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At least 865 records · Page 48
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

POST-SHOCK-REVIVAL EVOLUTION IN THE NEUTRINO-HEATING MECHANISM OF CORE-COLLAPSE SUPERNOVAE

We perform experimental simulations with spherical symmetry and axisymmetry to understand the post-shock-revival evolution of core-collapse supernovae. Assuming that the stalled shock wave is relaunched by neutrino heating and employing the so-called light bulb approximation, we induce shock revival by raising the neutrino luminosity up to the critical value, which is determined by dynamical simulations. A 15 M{sub Sun} progenitor model is employed. We incorporate nuclear network calculations with a consistent equation of state in the simulations to account for the energy release by nuclear reactions and their feedback to hydrodynamics. Varying the shock-relaunch time rather arbitrarily, we investigate the ensuing long-term evolutions systematically, paying particular attention to the explosion energy and nucleosynthetic yields as a function of relaunch time, or equivalently, the accretion rate at shock revival. We study in detail how the diagnostic explosion energy approaches the asymptotic value and which physical processes contribute in what proportions to the explosion energy. Furthermore, we study the dependence of physical processes on the relaunch time and the dimension of dynamics. We find that the contribution of nuclear reactions to the explosion energy is comparable to or greater than that of neutrino heating. In particular, recombinations are dominant over burnings in the contributions of nuclear reactions. Interestingly, one-dimensional (1D) models studied in this paper cannot produce the appropriate explosion energy and nickel mass simultaneously; nickels are overproduced. This problem is resolved in 2D models if the shock is relaunched at 300-400 ms after the bounce.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

Validation of gyrokinetic simulations with measurements of electron temperature fluctuations and density-temperature phase angles on ASDEX Upgrade

Measurements of turbulent electron temperature fluctuation amplitudes, $δT_e⊥/T_e$, frequency spectra, and radial correlation lengths, $L_r(T_{e⊥}$), have been performed at ASDEX Upgrade using a newly upgraded Correlation ECE diagnostic in the range of scales $k_⊥<1.4 cm^{-1}, k_r < 3.5 cm^{-1} (k_⊥ρ_s < 0.28$ and $k_rρ_s < 0.7$). The phase angle between turbulent temperature and density fluctuations, α nT , has also been measured by using an ECE radiometer coupled to a reflectometer along the same line of sight. These quantities are used simultaneously to constrain a set of ion-scale non-linear gyrokinetic turbulence simulations of the outer core ( ρ tor = 0.75) of a low density, electron heated L-mode plasma, performed using the gyrokinetic simulation code, GENE. The ion and electron temperature gradients were scanned within uncertainties. It is found that gyrokinetic simulations are able to match simultaneously the electron and ion heat flux at this radius within the experimental uncertainties. The simulations were performed based on a reference discharge for which $δT_{e⊥}/T_e$ measurements were available, and $L_r(T_{e⊥}$) and α nT were then predicted using synthetic diagnostics prior to measurements in a repeat discharge. While temperature fluctuation amplitudes are overestimated by >50% for all simulations within the sensitivity scans performed, good quantitative agreement is found for $L_r(T_{e⊥}$) and α nT . Finally, a validation metric is used to quantify the level of agreement of individual simulations with experimental measurements, and the best agreement is found close to the experimental gradient values.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Consistent evidence of increasing Antarctic accumulation with warming

Projections of changes in Antarctic Ice Sheet (AIS) surface mass balance indicate a negative contribution to sea level because of the expected increase in precipitation due to the higher moisture holding capacity of warmer air1. Observations over the past decades, however, are unable to constrain the relation between temperature and accumulation changes because both are dominated by strong natural variability. Here we derive a consistent continental-scale increase in accumulation of approximately 5 ± 1% K -1 , through the assessment of ice-core data (spanning the large temperature change during the last deglaciation, 21,000 to 10,000 years ago), in combination with palaeo-simulations, future projections by 35 general circulation models (GCMs), and one high-resolution future simulation. The ice-core data and modelling results for the last deglaciation agree, showing uniform local sensitivities of ~6% K -1 . The palaeo-simulation allows for a continental-scale aggregation of accumulation changes reaching 4.3% K -1 . Despite the different timescales, these sensitivities agree with the multi-model mean of 6.1 ± 2.6% K -1 (GCM projections) and the continental-scale sensitivity of 4.9% K -1 (high-resolution future simulation). Because some of the mass gain of the AIS is offset by dynamical losses induced by accumulation, we provide a response function allowing projections of sea-level fall in terms of continental-scale accumulation changes that compete with surface melting and dynamical losses induced by other mechanisms.

