427 research outputs found
Astrophysical fluid simulations of thermally ideal gases with non-constant adiabatic index: numerical implementation
An Equation of State (\textit{EoS}) closes the set of fluid equations.
Although an ideal EoS with a constant \textit{adiabatic index} is the
preferred choice due to its simplistic implementation, many astrophysical fluid
simulations may benefit from a more sophisticated treatment that can account
for diverse chemical processes. Here, we first review the basic thermodynamic
principles of a gas mixture in terms of its thermal and caloric EoS by
including effects like ionization, dissociation as well as temperature
dependent degrees of freedom such as molecular vibrations and rotations. The
formulation is revisited in the context of plasmas that are either in
equilibrium conditions (local thermodynamic- or collisional excitation-
equilibria) or described by non-equilibrium chemistry coupled to optically thin
radiative cooling. We then present a numerical implementation of thermally
ideal gases obeying a more general caloric EoS with non-constant adiabatic
index in Godunov-type numerical schemes.We discuss the necessary modifications
to the Riemann solver and to the conversion between total energy and pressure
(or vice-versa) routinely invoked in Godunov-type schemes. We then present two
different approaches for computing the EoS.The first one employs root-finder
methods and it is best suited for EoS in analytical form. The second one leans
on lookup table and interpolation and results in a more computationally
efficient approach although care must be taken to ensure thermodynamic
consistency. A number of selected benchmarks demonstrate that the employment of
a non-ideal EoS can lead to important differences in the solution when the
temperature range is K where dissociation and ionization occur. The
implementation of selected EoS introduces additional computational costs
although using lookup table methods can significantly reduce the overhead by a
factor .Comment: 17 pages, 10 figures, Accepted for publication in A&
Multi-D magnetohydrodynamic modelling of pulsar wind nebulae: recent progress and open questions
In the last decade, the relativistic magnetohydrodynamic (MHD) modelling of
pulsar wind nebulae, and of the Crab nebula in particular, has been highly
successful, with many of the observed dynamical and emission properties
reproduced down to the finest detail. Here, we critically discuss the results
of some of the most recent studies: namely the investigation of the origin of
the radio emitting particles and the quest for the acceleration sites of
particles of different energies along the termination shock, by using wisps
motion as a diagnostic tool; the study of the magnetic dissipation process in
high magnetization nebulae by means of new long-term three-dimensional
simulations of the pulsar wind nebula evolution; the investigation of the
relativistic tearing instability in thinning current sheets, leading to fast
reconnection events that might be at the origin of the Crab nebula gamma-ray
flares.Comment: 30 pages, 12 figure
TV-Centric technologies to provide remote areas with two-way satellite broadband access
October 1-2, 2007, Rome, Italy TV-Centric Technologies To Provide Remote Areas With Two-Way Satellite Broadband Acces
A two-way interactive broadband satellite architecture to break the digital divide barrier
September 24-26, 2007, Turin, Ital
A Particle Module for the PLUTO Code: I - an implementation of the MHD-PIC equations
We describe an implementation of a particle physics module available for the
PLUTO code, appropriate for the dynamical evolution of a plasma consisting of a
thermal fluid and a non-thermal component represented by relativistic charged
particles, or cosmic rays (CR). While the fluid is approached using standard
numerical schemes for magnetohydrodynamics, CR particles are treated
kinetically using conventional Particle-In-Cell (PIC) techniques.
The module can be used to describe either test particles motion in the fluid
electromagnetic field or to solve the fully coupled MHD-PIC system of equations
with particle backreaction on the fluid as originally introduced by
\cite{Bai_etal.2015}. Particle backreaction on the fluid is included in the
form of momentum-energy feedback and by introducing the CR-induced Hall term in
Ohm's law. The hybrid MHD-PIC module can be employed to study CR kinetic
effects on scales larger than the (ion) skin depth provided the Larmor gyration
scale is properly resolved. When applicable, this formulation avoids to resolve
microscopic scales offering a substantial computational saving with respect to
PIC simulations.
