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At least 1,063 records · Page 59

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DOE OSTI2015

Efficient surface plasmon amplification in gain-assisted silver nanotubes and associated dimers

SPASER (surface plasmon amplification by stimulated emission of radiation) properties in active SiO{sub 2}–Ag nanotubes and associated dimers have been investigated by using the scattering theory and the finite element method. In the active Ag nanotube, as the gain coefficient of the core increases to a critical value, a super-resonance occurs. The SPASER phenomenon also can be found in the active Ag nanotube dimer. The strong couplings between two nanotubes lead to larger gain threshold for the active Ag nanotube dimer compared with the active Ag nanotube. At the super-resonance, the maximal surface enhanced Raman scattering factor at the “hot spot” in the active Ag nanotube dimer can achieve about 8 × 10{sup 18}, which is large enough for single molecule detection. Furthermore, with increasing the separation between two Ag nanotubes, the gain threshold value for the super-resonance of the active Ag nanotube dimer decreases, while the corresponding super-resonance wavelength increases first and then decreases.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2018

Finite Element Simulation and Optimization of Gas-Quenching Process for Tool Steels

Various gas-quenching processes, including marquenching, were investigated to optimize gas-quenching processes in terms of distortion and hardness by means of numerical and experimental analyses. The temperature, microstructure, hardness, and distortion during the various gas-quenching processes of a tool steel block were simulated using a finite element method based on a coupled thermo-metallurgical–mechanical model. The predicted temperature, hardness, and distortion agreed well with the experimental data. The tool steel block (200 × 150 × 70 mm{sup 3}) quenched under 10 bar pressure of nitrogen gas (Case 2) had higher hardness due to the higher martensite fraction and larger distortion owing to the higher thermal stress induced by faster cooling, compared to the block quenched under 2 bar pressure of nitrogen gas (Case 1). The tool steel block marquenched under 10 bar pressure of nitrogen gas interrupted by isothermal holding at 500 °C (Case 3) had 30% smaller distortion with a negligible loss of hardness compared to Case 2. Furthermore, the simulation results could provide an optimized process condition to minimize distortion of the gas-quenched tool steel block while satisfying the hardness requirement.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Effect of Ag3Sn nanoparticles and temperature on Cu6Sn5 IMC growth in Sn-xAg/Cu solder joints

Highlights: • The number density of Ag{sub 3}Sn grains precipitated upon Cu{sub 6}Sn{sub 5}, increases with Ag content and reflow temperature (T). • Raising of wt. % of Ag in solder and lowering of T can cause an increase in grain diameter of Ag{sub 3}Sn nanoparticles. • The blockading effect of Ag{sub 3}Sn particles on Cu{sub 6}Sn{sub 5} IMC growth is more pronounced in Sn-3Ag and Sn-3.5Ag alloys. • IMC suppressing nanoparticles can be utilized for solder materials design. - Abstract: The growing importance of Pb-free Sn-xAg solders and high cost of Ag has attracted the attention of researchers to perform experiment with solders with varied content of Ag element. In this study, Sn-xAg(0,1,2,3.5) solder balls on Cu substrate have been reflowed at 250/275/300 °C for 60 s and air cooled. Finite element method has been utilized to simulate the concentration and temperature during cooling. The experimental results suggest that the Cu flux through grain boundary diffusion is the dominant mechanism for Cu{sub 6}Sn{sub 5} intermetallic compound growth in Pure Sn and Sn-1Ag solders. When the Ag content is increased, as in case of Sn-3Ag and Sn-3.5Ag alloys; the shielding effect of Ag{sub 3}Sn nanoparticles is the determinant factor for affecting the thickness of Cu{sub 6}Sn{sub 5} IMC. Both Cu influx rate and shielding effect of Ag{sub 3}Sn nanoparticles are enhanced with the raise in pre-cooled isothermal reflow temperature. Sn-1Ag solder is observed to have the thickest Cu{sub 6}Sn{sub 5} IMC.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2016

Multiscale Simulation Framework for Coupled Fluid Flow and Mechanical Deformation

