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Deuterium Hugoniot: Pitfalls of thermodynamic sampling beyond density functional theory

Physical Review B

Clay III, Raymond C.; Desjarlais, Michael P.; Shulenburger, Luke N.

Outstanding problems in the high-pressure phase diagram of hydrogen have demonstrated the need for more accurate ab initio methods for thermodynamic sampling. One promising method that has been deployed extensively above 100 GPa is coupled electron-ion Monte Carlo (CEIMC), which treats the electronic structure with quantum Monte Carlo (QMC). However, CEIMC predictions of the deuterium principal Hugoniot disagree significantly with experiment, overshooting the experimentally determined peak compression density by 7% and lower temperature gas-gun data by well over 20%. By deriving an equation relating the predicted Hugoniot density to underlying equation of state errors, we show that QMC and many-body methods can easily spoil the error cancellation properties inherent in the Rankine-Hugoniot relation, and very likely suffer from error addition. By cross validating QMC based on systematically improvable trial functions against post-Hartree-Fock many-body methods, we find that these methods introduce errors of the right sign and magnitude to account for much of the observed discrepancy between CEIMC and experiment. We stress that this is not just a CEIMC problem, but that thermodynamic sampling based on other many-body methods is likely to experience similar difficulties.

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TYPE Journal Article YEAR 2019
DOIOSTIScopus

A unified analysis of nano-to-microscale particle dispersion in tubular blood flow

Physics of Fluids

Liu, Z.; Clausen, Jonathan C.; Rao, Rekha R.; Aidun, C.K.

Transport of solid particles in blood flow exhibits qualitative differences in the transport mechanism when the particle varies from nanoscale to microscale size comparable to the red blood cell (RBC). The effect of microscale particle margination has been investigated by several groups. Also, the transport of nanoscale particles (NPs) in blood has received considerable attention in the past. This study attempts to bridge the gap by quantitatively showing how the transport mechanism varies with particle size from nano-to-microscale. Using a three-dimensional (3D) multiscale method, the dispersion of particles in microscale tubular flows is investigated for various hematocrits, vessel diameters, and particle sizes. NPs exhibit a nonuniform, smoothly dispersed distribution across the tube radius due to severe Brownian motion. The near-wall concentration of NPs can be moderately enhanced by increasing hematocrit and confinement. Moreover, there exists a critical particle size (∼1 μm) that leads to excessive retention of particles in the cell-free region near the wall, i.e., margination. Above this threshold, the margination propensity increases with the particle size. The dominance of RBC-enhanced shear-induced diffusivity (RESID) over Brownian diffusivity (BD) results in 10 times higher radial diffusion rates in the RBC-laden region compared to that in the cell-free layer, correlated with the high margination propensity of microscale particles. This work captures the particle size-dependent transition from Brownian-motion dominant dispersion to margination using a unified 3D multiscale computational approach and highlights the linkage between the radial distribution of RESID and the margination of particles in confined blood flows.

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TYPE Journal Article YEAR 2019
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Direct simulation Monte Carlo on petaflop supercomputers and beyond

Physics of Fluids

Plimpton, Steven J.; Moore, Stan G.; Borner, A.; Stagg, Alan K.; Koehler, T.P.; Torczynski, J.R.; Gallis, Michail A.

The gold-standard definition of the Direct Simulation Monte Carlo (DSMC) method is given in the 1994 book by Bird [Molecular Gas Dynamics and the Direct Simulation of Gas Flows (Clarendon Press, Oxford, UK, 1994)], which refined his pioneering earlier papers in which he first formulated the method. In the intervening 25 years, DSMC has become the method of choice for modeling rarefied gas dynamics in a variety of scenarios. The chief barrier to applying DSMC to more dense or even continuum flows is its computational expense compared to continuum computational fluid dynamics methods. The dramatic (nearly billion-fold) increase in speed of the largest supercomputers over the last 30 years has thus been a key enabling factor in using DSMC to model a richer variety of flows, due to the method's inherent parallelism. We have developed the open-source SPARTA DSMC code with the goal of running DSMC efficiently on the largest machines, both current and future. It is largely an implementation of Bird's 1994 formulation. Here, we describe algorithms used in SPARTA to enable DSMC to operate in parallel at the scale of many billions of particles or grid cells, or with billions of surface elements. We give a few examples of the kinds of fundamental physics questions and engineering applications that DSMC can address at these scales.

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TYPE Journal Article YEAR 2019
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Heterogeneities dominate mechanical performance of additively manufactured metal lattice struts

Additive Manufacturing

Dressler, Amber D.; Boyce, Brad B.; Moore, David G.; Miers, John C.; Jost, Elliott W.; Seepersad, Carolyn C.

