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DOE OSTI2017

Self-consistent Modeling of Reionization in Cosmological Hydrodynamical Simulations

The ultraviolet background (UVB) emitted by quasars and galaxies governs the ionization and thermal state of the intergalactic medium (IGM), regulates the formation of high-redshift galaxies, and is thus a key quantity for modeling cosmic reionization. The vast majority of cosmological hydrodynamical simulations implement the UVB via a set of spatially uniform photoionization and photoheating rates derived from UVB synthesis models. We show that simulations using canonical UVB rates reionize and, perhaps more importantly, spuriously heat the IGM, much earlier (z∼15) than they should. This problem arises because at z>6, where observational constraints are nonexistent, the UVB amplitude is far too high. We introduce a new methodology to remedy this issue, and we generate self-consistent photoionization and photoheating rates to model any chosen reionization history. Following this approach, we run a suite of hydrodynamical simulations of different reionization scenarios and explore the impact of the timing of reionization and its concomitant heat injection on the thermal state of the IGM. We present a comprehensive study of the pressure smoothing scale of IGM gas, illustrating its dependence on the details of both hydrogen and helium reionization, and argue that it plays a fundamental role in interpreting Lyα forest statistics and the thermal evolution of the IGM. The premature IGM heating we have uncovered implies that previous work has likely dramatically overestimated the impact of photoionization feedback on galaxy formation, which sets the minimum halo mass able to form stars at high redshifts. We make our new UVB photoionization and photoheating rates publicly available for use in future simulations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Parsec-scale Obscuring Accretion Disk with Large-scale Magnetic Field in AGNs

A magnetic field dragged from the galactic disk, along with inflowing gas, can provide vertical support to the geometrically and optically thick pc-scale torus in AGNs. Using the Soloviev solution initially developed for Tokamaks, we derive an analytical model for a rotating torus that is supported and confined by a magnetic field. We further perform three-dimensional magneto-hydrodynamic simulations of X-ray irradiated, pc-scale, magnetized tori. We follow the time evolution and compare models that adopt initial conditions derived from our analytic model with simulations in which the initial magnetic flux is entirely contained within the gas torus. Numerical simulations demonstrate that the initial conditions based on the analytic solution produce a longer-lived torus that produces obscuration that is generally consistent with observed constraints.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Galaxy Formation through Filamentary Accretion at z = 6.1

We present Atacama Large Millimeter/submillimeter Array observations of the dust continuum and [C ii] 158 μm line emission from the z = 6.0695 Lyman-Break Galaxy (LBG) WMH5. These observations at 0.″3 spatial resolution show a compact (∼3 kpc) main galaxy in dust and [C ii] emission, with a “tail” of emission extending to the east by about 5 kpc (in projection). The [C ii] tail is comprised predominantly of two distinct sub-components in velocity, separated from the core by ∼100 and 250 km s{sup −1}, with narrow intrinsic widths of about 80 km s{sup −1}, which we call “sub-galaxies.” The sub-galaxies themselves are extended east–west by about 3 kpc in individual channel images. The [C ii] tail joins smoothly into the main galaxy velocity field. The [C ii] line to continuum ratios are comparable for the main and sub-galaxy positions, within a factor two. In addition, these ratios are comparable to z∼5.5 LBGs. We conjecture that the WMH5 system represents the early formation of a galaxy through the accretion of smaller satellite galaxies, embedded in a smoother gas distribution, along a possibly filamentary structure. The results are consistent with current cosmological simulations of early galaxy formation and support the idea of very early enrichment with dust and heavy elements of the accreting material.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Growth of First Galaxies: Impacts of Star Formation and Stellar Feedback

Recent observations have detected galaxies at high-redshift z∼6--11, and revealed the diversity of their physical properties, from normal star-forming galaxies to starburst galaxies. To understand the properties of these observed galaxies, it is crucial to understand the star formation (SF) history of high-redshift galaxies under the influence of stellar feedback. In this work, we present the results of cosmological hydrodynamic simulations with zoom-in initial conditions, and investigate the formation of the first galaxies and their evolution toward observable galaxies at z∼6. We focus on three different galaxies that end up in halos with masses M{sub h}=2.4×10{sup 10} h{sup −1} M{sub ⊙} (Halo-10), 1.6×10{sup 11} h{sup −1} M{sub ⊙} (Halo-11), and 0.7×10{sup 12} h{sup −1} M{sub ⊙} (Halo-12) at z = 6. Our simulations also probe the impacts of different subgrid assumptions, i.e., SF efficiency and cosmic reionization, on SF histories in the first galaxies. We find that SF occurs intermittently due to supernova (SN) feedback at z≳10, and then it proceeds more smoothly as the halo mass grows at lower redshifts. Galactic disks are destroyed due to SN feedback, while galaxies in simulations with no feedback or lower SF efficiency models can sustain a galactic disk for long periods ≳10 Myr. The expulsion of gas at the galactic center also affects the inner dark matter density profile for a short period. Our simulated galaxies in Halo-11 and Halo-12 reproduce the SF rates and stellar masses of observed Lyα emitters at z∼7--8 fairly well given the observational uncertainties.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Two-dimensional Radiative Magnetohydrodynamic Simulations of Partial Ionization in the Chromosphere. II. Dynamics and Energetics of the Low Solar Atmosphere

