5,817 research outputs found
Numerical solution of a non-linear conservation law applicable to the interior dynamics of partially molten planets
The energy balance of a partially molten rocky planet can be expressed as a
non-linear diffusion equation using mixing length theory to quantify heat
transport by both convection and mixing of the melt and solid phases. In this
formulation the effective or eddy diffusivity depends on the entropy gradient,
, as well as entropy. First we present a simplified
model with semi-analytical solutions, highlighting the large dynamic range of
, around 12 orders of magnitude, for physically-relevant
parameters. It also elucidates the thermal structure of a magma ocean during
the earliest stage of crystal formation. This motivates the development of a
simple, stable numerical scheme able to capture the large dynamic range of
and provide a flexible and robust method for
time-integrating the energy equation.
We then consider a full model including energy fluxes associated with
convection, mixing, gravitational separation, and conduction that all depend on
the thermophysical properties of the melt and solid phases. This model is
discretised and evolved by applying the finite volume method (FVM), allowing
for extended precision calculations and using as the
solution variable. The FVM is well-suited to this problem since it is naturally
energy conserving, flexible, and intuitive to incorporate arbitrary non-linear
fluxes that rely on lookup data. Special attention is given to the numerically
challenging scenario in which crystals first form in the centre of a magma
ocean.
Our computational framework is immediately applicable to modelling high melt
fraction phenomena in Earth and planetary science research. Furthermore, it
provides a template for solving similar non-linear diffusion equations arising
in other disciplines, particularly for non-linear functional forms of the
diffusion coefficient
Finite element solution techniques for large-scale problems in computational fluid dynamics
Element-by-element approximate factorization, implicit-explicit and adaptive implicit-explicit approximation procedures are presented for the finite-element formulations of large-scale fluid dynamics problems. The element-by-element approximation scheme totally eliminates the need for formation, storage and inversion of large global matrices. Implicit-explicit schemes, which are approximations to implicit schemes, substantially reduce the computational burden associated with large global matrices. In the adaptive implicit-explicit scheme, the implicit elements are selected dynamically based on element level stability and accuracy considerations. This scheme provides implicit refinement where it is needed. The methods are applied to various problems governed by the convection-diffusion and incompressible Navier-Stokes equations. In all cases studied, the results obtained are indistinguishable from those obtained by the implicit formulations
Astrophysical turbulence modeling
The role of turbulence in various astrophysical settings is reviewed. Among
the differences to laboratory and atmospheric turbulence we highlight the
ubiquitous presence of magnetic fields that are generally produced and
maintained by dynamo action. The extreme temperature and density contrasts and
stratifications are emphasized in connection with turbulence in the
interstellar medium and in stars with outer convection zones, respectively. In
many cases turbulence plays an essential role in facilitating enhanced
transport of mass, momentum, energy, and magnetic fields in terms of the
corresponding coarse-grained mean fields. Those transport properties are
usually strongly modified by anisotropies and often completely new effects
emerge in such a description that have no correspondence in terms of the
original (non coarse-grained) fields.Comment: 88 pages, 26 figures, published in Reports on Progress in Physic
Two-dimensional hydrodynamic core-collapse supernova simulations with spectral neutrino transport. I. Numerical method and results for a 15 M_sun star
Supernova models with a full spectral treatment of the neutrino transport are
presented, employing the Prometheus/Vertex neutrino-hydrodynamics code with a
``ray-by-ray plus'' approximation for treating two- (or three-) dimensional
problems. The method is described in detail and critically assessed with
respect to its capabilities, limitations, and inaccuracies in the context of
supernova simulations. In this first paper of a series, 1D and 2D core-collapse
calculations for a (nonrotating) 15 M_sun star are discussed, uncertainties in
the treatment of the equation of state -- numerical and physical -- are tested,
Newtonian results are compared with simulations using a general relativistic
potential, bremsstrahlung and interactions of neutrinos of different flavors
are investigated, and the standard approximation in neutrino-nucleon
interactions with zero energy transfer is replaced by rates that include
corrections due to nucleon recoil, thermal motions, weak magnetism, and nucleon
correlations. Models with the full implementation of the ``ray-by-ray plus''
spectral transport were found not to explode, neither in spherical symmetry nor
in 2D with a 90 degree lateral wedge. The success of previous 2D simulations
with grey, flux-limited neutrino diffusion can therefore not be confirmed.
