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At least 1,297 records · Page 72
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

An iterative phase-space explicit discontinuous Galerkin method for stellar radiative transfer in extended atmospheres

In this work, a phase-space discontinuous Galerkin (PSDG) method is presented for the solution of stellar radiative transfer problems. It allows for greater adaptivity than competing methods without sacrificing generality. The method is extensively tested on a spherically symmetric, static, inverse-power-law scattering atmosphere. Results for different sizes of atmospheres and intensities of scattering agreed with asymptotic values. The exponentially decaying behavior of the radiative field in the diffusive-transparent transition region, and the forward peaking behavior at the surface of extended atmospheres were accurately captured. The integrodifferential equation of radiation transfer is solved iteratively by alternating between the radiative pressure equation and the original equation with the integral term treated as an energy density source term. In each iteration, the equations are solved via an explicit, flux-conserving, discontinuous Galerkin method. Finite elements are ordered in wave fronts perpendicular to the characteristic curves so that elemental linear algebraic systems are solved quickly by sweeping the phase space element by element. Two implementations of a diffusive boundary condition at the origin are demonstrated wherein the finite discontinuity in the radiation intensity is accurately captured by the proposed method. This allows for a consistent mechanism to preserve photon luminosity. The method was proved to be robust and fast, and a case is made for the adequacy of parallel processing. In addition to classical two-dimensional plots, results of normalized radiation intensity were mapped onto a log-polar surface exhibiting all distinguishing features of the problem studied.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Extreme-scale EV charging infrastructure planning for last-mile delivery using high-performance parallel computing

Here, this paper addresses stochastic charger location and allocation problems under queue congestion for last-mile delivery using electric vehicles (EVs). The objective is to decide where to open charging stations and how many chargers of each type to install, subject to budgetary and waiting-time constraints. We formulate the problem as a mixed-integer non-linear program, where each station-charger pair is modeled as a multiserver queue with stochastic arrivals and service times to capture the notion of waiting in fleet operations. The model is extremely large, with billions of variables and constraints for a typical metropolitan area; even loading the model in solver memory is difficult, let alone solving it. To address this challenge, we develop a Lagrangian-based dual decomposition framework that decomposes the problem by station and leverages parallelization on high-performance computing systems, where the subproblems are solved by using a cutting plane method and their solutions are collected at the master level. We also develop a three-step rounding heuristic to transform the fractional subproblem solutions into feasible integral solutions. Computational experiments on data from the Chicago metropolitan area with hundreds of thousands of households and thousands of candidate stations show that our approach produces high-quality solutions in cases where existing exact methods cannot even load the model in memory. We also analyze various policy scenarios, demonstrating that combining existing depots with newly built stations under multiagency collaboration substantially reduces costs and congestion. These findings offer a scalable and efficient framework for developing sustainable large-scale EV charging networks.

Capacity allocation↗
DOE OSTI2015

Critical Factors Driving the High Volumetric Uptake of Methane in Cu 3 (btc) 2

We report a thorough experimental and computational study has been carried out to elucidate the mechanistic reasons for the high volumetric uptake of methane in the metal–organic framework Cu 3 (btc) 2 (btc 3– = 1,3,5-benzenetricarboxylate; HKUST-1). Methane adsorption data measured at several temperatures for Cu 3 (btc) 2 , and its isostructural analogue Cr 3 (btc) 2 , show that there is little difference in volumetric adsorption capacity when the metal center is changed. In situ neutron powder diffraction data obtained for both materials were used to locate four CD 4 adsorption sites that fill sequentially. This data unequivocally shows that primary adsorption sites around, and within, the small octahedral cage in the structure are favored over the exposed Cu 2+ or Cr 2+ cations. These results are supported by an exhaustive parallel computational study, and contradict results recently reported using a time-resolved diffraction structure envelope (TRDSE) method. Moreover, the computational study reveals that strong methane binding at the open metal sites is largely due to methane–methane interactions with adjacent molecules adsorbed at the primary sites instead of an electronic interaction with the metal center. Simulated methane adsorption isotherms for Cu 3 (btc) 2 are shown to exhibit excellent agreement with experimental isotherms, allowing for additional simulations that show that modifications to the metal center, ligand, or even tuning the overall binding enthalpy would not improve the working capacity for methane storage over that measured for Cu 3 (btc) 2 itself.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Generation of large-scale, barrier-free diffuse plasmas in air at atmospheric pressure using array wire electrodes and nanosecond high-voltage pulses

