692 research outputs found
Collective Phenonema in the Interaction of Radiation with Matter
The aim of this communication is to present in a concentrated form the main
ideas of a method, developed by the author, for treating strongly
nonequilibrium collective phenomena typical of the interaction of radiation
with matter, as well as to give a survey of several applications of the method.
The latter is called the Scale Separation Approach since its basic techniques
rely on the possibility of separating different space-time scales in
nonequilibrium statistical systems. This approach is rather general and can be
applied to diverse physical problems, several of which are discussed here.
These problems are: Superradiance of nuclear spins, filamentation in resonant
media, semiconfinement of neutral atoms, negative electric current, and
collective liberation of light.Comment: 1 file, 16 pages, LaTe
A multigroup diffusion solver using pseudo transient continuation for a radiation-hydrodynamic code with patch-based AMR
We present a scheme to solve the nonlinear multigroup radiation diffusion
(MGD) equations. The method is incorporated into a massively parallel,
multidimensional, Eulerian radiation-hydrodynamic code with adaptive mesh
refinement (AMR). The patch-based AMR algorithm refines in both space and time
creating a hierarchy of levels, coarsest to finest. The physics modules are
time-advanced using operator splitting. On each level, separate level-solve
packages advance the modules. Our multigroup level-solve adapts an implicit
procedure which leads to a two-step iterative scheme that alternates between
elliptic solves for each group with intra-cell group coupling. For robustness,
we introduce pseudo transient continuation (PTC). We analyze the magnitude of
the PTC parameter to ensure positivity of the resulting linear system, diagonal
dominance and convergence of the two-step scheme. For AMR, a level defines a
subdomain for refinement. For diffusive processes such as MGD, the refined
level uses Dirichet boundary data at the coarse-fine interface and the data is
derived from the coarse level solution. After advancing on the fine level, an
additional procedure, the sync-solve (SS), is required in order to enforce
conservation. The MGD SS reduces to an elliptic solve on a combined grid for a
system of G equations, where G is the number of groups. We adapt the partial
temperature scheme for the SS; hence, we reuse the infrastructure developed for
scalar equations. Results are presented. (Abridged)Comment: 46 pages, 14 figures, accepted to JC
Kinetic approaches to particle acceleration at cosmic ray modified shocks
Kinetic approaches provide an effective description of the process of
particle acceleration at shock fronts and allow to take into account the
dynamical reaction of the accelerated particles as well as the amplification of
the turbulent magnetic field as due to streaming instability. The latter does
in turn affect the maximum achievable momentum and thereby the acceleration
process itself, in a chain of causality which is typical of non-linear systems.
Here we provide a technical description of two of these kinetic approaches and
show that they basically lead to the same conclusions. In particular we discuss
the effects of shock modification on the spectral shape of the accelerated
particles, on the maximum momentum, on the thermodynamic properties of the
background fluid and on the escaping and advected fluxes of accelerated
particles.Comment: 22 pages, 7 figures, accepted for publication in MNRA
Multigroup radiation hydrodynamics with flux-limited diffusion and adaptive mesh refinement
International audienceContext. Radiative transfer plays a crucial role in the star formation process. Because of the high computational cost, radiation-hydrodynamics simulations performed up to now have mainly been carried out in the grey approximation. In recent years, multifrequency radiation-hydrodynamics models have started to be developed in an attempt to better account for the large variations in opacities as a function of frequency.Aims. We wish to develop an efficient multigroup algorithm for the adaptive mesh refinement code RAMSES which is suited to heavy proto-stellar collapse calculations.Methods. Because of the prohibitive timestep constraints of an explicit radiative transfer method, we constructed a time-implicit solver based on a stabilized bi-conjugate gradient algorithm, and implemented it in RAMSES under the flux-limited diffusion approximation.Results. We present a series of tests that demonstrate the high performance of our scheme in dealing with frequency-dependent radiation-hydrodynamic flows. We also present a preliminary simulation of a 3D proto-stellar collapse using 20 frequency groups. Differences between grey and multigroup results are briefly discussed, and the large amount of information this new method brings us is also illustrated.Conclusions. We have implemented a multigroup flux-limited diffusion algorithm in the RAMSES code. The method performed well against standard radiation-hydrodynamics tests, and was also shown to be ripe for exploitation in the computational star formation context