Source record↗
DOE OSTI2019

Multimessenger asteroseismology of core-collapse supernovae

In this work we investigate correlated gravitational wave and neutrino signals from rotating core-collapse supernovae with simulations. Using an improved mode identification procedure based on mode function matching, we show that a linear quadrupolar mode of the core produces a dual imprint on gravitational waves and neutrinos in the early post-bounce phase of the supernova. The angular harmonics of the neutrino emission are consistent with the mode energy around the neutrinospheres, which points to a mechanism for the imprint on neutrinos. Thus, neutrinos carry information about the mode amplitude in the outer region of the core, whereas gravitational waves probe deeper in. We also find that the best-fit mode function has a frequency bounded above by ~420 Hz , and yet the mode’s frequency in our simulations is ~15 % higher, due to the use of Newtonian hydrodynamics and a widely used pseudo-Newtonian gravity approximation. This overestimation is particularly important for the analysis of gravitational wave detectability and asteroseismology, pointing to limitations of pseudo-Newtonian approaches for these purposes, possibly even resulting in excitation of incorrect modes. In addition, mode frequency matching (as opposed to mode function matching) could be resulting in mode misidentification in recent work. Lastly, we evaluate the prospects of a multimessenger detection of the mode using current technology. The detection of the imprint on neutrinos is most challenging, with a maximum detection distance of ~ 1 kpc using the IceCube Neutrino Observatory. The maximum distance for detecting the complementary gravitational wave imprint is ~ 5 kpc using Advanced LIGO at design sensitivity.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

Deploy threading in Nalu solver stack

The goal of the ExaWind project is to enable predictive simulations of wind farms composed of many MW-scale turbines situated in complex terrain. Predictive simulations will require computational fluid dynamics (CFD) simulations for which the mesh resolves the geometry of the turbines, and captures the rotation and large deflections of blades. Whereas such simulations for a single turbine are arguably petascale class, multi-turbine wind farm simulations will require exascale-class resources. The primary code in the ExaWind project is Nalu, which is an unstructured-grid solver for the acousticallyincompressible Navier-Stokes equations, and mass continuity is maintained through pressure projection. The model consists of the mass-continuity Poisson-type equation for pressure and a momentum equation for the velocity. For such modeling approaches, simulation times are dominated by linear-system setup and solution for the continuity and momentum systems. For the ExaWind challenge problem, the moving meshes greatly affect overall solver costs as re-initialization of matrices and re-computation of preconditioners is required at every time step In this Milestone, we examine the effect of threading on the solver stack performance against flat-MPI results obtained from previous milestones using Haswell performance data full-turbine simulations. Whereas the momentum equations are solved only with the Trilinos solvers, we investigate two algebraic-multigrid preconditioners for the continuity equations: Trilinos/Muelu and HYPRE/BoomerAMG. These two packages embody smoothed-aggregation and classical Ruge-Stiiben AMG methods, respectively. In our FY18 Q2 report, we described our efforts to improve setup and solve of the continuity equations under flat-MPI parallelism. While significant improvement was demonstrated in the solve phase, setup times remained larger than expected. Starting with the optimized settings described in the Q2 report, we explore here simulation performance where OpenMP threading is employed in the solver stack. For Trilinos, threading is acheived through the Kokkos abstraction where, whereas HYPRE/BoomerAMG employs straight OpenMP. We examined results for our mid-resolution baseline turbine simulation configuration (229M DOF). Simulations on 2048 Haswell cores explored the effect of decreasing the number of MPI ranks while increasing the number of threads. Both HYPRE and Trilinos exhibited similar overal solution times, and both showed dramatic increases in simulation time in the shift from MPI ranks to OpenMP threads. This increase is attributed to the large amount of work per MPI rank starting at the single-thread configuration. Decreasing MPI ranks, while increasing threads, may be increasing simulation time due to thread synchronization and start-up overhead contributing to the latency and serial time in the model. These result showed that an MPI+OpenMP parallel decomposition will be more effective as the amount per MPI rank computation per MPI rank decreases and the communication latency increases. This idea was demonstrated in a strong scaling study of our low-resolution baseline model (29M DOF) with the Trilinos-HYPRE configuration. While MPI-only results showed scaling improvement out to about 1536 cores, engaging threading carried scaling improvements out to 4128 cores — roughly 7000 DOF per core. This is an important result as improved strong scaling is needed for simulations to be executed over sufficiently long simulated durations (i.e., for many timesteps). In addition to threading work described above, the team examined solver-performance improvements by exploring communication-overhead in the HYPRE-GMRES implementation through a communicationoptimal- GMRE algorithm (CO-GMRES), and offloading compute-intensive solver actions to GPUs. To those ends, a HYPRE mini-app was allow us to easily test different solver approaches and HYPRE parameter settings without running the entire Nalu code. With GPU acceleration on the Summitdev supercomputer, a 20x speedup was achieved for the overall preconditioner and solver execution time for the mini-app. A study on Haswell processors showed that CO-GMRES provides benefits as one increases MPI ranks.