This paper introduces a method to generate large-scale diffuse plasmas by using a repetition nanosecond pulse generator and a parallel array wire-electrode configuration. We investigated barrier-free diffuse plasmas produced in the open air in parallel and cross-parallel array line-line electrode configurations. We found that, when the distance between the wire-electrode pair is small, the discharges were almost extinguished. Also, glow-like diffuse plasmas with little discharge weakening were obtained in an appropriate range of line-line distances and with a cathode-grounding cross-electrode configuration. As an example, we produced a large-scale, stable diffuse plasma with volumes as large as 18 × 15 × 15 cm{sup 3}, and this discharge region can be further expanded. Additionally, using optical and electrical measurements, we showed that the electron temperature was higher than the gas temperature, which was almost the same as room temperature. Also, an array of electrode configuration with more wire electrodes had helped to prevent the transition from diffuse discharge to arc discharge. Comparing the current waveforms of configurations with 1 cell and 9 cells, we found that adding cells significantly increased the conduction current and the electrical energy delivered in the electrode gaps.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Implementation of analytic gradients for CCSD and EOM-CCSD using Cholesky decomposition of the electron-repulsion integrals and their derivatives: Theory and benchmarks

We present a general formulation of analytic nuclear gradients for the coupled-cluster with single and double substitution (CCSD) and equation-of-motion (EOM) CCSD energies computed using Cholesky decomposition (CD) representations of the electron repulsion integrals. By rewriting the correlated energy and response equations such that the storage of the largest four-index intermediates is eliminated, CD leads to a significant reduction in disk storage requirements, reduced I/O penalties, and an improved parallel performance. CD thus extends the scope of the systems that can be treated by (EOM-)CCSD methods, although analytic gradients in the framework of CD are needed to extend the applicability of (EOM-)CCSD methods in the context of geometry optimizations. This paper presents a formulation of analytic (EOM-)CCSD gradient within the CD framework and reports on the salient details of the corresponding implementation. The accuracy and the capabilities of analytic CD-based (EOM-)CCSD gradients are illustrated by benchmark calculations and several illustrative examples.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

High-Fidelity Velocity and Concentration Measurements of Turbulent Buoyant Jets

Accurate models of turbulent buoyant flows are essential for the design of the cooling circuit of nuclear reactors and passive safety systems. However, available models fail to fully capture the physics of turbulent mixing when buoyancy becomes predominant with respect to momentum. Therefore, high-fidelity experiments of well-controlled fundamental flows are needed to develop and validate more accurate models. We analyze experiments of positive and negative turbulent buoyant jets, both in uniform and stratified environments, with the aim of understanding the thermal hydraulics of turbulent mixing with variable density and providing high-fidelity data for the development and validation of turbulence models. Non-intrusive, simultaneous particle image velocimetry and laser-induced fluorescence measurements were carried out to acquire instantaneous velocity and concentration fields on a vertical section parallel to the axis of a jet in the self-similar region. The refractive index matching method was applied to measure high-resolution buoyant jets with up to 8.6% density difference. These data are free of the typical errors that characterize optical measurements of buoyancy-driven flows (e.g. natural and mixed convection) where the refractive index of the fluid is inhomogeneous throughout the measurement domain. Turbulent statistics and entrainment of buoyant jets in uniform and stratified environments are presented. These data are compared with non-buoyant jets in a uniform environment, as a reference to investigate the effects of buoyancy and stratification on turbulent mixing. The results will be used for the assessment of current turbulence models and as a basis for the development of a new model that captures turbulent mixing.

laser induced fluorescence↗
DOE OSTI2020

Diffusion Synthetic Acceleration for Heterogeneous Domains, Compatible with Voids

A standard approach to solving the S N transport equations is to use source iteration with diffusion synthetic acceleration (DSA). Although this approach is widely used and effective on many problems, there remain some practical issues with DSA preconditioning, particularly on highly heterogeneous domains. For large-scale parallel simulation, it is critical that both (a) preconditioned source iteration converges rapidly and (b) the action of the DSA preconditioner can be applied using fast, scalable solvers, such as algebraic multigrid (AMG). For heterogeneous domains, these two interests can be at odds. In particular, there exist DSA diffusion discretizations that can be solved rapidly using AMG, but they do not always yield robust/fast convergence of the larger source iteration. Conversely, there exist robust DSA discretizations where source iteration converges rapidly on difficult heterogeneous problems, but fast parallel solvers like AMG tend to struggle applying the action of such operators. Moreover, very few current methods for the solution of deterministic transport are compatible with voids. This paper develops a new heterogeneous DSA preconditioner based on only preconditioning the optically thick subdomains. The resulting method proves robust on a variety of heterogeneous transport problems, including a linearized hohlraum mesh related to inertial confinement fusion. Moreover, the action of the preconditioner is easily computed using O(1) AMG iterations, convergence of the transport iteration typically requires 2 to 5× fewer iterations than current state-of-the-art “full” DSA, and the proposed method is trivially compatible with voids. On the hohlraum problem, rapid convergence is obtained by preconditioning less than 3% of the mesh elements with five to ten AMG iterations.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2025