Our work in this project is aimed at making fundamental advances in multiscale methods for flow and transport in highly heterogeneous porous media. The main thrust of this research is to develop a systematic multiscale analysis and efficient coarse-scale models that can capture global effects and extend existing multiscale approaches to problems with additional physics and uncertainties. A key emphasis is on problems without an apparent scale separation. Multiscale solution methods are currently under active investigation for the simulation of subsurface flow in heterogeneous formations. These procedures capture the effects of fine-scale permeability variations through the calculation of specialized coarse-scale basis functions. Most of the multiscale techniques presented to date employ localization approximations in the calculation of these basis functions. For some highly correlated (e.g., channelized) formations, however, global effects are important and these may need to be incorporated into the multiscale basis functions. Other challenging issues facing multiscale simulations are the extension of existing multiscale techniques to problems with additional physics, such as compressibility, capillary effects, etc. In our project, we explore the improvement of multiscale methods through the incorporation of additional (single-phase flow) information and the development of a general multiscale framework for flows in the presence of uncertainties, compressible flow and heterogeneous transport, and geomechanics. We have considered (1) adaptive local-global multiscale methods, (2) multiscale methods for the transport equation, (3) operator-based multiscale methods and solvers, (4) multiscale methods in the presence of uncertainties and applications, (5) multiscale finite element methods for high contrast porous media and their generalizations, and (6) multiscale methods for geomechanics.

97 MATHEMATICS AND COMPUTING↗

DOE OSTI2021

Effects of pillar design on the thermal performance of vacuum-insulated glazing

Vacuum-insulated glazing (VIG) with a low-emittance coating has a great market potential as an effective transparent insulator. The thermal insulating performance of VIG is determined by its design, including material selection and configuration of different components. Thermal conductance of the vacuum gap as a transport bottleneck is one of the primary factors controlling the thermal transport across VIG. In particular, because support pillars provide the main thermal transport channels across the vacuum gap, increasing the pillar thermal resistance is a key strategy for creating effective thermal insulation while maintaining the vacuum space. Here, the effects of various pillar design parameters, such as thermal conductivity, geometry, and arrangement, on the VIG thermal performance were comprehensively investigated via the finite element method. In addition, analytical models for thermal transport were examined and thermal conductance across the VIG unit was experimentally measured for validation. The pillar design parameters, especially the height, shape, spacing, and arrangement of the pillars, showed significant effects on the thermal performance of VIG. This research also shows that the smaller contact area for horizontal pillars can effectively decrease the heat loss by more than 30%. Because current VIG analytical equations of thermal performance are only applicable to cylindrical pillars, an analytical equation that can better describe the thermal performance of rectangular parallelepiped pillars is presented, along with a discussion about the mechanism of thermal transfer for different pillar shapes. Thermo-mechanical analyses based on 3D FEM simulations can provide valuable insights into the effect of various design parameters on the overall performance the VIG, allowing for the development of an optimal VIG design.

36 MATERIALS SCIENCE↗

DOE OSTI2016

Tailoring surface plasmon resonance and dipole cavity plasmon modes of scattering cross section spectra on the single solid-gold/gold-shell nanorod

Tunable surface plasmon resonance (SPR) and dipole cavity plasmon modes of the scattering cross section (SCS) spectra on the single solid-gold/gold-shell nanorod have been numerically investigated by using the finite element method. Various effects, such as the influence of SCS spectra under x- and y-polarizations on the surface of the single solid-gold/gold-shell nanorod, are discussed in detail. With the single gold-shell nanorod, one can independently tune the relative SCS spectrum width by controlling the rod length and rod diameter, and the surface scattering by varying the shell thickness and polarization direction, as well as the dipole peak energy. These behaviors are consistent with the properties of localized SPRs and offer a way to optically control and produce selected emission wavelengths from the single solid-gold/gold-shell nanorod. The electric field and magnetic distributions provide us a qualitative idea of the geometrical properties of the single solid-gold/gold-shell nanorod on plasmon resonance.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2019

Effect of α/β Forging on Microstructure and Texture Inhomogeneity in a Ti-1023 Forged Disk

The microstructure and texture distribution of a Ti-1023 forged disk were investigated by scanning electron microscopy and synchrotron-based high-energy X-ray diffraction. The finite element method was used to simulate temperature and strain distribution in order to investigate the relationship of the α/β forging process with microstructure and texture distribution. A bimodal microstructure and rolling textures with large inhomogeneity were observed in the disk. A plate-like and a necklace-like morphology and volume fraction variations of the primary α phase were observed in different regions with different forging conditions, such as temperature, deformation, position, and high density of flow lines. Texture sharpness distribution of the β phase was in good agreement with the strain distribution, which suggests deformation may play the most important part in the texture inhomogeneity. A weak cube texture was obtained near the center of the disk, and fiber textures were found near the rim of the disk. The primary a phase also exhibited a transverse texture, which is favored by large deformation.

