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At least 1,675 records · Page 93
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DOE OSTI2025

Realizability-preserving discontinuous Galerkin method for spectral two-moment radiation transport in special relativity

Here we present a realizability-preserving numerical method for solving a spectral two-moment model to simulate the transport of massless, neutral particles interacting with a steady background material moving with relativistic velocities. The model is obtained as the special relativistic limit of a four-momentum-conservative general relativistic two-moment model. Using a maximum-entropy closure, we solve for the Eulerian-frame energy and momentum. The proposed numerical method is designed to preserve moment realizability, which corresponds to moments defined by a nonnegative phase-space density. The realizability-preserving method is achieved with the following key components: (i) a discontinuous Galerkin phase-space discretization with specially constructed numerical fluxes in the spatial and energy dimensions; (ii) a strong stability-preserving implicit-explicit time-integration method; (iii) a realizability-preserving conserved to primitive moment solver; (iv) a realizability-preserving implicit collision solver; and (v) a realizability-enforcing limiter. Component (iii) is necessitated by the closure procedure, which closes higher order moments nonlinearly in terms of primitive moments. The nonlinear conserved to primitive and the implicit collision solves are formulated as fixed-point problems, which are solved with custom iterative solvers designed to preserve the realizability of each iterate. With a series of numerical tests, we demonstrate the accuracy and robustness of this discontinuous-Galerkin-implicit-explicit method.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Unitarity and the three flavor neutrino mixing matrix

Unitarity is a fundamental property of any theory required to ensure we work in a theoretically consistent framework. In comparison with the quark sector, experimental tests of unitarity for the 3x3 neutrino mixing matrix are considerably weaker. It must be remembered that the vast majority of our information on the neutrino mixing angles originates from v - e and v μ disappearance experiments, with the assumption of unitarity being invoked to constrain the remaining elements. New physics can invalidate this assumption for the 3x3 subset and thus modify our precision measurements. We also perform a reanalysis to see how global knowledge is altered when one refits oscillation results without assuming unitarity, and present 3σ ranges for allowed U PMNS elements consistent with all observed phenomena. We calculate the bounds on the closure of the six neutrino unitarity triangles, with the closure of the v - e and v μ triangle being constrained to be ≤0.03, while the remaining triangles are significantly less constrained to be ≤ 0.1 - 0.2. Similarly for the row and column normalization, we find their deviation from unity is constrained to be ≤ 0.2 - 0.4, for four out of six such normalizations, while for the v μ and v e row normalization the deviations are constrained to be ≤0.07, all at the 3σCL. Additionally, we emphasize that there is significant room for new low energy physics, especially in the v τ sector which very few current experiments constrain directly.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2020

Integral equation theory of thermodynamics, pair structure, and growing static length scale in metastable hard sphere and Weeks-Chandler-Andersen fluids

Here, we employ the Ornstein-Zernike integral equation theory with the Percus-Yevick (PY) and modified-Verlet (MV) closures to study the equilibrium structural and thermodynamic properties of metastable monodisperse hard sphere and continuous repulsion Weeks-Chandler-Andersen (WCA) fluids under density and temperature conditions where the system is strongly overcompressed or supercooled, respectively. The theoretical results are compared to crystal-avoiding simulations of these dense monodisperse model one-component fluids. The equation of state (EOS) and dimensionless compressibility are computed using both the virial and compressibility routes. For hard spheres, the MV-based virial route EOS and dimensionless compressibility are in very good agreement with simulation for all packing fractions, much better than the PY analogs. The corresponding MV-based predictions for the static structure factor are also very good. The amplitude of density fluctuations on the local cage scale and in the long wavelength limit, and three technically different measures of the density correlation length, are studied with both closures. All five properties grow in a roughly exponential manner with density in the metastable regime up to packing fractions of 58% with no sign of saturation. The MV-based results are in good agreement with our crystal-avoiding simulations. Interestingly, the density dependences of long and short wavelength quantities are closely related. The MV-based theory is also quite accurate for the thermodynamics and structure of supercooled monodisperse WCA fluids. Overall our findings are also relevant as critical input to microscopic theories that relate the equilibrium pair correlation function or static structure factor to dynamical constraints, barriers, and activated relaxation in glass-forming liquids.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Interface learning of multiphysics and multiscale systems

