5,383 research outputs found
Time Dependent Radiative Transfer Calculations for Supernovae
In previous papers we discussed results from fully time-dependent radiative
transfer models for core-collapse supernova (SN) ejecta, including the Type
II-peculiar SN 1987A, the more "generic" SN II-Plateau, and more recently Type
IIb/Ib/Ic SNe. Here we describe the modifications to our radiative modeling
code, CMFGEN, which allowed those studies to be undertaken. The changes allow
for time-dependent radiative transfer of SN ejecta in homologous expansion. In
the modeling we treat the entire SN ejecta, from the innermost layer that does
not fall back on the compact remnant out to the progenitor surface layers. From
our non-LTE time-dependent line-blanketed synthetic spectra, we compute the
bolometric and multi-band light curves: light curves and spectra are thus
calculated simultaneously using the same physical processes and numerics. These
upgrades, in conjunction with our previous modifications which allow the
solution of the time dependent rate equations, will improve the modeling of SN
spectra and light curves, and hence facilitate new insights into SN ejecta
properties, the SN progenitors and the explosion mechanism(s). CMFGEN can now
be applied to the modeling of all SN typesComment: 20 pages, 10 figures, to appear in MNRA
Linking 1D Evolutionary to 3D Hydrodynamical Simulations of Massive Stars
Stellar evolution models of massive stars are important for many areas of
astrophysics, for example nucleosynthesis yields, supernova progenitor models
and understanding physics under extreme conditions. Turbulence occurs in stars
primarily due to nuclear burning at different mass coordinates within the star.
The understanding and correct treatment of turbulence and turbulent mixing at
convective boundaries in stellar models has been studied for decades but still
lacks a definitive solution. This paper presents initial results of a study on
convective boundary mixing (CBM) in massive stars. The 'stiffness' of a
convective boundary can be quantified using the bulk Richardson number
(), the ratio of the potential energy for restoration of the
boundary to the kinetic energy of turbulent eddies. A 'stiff' boundary
() will suppress CBM, whereas in the opposite case a
'soft' boundary () will be more susceptible to CBM. One
of the key results obtained so far is that lower convective boundaries (closer
to the centre) of nuclear burning shells are 'stiffer' than the corresponding
upper boundaries, implying limited CBM at lower shell boundaries. This is in
agreement with 3D hydrodynamic simulations carried out by Meakin and Arnett
[The Astrophysical Journal 667:448-475, 2007]. This result also has
implications for new CBM prescriptions in massive stars as well as for nuclear
burning flame front propagation in Super-Asymptotic Giant Branch stars and also
the onset of novae.Comment: Accepted for publication (12/12/15) in the Physica Scripta focus
issue on Turbulent Mixing and Beyon
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
On Computational Modelling of Strain-Hardening Material Dynamics
In this paper we show that entropy can be used within a functional for the stress relaxation time of solid materials to parametrise finite viscoplastic strain-hardening deformations. Through doing so the classical empirical recovery of a suitable irreversible scalar measure of work-hardening from the three-dimensional state parameters is avoided. The success of the proposed approach centres on determination of a rate-independent relation between plastic strain and entropy, which is found to be suitably simplistic such to not add any significant complexity to the final model. The result is sufficiently general to be used in combination with existing constitutive models for inelastic deformations parametrised by one-dimensional plastic strain provided the constitutive models are thermodynamically consistent. Here a model for the tangential stress relaxation time based upon established dislocation mechanics theory is calibrated for OFHC copper and subsequently integrated within a two-dimensional moving-mesh scheme. We address some of the numerical challenges that are faced in order to ensure successful implementation of the proposed model within a hydrocode. The approach is demonstrated through simulations of flyer-plate and cylinder impacts
AMRA: An Adaptive Mesh Refinement Hydrodynamic Code for Astrophysics
Implementation details and test cases of a newly developed hydrodynamic code,
AMRA, are presented. The numerical scheme exploits the adaptive mesh refinement
technique coupled to modern high-resolution schemes which are suitable for
relativistic and non-relativistic flows. Various physical processes are
incorporated using the operator splitting approach, and include self-gravity,
nuclear burning, physical viscosity, implicit and explicit schemes for
conductive transport, simplified photoionization, and radiative losses from an
optically thin plasma. Several aspects related to the accuracy and stability of
the scheme are discussed in the context of hydrodynamic and astrophysical
flows.Comment: 41 pages, 21 figures (9 low-resolution), LaTeX, requires elsart.cls,
submitted to Comp. Phys. Comm.; additional documentation and high-resolution
figures available from http://www.camk.edu.pl/~tomek/AMRA/index.htm
Scalable explicit implementation of anisotropic diffusion with Runge-Kutta-Legendre super-time-stepping
An important ingredient in numerical modelling of high temperature magnetised
astrophysical plasmas is the anisotropic transport of heat along magnetic field
lines from higher to lower temperatures.Magnetohydrodynamics (MHD) typically
involves solving the hyperbolic set of conservation equations along with the
induction equation. Incorporating anisotropic thermal conduction requires to
also treat parabolic terms arising from the diffusion operator. An explicit
treatment of parabolic terms will considerably reduce the simulation time step
due to its dependence on the square of the grid resolution () for
stability. Although an implicit scheme relaxes the constraint on stability, it
is difficult to distribute efficiently on a parallel architecture. Treating
parabolic terms with accelerated super-time stepping (STS) methods has been
discussed in literature but these methods suffer from poor accuracy (first
order in time) and also have difficult-to-choose tuneable stability parameters.