36 MATERIALS SCIENCE↗

DOE OSTI2015

High-speed GPU-based finite element simulations for NDT

The finite element method solved with explicit time increments is a general approach which can be applied to many ultrasound problems. It is widely used as a powerful tool within NDE for developing and testing inspection techniques, and can also be used in inversion processes. However, the solution technique is computationally intensive, requiring many calculations to be performed for each simulation, so traditionally speed has been an issue. For maximum speed, an implementation of the method, called Pogo [Huthwaite, J. Comp. Phys. 2014, doi: 10.1016/j.jcp.2013.10.017], has been developed to run on graphics cards, exploiting the highly parallelisable nature of the algorithm. Pogo typically demonstrates speed improvements of 60-90x over commercial CPU alternatives. Pogo is applied to three NDE examples, where the speed improvements are important: guided wave tomography, where a full 3D simulation must be run for each source transducer and every different defect size; scattering from rough cracks, where many simulations need to be run to build up a statistical model of the behaviour; and ultrasound propagation within coarse-grained materials where the mesh must be highly refined and many different cases run.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Generalized Multiscale Inversion for Heterogeneous Problems

In this work, we propose a generalized multiscale inversion algorithm for heterogeneous problems that aims at solving an inverse problem on a computational coarse grid. Previous inversion techniques for multiscale problems seek a coarse-grid medium properties, e.g., permeability and conductivity, and by doing so, they assume that there exists a homogenized representation of the underlying fine-scale permeability field on a coarse grid. Generally such assumptions do not hold for highly heterogeneous fields, e.g., fracture media or channelized fields, where the width of channels are very small compared to the scale of coarse grids. In these cases, grid refinement can lead to many degrees of freedom, and thus numerically unattractive to apply. The proposed algorithm is based on the Generalized Multiscale Finite Element Method (GMsFEM), which uses local spectral problems to identify non-localized features, i.e., channels (high-conductivity inclusions that connect the boundaries of the coarse-grid block). The inclusion of these features in the coarse space enables one to achieve a good accuracy. The approach is valid under the assumption that the solution can be well represented in a reduced-dimensional space spanned by multiscale basis functions. In practice, these basis functions are non-observable as we do not identify the fine-scale features of the permeability field. Our inversion algorithm finds the discretization parameters of the resulting system on the coarse grid. By doing so, we identify the appropriate coarse-grid parameters representing the permeability field instead of fine-grid permeability field. We illustrate the potential of the approach by numerical results for fractured media.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2019

Novel refractive index sensor based on fiber bragg grating in nano-bore optical fiber

In this paper, we propose a novel refractive index sensor with large dynamic range based on fiber Bragg grating in the nano-bore optical fiber. The spectral responses of grating on the filling index in the bore with different bore diameters and index ranges are numerically studied by the finite element method and coupled mode theory based transmission matrix method. It is found that the Bragg wavelength of grating is sensitive to the filling index in the bore. The influences of bore diameter on the wavelength sensitivity, resolution and dynamic range of the index sensor are analyzed. There exists a trade off between the linear dynamic range and sensitivity of index when the bore diameter varies. The optimized wavelength sensitivity and resolution of filling index are 53.6923 nm/index unit and 9.3123 × 10{sup −6} index under a linear index range of 1.0–1.4 using 2 µm bore diameter. The optimized linear dynamic range of index is 1.0–1.48 with wavelength sensitivity of 13.3056 nm/index unit and resolution of 3.7578 × 10{sup −5} index using 1 µm bore diameter. Our proposed index sensor has the technical advantages of simple all-fiber structure, large index range, and potential capability to synchronously detect index, temperature and strain.

36 MATERIALS SCIENCE↗

DOE OSTI2019

Residual Stress Asymmetry in Thin Sheets of Double-Sided Shot Peened Aluminum

The local mechanical properties and residual stresses of a double-sided shot-peened aluminum alloy AA7050-T7451 sheet were measured by nanoindentation and simulated by the finite element method (FEM). The variation of the pileup area around the indentation due to residual stresses, as well as changes in hardness as a function of cold working, was assessed. Numerical simulations were conducted to investigate the residual stress profile. When the indentation contact area was corrected to account for pileup induced from residual stresses, the hardness increased about 10% over the range of depths at which compressive residual stresses exist, indicating some strain hardening did occur. The maximum compressive residual stress for the second shot-peened surface is on the order of 10% higher than the first surface; this is ascribed to the sequencing during the peening process, in which the first side peening influenced the stress throughout the part thickness for these thin plate structures. The ability to couple FEM simulations and rapid experimental assessments of residual stresses and hardness allows for future prediction of part thickness effects in double-sided peening processes.