Complex natural or engineered systems comprise multiple characteristic scales, multiple spatiotemporal domains, and even multiple physical closure laws. To address such challenges, we introduce an interface learning paradigm and put forth a data-driven closure approach based on memory embedding to provide physically correct boundary conditions at the interface. To enable the interface learning for hyperbolic systems by considering the domain of influence and wave structures into account, we put forth the concept of upwind learning toward a physics-informed domain decomposition. The promise of the proposed approach is shown for a set of canonical illustrative problems. Here, we highlight that high-performance computing environments can benefit from this methodology to reduce communication costs among processing units in emerging machine-learning-ready heterogeneous platforms toward exascale era.

42 ENGINEERING↗
DOE OSTI2022

Simulation and Reynolds-averaged Navier-Stokes modeling of a three-component Rayleigh-Taylor mixing problem with thermonuclear burn

Rayleigh-Taylor mixing in the presence of a third component with intermediate density is investigated through three-dimensional large-eddy simulation (LES) with a high-order compact finite-difference code. Two configurations are considered: (1) a symmetric configuration in which the Atwood number between the heavy and intermediate components matches the Atwood number between the intermediate and light components and (2) an asymmetric configuration in which the Atwood number between the heavy and intermediate components is an order of magnitude greater than the Atwood number between the intermediate and light components. Mass fraction covariances are extracted, and proposed Reynolds-averaged Navier-Stokes (RANS) closures for density-specific-volume and density-mass-fraction covariances are evaluated in an a priori fashion. Additionally, a multicomponent extension of the k - Φ - L - a - V RANS model [Morgan, Phys. Rev. E 104, 015107 (2021)] is presented which includes model equations for the upper-triangular elements of the mass fraction covariance matrix. This model, referred to as the k - Φ - L - a - C model, is compared against results from LES and against other RANS models. Profiles of average mass fraction, mass-fraction covariance, and density-specific-volume covariance obtained with the k - Φ - L - a - C model are found to agree well with LES data. Finally, the impact of three-component turbulent mixing on average reaction rate is investigated in both premixed and nonpremixed cases by heating the mixing layer and allowing it to undergo thermonuclear (TN) burn. A closure model for average reaction rate is proposed for use with the k - Φ - L - a - C model, and when this model is applied, improved agreement is obtained between LES and RANS in total TN neutron production.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

Large-eddy simulation and Reynolds-averaged Navier-Stokes modeling of three Rayleigh-Taylor mixing configurations with gravity reversal

In this work, high-fidelity large-eddy simulation (LES) is performed of Rayleigh-Taylor (RT) mixing in three different configurations involving gravity reversal. In each configuration, LES results are compared with one-dimensional Reynolds-averaged Navier-Stokes (RANS) results, and a deficiency in a commonly used transport equation for the mass-flux velocity, $a_j$, is identified. In the first configuration, a classical two-component RT mixing layer is allowed to develop before it is subjected to rapid acceleration reversal. In the second configuration, a three-component RT mixing layer with an intermediate density layer is allowed to develop before being subjected to rapid acceleration reversal. Finally, in the third configuration, a light layer is interposed between two heavy layers; in this configuration, only one interface is RT-unstable at a time as it undergoes rapid acceleration reversal. In all cases, a commonly used buoyancy production closure in the $a_j$ transport equation is shown to lead to significant over-prediction of mixing layer growth after gravity reversal. An alternative formulation for this closure is then presented which is shown to more accurately capture the stabilization effect of gravity reversal.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2023

Generation of momentum transport in weakly turbulent β -plane magnetohydrodynamics

Magnetohydrodynamic turbulence on a β plane with an in-plane mean field, a system which serves as a simple model for the solar tachocline, is investigated analytically and computationally. We first derive two useful analytic constraints: We express the mean turbulent cross-helicity in terms of the mean turbulent magnetic energy, and then show that (for weak turbulence) the time-averaged momentum transport in the system can be expressed in terms of the cross-helicity spectrum. Here, we then complete a closure of the system using weak turbulence theory, appropriately extended to a system with multiple interacting eigenmodes. We use this closure to perturbatively solve for the spectra at lowest order in the Rossby parameter β and thereby show that the momentum transport in the system is O ⁡(β 2 ), thus quantifying the transition away from Alfvénized turbulence. Finally, we verify our theoretical results by performing direct numerical simulations of the system over a broad range of β.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Temporal slow-growth formulation for direct numerical simulation of compressible wall-bounded flows