In this work we highlight a second order (in time) Runge Kutta Legendre (RKL)
scheme (first described by Meyer et. al. 2012) that is robust, fast and
accurate in treating parabolic terms alongside the hyperbolic conversation
laws. We demonstrate its superiority over the first order super time stepping
schemes with standard tests and astrophysical applications. We also show that
explicit conduction is particularly robust in handling saturated thermal
conduction. Parallel scaling of explicit conduction using RKL scheme is
demonstrated up to more than processors.Comment: 15 pages, 9 figures, incorporated comments from the referee. This
version is now accepted for publication in MNRA
Modelling of explosion deflagrating flames using Large Eddy Simulation
Encouraged by the recent demand for eco-friendly combustion systems, advancements in the predictive capability of turbulent premixed combustion are considered to be essential. The explosion and deflagrating flame are modelled with the numerical method by applying the Large Eddy Simulation (LES) technique. It has evolved itself as a powerful tool for the prediction of turbulent premixed flames. In the LES, Sub-Grid Scale (SGS) modelling plays a pivotal role in accounting for various SGS effects. The chemical reaction rate in LES turbulent premixed flames is a SGS phenomenon and must be accounted for accurately. The Dynamical Flame Surface Density (DFSD) model which is based on the classical laminar flamelet theory is a prominent and well accepted choice in predicting turbulent premixed flames in RANS modelling.
The work presented in this thesis is mainly focused upon the implementation of a dynamic flame surface density (DFSD) model for the calculation of transient, turbulent premixed propagating flames using the LES technique. The concept of the dynamism is achieved by the application of a test filter in combination with Germano identity, which provides unresolved SGS flame surface density information. The DFSD model is coupled with the fractal theory in order to evaluate the instantaneous fractal dimension of the propagating turbulent flame front.
LES simulations are carried out to simulate stoichiometric propane/air flame propagating past solid obstacles in order to validate the model developed in this work with the experiments conducted by the combustion group at The University of Sydney. Various numerical tests were carried out to establish the confidence of LES. A detailed analysis has been carried out to determine the regimes of combustion at different stages of flame propagation inside the chamber. LES predictions using the DFSD model are evaluated and validated against experimental measurements for various flow configurations. The LES predictions were identified to be in strong agreement with experimental measurements. The impact of the number and position of the baffles with respect to ignition origin has also been studied. LES results were found to be in very good agreement with experimental measurements in all these cases
Beam-Induced Damage Mechanisms and their Calculation
The rapid interaction of highly energetic particle beams with matter induces
dynamic responses in the impacted component. If the beam pulse is sufficiently
intense, extreme conditions can be reached, such as very high pressures,
changes of material density, phase transitions, intense stress waves, material
fragmentation and explosions. Even at lower intensities and longer time-scales,
significant effects may be induced, such as vibrations, large oscillations, and
permanent deformation of the impacted components. These lectures provide an
introduction to the mechanisms that govern the thermomechanical phenomena
induced by the interaction between particle beams and solids and to the
analytical and numerical methods that are available for assessing the response
of impacted components. An overview of the design principles of such devices is
also provided, along with descriptions of material selection guidelines and the
experimental tests that are required to validate materials and components
exposed to interactions with energetic particle beams.Comment: 69 pages, contribution to the 2014 Joint International Accelerator
School: Beam Loss and Accelerator Protection, Newport Beach, CA, USA , 5-14
Nov 201
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