17 WIND ENERGY↗
DOE OSTI2018

Real-Time Simulation of Modular Multilevel Converters

Modular multilevel converters (MMCs) are one of the most popular configurations of voltage-source converters (VSCs) used in applications like high-voltage direct current (HVdc) transmission systems and flexible alternate current transmission systems (FACTS). They have a large number of modules with thousands of capacitors and semiconductor devices, which make it hard to simulate in real-time. In this paper, a low-cost high-performance computing (HPC) configuration to simulate MMCs in real-time is proposed. The configuration uses a HPC digital signal processor (DSP) with 8 cores. An advanced MMC simulation algorithm is implemented using HPC techniques to optimize the real-time performance in the DSP. A comparison is performed with existing solutions to show the superiority of the proposed configuration and algorithms.

Debnath, Suman↗
DOE OSTI2021

Simulating Magnetized Electron Cooling for EIC with JSPEC

We present a possible electron cooling configuration for the proposed Electron Ion Collider (EIC) facility, developed using a Nelder-Mead Simplex optimization procedure built into JSPEC, an electron cooling code developed at Jefferson Lab. We show the time evolution of the emittance of the ion beam in the presence of this cooler evaluated assuming the ion distribution remains Gaussian. We also show that bi-gaussian distributions emerge in simulations of ion macro-particles. We show how intra-beam scattering can be treated with a core-tail model in simulations of ion macro-particles. The Sirepo/JSPEC* and Sirepo/Jupyter** apps will be presented, with instructions enabling the community to reproduce our simulations.