We investigate the effects of interactions between ions and neutrals on the chromosphere and overlying corona using 2.5D radiative MHD simulations with the Bifrost code. We have extended the code capabilities implementing ion–neutral interaction effects using the generalized Ohm’s law, i.e., we include the Hall term and the ambipolar diffusion (Pedersen dissipation) in the induction equation. Our models span from the upper convection zone to the corona, with the photosphere, chromosphere, and transition region partially ionized. Our simulations reveal that the interactions between ionized particles and neutral particles have important consequences for the magnetothermodynamics of these modeled layers: (1) ambipolar diffusion increases the temperature in the chromosphere; (2) sporadically the horizontal magnetic field in the photosphere is diffused into the chromosphere, due to the large ambipolar diffusion; (3) ambipolar diffusion concentrates electrical currents, leading to more violent jets and reconnection processes, resulting in (3a) the formation of longer and faster spicules, (3b) heating of plasma during the spicule evolution, and (3c) decoupling of the plasma and magnetic field in spicules. Our results indicate that ambipolar diffusion is a critical ingredient for understanding the magnetothermodynamic properties in the chromosphere and transition region. The numerical simulations have been made publicly available, similar to previous Bifrost simulations. This will allow the community to study realistic numerical simulations with a wider range of magnetic field configurations and physics modules than previously possible.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Particle-in-cell simulations of electron and ion dissipation by whistler turbulence: variations with electron β

Two ensembles of three-dimensional particle-in-cell (PIC) simulations of the forward cascade of decaying whistler turbulence have been carried out on a model of collisionless, homogeneous, magnetized plasma with parameters similar to those of the solar wind near Earth. Initial, relatively isotropic, narrowband spectra of relatively long wavelength modes cascade to anisotropic, broadband spectra of magnetic fluctuations at shorter wavelengths. Electron and ion dissipation rates are computed as functions of the initial electron beta, β{sub e}, over the range 0.1 ≤ β{sub e} ≤ 5.0, where this quantity is varied by changes in the background magnetic field magnitude B{sub o}. Ensemble One holds the value of the dimensionless initial magnetic fluctuation energy density ϵ{sub o} ≡ Σ{sub k} |δB{sub k}|{sup 2}/B{sub o}{sup 2} constant; Ensemble Two follows solar wind observations, imposing the initial condition ϵ{sub o} = 0.20 β{sub e}. In both ensembles, the maximum dissipation rate of the electrons, Q{sub e}, and the maximum dissipation rate of the ions, Q{sub i}, satisfy Q{sub e} ≫ Q{sub i}. In Ensemble One, both dissipation rates scale approximately as β{sub e}{sup −1}, whereas over 0.1 ≤ β{sub e} ≤ 1.0 in Ensemble Two, Q{sub e} is approximately constant while Q{sub i} scales approximately as β{sub e}{sup 1/2}. These results, when combined with conclusions from earlier PIC simulations, suggest that sufficiently long wavelength and sufficiently large-amplitude magnetosonic-whistler turbulence at sufficiently large β{sub e} may heat ions more rapidly than electrons.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

Synergism of the method of characteristics and CAD technology for neutron transport calculation

The method of characteristics (MOC) is a very popular methodology in neutron transport calculation and numerical simulation in recent decades for its unique advantages. One of the key problems determining whether the MOC can be applied in complicated and highly heterogeneous geometry is how to combine an effective geometry processing method with MOC. Most of the existing MOC codes describe the geometry by lines and arcs with extensive input data, such as circles, ellipses, regular polygons and combination of them. Thus they have difficulty in geometry modeling, background meshing and ray tracing for complicated geometry domains. In this study, a new idea making use of a CAD solid modeler MCAM which is a CAD/Image-based Automatic Modeling Program for Neutronics and Radiation Transport developed by FDS Team in China was introduced for geometry modeling and ray tracing of particle transport to remove these geometrical limitations mentioned above. The diamond-difference scheme was applied to MOC to reduce the spatial discretization error of the flat flux approximation in theory. Based on MCAM and MOC, a new MOC code was developed and integrated into SuperMC system, which is a Super Multi-function Computational system for neutronics and radiation simulation. The numerical testing results demonstrated the feasibility and effectiveness of the new idea for geometry treatment in SuperMC. (authors)