We present a fully-conservative formulation which is second-order accurate in
time and space and extends to either Runge-Kutta (RK) or
corner-transport-upwind (CTU) time-stepping schemes (for the fluid) while a
standard Boris integrator is employed for the particles. For highly-energetic
relativistic CRs and in order to overcome the time step restriction a novel
sub-cycling strategy that retains second-order accuracy in time is presented.
Numerical benchmarks and applications including Bell instability, diffusive
shock acceleration and test particle acceleration in reconnecting layers are
discussed.Comment: 27 pages, 16 figures. Accepted for publication in ApJ Supplement
serie
A multidimensional grid-adaptive relativistic magnetofluid code
A robust second order, shock-capturing numerical scheme for multi-dimensional
special relativistic magnetohydrodynamics on computational domains with
adaptive mesh refinement is presented. The base solver is a total variation
diminishing Lax-Friedrichs scheme in a finite volume setting and is combined
with a diffusive approach for controlling magnetic monopole errors. The
consistency between the primitive and conservative variables is ensured at all
limited reconstructions and the spatial part of the four velocity is used as a
primitive variable. Demonstrative relativistic examples are shown to validate
the implementation. We recover known exact solutions to relativistic MHD
Riemann problems, and simulate the shock-dominated long term evolution of
Lorentz factor 7 vortical flows distorting magnetic island chains.Comment: accepted for publication in Computer Physics Communication
The PLUTO Code for Adaptive Mesh Computations in Astrophysical Fluid Dynamics
We present a description of the adaptive mesh refinement (AMR) implementation
of the PLUTO code for solving the equations of classical and special
relativistic magnetohydrodynamics (MHD and RMHD). The current release exploits,
in addition to the static grid version of the code, the distributed
infrastructure of the CHOMBO library for multidimensional parallel computations
over block-structured, adaptively refined grids. We employ a conservative
finite-volume approach where primary flow quantities are discretized at the
cell-center in a dimensionally unsplit fashion using the Corner Transport
Upwind (CTU) method. Time stepping relies on a characteristic tracing step
where piecewise parabolic method (PPM), weighted essentially non-oscillatory
(WENO) or slope-limited linear interpolation schemes can be handily adopted. A
characteristic decomposition-free version of the scheme is also illustrated.
The solenoidal condition of the magnetic field is enforced by augmenting the
equations with a generalized Lagrange multiplier (GLM) providing propagation
and damping of divergence errors through a mixed hyperbolic/parabolic explicit
cleaning step. Among the novel features, we describe an extension of the scheme
to include non-ideal dissipative processes such as viscosity, resistivity and
anisotropic thermal conduction without operator splitting. Finally, we
illustrate an efficient treatment of point-local, potentially stiff source
terms over hierarchical nested grids by taking advantage of the adaptivity in
time. Several multidimensional benchmarks and applications to problems of
astrophysical relevance assess the potentiality of the AMR version of PLUTO in
resolving flow features separated by large spatial and temporal disparities.Comment: 34 pages, 34 figures, accepted for publication in ApJ
AZEuS: An Adaptive Zone Eulerian Scheme for Computational MHD
A new adaptive mesh refinement (AMR) version of the ZEUS-3D astrophysical
magnetohydrodynamical (MHD) fluid code, AZEuS, is described. The AMR module in
AZEuS has been completely adapted to the staggered mesh that characterises the
ZEUS family of codes, on which scalar quantities are zone-centred and vector
components are face-centred. In addition, for applications using static grids,
it is necessary to use higher-order interpolations for prolongation to minimise
the errors caused by waves crossing from a grid of one resolution to another.
Finally, solutions to test problems in 1-, 2-, and 3-dimensions in both
Cartesian and spherical coordinates are presented.Comment: 52 pages, 17 figures; Accepted for publication in ApJ
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