Coleman, S. J.↗
DOE OSTI2015

VERA 3.3 Release Notes

The Virtual Environment for Reactor Applications components included in this distribution include selected computational tools and supporting infrastructure that solve neutronics, thermal-hydraulics, fuel performance, and coupled neutronics-thermal hydraulics-fuel performance problems. The infrastructure components provide a simplified common user input capability and provide for the physics integration with data transfer and coupled-physics iterative solution algorithms. Neutronics analysis can be performed for 2D lattices, 2D core and 3D core problems for pressurized water reactor geometries that can be used to calculate criticality and fission rate distributions by pin for input fuel compositions. Two neutronics capabilities are provided in this distribution including MPACT (from the University of Michigan) that supports 2D lattices and Insilico (from Oak Ridge National Laboratory) that supports 2D lattices, 2D core planes, and 3D core geometries. Both codes include integrated cross section capabilities that provide problem-specific cross sections for the problems defined. A thermal-hydraulics capability is provided with CTF (an updated version of the COBRA-TF code developed by the Pennsylvania State University) that allows thermal-hydraulics analyses for single and multiple assemblies using the simplified VERA common input. This distribution also includes an initial coupled neutronics/thermal-hydraulics capability to allow calculations of single assemblies using Insilico or MPACT coupled with CTF. The VERA fuel rod performance subcomponent BISON-CASL calculates, on a 3D basis, fuel rod temperature, fuel rod internal pressure, free gas volume, clad integrity and fuel rod waterside diameter. These capabilities allow simulation of power cycling, fuel conditioning and deconditioning, high burnup performance, power uprate scoping studies, and accident performance. Specifically targeted at VERA’s core simulator functionality (VERA-CS), the existing VERA subchannel-neutronics capability based on CTF and Insilico has been extended to use the axisymmetric 2D R-Z fuel rod modeling functionality of BISON-CASL to produce a three-way coupled neutronics, thermal-hydraulics and fuel performance capability known as Tiamat. This distribution is intended to be used for testing and evaluation of the released components of VERA. Testing within CASL has focused specifically on Westinghouse four-loop reactor geometries and conditions with example problems included in the distribution.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
NASA NTRS1986

Composite sandwich thermostructural behavior - Computational simulation

Computational methods have been developed for simulating the thermomechanical behavior of composite sandwiches, in which the analyses with several levels of progressive sophistication were used in conjunction with composite hygrothermomechanical theory. The sophistication levels include: (1) three-dimensional detailed finite element modeling of the honeycomb, the adhesive, and the composite faces; (2) three-dimensional finite element modeling assuming a homogeneous core; (3) laminate theory simulation; and (4) simple equations for predicting the equivalent properties of the honeycomb core. These levels have been packaged into a procedure embedded in a computer code streamlined for the simulation of the composite sandwich hygrothermal and structural behavior. It is shown that in order to properly simulate the thermomechanical response of the composite sandwich, all the honeycomb thermal and mechanical properties must be used.

Chamis, C. C.↗
DOE OSTI2015

Measurement of neutron spectra in the experimental reactor LR-0

The measurement of fast neutron fluxes is important in many areas of nuclear technology. It affects the stability of the reactor structural components, performance of fuel, and also the fuel manner. The experiments performed at the LR-0 reactor were in the past focused on the measurement of neutron field far from the core, in reactor pressure vessel simulator or in biological shielding simulator. In the present the measurement in closer regions to core became more important, especially measurements in structural components like reactor baffle. This importance increases with both reactor power increase and also long term operation. Other important task is an increasing need for the measurement close to the fuel. The spectra near the fuel are aimed due to the planned measurements with the FLIBE salt, in FHR / MSR research, where one of the task is the measurement of the neutron spectra in it. In both types of experiments there is strong demand for high working count rate. The high count rate is caused mainly by high gamma background and by high fluxes. The fluxes in core or in its vicinity are relatively high to ensure safe reactor operation. This request is met in the digital spectroscopic apparatus. All experiments were realized in the LR-0 reactor. It is an extremely flexible light water zero-power research reactor, operated by the Research Center Rez (Czech Republic). (authors)

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
DOE OSTI2014

LDRD HPC4Energy Wrapup Report - LDRD 12-ERD-074

High-performance computing and simulation has the potential to optimize production, distribution, and conversion of energy. Although a number of concepts have been discussed, a comprehensive research project to establish and quantify the effectiveness of computing and simulation at scale to core energy problems has not been conducted. We propose to perform the basic research to adapt existing high-performance computing tools and simulation approaches to two selected classes of problems common across the energy sector. The first, applying uncertainty quantification and contingency analysis techniques to energy optimization, allows us to assess the effectiveness of LLNL core competencies to problems such as grid optimization and building-system efficiency. The second, applying adaptive meshing and numerical analysis techniques to physical problems at fine scale, could allow immediate impacts in key areas such as efficient combustion and fracture and spallation. By creating an integrated project team with the necessary expertise, we can efficiently address these issues, delivering both near-term results as well as quantifying developments needed to address future energy challenges.