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2015

Magnetic fields and galactic star formation rates

The regulation of galactic-scale star formation rates (SFRs) is a basic problem for theories of galaxy formation and evolution: which processes are responsible for making observed star formation rates so inefficient compared to maximal rates of gas content divided by dynamical timescale? Here we study the effect of magnetic fields of different strengths on the evolution of giant molecular clouds (GMCs) within a kiloparsec patch of a disk galaxy and resolving scales down to ≃0.5 pc. Including an empirically motivated prescription for star formation from dense gas (n{sub H}>10{sup 5} cm{sup −3}) at an efficiency of 2% per local free-fall time, we derive the amount of suppression of star formation by magnetic fields compared to the nonmagnetized case. We find GMC fragmentation, dense clump formation, and SFR can be significantly affected by the inclusion of magnetic fields, especially in our strongest investigated B-field case of 80 μG. However, our chosen kiloparsec-scale region, extracted from a global galaxy simulation, happens to contain a starbursting cloud complex that is only modestly affected by these magnetic fields and likely requires internal star formation feedback to regulate its SFR.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Preliminary Logistics Modeling of the Integrated Wash Aid Treatment Emergency Reuse System - 19495

We employed Analysis of Mobility Platform (AMP) and GoldSim software to model the logistics of deploying materials to implement the Integrated Wash Aid Treatment Emergency Reuse System (IWATERS) in a hypothetical cesium-contamination scenario covering a four-block strip of downtown Chicago, IL. In this deployment of IWATERS, buildings were washed down via fire-hosing and contaminated wash solution was collected in the city's waste/stormwater collection system. The contaminated wash water was treated to remove Cs via ad hoc sand/clay filtration beds, and the treated wash solution was recycled during continuing decontamination operations. A detailed task timeline for the IWATERS deployment and information (or assumptions) on the availability and location of required resources for operations provided input parameters, as well as a framework, to enable the AMP and GoldSim simulations. From the decision to deploy IWATERS to the end of operations, including the completion of processing all contaminated wash water, took 49 days with decontamination teams active for 35 days and treatment bed teams active for 37 days. The modeling results showed the availability of vermiculite clay (based on data supplied by its commercial vendors/distributors) as the limiting factor for deploying treatment bed teams and, consequentially, the recycling rate of wash water and the timeline for decontamination operations. These simulations represent the first attempt at understanding the material and personnel requirements for large-scale IWATERS operation. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2017

Computational Study of Droplet Dynamics in Dispersed Flow Film Boiling Regime in PWR Geometries

Dispersed flow film boiling (DFFB) regime is exhibited during low-flooding rate conditions following the postulated Loss-of-Coolant Accident (LOCA) in nuclear reactor flow channels. LOCA is a type of design-basis accident scenario, which is crucial for reactor design and licensing. Accurate models to predict the heat transfer in DFFB regime is critical for the reactor to maintain safety limits. The nominal operating linear heat rate is also set based on the maximum linear heat rate determined for a LOCA transient. It is understood that the behavior of entrained droplets in DFFB regime greatly affects the heat transfer by evaporating and desuperheating the surrounding steam. Furthermore, interaction with flow structures also aids in increasing the evaporation rate of entrained droplets. In order to quantify this enhancement in heat transfer, the size of droplets downstream of flow structures (Spacer Grids and Mixing Vanes) is of crucial importance. Thermal hydraulic analysis codes like COBRA-TF, CATHARE-3 and adopt a three-field equation model that allows them to model the droplet field separately. However, to ensure the accuracy of these field equations, various correlations and physical models are required. This paper proposes a method for characterizing the mean Sauter Diameter of droplets across spacer-grids and mixing vanes in PWR sub-channels using PHASTA - a high fidelity flow solver with multiphase capabilities using the level-set method. The PHASTA code is a finite element based computational fluid dynamics code designed for solving compressible and incompressible turbulent flows. It has been demonstrated to be massively scalable (up to 786,432 cores), which is crucial for simulating large realistic domains. The code incorporates the level-set method for interface tracking of the gas-liquid interface.