LATTE: open-source, high-performance traveltime computation, tomography and source location in acoustic and elastic media

Traveltime-based tomography and source location are fundamental approaches for imaging subsurface structures and understanding the spatiotemporal distribution of seismicity from local to global scales. We present an open-source, high-performance framework integrating eikonal equation solvers and adjoint-state theory for traveltime computation, velocity tomography, source location and joint tomography-location in 2-D/3-D acoustic and elastic media. We introduce novel regularization schemes based on total generalized p-variation, structural similarity and multitask machine learning to enhance the fidelity and interpretability of inverted models and source locations. Key features of our implementation also include the ability to leverage both absolute-difference and double-difference traveltime misfits for high-fidelity velocity tomography and source parameter estimation; support for traveltime computation and inversion in diverse 2-D/3-D scenarios with arbitrary source and receiver distributions; and a perturbation-based optimal step-size estimation method to reduce computational costs. In addition, our implementation employs shared-memory and distributed-memory parallelization to provide an efficient solution for traveltime computation, tomography, and source location. In conclusion, we validate the efficacy and accuracy of our approach through multiple synthetic data examples.

58 GEOSCIENCES↗
DOE OSTI2014

TH-C-19A-09: Quantification of Transmission and Backscatter Factors as a function of Distance to Inhomogeneity Interface for Three Types of Surgical Implant Plates

Purpose: Carbon fiber materials have been increasingly used clinically, mainly in orthopedics, as an alternative to metallic implants because of their minimal artifacts on CT and MRI images. This study characterizes the transmission and backscatter property of carbon fiber plates (CarboFix Orthopedics, Herzeliya, Israel) with measurements for radiation therapy applications, and compares them to traditional Stainless Steel (SS) and Titanium (Ti) metal materials. Methods: For the transmission measurements, 1-mm-thick test plate was placed upstream from a plane parallel Markus chamber, separated by various thicknesses of polystyrene plates in 0.5 cm increments between 0 and 5 cm. With this setup, we quantified the radiation transmission as a function of distance to the inhomogeneity interface. The LINAC source to detector distance was maintained at 100 cm and 200 MU was delivered for each measurement. Two 3-cm solid water phantoms were placed at the top and bottom to provide build up. All the measurements were performed for 6 MV and 18 MV photons. The backscatter measurements had the identical setup, except that the test plate was downstream of the chamber from radiation. Results: The carbon fiber plates did not introduce any measureable inhomogeneity effect on the transmission and backscatter factor because of its low atomic number. In contrast, traditional metal implant materials caused up to 15% dose difference at upstream and 25% backscatter at downstream from radiation. Such differences decrease as the distance to the inhomogeneity interface increases and become unmeasurable at distance of 3 cm and 1 cm for upstream and downstream, respectively. Conclusion: A new type of carbon fiber implant plate was evaluated and found to have minimal inhomogeneity effect in MV radiation beams. Patients would benefit from a carbon based implant over metal for radiation therapy due to their minimal backscatter and imaging artifacts.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2015

SU-D-210-07: The Dependence On Acoustic Velocity of Medium On the Needle Template and Electronic Grid Alignment in Ultrasound QA for Prostate Brachytherapy

Purpose: To analyze the impact on acoustic velocity (AV) of two different media (water and milk) using the needle template/electronic grid alignment test. Water, easily available, makes a good material to test the alignment of the template and grid although water’s AV (1498 m/s at 25°C) is significantly different from tissue (1540 m/s). Milk, with an AV much closer (1548 m/s) to prostate tissue, may be a good substitute for water in ultrasound quality assurance testing. Methods: Tests were performed using a Hitachi ultrasound unit with a mechanical arrangement designed to position needles parallel to the transducer. In this work, two materials – distilled water and homogenized whole milk (AVs of 1498 and 1548 m/s at 25°C) were used in a phantom to test ultrasound needle/grid alignment. The images were obtained with both materials and analyzed for their placement accuracy. Results: The needle template/electronic grid alignment tests showed displacement errors between measured and calculated values. The measurements showed displacements of 2.3mm (water) and 0.4mm (milk), and 1.6mm (water) and 0.3mm (milk) at depths of 7cm and 5cm respectively from true needle positions. The calculated results showed a displacement of 2.36 mm (water); 0.435mm (milk), and 1.66mm (water) and 0.31mm (milk) at a depth of 7cm and 5cm respectively. The displacements in the X and Y directions were also calculated. At depths of 7cm and 5cm, the (ΔX,ΔY) displacements in water were (0.829mm, 2.21mm) and (0.273mm, 1.634mm) and for milk were (0.15mm, 0.44mm) and (0.05mm, 0.302mm) respectively. Conclusion: The measured and calculated values were in good agreement for all tests. They show that milk provides superior results when performing needle template and electronic grid alignment tests for ultrasound units used in prostate brachytherapy.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2022