A slow-growth formulation for DNS of wall-bounded turbulent flow is developed and demonstrated to enable extension of slow-growth modeling concepts to wall-bounded flows with complex physics. As in previous slow-growth approaches, the formulation assumes scale separation between the fast scales of turbulence and the slow evolution of statistics such as the mean flow. This separation enables the development of approaches where the fast scales of turbulence are directly simulated while the forcing provided by the slow evolution is modeled. The resulting model admits periodic boundary conditions in the streamwise direction, which avoids the need for extremely long domains and complex inflow conditions that typically accompany spatially developing simulations. Further, it enables the use of efficient Fourier numerics. Unlike previous approaches [Guarini, Moser, Shariff, and Wray, J. Fluid Mech. 414, 1 (2000); Maeder, Adams, and Kleiser, J. Fluid Mech. 429, 187 (2001); Spalart, J. Fluid Mech. 187, 61 (1988)], the present approach is based on a temporally evolving boundary layer and is specifically tailored to give results for calibration and validation of Reynolds-averaged Navier–Stokes (RANS) turbulence models. The use of a temporal homogenization simplifies the modeling, enabling straightforward extension to flows with complicating features, including cold and blowing walls. To generate data useful for calibration and validation of RANS models, special care is taken to ensure that the mean slow-growth forcing is closed in terms of the mean and other quantities that appear in standard RANS models, ensuring that there is no confounding between typical RANS closures and additional closures required for the slow-growth problem. The performance of the method is demonstrated on two problems: an essentially incompressible, zero-pressure-gradient boundary layer and a transonic boundary layer over a cooled, transpiring wall. The results show that the approach produces flows that are qualitatively similar to other slow-growth methods as well as spatially developing simulations and that the method can be a useful tool in investigating wall-bounded flows with complex physics.

42 ENGINEERING↗
DOE OSTI2017

Self-contained filtered density function

The filtered density function (FDF) closure is extended to a “self-contained” format to include the subgrid-scale (SGS) statistics of all of the hydro-thermo-chemical variables in turbulent flows. These are the thermodynamic pressure, the specific internal energy, the velocity vector, and the composition field. In this format, the model is comprehensive and facilitates large-eddy simulation (LES) of flows at both low and high compressibility levels. A transport equation is developed for the joint pressure-energy-velocity-composition filtered mass density function (PEVC-FMDF). In this equation, the effect of convection appears in closed form. The coupling of the hydrodynamics and thermochemistry is modeled via a set of stochastic differential equation for each of the transport variables. This yields a self-contained SGS closure. We demonstrated how LES is conducted of a turbulent shear flow with transport of a passive scalar. Finally, the consistency of the PEVC-FMDF formulation is established, and its overall predictive capability is appraised via comparison with direct numerical simulation (DNS) data.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Ab-initio coupled-cluster effective interactions for the shell model: Application to neutron-rich oxygen and carbon isotopes

We derive and compute effective valence-space shell-model interactions from ab initio coupled-cluster theory and apply them to open-shell and neutron-rich oxygen and carbon isotopes. Our shell-model interactions are based on nucleon-nucleon and three-nucleon forces from chiral effective-field theory. We compute the energies of ground and low-lying states, and find good agreement with experiment. In particular, our computed 2 + states are consistent with N=14,16 shell closures in 22,24 O, and a weaker N=14 shell closure in 20 C. We find good agreement between our coupled-cluster effective-interaction results with those obtained from standard single-reference coupled-cluster calculations for up to eight valence neutrons.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2020

Early Signal of Emerging Nuclear Collectivity in Neutron-Rich 129Sb

Radioactive 129Sb, which can be treated as a proton plus semimagic 128Sn core within the particle-core coupling scheme, was studied by Coulomb excitation. Reduced electric quadrupole transition probabilities, B(E2), for the 2+Ⓧπg7/2 multiplet members and candidate πd5/2 state were measured. The results indicate that the total electric quadrupole strength of 129Sb is a factor of 1.39(11) larger than the 128Sn core, which is in stark contrast to the expectations of the empirically successful particle-core coupling scheme. Shell-model calculations performed with two different sets of nucleon-nucleon interactions suggest that this enhanced collectivity is due to constructive quadrupole coherence in the wave functions stemming from the proton-neutron residual interactions, where adding one nucleon to a core near a double-shell closure can have a pronounced effect. The enhanced electric quadrupole strength is an early signal of the emerging nuclear collectivity that becomes dominant away from the shell closure.