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
DOE OSTI2014

Induced Radioactivity Analysis for the NSRL Linac in China Using Monte Carlo Simulations and Gamma Spectroscopy - Paper 141

The Synchrotron Radiation Facility of the National Synchrotron Radiation Laboratory (NSRL) located in Hefei, China consists of a 200-MeV electron linear accelerator and an 800-MeV electronic storage ring. Constructed in 1989, the 200-MeV Electron linac is one of the earliest high-energy electron linear accelerators in China. The NSRL is a large-scale experiment device, having been operated more than twenty years, so it is hard to give a historical recount about the injection of NSRL Linac. Lower-energy electrons emitted from the accelerator electron gun are progressively accelerated to about 200 MeV in five acceleration tubes, and then are injected into the electron storage ring, reaching the final energy of 800 MeV. The linear accelerator is a traveling wave linear accelerator, consisting of pre-injection, four 6-m acceleration segment, some beam sensing elements, and focusing inserts. The electrons are accelerated in the acceleration tubes to 32 MeV, 74 MeV, 116 MeV, 158 MeV, respectively, and the electronic energy reaches 200 MeV at the end of the acceleration trip. A scraper is used for beam collimation and is located at the end of each acceleration tube, with a length that increases with the energy to provide sufficient collimation. The NSRL Linac was retired in 2012 due to a plan to upgrade the facility. The decommissioning produced many real challenges related to the methods to handle the large size of the device and the time required to store structural materials until the radioactivity is reduced in the waste repository. These problems should be solved as soon as possible in order for the upgrade to proceed smoothly. At present, no prior decommissioning experience existed in China on such a high-energy electron linear accelerator. Therefore, the decommission project provided an opportunity to study and evaluate the challenges, in particular, involving the induced radioactivity for the purposes of radiation protection. For a well-shielded accelerator facility, the induced radioactivity is an important source of radiation for people who perform the decommissioning. In addition, information on the level of induced radioactivity is needed for disposing of the retired accelerator materials. Therefore it is significant to predict such radioactivity level accurately. However, the induced radioactivity study for such type high-energy electron linear accelerator is very seldom. Particle accelerators that operate at energy greater than 10 MeV will produce induced radioactivity due to well-known photonuclear interactions. However, the assessment of such as induced radioactivity of large-scale installations is a time-consuming process. Experimental methods are time-consuming and require special radiation safety measures to protection the investigators. On the other hand, empirical formula cannot yield sufficiently accurate results. Therefore, Monte Carlo simulation method is the primary means to study such problems. In the decommissioning process of the NSRL Linac, the induced radioactivity assessment will be based on Monte Carlo simulation method as the main auxiliary means to expand a series of studies. (authors)

43 PARTICLE ACCELERATORS↗
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

Optimized Gating System for Steel Castings

Computer modeling using a commercially available software package was used to explore the optimization of filling systems based on the relatively new concepts of avoiding entrainment of air bubbles and oxides by avoiding surface turbulence. The test casting was based on a pattern for a traditional top poured test bar, whose cross section was a tri-lobed clover-like shape. The study illustrates clearly that the detailed design of the filling system has a major influence on the conditions for defect generation during filling. Traditional steel casting systems using the widely popular assembly of preformed refractory tubes were found to behave poorly. Systems were demonstrated which were capable of delivering highly controlled filling behavior. The latest systems to be developed employed (1) a naturally pressurized filling system and (2) the use of filters placed flush on the top of the runner to act as bubble diverters, together with (3) terminal spin traps. These novel filling systems demonstrated excellent performance in simulation, in agreement with practical experience of the capability (of the trident gate in particular) to produce, for the first time in the history of casting, defect-free castings on a routine basis.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Ettringite hysteresis under sorption from molecular simulations

Highlights: • Hybrid Grand Canonical Monte Carlo (GCMC) and Molecular Dynamics (MD) simulations are performed. • Anisotropic large strain coefficient of crystal shrinkage of ettringite is provided. • Entropy increases when ettringite converts into metaettringite. • Sorption-induced stress may lead to columns collapse upon drying. • H-bonds involving OH groups in columns play a role in hysteresis. The response of ettringite under sorption is critical for the utilization of this mineral as an energy storage material and to understand the concrete durability problems associated with ettringite formation. We report molecular simulations directly sampling the osmotic ensemble to understand the sorption processes and structural changes in ettringite-metaettringite transition. Desorption branch and sorption-induced volume changes from simulations agree with the experiments. The structural changes at low relative humidity are analyzed, revealing for the first time details of metaettringite structure. The reversibility under sorption observed experimentally is captured by simulation in the ettringite domain. The reversibility in the conversion of metaettringite into ettringite is not captured by the direct simulations in the osmotic ensemble due to the large volume changes associated with ettringite desorption. Finally, we discuss the role of hydrogen bonds on the hysteresis in ettringite.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Coupling scales for modelling heavy metal vaporization from municipal solid waste incineration in a fluid bed by CFD