36 MATERIALS SCIENCE↗

DOE OSTI2016

New developments in ALCYONE 2.0 fuel performance code

ALCYONE is a multi-dimensional (1D, 2D or 3D) fuel performance code dedicated to pressurized water reactors (PWR). This code is co-developed in the PLEIADES platform by CEA, EDF and AREVA and simulates fuel pellets and cladding evolutions during base irradiation, power ramps and accidental situations. Sophisticated chemical and physical models are used. The thermal and mechanical behavior is solved with the finite element method based code CAST3M. ALCYONE 2.0 is the next release of the code and it includes several new features: improvements in incidental and accidental situations modeling, new models available for 2D and 3D schemes and an evolution of neutron model to simulate gadolinium fuel. ALCYONE 2.0 uses a new version of PLEIADES platform which allows the parallelized computation of rod axial slices in order to decrease computation times. To better simulate accidental situations, an axial transfer model for gases and a modeling of heaters for Loss Of Coolant Accident (LOCA) experiments have been added to the code. The axial transfer model can simulate an axial internal pressure gradient in the fuel rod. For power ramps simulations in the 1D scheme, a new model for dish filling is available. This version will be validated on more than 400 rods covering a wide range of irradiation conditions.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗

DOE OSTI2017

Metal Big Area Additive Manufacturing: Process Modeling and Validation

Metal Big Area Additive Manufacturing (mBAAM) is a new additive manufacturing (AM) technology for printing large-scale 3D objects. mBAAM is based on the gas metal arc welding process and uses a continuous feed of welding wire to manufacture an object. An electric arc forms between the wire and the substrate, which melts the wire and deposits a bead of molten metal along the predetermined path. In general, the welding process parameters and local conditions determine the shape of the deposited bead. The sequence of the bead deposition and the corresponding thermal history of the manufactured object determine the long range effects, such as thermal-induced distortions and residual stresses. Therefore, the resulting performance or final properties of the manufactured object are dependent on its geometry and the deposition path, in addition to depending on the basic welding process parameters. Physical testing is critical for gaining the necessary knowledge for quality prints, but traversing the process parameter space in order to develop an optimized build strategy for each new design is impractical by pure experimental means. Computational modeling and optimization may accelerate development of a build process strategy and saves time and resources. Because computational modeling provides these opportunities, we have developed a physics-based Finite Element Method (FEM) simulation framework and numerical models to support the mBAAM process s development and design. In this paper, we performed a sequentially coupled heat transfer and stress analysis for predicting the final deformation of a small rectangular structure printed using the mild steel welding wire. Using the new simulation technologies, material was progressively added into the FEM simulation as the arc weld traversed the build path. In the sequentially coupled heat transfer and stress analysis, the heat transfer was performed to calculate the temperature evolution, which was used in a stress analysis to evaluate the residual stresses and distortions. In this formulation, we assume that physics is directionally coupled, i.e. the effect of stress of the component on the temperatures is negligible. The experiment instrumentation (measurement types, sensor types, sensor locations, sensor placements, measurement intervals) and the measurements are presented. The temperatures and distortions from the simulations show good correlation with experimental measurements. Ongoing modeling work is also briefly discussed.

Additive Manufacturing↗

DOE OSTI2018

Numerical Simulation of Phase Transitions in Type-II Annular Superconductor Using Time-dependent Ginzburg-Landau Equations

Time-dependent Ginzburg-Landau equations were solved by finite element method in two-dimensional space for order parameter and energy components of the annular superconducting sample in steady magnetic fields. Vortices preferred to penetrate from the inner surface of the annulus due to lesser energy required at the concave surface. A transition magnetic field strength was observed in spatial averages of carrier concentration and energy components, showing small bumps and abrupt variations, indicating phase transition from a non-vortex to vortex state. These effects were observed to repeat with every subsequent entry of a set of vortices into the sample; transition magnetic field strength was found to depend inversely on the annular width of the sample. The present work gives a better understanding of energy variations during phase transition from non-vortex to vortex state and predicts that vortex state can be avoided by tuning the wire thickness in practical applications, e.g., superconducting electromagnets.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2019

Multi-scale simulation of reaction, transport and deactivation in a SBA-16 supported catalyst for the conversion of ethanol to butadiene