29 ENERGY PLANNING, POLICY, AND ECONOMY↗
DOE OSTI2022

Dark matter from axion strings with adaptive mesh refinement

Abstract Axions are hypothetical particles that may explain the observed dark matter density and the non-observation of a neutron electric dipole moment. An increasing number of axion laboratory searches are underway worldwide, but these efforts are made difficult by the fact that the axion mass is largely unconstrained. If the axion is generated after inflation there is a unique mass that gives rise to the observed dark matter abundance; due to nonlinearities and topological defects known as strings, computing this mass accurately has been a challenge for four decades. Recent works, making use of large static lattice simulations, have led to largely disparate predictions for the axion mass, spanning the range from 25 microelectronvolts to over 500 microelectronvolts. In this work we show that adaptive mesh refinement simulations are better suited for axion cosmology than the previously-used static lattice simulations because only the string cores require high spatial resolution. Using dedicated adaptive mesh refinement simulations we obtain an over three order of magnitude leap in dynamic range and provide evidence that axion strings radiate their energy with a scale-invariant spectrum, to within ~5% precision, leading to a mass prediction in the range (40,180) microelectronvolts.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2022

Thermal performance sensitivity to radial power flattening considerations in Nuclear Thermal Propulsion Reactors

Nuclear thermal propulsion is currently being developed for manned Mars and Lunar missions by the National Aeronautics and Space Administration due to its increased specific impulse and significant thrust to reduce the total trip time and cost of deep space missions. To maximize the performance of the engine the exit hydrogen propellant temperature should, ideally, be uniformly distributed among all the fuel elements. Uniform exit temperatures imply that the power must be flattened as much as possible. This paper leverages decoupled neutronic and thermal-hydraulic models to assess the thermal performance and reactivity tradeoffs associated with flattening the radial power distribution by adjusting the fuel kernel loading for each fuel element within the core. The results suggest that a uniform radial power distribution for a single control drum position will not necessarily yield an optimal core design due to the higher variance in power and thus maximum fuel and exit propellant temperature as the control drums rotate out during operation. The results presented here only point to the design characteristics be considered when core-level multi-physics simulations are applied in the context of a specific mission. (authors)

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
DOE OSTI2025

GEANT3 simulations for a beryllium Moderator for the Neutron Target Demonstrator (NTD)

Measurements of neutron-induced reactions, particularly on short-lived nuclei, are crucial for nuclear astrophysics and applications. The Neutron Target Demonstrator (NTD) aims to address this by using a strong mod erated spallation neutron source coupled with an ion storage ring. This work investigates the potential of improving the neutron moderator, a key component of the NTD. Using GEANT3 simulations, we compare the performance of the existing 80 cm carbon moderator with a new moder ator design, which utilizes beryllium pieces available at LANL to form a core surrounded by carbon. Simulations were performed 800 MeV pro tons incident on a tungsten target. The results demonstrate that the beryllium-core moderator increases the density of moderated neutrons within the central beam pipe by roughly a factor 4 compared to the pure carbon moderator. This enhancement makes the beryllium-core design a very promising option for future experiments focused on neutron-induced measurements in inverse kinematics.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2021

Three-dimensional Hydrodynamic Simulations of Convective Nuclear Burning in Massive Stars Near Iron Core Collapse

Nonspherical structure in massive stars at the point of iron core collapse can have a qualitative impact on the properties of the ensuing core-collapse supernova explosions and the multimessenger signals they produce. Strong perturbations can aid successful explosions by strengthening turbulence in the postshock region. Here we report on a set of 4π 3D hydrodynamic simulations of O- and Si-shell burning in massive star models of varied initial masses using MESA and the FLASH simulation framework. We evolve four separate 3D models for roughly the final 10 minutes prior to and including iron core collapse. We consider initial 1D MESA models with masses of 14, 20, and 25 M ⊙ to survey a range of O/Si-shell density and compositional configurations. Further, we characterize the convective shells in our 3D models and compare them to the corresponding 1D models. In general, we find that the angle-average convective speeds in our 3D simulations near collapse are three to four times larger than the convective speeds predicted by MESA at the same epoch for our chosen mixing length parameter of α MLT = 1.5. In three of our simulations, we observe significant power in the spherical harmonic decomposition of the radial velocity field at harmonic indices of ℓ = 1–3 near collapse. Our results suggest that large-scale modes are common in massive stars near collapse and should be considered a key aspect of presupernova progenitor models.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2025