When Do Extended Physics-Informed Neural Networks (XPINNs) Improve Generalization?

Physics-informed neural networks (PINNs) have become a popular choice for solving high-dimensional partial differential equations (PDEs) due to their excellent approximation power and generalization ability. Recently, extended PINNs (XPINNs) based on domain decomposition methods have attracted considerable attention due to their effectiveness in modeling multiscale and multiphysics problems and their parallelization. However, theoretical understanding of their convergence and generalization properties remains unexplored. In this study, we take an initial step towards understanding how and when XPINNs outperform PINNs. Specifically, for general multilayer PINNs and XPINNs, we first provide a prior generalization bound via the complexity of the target functions in the PDE problem and a posterior generalization bound via the posterior matrix norms of the networks after optimization. Moreover, based on our bounds, we analyze the conditions under which XPINNs improve generalization. Concretely, our theory shows that the key building block of XPINN, namely, the domain decomposition, introduces a tradeoff for generalization. On the one hand, XPINNs decompose the complex PDE solution into several simple parts, which decreases the complexity needed to learn each part and boosts generalization. On the other hand, decomposition leads to less training data being available in each subdomain, and hence such a model is typically prone to overfitting and may become less generalizable. Empirically, we choose five PDEs to show when XPINNs perform better than, similar to, or worse than PINNs, hence demonstrating and justifying our new theory.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2019

Efficient and Effective Sparse Tensor Reordering

This paper proposed two reordering schemes for sparse tensors: BFS-MCS and LEXI-ORDER. BFS-MCS is a Breadth First Search (BFS)-like heuristic approach based on the maximum cardinality search family; LEXI-ORDER is an extension of doubly lexical ordering of matrices to tensors. CANDECOMP/PARAFAC decomposition (CPD) is taken as an example to show their effect on existing three sparse tensor formats for CPUs: coordinate (COO), compressed sparse fiber (CSF), and hierarchical coordinate (HICOO). LEXI-ORDER obtains up to 4.14× speedup on sequential HICOO-MTTKRP and 11.88× speedup on its parallel case. COO- and CSF-MTTKRP also achieves performance improvement in different degree. Our two reordering methods are more efficient and effective than other state-of-the-art reordering methods used in SPLATT.

HPC↗
DOE OSTI2023

Brief Announcement: Communication Optimal Sparse LU Factorization for Planar Matrices

We introduce a new parallel algorithm for solving sparse LU factorization of planar matrices, which commonly arise in the finite element method for 2D PDEs. Existing scalable methods, such as the multifrontal approach with subtree-to-subcube mapping by Gupta et al. [1] and right-looking with 3D mapping by Sao et al. [2] fail to achieve optimal communication costs for these matrices. Our new algorithm combines 3D mapping and subtree-to-subcube mapping to minimize communication costs while allowing trade-offs between extra memory and reduced communication. We demonstrate that our proposed algorithm attains the communication lower bound up to a factor of O(log log n) in the memory-optimal case and up to a factor of O(log P) in the memory-independent case for an n-dimensional planar sparse matrix on P processors.

Sao, Piyush↗
DOE OSTI2023

PNNL-CompBio/BoltzmannMFX

BoltzmannMFX is a biological simulation code that solves chemical reaction networks using maximum entropy methods. It uses modules from MFiX-Exa and is based on the AMReX framework for massively parallel block-structured adaptive mesh applications.

Palmer, Bruce↗
DOE OSTI2025

Matrix-based Parallel Redistribution

MatRed is a parallel redistribution tool for HPC applications. It provides a simple approach that only requires a few relation matrices between entities to build redistribution matrices in parallel simulation codes. In particular, MatRed is well-suited for simulation codes based on finite element/volume methods.