Gray, Tim↗
DOE OSTI2015

Requirements for self-magnetically insulated transmission lines

Self-magnetically insulated transmission lines (MITLs) connect pulsed-power drivers with a load. Although the technology was originally developed in the 1970s and is widely used today in super power generators, failure of the technology is the principal limitation on the power that can be delivered to an experiment. We address issues that are often overlooked, rejected after inadequate simulations, or covered by overly conservative assumptions: (i) electron retrapping in coupling MITLs to loads, (ii) the applicability of collisionless versus collisional electron flow, (iii) power transport efficiency as a function of the geometry at the beginning of the MITL, (iv) gap closure and when gap closure can be neglected, and (v) the role of negative ions in causing anode plasmas and enhancing current losses. We suggest a practical set of conservative design requirements for self-magnetically insulated electron flow based on the results discussed in this paper and on previously published results. The requirements are not necessarily severe constraints in all MITL applications; however, each of the 18 suggested requirements should be examined in the design of a MITL and in the investigation of excessive losses.

43 PARTICLE ACCELERATORS↗
DOE OSTI2025

Scalar flux transport models for self-similar turbulent mixing

A common approach to closing turbulent species flux in multicomponent Reynolds-averaged Navier-Stokes models is to use the standard gradient diffusion approximation. While such an approach has been shown to work well when applied to many canonical turbulent mixing configurations, a gradient diffusion approach is fundamentally limited in its ability to capture complex phenomena such as countergradient transport. For this reason, complicated mixing applications may benefit by treating the turbulent diffusivity with a model transport equation in a manner analogous to second-moment momentum closure in Reynolds-stress transport models. Here, the present work explores the development and application of two different scalar flux transport (SFT) models. Self-similarity constraints are derived for these models, and they are evaluated against gradient-diffusion-based models in several one- and two-dimensional problems of turbulent mixing. It is found that the new SFT models out-perform gradient diffusion models in problems involving rapid acceleration reversal and in problems involving anisotropic transport of materials. In addition, it is found that even a hybrid-SFT approach, in which an SFT equation is utilized along with a gradient diffusion closure, provides some measure of improvement over models that transport the mass flux rather than the scalar flux.

Reynolds-averaged Navier Stokes↗
DOE OSTI2014

Structure of human farnesyl pyrophosphate synthase in complex with an aminopyridine bisphosphonate and two molecules of inorganic phosphate

A co-crystal structure of human farnesyl pyrophosphate synthase in complex with an aminopyridine bisphosphonate, YS0470, and two molecules of inorganic phosphate has been determined. The identity of the phosphate ligands was confirmed by anomalous diffraction data. Human farnesyl pyrophosphate synthase (hFPPS) produces farnesyl pyrophos@@phate, an isoprenoid essential for a variety of cellular processes. The enzyme has been well established as the molecular target of the nitrogen-containing bisphosphonates (N-BPs), which are best known for their antiresorptive effects in bone but are also known for their anticancer properties. Crystal structures of hFPPS in ternary complexes with a novel bisphosphonate, YS0470, and the secondary ligands inorganic phosphate (P{sub i}), inorganic pyrophosphate (PP{sub i}) and isopentenyl pyrophosphate (IPP) have recently been reported. Only the co-binding of the bisphosphonate with either PP{sub i} or IPP resulted in the full closure of the C-@@terminal tail of the enzyme, a conformational change that is required for catalysis and that is also responsible for the potent in vivo efficacy of N-BPs. In the present communication, a co-crystal structure of hFPPS in complex with YS0470 and two molecules of P{sub i} is reported. The unusually close proximity between these ligands, which was confirmed by anomalous diffraction data, suggests that they interact with one another, with their anionic charges neutralized in their bound state. The structure also showed the tail of the enzyme to be fully disordered, indicating that simultaneous binding of two P{sub i} molecules with a bisphosphonate cannot induce the tail-closing conformational change in hFPPS. Examination of homologous FPPSs suggested that this ligand-dependent tail closure is only conserved in the mammalian proteins. The prevalence of P{sub i}-bound hFPPS structures in the PDB raises a question regarding the in vivo relevance of P{sub i} binding to the function of the enzyme.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

