) (with a minor modification), and requires that the residue of the island centre be small. Therefore, the gradient of the residue is calculated in addition. Both the island width and the gradient calculations are verified using an analytical magnetic field configuration introduced by Reiman & Greenside ( Comput. Phys. Commun. , vol. 43, issue 1, 1986, pp. 157–167). The method is also applied to the calculation of the shape gradient of the width of a magnetic island in a National Compact Stellarator Experiment (NCSX) vacuum configuration with respect to positions on a coil. A gradient-based optimization is applied to a magnetic field configuration studied by Hanson & Cary ( Phys. Fluids , vol. 27, issue 4, 1984, pp. 767–769) to minimize stochasticity by adding perturbations to a pair of helical coils. Although only vacuum magnetic fields and an analytical magnetic field model are considered in this work, the adjoint calculation of the island width gradient could also be applied to a magnetohydrodynamic (MHD) equilibrium if the derivative of the magnetic field, with respect to the equilibrium parameters, is known. Using the island width gradient calculation presented here, more general gradient-based optimization methods can be applied to design stellarators with small magnetic islands. Moreover, the sensitivity of the island size may itself be optimized to ensure that coil tolerances, with respect to island size, are kept as high as possible.

Physics↗
DOE OSTI2022

Synthetic genetic circuits as a means of reprogramming plant roots

We report the shape of a plant’s root system influences its ability to reach essential nutrients in the soil and to acquire water during drought. Progress in engineering plant roots to optimize water and nutrient acquisition has been limited by our capacity to design and build genetic programs that alter root growth in a predictable manner. We developed a collection of synthetic transcriptional regulators for plants that can be compiled to create genetic circuits. These circuits control gene expression by performing Boolean logic operations and can be used to predictably alter root structure. This work demonstrates the potential of synthetic genetic circuits to control gene expression across tissues and reprogram plant growth.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2024

Missing Wedge Completion via Unsupervised Learning with Coordinate Networks

Cryogenic electron tomography (cryoET) is a powerful tool in structural biology, enabling detailed 3D imaging of biological specimens at a resolution of nanometers. Despite its potential, cryoET faces challenges such as the missing wedge problem, which limits reconstruction quality due to incomplete data collection angles. Recently, supervised deep learning methods leveraging convolutional neural networks (CNNs) have considerably addressed this issue; however, their pretraining requirements render them susceptible to inaccuracies and artifacts, particularly when representative training data is scarce. To overcome these limitations, we introduce a proof-of-concept unsupervised learning approach using coordinate networks (CNs) that optimizes network weights directly against input projections. This eliminates the need for pretraining, reducing reconstruction runtime by 3–20× compared to supervised methods. Our in silico results show improved shape completion and reduction of missing wedge artifacts, assessed through several voxel-based image quality metrics in real space and a novel directional Fourier Shell Correlation (FSC) metric. Our study illuminates benefits and considerations of both supervised and unsupervised approaches, guiding the development of improved reconstruction strategies.

42 ENGINEERING↗
DOE OSTI2018

Chapter 4 - Mechanical Fractionations of Biomass Feedstocks for Enhanced Pretreatment and Conversion. In the book Biomass Preprocessing and Pretreatments for Production of Biofuels: Mechanical, Chemical and Thermal Methods.

Living plant biomass consists of multiple types of tissues and organs, each performing unique and essential physical or biological functions in the living plant. Because of these differences in form and function, these tissues and organs can retain these unique physical and chemical properties after senescence or harvest. Properties such as ash composition, ash content, carbohydrate content, lignin content, density, and recalcitrance can vary widely between plant parts and tissues. Mechanical fractionation of biomass feedstocks has the potential to take advantage of these naturally occurring differences to produce physically or chemically unique fractions that have higher or lower value within specific biochemical or thermochemical conversion platforms. Mechanical fractionation techniques that have shown promise include density separations in air or liquid, size separations, and automated sorting. Density separations, such as air classification, separate based on particle density, size, and shape. Size separations are highly dependent upon size reduction techniques, however anatomical differences can affect particle size. The chemical and physical properties of the resulting fractions help determine their optimal use. These unique fractions can be used directly or treated as ingredients and blended with other feedstocks or fractions of feedstocks to meet cost, quality, or conversion specifications.