Kalchev, DelyanZ [Lawrence Livermore National Labo↗
DOE OSTI2014

Ponderomotive Forces On Waves In Modulated Media

Nonlinear interactions of waves via instantaneous cross-phase modulation can be cast in the same way as ponderomotive wave-particle interactions in high-frequency electromagnetic fi eld. The ponderomotive effect arises when rays of a probe wave scatter off perturbations of the underlying medium produced by a second, modulation wave, much like charged particles scatter off a quasiperiodic field. Parallels with the point-particle dynamics, which itself is generalized by this theory, lead to new methods of wave manipulation, including asymmetric barriers for light.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Non-iterative Voltage Stability

This report demonstrates promising capabilities and performance characteristics of the proposed method using several power systems models. The new method will help to develop a new generation of highly efficient tools suitable for real-time parallel implementation. The ultimate benefit obtained will be early detection of system instability and prevention of system blackouts in real time.

24 POWER TRANSMISSION AND DISTRIBUTION↗
DOE OSTI2016

Poly(p-Phenylene Sulfonic Acids). PEMs with frozen-in free volume

Early work with rigid rod aromatic polyelectrolytes implied that steric hindrance in packing of the rigid rods left unoccupied volumes that could absorb and hold water molecules strongly. We called this “frozen in free volume). It is illustrated and contrasted with the packing of flexible backbone polyelectrolytes (Reference 5 of this report). This was quantified for poly(biphenylene disulfonic acid) (PBDSA) and poly(phenylene disulfonic acid) (PPDSA). We found that PPDSA held three water molecules per acid group down to 11% relative humidity (RH) and had very high conductivity even at these low RHs. (Reference 1 of report.) The frozen-in free volume was calculated to be equivalent to a λ of 3.5. The work reported below concentrated on studying these polymers and their copolymers with biphenylene disulfonic acid. As expected, the polyelectrolytes are water soluble. Several approaches towards making water stable films were studied. Grafting alkyl benzene substituents on sulfonic acid groups had worked for PBPDSA (1) so it was tried with PPDSA and a 20%/80% copolymer of BPDSA and PDSA (B20P80). T-butyl, n-octyl and n-dodecyl benzene were grafted. Good films could be made. Water absorption and conductivity were studied as a function of RH and temperature (Reference 2). When less than 20% of the sulfonic acid groups were grafted, conductivity was much higher than that of Nafion NR212 at all RHs. At low graft levels, conductivity was ten times higher. Mechanical properties and swelling were acceptable below 90% RH. However, all the films were unstable in water and slowly disintegrated. The proposed explanation was that the molecules formed nano-aggregates in solution held together by hydrophobic bonding. Their cast films disintegrated when placed in water since hydrophobic bonding between the nano-aggregates was poor. We then shifted to crosslinking as a method to produce water stable films (References 3 and 4). Biphenyl could easily be reacted with the polymer, generating biphenyl sulfone grafts. Films were cast and then crosslinked by heating to 210°C for at least one hour. There was no loss of acid groups even after heating at 225°C for two hours. PPDSA and B20P80 polymers were grafted and crosslinked. Conductivity and water absorption were measured on polymers with grafting degrees from 3 to 16%. As before they all had much higher conductivity than Nafion NR212. One sample was tested as a fuel cell membrane. Unfortunately it tore; the tear was mended with a 3M ionomer. There was some hydrogen leakage. However, the current at a given voltage was about 95% that of a Nafion NR212 membrane over the whole useful range. Considering that this was the first test for such a system, this is remarkable performance. A mechanically better membrane was tested at the end of the grant period. This did not have tears or micro cracks (Reference 5). Testing confirmed earlier data (Sergio Granados-Focil, Ph. D. Thesis, CWRU. 2006) that such very polar membranes were excellent barriers to hydrogen diffusion. Standard measurements showed no hydrogen leakage. This was confirmed by open circuit voltage measurements. They were higher than those of the Nafion NR212 cells. Again the single cell performance was about 95% that of the Nafion SR212 cell over the whole range. These films show great promise. They are very stable and perform very well at very low humidity. They should be able to outperform the present cells with relatively little further development. A second project is reported fully in the final report since it was not published. Its goal was to make membranes with high conductivity and good mechanical strength that would have a constant volume at all RHs. We made large planar 2-D polymers containing 1.5 nm holes lined with sulfonic acid groups. These should hold water very strongly. The largest made was calculated to have a number average diameter of 29 nm. It was expected that when they were cast, they would aggregate and create channels for proton conduction. However, they were so large that they aggregated in solution during preparation. Measurements showed that the aggregates developed long channels. This was demonstrated and is reported. Unfortunately, the aggregates could not be cast with the channels perpendicular to the film plane. If methods are developed to keep the molecules in solution until they are cast, their planes should be parallel and the channels perpendicular to the film direction. They could be very interesting and perhaps important materials.

25 ENERGY STORAGE↗