An active site-tail interaction in the structure of hexahistidine-tagged Thermoplasma acidophilum citrate synthase

Citrate synthase (CS) plays a central metabolic role in aerobes and many other organisms. The CS reaction comprises two half-reactions: a Claisen aldol condensation of acetyl-CoA (AcCoA) and oxaloacetate (OAA) that forms citryl-CoA (CitCoA), and CitCoA hydrolysis. Protein conformational changes that `close' the active site play an important role in the assembly of a catalytically competent condensation active site. CS from the thermoacidophile Thermoplasma acidophilum (TpCS) possesses an endogenous Trp fluorophore that can be used to monitor the condensation reaction. The 2.2 Å resolution crystal structure of TpCS fused to a C-terminal hexahistidine tag (TpCSH6) reported here is an `open' structure that, when compared with several liganded TpCS structures, helps to define a complete path for active-site closure. One active site in each dimer binds a neighboring His tag, the first nonsubstrate ligand known to occupy both the AcCoA and OAA binding sites. Solution data collectively suggest that this fortuitous interaction is stabilized by the crystalline lattice. In conclusion, as a polar but almost neutral ligand, the active site-tail interaction provides a new starting point for the design of bisubstrate-analog inhibitors of CS.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Dual effects of atmospheric pressure plasma jet on skin wound healing of mice

ABSTRACT Cold plasma has become an attractive tool for promoting wound healing and treating skin diseases. This article presents an atmospheric pressure plasma jet (APPJ) generated in argon gas through dielectric barrier discharge, which was applied to superficial skin wounds in BALB/c mice. The mice ( n = 50) were assigned randomly into five groups (named A, B, C, D, E) with 10 animals in each group. Natural wound healing was compared with stimulated wound healing treated daily with APPJ for different time spans (10, 20, 30, 40, and 50 seconds) on 14 consecutive days. APPJ emission spectra, morphological changes in animal wounds, and tissue histological parameters were analyzed. Statistical results revealed that wound size changed over the duration of the experimental period and there was a significant interaction between experimental day and group. Differences between group C and other groups at day 7 were statistically significant ( p < 0.05). All groups had nearly achieved closure of the untreated control wounds at day 14. The wounds treated with APPJ for 10, 20, 30, and 40 seconds showed significantly enhanced daily improvement compared with the control and almost complete closure at day 12, 10, 7, and 13, respectively. The optimal results of epidermal cell regeneration, granulation tissue hyperplasia, and collagen deposition in histological aspect were observed at day 7. However, the wounds treated for 50 seconds were less well healed at day 14 than those of the control. It was concluded that appropriate doses of cold plasma could inactivate bacteria around the wound, activate fibroblast proliferation in wound tissue, and eventually promote wound healing. Whereas, over doses of plasma suppressed wound healing due to causing cell death by apoptosis or necrosis. Both positive and negative effects may be related to the existence of reactive oxygen and nitrogen species (ROS and RNS) in APPJ.

Xu, Gui‐Min↗
DOE OSTI2017

Direct Numerical Simulation of Pebble Bed Flows: Database Development and Investigation of Low-Frequency Temporal Instabilities

Computational analyses of fluid flow through packed pebble bed domains using the Reynolds-averaged NavierStokes framework have had limited success in the past. Because of a lack of high-fidelity experimental or computational data, optimization of Reynolds-averaged closure models for these geometries has not been extensively developed. In the present study, direct numerical simulation was employed to develop a high-fidelity database that can be used for optimizing Reynolds-averaged closure models for pebble bed flows. A face-centered cubic domain with periodic boundaries was used. Flow was simulated at a Reynolds number of 9308 and cross-verified by using available quasi-DNS data. During the simulations, low-frequency instability modes were observed that affected the stationary solution. Furthermore, these instabilities were investigated by using the method of proper orthogonal decomposition, and a correlation was found between the time-dependent asymmetry of the averaged velocity profile data and the behavior of the highest energy eigenmodes.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