09 BIOMASS FUELS↗
DOE OSTI2023

Concept of a polarized positron source for CEBAF

This thesis focuses on the development of polarized and unpolarized positron beams for the future experimental pro- gram of hadronic physics at the Thomas Jefferson National Accelerator Facility (JLab). The primary challenge is to produce high-duty-cycle and high-intensity polarized positron beams. The JLab positron source, which is based on the PEPPo (Polarized Electrons for Polarized Positrons) technique, aims to use a high intensity (1 mA) and highly polarized (90%) continuous electron beam of moderate energy (120 MeV) to produce either a low intensity (>50 nA), highly polarized (60%) positron beam or a high intensity (>1 ?A), unpolarized positron beam. The optimization of the layout and performance of the positron source is examined in this thesis. The source is designed with a specialized second injector to generate, transport, accelerate, and shape positron beams. It is compatible with acceleration at the Continuous Electron Beam Accelerator Facility (CEBAF), and the investigation results are presented in this document.

Habet, Sami↗
DOE OSTI2024

Piston geometry and stroke optimization for high efficiency propane spark ignition engines

Propane has unique properties and offers interesting characteristics for high-efficiency spark ignition engines. Its high volatility reduces or completely eliminates fuel-wall wetting and facilitates fuel air mixing. Furthermore, propane has a research octane number of 112 and a high octane sensitivity of 15. Finally, its laminar flame speed is on the same order as that of conventional gasoline, and it exhibits high dilution tolerance. Modern spark ignition internal combustion engines rely on fast combustion rates and high dilution to achieve high brake thermal efficiencies. To accomplish this, high stroke-to-bore ratios and high geometric compression ratios have been used in new engine designs. Therefore, propane’s relatively high laminar flame speeds, high knock resistance, and dilution tolerance make it an excellent candidate fuel for modern spark ignition engines. The objective of this work is to co-optimize the piston geometry and the engine stroke to maximize the efficiency of a spark-ignition engine fueled with propane. 3D computational fluid dynamics (CFD) simulations employing the extended coherent flamelet model were used to study the parametric effects of piston shape and stroke length. A piston geometry based on high performing pistons was parameterized using four controlling parameters. The piston geometry and engine stroke design space was explored using deterministic and quasi-random sampling techniques. In conclusion, a Gaussian process regression model was built using the simulation data to explain the results observed.

33 ADVANCED PROPULSION SYSTEMS↗
DOE OSTI2020

Shoulder fillet effects in strength distributions of microelectromechanical system components

The failure forces and fracture strengths of polysilicon microelectromechanical system (MEMS) components in the form of stepped tensile bars with shoulder fillets were measured using a sequential failure chain methodology. Approximately 150 specimens for each of four fillet geometries with different stress concentration factors were tested. The resulting failure force and strength distributions of the four geometries were related by a common sidewall flaw population existing within different effective stressed lengths. The failure forces, strengths, and flaw population were well described by a weakest-link based analytical framework. Finite element analysis was used to verify body-force based expressions for the stress concentration factors and to provide insight into the variation of specimen effective length with fillet geometry. Monte Carlo simulations of flaw size and location, based on the strength measurements, were also used to provide insight into fillet shape and size effects. The successful description of the shoulder fillet specimen strengths provides further empirical support for application of the strength and flaw framework in MEMS fabrication and design optimization.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Parametric Study of Two Stable Forms of Discharge Burning in a Hall-Effect Thruster

In this paper, we experimentally studied the behavior of the integral operation parameters of a Hall-effect thruster (thrust and specific thrust impulse) in two stable discharge glow modes significantly different from each other in anodic efficiency. The studies were conducted using a laboratory model of a Hall-effect thruster with an extended layer with an average discharge channel diameter of 77 mm in the discharge voltage range of 500–900 V and in the gas-flow rate range from 2 to 5 mg/s. The most striking distinguishing features of the observed glow regimes are the plasma jet shape and the discharge current value at almost identical input parameters (gas flow, discharge voltage, and magnetic field). In the entire studied range of input parameters when changing from the optimal mode (in terms of efficiency) to suboptimal, the main integral characteristics of the engine are shown to undergo a stepwise change: the discharge current increases by 10‒30% with a simultaneous relative drop in thrust by 5–15% and in efficiency by 20–40%. A detailed study of the anodic efficiency structure showed that, at abrupt rebuilding, the efficiency of using the electron current (the ratio of the ion current to the discharge current) changes, i.e., the electron conductivity in the engine discharge channel.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2020

Optimization of anionic conductivity through the coexistence of ionomer cluster and backbone-backbone morphologies in anion exchange membranes