A quasi-linear model of electromagnetic turbulent transport and its application to flux-driven transport predictions for STEP

A quasi-linear reduced transport model is developed from a database of high-β electromagnetic nonlinear gyrokinetic simulations performed with spherical tokamak for energy production (STEP) relevant parameters. The quasi-linear model is fully electromagnetic and accounts for the effect of equilibrium flow shear using a novel approach. Its flux predictions are shown to agree quantitatively with predictions from local nonlinear gyrokinetic simulations across a broad range of STEP-relevant local equilibria. This reduced transport model is implemented in the T3D transport solver that is used to perform the first flux-driven simulations for STEP to account for transport from hybrid kinetic ballooning mode turbulence, which dominates over a wide region of the core plasma. Nonlinear gyrokinetic simulations of the final transport steady state from T3D return turbulent fluxes that are consistent with the reduced model, indicating that the quasi-linear model may also be appropriate for describing the transport steady state. Within the assumption considered here, our simulations support the existence of a transport steady state in STEP with a fusion power comparable to that in the burning flat top of the conceptual design, but do not demonstrate how this state can be accessed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

A comparative study of hyperon equations of state in supernova simulations

A comparative study of the Λ hyperon equations of state of Banik, Hempel and Banyopadhyay (BHB), Banik et al., and Shen et al. (denoted as HShen Λ) for core-collapse supernova (CCSN) simulations is carried out in this work. The dynamical evolution of a protoneutron star (PNS) into a black hole is investigated in CCSN simulations in the general relativistic one-dimensional code using the BHBΛϕ and HShen Λ equation of state (EOS) tables and different progenitor models from Woosley and Heger. Radial profiles of the mass fractions of baryons, the density, as well as the temperature in the PNS at different moments in time, are compared for both EOS tables. The behavior of the central density of the PNS with time is demonstrated for these two Λ hyperon EOS tables and compared with their corresponding nuclear EOS tables. It is observed that the black hole formation time is higher in the BHBΛϕ case than in the HShen Λ EOS case for the entire set of progenitor models adopted here, because the repulsive Λ–Λ interaction makes the BHBΛϕ EOS stiffer. Neutrino emission with the Λ hyperon EOS ceases earlier than that of its nuclear counterpart. The long-duration evolution of the shock radius and the gravitational mass of the PNS after a successful supernova explosion with enhanced neutrino heating are studied with the BHBΛϕ EOS and s20WH07 progenitor model. The PNS is found to remain stable for 4 s and might evolve into a cold neutron star.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

Supernova Shock Breakout/Emergence Detection Predictions for a Wide-field X-Ray Survey

There are currently many large-field surveys that are operational and are being planned including the powerful Vera C. Rubin Observatory Legacy Survey of Space and Time. These surveys will increase the number and diversity of transients dramatically. However, for some transients, like supernovae (SNe), we can gain more understanding by directed observations (e.g., shock breakout and γ-ray detections) than by simply increasing the sample size. For example, the initial emission from these transients can be a powerful probe of these explosions. Upcoming ground-based detectors are not ideally suited to observing the initial emission (shock emergence) of these transients. These observations require a large field-of-view X-ray mission with a UV follow-up within the first hour of shock breakout. The emission in the first 1 hr to even 1 day provides strong constraints on the stellar radius and asymmetries in the outer layers of stars, the properties of the circumstellar medium (e.g., inhomogeneities in the wind for core-collapse SNe and accreting companions in thermonuclear SNe), and the transition region between these two areas. This paper describes a simulation for the number of SNe that could be seen by a large field-of-view lobster-eye X-ray and UV observatory.

79 ASTRONOMY AND ASTROPHYSICS↗