Architected structural metamaterials, also known as lattice, truss, or acoustic materials, provide opportunities to produce tailored effective properties that are not achievable in bulk monolithic materials. These topologies are typically designed under the assumption of uniform, isotropic base material properties taken from reference databases and without consideration for sub-optimal as-printed properties or off-nominal dimensional heterogeneities. However, manufacturing imperfections such as surface roughness are present throughout the lattices and their constituent struts create significant variability in mechanical properties and part performance. This study utilized a customized tensile bar with a gauge section consisting of five parallel struts loaded in a stretch (tensile) orientation to examine the impact of manufacturing heterogeneities on quasi-static deformation of the struts, with a focus on ultimate tensile strength and ductility. The customized tensile specimen was designed to prevent damage during handling, despite the sub-millimeter thickness of each strut, and to enable efficient, high-throughput mechanical testing. The strut tensile specimens and reference monolithic tensile bars were manufactured using a direct metal laser sintering (also known as laser powder bed fusion or selective laser melting) process in a precipitation hardened stainless steel alloy, 17-4PH, with minimum feature sizes ranging from 0.5-0.82 mm, comparable to minimum allowable dimensions for the process. Over 70 tensile stress-strain tests were performed revealing that the effective mechanical properties of the struts were highly stochastic, considerably inferior to the properties of larger as-printed reference tensile bars, and well below the minimum allowable values for the alloy. Pre- and post-test non-destructive analyses revealed that the primary source of the reduced properties and increased variability was attributable to heterogeneous surface topography with stress-concentrating contours and commensurate reduction in effective load-bearing area.

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TYPE Journal Article YEAR 2019
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Asynchronous Ballistic Reversible Fluxon Logic

IEEE Transactions on Applied Superconductivity

Frank, Michael P.; Lewis, Rupert; Missert, Nancy A.; Wolak, Matthaeus W.; Henry, Michael D.

In a previous paper, we described a new abstract circuit model for reversible computation called asynchronous ballistic reversible computing (ABRC), in which localized information-bearing pulses propagate ballistically along signal paths between stateful abstract devices and elastically scatter off those devices serially, while updating the device state in a logically-reversible and deterministic fashion. The ABRC model has been shown to be capable of universal computation. In the research reported here, we begin exploring how the ABRC model might be realized in practice using single flux quantum solitons (fluxons) in superconducting Josephson junction (JJ) circuits. One natural family of realizations could utilize fluxon polarity to represent binary data in individual pulses propagating near-ballistically, along discrete or continuous long Josephson junctions or microstrip passive transmission lines, and utilize the flux charge (-1, 0, +1) of a JJ-containing superconducting loop with Φ0 < IcL < 2Φ0 to encode a ternary state variable internal to a device. A natural question then arises as to which of the definable abstract ABRC device functionalities using this data representation might be implementable using a JJ circuit that dissipates only a small fraction of the input fluxon energy. We discuss conservation rules and symmetries considered as constraints to be obeyed in these circuits, and begin the process of classifying the possible ABRC devices in this family having up to three bidirectional I/O terminals, and up to three internal states.

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TYPE Conference Poster YEAR 2019
DOIOSTIScopus

Revenue Opportunities for Electric Storage Resources in the Southwest Power Pool Integrated Marketplace

IEEE Power and Energy Society General Meeting

Concepcion, Ricky J.; Wilches-Bernal, Felipe; Byrne, Raymond H.

This paper explores the revenue potential for electric storage resources (ESRs), also referred to as electrical energy storage, in the Southwest Power Pool Integrated Marketplace. In particular, opportunities in the day-ahead market with the energy and frequency regulation products are considered. The revenue maximization problem is formulated as a linear program model, where an ESR seeks to maximize its revenue through the available revenue streams. The ESR has perfect foresight of historical prices and determines the optimal policy accordingly. A case study using FY2018 data shows that frequency regulation services are the most lucrative for revenue potential. This paper also explores different methods of using area control error data to infer the regulation control signal and the consequent effect on the optimization. Finally, the paper conducts a sensitivity analysis of ESR payback period to energy capacity and power rating.

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TYPE Conference Poster YEAR 2019
OSTIScopus

Thermal atom-ion collisions in a K- Yb+ hybrid system

Physical Review A

Jau, Yuan-Yu J.

We present experimental studies of atom-ion collisions using buffer-gas cooled, trapped ytterbium (Yb+) ions immersed in potassium (K) vapor. The range of the collisional temperature is on the order of several hundred kelvin (thermal regime). We have determined various collisional rate coefficients of the Yb+ ion per K-atom number density. We find the upper bounds of charge-exchange rate coefficients κce to be (12.7±1.6)×10-14cm3s-1 for K-Yb+171 and (5.3±0.7)×10-14cm3s-1 for K-Yb+172. For both isotopes, the spin-destruction rate coefficient κsd has an upper bound at (1.46±0.77)×10-9cm3s-1. The spin-exchange rate coefficient κse is measured to be (1.64±0.51)×10-9cm3s-1. The relatively low charge-exchange rate reported here demonstrates the advantage of using K atoms to sympathetically cool Yb+ ions and the relatively high spin-exchange rate may benefit research work in quantum metrology and quantum information processing on an atom-ion platform using K atoms and Yb+ ions.