Random copolymers of poly(4-vinylpyridine) and polyisoprene were synthesized, and subsequently quaternized with 1-alkylbromides. The number of carbons on the pendant side-chain of the resultant comb-shaped polymer, n, ranged from 2-8. The comb-shaped polymers were crosslinked employing thiol-ene chemistry to give mechanically robust ion conducting membranes. Analysis by wide and medium-angle X-ray scattering show three morphology regimes that are dependent on the number of carbons on the pendant side-chains. When n = 2, ionomer cluster morphology was dominant, when n = 8 backbone-backbone morphology was dominant, and when n = 3-6, the membrane showed a coexistence of both ionomer cluster and backbone-backbone morphologies. Evaluation of the water uptake of the membranes showed a maximum water uptake per cation of 9.5 when n = 5 at 95% relative humidity (RH) and 60 degrees C. Finally, conductivity of the samples characterized by electrochemical impedance spectroscopy showed bromide conductivity as high as 110 mS/cm when n = 3 at 95% RH and 90 degrees C.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Packing Optimization for Dismantled Parts of Nuclear Facilities - 18625

When dismantling nuclear facilities, the removed large components must be segmented into smaller pieces and then be stored in the containers. With good cutting and packing plans, the required quantity of the containers can be decreased, and the cost spent in preparing, handling, and disposing these containers can also be reduced significantly. To accomplish this, this article developed a methodology which integrates voxelization and metaheuristic to discretize the irregular shape of various components and put them into the containers with greatest efficiency. The proposed methodology is not limited to certain component types or shapes and can be used for all types of containers. This makes the proposed methodology useful and practical in the nuclear facility dismantling applications. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2023

THE ROLE OF FRACTURE PROPERTIES ON LAP JOINT STRENGTH OF FRICTION STIR WELDED AA7055-T6 SHEETS

Friction stir lap welded (FSLW) joints have weight-saving potential in aluminum-intensive automotive assembly. However, FSLW also modifies the material microstructure close to the joint. Optimizing the FSLW process requires understanding the relationship between the strength and the joint's microstructure. In previous studies, efforts have been dedicated to determining the effects of local softening, the shape of the oxide line, and porosity. However, the impact of changes to the fracture properties on the joint's strength has not been studied. In this work, strength testing, and simulations, aided by material characterization, were used to determine the role of fracture properties on the shear strength of a 3-sheet (aluminum alloys 7055-7055-6022) lap joint. Characterization involved testing for fracture properties in the weld region. This data was then implemented into finite element simulations. As a result, the joint strength was predicted with a deviation of less than 6% from the experimental value. Comparison with strength prediction using only the base metal properties indicates that fracture property in the nugget region determines the strength of AA7055 FSLW.

Finite element analysis, Friction stir welding, ma↗
DOE OSTI2019

Strong Effect of B-Site Substitution on the Reactivity of Layered Perovskite Oxides Probed via Isopropanol Conversion

Perovskite oxides have exhibited promising catalytic performance in photocatalysis, electrocatalysis, and thermocatalysis, and the performance can be potentially enhanced by tuning their flexible electronic and crystal structure. The B cations play essential roles in altering the inherent electronic structure and catalytic activity of these perovskite and related layered perovskite oxides. In this work, we establish the relationship of composition (electronic structure) and activity/selectivity of layered perovskites for pure and B-site-substituted Sr 2 Sn 1–x Ru x O 4 (0 ≤ x ≤ 1). The isopropanol decomposition was chosen as the probe reaction because its divergent reaction pathways through either dehydration or dehydrogenation are sensitive to the solid-state properties of the metal oxide surface. Both parent layered perovskite oxides of Sr 2 SnO 4 and Sr 2 RuO 4 exhibited decent conversion (20%–30%) and selectivity (>80%) to acetone through dehydrogenation pathways over a temperature range of 280–320 °C under atmospheric pressure, while the substituted perovskites, even with 10 at. % substitution of Ru at the B-site showed remarkable enhancement for both activity and selectivity toward acetone. Further, by varying the dopant concentration linearly, both activity and selectivity showed a volcano-shaped dependency on the atomic concentration of the substituent. Overall, the Sr 2 Sn 0.3 Ru 0.7 O 4 had the best yield of acetone, which may be ascribed to the optimal eg filling state of the B site for appropriate B–O bond strength during oxygenate adsorption. The current research provides a guide to the rational design of perovskite oxide catalysts for efficient chemical transformation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Periodic corrugations to increase efficiency of thermophotovoltaic emitting structures