Stability of explicit radiation-material coupling in radiative transfer calculations
a b s t r a c t We show that explicit radiation-material coupling, which is essentially always stable for infrared radiative transfer is conditionally stable in the high energy density regime. A linearized stability analysis is first performed for a simple infinite-medium problem that yields both a criterion for unconditional stability, a time-step restriction that applies for conditional stability, and a time-step criterion that always applies for nonoscillatory solutions. This analysis is then extended to include space dependence with the result that the system is always conditionally stable, but with a time step restriction somewhat different from the infinite-medium case. Nonetheless, the time step restriction for non-oscillatory solutions remains the same. Computations are presented that confirm the predictions of our analysis, and conclusions are given
A Radiation Transfer Solver for Athena using Short Characteristics
We describe the implementation of a module for the Athena
magnetohydrodynamics (MHD) code which solves the time-independent,
multi-frequency radiative transfer (RT) equation on multidimensional Cartesian
simulation domains, including scattering and non-LTE effects. The module is
based on well-known and well-tested algorithms developed for modeling stellar
atmospheres, including the method of short characteristics to solve the RT
equation, accelerated Lambda iteration to handle scattering and non-LTE
effects, and parallelization via domain decomposition. The module serves
several purposes: it can be used to generate spectra and images, to compute a
variable Eddington tensor (VET) for full radiation MHD simulations, and to
calculate the heating and cooling source terms in the MHD equations in flows
where radiation pressure is small compared with gas pressure. For the latter
case, the module is combined with the standard MHD integrators using
operator-splitting and we describe this approach in detail. Implementation of
the VET method for radiation pressure dominated flows is described in a
companion paper. We present results from a suite of test problems for both the
RT solver itself, and for dynamical problems that include radiative heating and
cooling. These tests demonstrate that the radiative transfer solution is
accurate, and confirm that the operator split method is stable, convergent, and
efficient for problems of interest. We demonstrate there is no need to adopt
ad-hoc assumptions of questionable accuracy to solve RT problems in concert
with MHD: the computational cost for our general-purpose module for simple
(e.g. LTE grey) problems can be comparable to or less than a single timestep of
Athena's MHD integrators, and only few times more expensive than that for more
general problems. (Abridged)Comment: 20 pages, 13 figures, accepted for publication in ApJ Supplement
Serie
Computational Inverse Problems for Partial Differential Equations
The problem of determining unknown quantities in a PDE from measurements of (part of) the solution to this PDE arises in a wide range of applications in science, technology, medicine, and finance. The unknown quantity may e.g. be a coefficient, an initial or a boundary condition, a source term, or the shape of a boundary. The identification of such quantities is often computationally challenging and requires profound knowledge of the analytical properties of the underlying PDE as well as numerical techniques. The focus of this workshop was on applications in phase retrieval, imaging with waves in random media, and seismology of the Earth and the Sun, a further emphasis was put on stochastic aspects in the context of uncertainty quantification and parameter identification in stochastic differential equations. Many open problems and mathematical challenges in application fields were addressed, and intensive discussions provided an insight into the high potential of joining deep knowledge in numerical analysis, partial differential equations, and regularization, but also in mathematical statistics, homogenization, optimization, differential geometry, numerical linear algebra, and variational analysis to tackle these challenges
Sonochemical degradation of triclosan, benzophenone-1 and benzophenone -3 in Water
RESUMEN: Degradation of triclosan, benzophenone 1, and benzophenone 3 by multifrequency ultrasound was studied at high frequencies. Frequency effect on initial degradation rates was analyzed and an optimum frequency was found. Power density effect was also analyzed. A reaction mechanism was proposed and reaction kinetics were found. Pulsed US mode effect was analyzed too. Simultaneous analysis of the effect of power, frequency, pulse time, and silent time was made and optimization by response surface methodology was made for benzophenone 3 degradation. Analysis of this and radical scavengers, let conclude if degradation of molecules was taking place in the bubble/liquid interface, bubble, or bulk liquid. Toxicity tests were conducted by Microtox®. Degradation byproducts and degradation pathways were proposed
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