Effective design of heterogeneous catalytic systems requires careful coordination of physiochemical phenomena that span orders or magnitude in length and time scales. Multiscale modelling and simulation tools can provide useful insight for understanding and improving the performance of such systems. Mesoporous silica catalyst support materials present a versatile platform wherein support attributes may be tuned to control transport phenomena throughout the system. In this study we develop an integrated multiscale modelling approach for the conversion of ethanol to butadiene over a SBA-16 mesoporous silica supported catalyst. We use molecular dynamics simulations to calculate domain specific diffusivities of reactants and products in the various domains of SBA-16's nanostructure. This is used to calculate a resultant effective diffusivity through its microstructure using finite element method (FEM) models of the tessellated unit cell. The resultant intraparticle effective diffusivity enables the development of a reduced-order reactor level FEM model that is able to implicitly account for the SBA-16's transport effects without the explicit consideration of its detailed geometry. Experimental bench-scale conversion data for the ethanol to butadiene process over SBA-16 is used to extract intrinsic kinetic rate constants for a simplified reaction mechanism using this reduced order bench-scale reactor FEM model. The model is able to reproduce experimental trends in conversion and activity lifetimes. The impact of varying key support attributes such as pore-size and particle size on catalytic activity lifetimes is then explored. We demonstrate the utility of this multi-scale modelling strategy in guiding catalyst design for the development of rational strategies to improve the performance of heterogeneous supported catalytic systems.

09 BIOMASS FUELS↗

DOE OSTI2019

Investigation on Microsegregation of IN718 Alloy During Additive Manufacturing via Integrated Phase-Field and Finite-Element Modeling

In this work, we apply a multi-scale model combining finite-element method (FEM) and phase-field model (PFM) to simulate the evolution of solidification microstructures at different locations within a molten pool of an additively manufactured IN718 alloy. Specifically, the FEM is used to calculate the shape of molten pool and the relative thermal gradient G at the macroscale. Then, the calculated thermal information is input into PFM for microstructure simulation. Finally, the morphology of solidification structures and formation of Laves phase at different sites are studied and compared. We found that the solidification site with a large angle between the temperature gradient and the preferred crystalline orientation could build up a high niobium (Nb) concentration in the liquid during solidification but has less possibility of forming continuous long chain morphology of Laves phase particles. This finding provides an understanding of the microstructure evolution inside the molten pool of IN718 alloy during solidification. Further, the finding indicates that the site with a large misorientation angle will have a good hot cracking resistance after solidification.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Asynchronous distributed-memory task-parallel algorithm for compressible flows on unstructured 3D Eulerian grids

Here, we discuss the implementation of a finite element method, used to numerically solve the Euler equations of compressible flows, using an asynchronous runtime system (RTS). The algorithm is implemented for distributed-memory machines, using stationary unstructured 3D meshes, combining data-, and task-parallelism on top of the Charm++ RTS. Charm++’s execution model is asynchronous by default, allowing arbitrary overlap of computation and communication. Task-parallelism allows scheduling parts of an algorithm independently of, or dependent on, each other. Built-in automatic load balancing enables continuous redistribution of computational load by migration of work units based on real-time CPU load measurement. The RTS also features automatic checkpointing, fault tolerance, resilience against hardware failure, and supports power-, and energy-aware computation. We demonstrate scalability up to 25 x 10 9 cells at $\mathscr{O}$10 4 compute cores and the benefits of automatic load balancing for irregular workloads. The full source code with documentation is available at https://quinoacomputing.org.

42 ENGINEERING↗

DOE OSTI2016

Deformation of contact surfaces in a vacuum interrupter after high-current interruptions

In a high-current interruption, the contact surface in a vacuum interrupter might be severely damaged by constricted vacuum arcs causing a molten area on it. As a result, a protrusion will be initiated by a transient recovery voltage after current zero, enhancing the local electric field and making breakdowns occur easier. The objective of this paper is to simulate the deformation process on the molten area under a high electric field by adopting the finite element method. A time-dependent Electrohydrodynamic model was established, and the liquid-gas interface was tracked by the level-set method. From the results, the liquid metal can be deformed to a Taylor cone if the applied electric field is above a critical value. This value is correlated to the initial geometry of the liquid metal, which increases as the size of the liquid metal decreases. Moreover, the buildup time of a Taylor cone obeys the power law t = k × E{sup −3}, where E is the initial electric field and k is a coefficient related to the material property, indicating a temporal self-similar characteristic. In addition, the influence of temperature has little impact on the deformation but has great impact on electron emission. Finally, the possible reason to initiate a delayed breakdown is associated with the deformation. The breakdown does not occur immediately when the voltage is just applied upon the gap but is postponed to several milliseconds later when the tip is formed on the liquid metal.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