24 research outputs found
An Application of Gaussian Process Modeling for High-order Accurate Adaptive Mesh Refinement Prolongation
We present a new polynomial-free prolongation scheme for Adaptive Mesh
Refinement (AMR) simulations of compressible and incompressible computational
fluid dynamics. The new method is constructed using a multi-dimensional
kernel-based Gaussian Process (GP) prolongation model. The formulation for this
scheme was inspired by the GP methods introduced by A. Reyes et al. (A New
Class of High-Order Methods for Fluid Dynamics Simulation using Gaussian
Process Modeling, Journal of Scientific Computing, 76 (2017), 443-480; A
variable high-order shock-capturing finite difference method with GP-WENO,
Journal of Computational Physics, 381 (2019), 189-217). In this paper, we
extend the previous GP interpolations and reconstructions to a new GP-based AMR
prolongation method that delivers a high-order accurate prolongation of data
from coarse to fine grids on AMR grid hierarchies. In compressible flow
simulations special care is necessary to handle shocks and discontinuities in a
stable manner. To meet this, we utilize the shock handling strategy using the
GP-based smoothness indicators developed in the previous GP work by A. Reyes et
al. We demonstrate the efficacy of the GP-AMR method in a series of testsuite
problems using the AMReX library, in which the GP-AMR method has been
implemented
The Post-Merger Magnetized Evolution of White Dwarf Binaries: The Double-Degenerate Channel of Sub-Chandrasekhar Type Ia Supernovae and the Formation of Magnetized White Dwarfs
Type Ia supernovae (SNe Ia) play a crucial role as standardizable
cosmological candles, though the nature of their progenitors is a subject of
active investigation. Recent observational and theoretical work has pointed to
merging white dwarf binaries, referred to as the double-degenerate channel, as
the possible progenitor systems for some SNe Ia. Additionally, recent
theoretical work suggests that mergers which fail to detonate may produce
magnetized, rapidly-rotating white dwarfs. In this paper, we present the first
multidimensional simulations of the post-merger evolution of white dwarf
binaries to include the effect of the magnetic field. In these systems, the two
white dwarfs complete a final merger on a dynamical timescale, and are tidally
disrupted, producing a rapidly-rotating white dwarf merger surrounded by a hot
corona and a thick, differentially-rotating disk. The disk is strongly
susceptible to the magnetorotational instability (MRI), and we demonstrate that
this leads to the rapid growth of an initially dynamically weak magnetic field
in the disk, the spin-down of the white dwarf merger, and to the subsequent
central ignition of the white dwarf merger. Additionally, these magnetized
models exhibit new features not present in prior hydrodynamic studies of white
dwarf mergers, including the development of MRI turbulence in the hot disk,
magnetized outflows carrying a significant fraction of the disk mass, and the
magnetization of the white dwarf merger to field strengths
G. We discuss the impact of our findings on the origins, circumstellar media,
and observed properties of SNe Ia and magnetized white dwarfs.Comment: Accepted ApJ version published on 8/20/13, with significant
additional text added discussing the nature of the magnetized outflows, and
possible CSM observational features relevant to NaID detection
Feasibility and Performance of the Staged Z-Pinch: A One-dimensional Study with FLASH and MACH2
Z-pinch platforms constitute a promising pathway to fusion energy research.
Here, we present a one-dimensional numerical study of the staged Z-pinch (SZP)
concept using the FLASH and MACH2 codes. We discuss the verification of the
codes using two analytical benchmarks that include Z-pinch-relevant physics,
building confidence on the codes' ability to model such experiments. Then,
FLASH is used to simulate two different SZP configurations: a xenon gas-puff
liner (SZP1*) and a silver solid liner (SZP2). The SZP2 results are compared
against previously published MACH2 results, and a new code-to-code comparison
on SZP1* is presented. Using an ideal equation of state and analytical
transport coefficients, FLASH yields a fuel convergence ratio (CR) of
approximately 39 and a mass-averaged fuel ion temperature slightly below 1 keV
for the SZP2 scheme, significantly lower than the full-physics MACH2
prediction. For the new SZP1* configuration, full-physics FLASH simulations
furnish large and inherently unstable CRs (> 300), but achieve fuel ion
temperatures of many keV. While MACH2 also predicts high temperatures, the fuel
stagnates at a smaller CR. The integrated code-to-code comparison reveals how
magnetic insulation, heat conduction, and radiation transport affect platform
performance and the feasibility of the SZP concept
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Strong suppression of heat conduction in a laboratory replica of galaxy-cluster turbulent plasmas
In conventional gases and plasmas, it is known that heat fluxes are proportional to temperature gradients, with collisions between particles mediating energy flow from hotter to colder regions and the coefficient of thermal conduction given by Spitzer's theory. However, this theory breaks down in magnetized, turbulent, weakly collisional plasmas, although modifications are difficult to predict from first principles due to the complex, multiscale nature of the problem. Understanding heat transport is important in astrophysical plasmas such as those in galaxy clusters, where observed temperature profiles are explicable only in the presence of a strong suppression of heat conduction compared to Spitzer's theory. To address this problem, we have created a replica of such a system in a laser laboratory experiment. Our data show a reduction of heat transport by two orders of magnitude or more, leading to large temperature variations on small spatial scales (as is seen in cluster plasmas)