Highlights: • A CFD two-scale model is formulated to simulate heavy metal vaporization from waste incineration in fluidized beds. • MSW particle is modelled with the macroscopic particle model. • Influence of bed dynamics on HM vaporization is included. • CFD predicted results agree well with experimental data reported in literature. • This approach may be helpful for fluidized bed reactor modelling purposes. - Abstract: Municipal Solid Waste Incineration (MSWI) in fluidized bed is a very interesting technology mainly due to high combustion efficiency, great flexibility for treating several types of waste fuels and reduction in pollutants emitted with the flue gas. However, there is a great concern with respect to the fate of heavy metals (HM) contained in MSW and their environmental impact. In this study, a coupled two-scale CFD model was developed for MSWI in a bubbling fluidized bed. It presents an original scheme that combines a single particle model and a global fluidized bed model in order to represent the HM vaporization during MSW combustion. Two of the most representative HM (Cd and Pb) with bed temperatures ranging between 923 and 1073 K have been considered. This new approach uses ANSYS FLUENT 14.0 as the modelling platform for the simulations along with a complete set of self-developed user-defined functions (UDFs). The simulation results are compared to the experimental data obtained previously by the research group in a lab-scale fluid bed incinerator. The comparison indicates that the proposed CFD model predicts well the evolution of the HM release for the bed temperatures analyzed. It shows that both bed temperature and bed dynamics have influence on the HM vaporization rate. It can be concluded that CFD is a rigorous tool that provides valuable information about HM vaporization and that the original two-scale simulation scheme adopted allows to better represent the actual particle behavior in a fluid bed incinerator.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2013

Simulations of material mixing in laser-driven reshock experiments

We perform simulations of a laser-driven reshock experiment [Welser-Sherrill et al., High Energy Density Phys. (unpublished)] in the strong-shock high energy-density regime to better understand material mixing driven by the Richtmyer-Meshkov instability. Validation of the simulations is based on direct comparison of simulation and radiographic data. Simulations are also compared with published direct numerical simulation and the theory of homogeneous isotropic turbulence. Despite the fact that the flow is neither homogeneous, isotropic nor fully turbulent, there are local regions in which the flow demonstrates characteristics of homogeneous isotropic turbulence. We identify and isolate these regions by the presence of high levels of turbulent kinetic energy (TKE) and vorticity. After reshock, our analysis shows characteristics consistent with those of incompressible isotropic turbulence. Self-similarity and effective Reynolds number assessments suggest that the results are reasonably converged at the finest resolution. Our results show that in shock-driven transitional flows, turbulent features such as self-similarity and isotropy only fully develop once de-correlation, characteristic vorticity distributions, and integrated TKE, have decayed significantly. Finally, we use three-dimensional simulation results to test the performance of two-dimensional Reynolds-averaged Navier-Stokes simulations. In this context, we also test a presumed probability density function turbulent mixing model extensively used in combustion applications.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Infrared laser induced rupture of thin liquid films on stationary substrates

We studied the deformation and destabilization of thin liquid films on stationary substrates via infrared illumination. The film thickness evolution was measured using interference microscopy. We developed numerical models for the temperature evolution and the liquid redistribution. The substrate wettability is explicitly accounted for via a phenomenological expression for the disjoining pressure. We systematically measured the film thinning- and rupture dynamics as a function of laser power, which are accurately reproduced by the simulations. While smaller laser spots generally lead to shorter rupture times, the latter can become independent of the spotsize for very narrow beams due to capillary suppression.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Trapping of electrons in troughs of self generated electromagnetic standing waves in a bounded plasma column

Observations and measurements are reported on electron trapping in troughs of self-generated electromagnetic standing waves in a bounded plasma column confined in a minimum-B field. The boundaries are smaller than the free space wavelength of the waves. Earlier work of researchers primarily focused upon electron localization effects induced by purely electrostatic perturbation. We demonstrate the possibility in the presence of electromagnetic standing waves generated in the bounded plasma column. The electron trapping is verified with electrostatic measurements of the plasma floating potential, electromagnetic measurements of the wave field profile, and optical intensity measurements of Argon ionic line at 488 nm. The experimental results show a reasonably good agreement with predictions of a Monte Carlo simulation code that takes into account all kinematical and dynamical effects in the plasma in the presence of bounded waves and external fields.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