In this letter we consider the question of designing insulator/metal thermovoltaic structures with periodically corrugated interfaces that give optimal performance based on the metric of useful power density. Using a Monte Carlo approach in a robust, rapid, and high-accuracy numerical simulation strategy, we have identified such interface shapes. Here, we searched among the class of sinusoids and found that a flat-interface configuration could be significantly improved in transverse magnetic polarization. More specifically, we found that (i.) the performance improves with increasing corrugation amplitude (ii.) up to a maximum, (iii.) the shape of the corrugation is largely irrelevant, and (iv.) the period of the corrugation should be chosen in connection to the bandgap energy of the photovoltaic cell. For the latter we provide a simple expression as a starting point for practitioners interested in fabricating such structures.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Tailored laser pulse chirp to maintain optimum radiation pressure acceleration of ions

Ion beams generated with ultra-intense laser-plasma accelerators hold promises to provide compact and affordable beams of relativistic ions. One of the most efficient acceleration setups was demonstrated to be direct acceleration by the laser's radiation pressure. Due to plasma instabilities developing in the ultra-thin foils required for radiation pressure acceleration, however, it is challenging to maintain stable acceleration over long distances. Recent studies demonstrated, on the other hand, that specially tailored laser pulses can shorten the required acceleration distance suppressing the onset of plasma instabilities. Here, we extend the concept of specific laser pulse shapes to the experimentally accessible parameter of a frequency chirp. We present a novel analysis of how a laser pulse chirp may be used to drive a foil target constantly maintaining optimal radiation pressure acceleration conditions for in dependence on the target's areal density and the laser's local field strength. Our results indicate that an appropriately frequency chirped laser pulse yields a significantly enhanced acceleration to higher energies and over longer distances suppressing the onset of plasma instabilities.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

The POLARBEAR Fourier transform spectrometer calibrator and spectroscopic characterization of the POLARBEAR instrument

Here we describe the Fourier Transform Spectrometer (FTS) used for in-field testing of the POLARBEAR receiver, an experiment located in the Atacama Desert of Chile which measures the cosmic microwave background (CMB) polarization. The POLARBEAR-FTS (PB-FTS) is a Martin-Puplett interferometer designed to couple to the Huan Tran Telescope (HTT) on which the POLARBEAR receiver is installed. The PB-FTS measured the spectral response of the POLARBEAR receiver with signal-to-noise ratio >20 for ~69% of the focal plane detectors due to three features: a high throughput of 15.1 sr cm 2 , optimized optical coupling to the POLARBEAR optics using a custom designed output parabolic mirror, and a continuously modulated output polarizer. The PB-FTS parabolic mirror is designed to mimic the shape of the 2.5 m-diameter HTT primary reflector, which allows for optimum optical coupling to the POLARBEAR receiver, reducing aberrations and systematics. One polarizing grid is placed at the output of the PB-FTS and modulated via continuous rotation. This modulation allows for decomposition of the signal into different harmonics that can be used to probe potentially pernicious sources of systematic error in a polarization-sensitive instrument. The high throughput and continuous output polarizer modulation features are unique compared to other FTS calibrators used in the CMB field. In-field characterization of the POLARBEAR receiver was accomplished using the PB-FTS in April 2014. We discuss the design, construction, and operation of the PB-FTS and present the spectral characterization of the POLARBEAR receiver. We introduce future applications for the PB-FTS in the next-generation CMB experiment, the Simons Array.

47 OTHER INSTRUMENTATION↗
DOE OSTI2023

Effect of aspect ratio on plasma response to resonant magnetic perturbations in tokamak devices

A systematic numerical study is carried out, computing and comparing the plasma response to the resonant magnetic perturbation (RMP) field, applied for controlling edge localized modes (ELMs), in a series of tokamak plasmas with varying aspect ratio and utilizing the MARS-F code. The aspect ratio is scanned either by varying the plasma major radius at a fixed minor radius or by varying the latter while fixing the former. Both approaches yield similar results when compared in terms of quantities with proper normalizations. In general, a non-monotonic dependence of the resonant response field (normalized by the vacuum counterpart) near the plasma edge is found with varying aspect ratio, indicating that a given ELM control coil current configuration strongly favors plasmas with a certain aspect ratio. This optimal aspect ratio, on the other hand, depends on the toroidal as well as poloidal (i.e., coil phasing) spectra of the applied RMP field. The equilibrium (edge) safety factor, the plasma shape, and the plasma toroidal flow are all fixed to ensure that the effects identified here are predominantly due to the plasma aspect ratio.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