Omitting the radial velocity terms in the neutrino momentum equation leads to
``artificial'' explosions by increasing the neutrino energy density in the
convective gain layer by about 20--30% and thus the integral neutrino energy
deposition in this region by about a factor of two. (abbreviated)Comment: 46 pages plus 13 pages online material; 49 figures; referee's
comments included, version accepted by Astronomy & Astrophysic
Anelastic Versus Fully Compressible Turbulent Rayleigh-B\'enard Convection
Numerical simulations of turbulent Rayleigh-B\'enard convection in an ideal
gas, using either the anelastic approximation or the fully compressible
equations, are compared. Theoretically, the anelastic approximation is expected
to hold in weakly superadiabatic systems with , where denotes the superadiabatic temperature drop over the
convective layer and the bottom temperature. Using direct numerical
simulations, a systematic comparison of anelastic and fully compressible
convection is carried out. With decreasing superadiabaticity , the
fully compressible results are found to converge linearly to the anelastic
solution with larger density contrasts generally improving the match. We
conclude that in many solar and planetary applications, where the
superadiabaticity is expected to be vanishingly small, results obtained with
the anelastic approximation are in fact more accurate than fully compressible
computations, which typically fail to reach small for numerical
reasons. On the other hand, if the astrophysical system studied contains
regions, such as the solar photosphere, fully compressible
simulations have the advantage of capturing the full physics. Interestingly,
even in weakly superadiabatic regions, like the bulk of the solar convection
zone, the errors introduced by using artificially large values for
for efficiency reasons remain moderate. If quantitative errors of the order of
are acceptable in such low regions, our work suggests that
fully compressible simulations can indeed be computationally more efficient
than their anelastic counterparts.Comment: 24 pages, 9 figure
Rayleigh-B\'enard convection with a melting boundary
We study the evolution of a melting front between the solid and liquid phases
of a pure incompressible material where fluid motions are driven by unstable
temperature gradients. In a plane layer geometry, this can be seen as classical
Rayleigh-B\'enard convection where the upper solid boundary is allowed to melt
due to the heat flux brought by the fluid underneath. This free-boundary
problem is studied numerically in two dimensions using a phase-field approach,
classically used to study the melting and solidification of alloys, which we
dynamically couple with the Navier-Stokes equations in the Boussinesq
approximation. The advantage of this approach is that it requires only moderate
modifications of classical numerical methods. We focus on the case where the
solid is initially nearly isothermal, so that the evolution of the topography
is related to the inhomogeneous heat flux from thermal convection, and does not
depend on the conduction problem in the solid. From a very thin stable layer of
fluid, convection cells appears as the depth -- and therefore the effective
Rayleigh number of the layer increases. The continuous melting of the solid
leads to dynamical transitions between different convection cell sizes and
topography amplitudes. The Nusselt number can be larger than its value for a
planar upper boundary, due to the feedback of the topography on the flow, which
can stabilize large-scale laminar convection cells.Comment: 36 pages, 16 figure
Modules for Experiments in Stellar Astrophysics (MESA): Convective Boundaries, Element Diffusion, and Massive Star Explosions
We update the capabilities of the software instrument Modules for Experiments
in Stellar Astrophysics (MESA) and enhance its ease of use and availability.
Our new approach to locating convective boundaries is consistent with the
physics of convection, and yields reliable values of the convective core mass
during both hydrogen and helium burning phases. Stars with
become white dwarfs and cool to the point where the electrons are degenerate
and the ions are strongly coupled, a realm now available to study with MESA due
to improved treatments of element diffusion, latent heat release, and blending
of equations of state. Studies of the final fates of massive stars are extended
in MESA by our addition of an approximate Riemann solver that captures shocks
and conserves energy to high accuracy during dynamic epochs. We also introduce
a 1D capability for modeling the effects of Rayleigh-Taylor instabilities that,
in combination with the coupling to a public version of the STELLA radiation
transfer instrument, creates new avenues for exploring Type II supernovae
properties. These capabilities are exhibited with exploratory models of
pair-instability supernova, pulsational pair-instability supernova, and the
formation of stellar mass black holes. The applicability of MESA is now widened
by the capability of importing multi-dimensional hydrodynamic models into MESA.
We close by introducing software modules for handling floating point exceptions
and stellar model optimization, and four new software tools -- MESAWeb,
MESA-Docker, pyMESA, and mesastar.org -- to enhance MESA's education and
research impact.Comment: 64 pages, 61 figures; Accepted to AAS Journal
Large time behavior of nonlinear finite volume schemes for convection-diffusion equations
In this contribution we analyze the large time behavior of a family of nonlinear finite volume schemes for anisotropic convection-diffusion equations set in a bounded bidimensional domain and endowed with either Dirichlet and / or no-flux boundary conditions. We show that solutions to the two-point flux approximation (TPFA) and discrete duality finite volume (DDFV) schemes under consideration converge exponentially fast toward their steady state. The analysis relies on discrete entropy estimates and discrete functional inequalities. As a biproduct of our analysis, we establish new discrete Poincaré-Wirtinger, Beckner and logarithmic Sobolev inequalities. Our theoretical results are illustrated by numerical simulations
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