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TYPE Journal Article YEAR 2019
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Relating microstructure to defect behavior in AA6061 using a combined computational and multiscale electron microscopy approach

Acta Materialia

Lim, Hojun L.; Yoo, Yung S.J.; Carroll, Jay D.; Emery, John M.; Kacher, Josh

In this study, a multiscale electron microscopy-based approach is applied to understanding how different aspects of the microstructure in a notched AA6061-T6, including grain boundaries, triple junctions, and intermetallic particles, promote localized dislocation accumulation as a function of applied tensile strain and depth from the sample surface. Experimental measurements and crystal plasticity simulations of dislocation distributions as a function of distance from specified microstructural features both showed preferential dislocation accumulation near intermetallic particles relative to grain boundaries and triple junctions. High resolution electron backscatter diffraction and site-specific transmission electron microscopy characterization showed that high levels of dislocation accumulation near intermetallic particles led to the development of an ultrafine sub-grain microstructure, indicative of a much higher level of local plasticity than predicted from the coarser measurements and simulations. In addition, high resolution measurements in front of a crack tip suggested a compounding influence of intermetallic particles and grain boundaries in dictating crack propagation pathways.

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TYPE Journal Article YEAR 2019
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TATB Sensitivity to Shocks from Electrical Arcs

Propellants, Explosives, Pyrotechnics

Chen, Kenneth C.; Warne, Larry K.; Jorgenson, Roy E.; Niederhaus, John H.

Use of insensitive high explosives (IHEs) has significantly improved ammunition safety because of their remarkable insensitivity to violent cook-off, shock and impact. Triamino-trinitrobenzene (TATB) is the IHE used in many modern munitions. Previously, lightning simulations in different test configurations have shown that the required detonation threshold for standard density TATB at ambient and elevated temperatures (250 C) has a sufficient margin over the shock caused by an arc from the most severe lightning. In this paper, the Braginskii model with Lee-More channel conductivity prescription is used to demonstrate how electrical arcs from lightning could cause detonation in TATB. The steep rise and slow decay in typical lightning pulse are used in demonstrating that the shock pressure from an electrical arc, after reaching the peak, falls off faster than the inverse of the arc radius. For detonation to occur, two necessary detonation conditions must be met: the Pop-Plot criterion and minimum spot size requirement. The relevant Pop-Plot for TATB at 250 C was converted into an empirical detonation criterion, which is applicable to explosives subject to shocks of variable pressure. The arc cross-section was required to meet the minimum detonation spot size reported in the literature. One caveat is that when the shock pressure exceeds the detonation pressure the Pop-Plot may not be applicable, and the minimum spot size requirement may be smaller.

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TYPE Journal Article YEAR 2019
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Investigation of assumptions and approximations in the virtual fields method for a viscoplastic material model

Strain

Jones, Elizabeth M.; Karlson, Kyle N.; Reu, Phillip L.

The Virtual Fields Method (VFM) is an inverse technique used for parameter estimation and calibration of constitutive models. Many assumptions and approximations—such as plane stress, incompressible plasticity, and spatial and temporal derivative calculations—are required to use VFM with full-field deformation data, for example, from Digital Image Correlation (DIC). This work presents a comprehensive discussion of the effects of these assumptions and approximations on parameters identified by VFM for a viscoplastic material model for 304L stainless steel. We generated synthetic data from a Finite-Element Analysis (FEA) in order to have a reference solution with a known material model and known model parameters, and we investigated four cases in which successively more assumptions and approximations were included in the data. We found that VFM is tolerant to small deviations from the plane stress condition in a small region of the sample, and that the incompressible plasticity assumption can be used to estimate thickness changes with little error. A local polynomial fit to the displacement data was successfully employed to compute the spatial displacement gradients. The choice of temporal derivative approximation (i.e., backwards difference versus central difference) was found to have a significant influence on the computed rate of deformation and on the VFM results for the rate-dependent model used in this work. Finally, the noise introduced into the displacement data from a stereo-DIC simulator was found to have negligible influence on the VFM results. Evaluating the effects of assumptions and approximations using synthetic data is a critical first step for verifying and validating VFM for specific applications. The results of this work provide the foundation for confidently using VFM for experimental data.

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TYPE Journal Article YEAR 2019
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High temperature synthesis and characterization of ultrathin tellurium nanostructures

APL Materials

Sapkota, Keshab R.; Lu, Ping L.; Medlin, Douglas L.; Wang, George T.

Thin tellurium (Te) has been predicted as a potential two dimensional system exhibiting superior thermoelectric and electrical properties. Here, we report the synthesis of high quality ultrathin Te nanostructures and the study of their electrical properties at room temperature. High quality ultrathin Te nanostructures are obtained by high temperature vapor phase deposition on c-plane sapphire substrates. The obtained nanostructures are as thin as 3 nm and exhibit α-Te phase with trigonal crystal structure. Room temperature electrical measurements show significantly higher electrical conductivity compared to prior reports of Te in bulk form or in nanostructure form synthesized by low temperature vapor deposition or wet chemical methods. Additionally, these nanostructures exhibit high field effect hole mobility comparable to black-phosphorous measured previously under similar conditions.

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TYPE Journal Article YEAR 2019
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Results 20201–20400 of 96,771
Results 20201–20400 of 96,771