56 research outputs found
Gradient discretization of Hybrid Dimensional Darcy Flows in Fractured Porous Media with discontinuous pressures at the matrix fracture interfaces
We investigate the discretization of Darcy flow through fractured porous
media on general meshes. We consider a hybrid dimensional model, invoking a
complex network of planar fractures. The model accounts for matrix-fracture
interactions and fractures acting either as drains or as barriers, i.e. we have
to deal with pressure discontinuities at matrix-fracture interfaces. The
numerical analysis is performed in the general framework of gradient
discretizations which is extended to the model under consideration. Two
families of schemes namely the Vertex Approximate Gradient scheme (VAG) and the
Hybrid Finite Volume scheme (HFV) are detailed and shown to satisfy the
gradient scheme framework, which yields, in particular, convergence. Numerical
tests confirm the theoretical results. Gradient Discretization; Darcy Flow,
Discrete Fracture Networks, Finite Volum
3D radiative transfer: Continuum and line scattering in non-spherical winds from OB stars
Context: State of the art quantitative spectroscopy of OB-stars compares
synthetic spectra (calculated by means of 1D, spherically symmetric computer
codes) with observations. Certain stellar atmospheres, however, show strong
deviations from spherical symmetry, and need to be treated in 3D. Aims: We
present a newly developed 3D radiative transfer code, tailored to the solution
of the radiation field in rapidly expanding stellar atmospheres. We apply our
code to the continuum transfer in wind-ablation models, and to the UV resonance
line formation in magnetic winds. Methods: We have used a 3D finite-volume
method for the solution of the equation of radiative transfer, to study
continuum- and line-scattering problems. Convergence has been accelerated by a
non-local approximate Lambda-iteration scheme. Particular emphasis has been put
on careful (spherically symmetric) test cases. Results: Typical errors of the
source functions, when compared to 1D solutions, are of the order of 10-20 %,
and increase for optically thick continua. In circumstellar discs, the
radiation temperatures in the (optically thin) transition region from wind to
disc are quite similar to corresponding values in the wind. For MHD simulations
of dynamical magnetospheres, the line profiles, calculated with our 3D code,
agree well with previous solutions using a 3D-SEI method. When compared with
profiles resulting from the `analytic dynamical magnetosphere' (ADM) model,
significant differences become apparent. Conclusions: Due to similar radiation
temperatures in the wind and the transition region to the disc, the same
line-strength distribution can be applied within radiation hydrodynamic
calculations for circumstellar discs in `accreting high-mass stars'. To
properly describe the UV line formation in dynamical magnetospheres, the ADM
model needs to be further developed, at least in a large part of the outer
wind
A 3D short-characteristics method for continuum and line scattering problems in the winds of hot stars
Context: Knowledge about hot, massive stars is usually inferred from
quantitative spectroscopy. To analyse non-spherical phenomena, the existing 1D
codes must be extended to higher dimensions, and corresponding tools need to be
developed. Aims: We present a 3D radiative transfer code that is capable of
calculating continuum and line scattering problems in the winds of hot stars.
By considering spherically symmetric test models, we discuss potential error
sources, and indicate advantages and disadvantages by comparing different
solution methods. Further, we analyse the UV resonance line formation in the
winds of rapidly rotating O stars. Methods: We consider both a (simplified)
continuum model including scattering and thermal sources, and a UV resonance
line transition approximated by a two-level-atom. We applied the
short-characteristics (SC) method, using linear or monotonic B\'ezier
interpolations, to solve the equation of radiative transfer on a non-uniform
Cartesian grid. To calculate scattering dominated problems, our solution method
is supplemented by an accelerated -iteration scheme. Results: For the
spherical test models, the mean relative error of the source function is on the
level, depending on the applied interpolation technique and the
complexity of the considered model. All calculated line profiles are in
excellent agreement with corresponding 1D solutions. The predicted line
profiles from fast rotating stars show a distinct behaviour as a function of
rotational speed and inclination. This behaviour is tightly coupled to the wind
structure and the description of gravity darkening and stellar surface
distortion. Conclusions: Our SC methods are ready to be used for quantitative
analyses of UV resonance line profiles. When calculating optically thick
continua, both SC methods give reliable results, in contrast to the alternative
finite-volume method
Binary-object spectral-synthesis in 3D (BOSS-3D) -- Modelling H-alpha emission in the enigmatic multiple system LB-1
Context: To quantitatively decode the information stored within an observed
spectrum, detailed modelling of the physical state and accurate radiative
transfer solution schemes are required. In the analysis of stellar spectra, the
numerical model often needs to account for binary companions and 3D structures
in the stellar envelopes. The enigmatic binary (or multiple) system LB-1
constitutes a perfect example of such a complex multi-D problem. Aims: To
improve our understanding of the LB-1 system, we directly modelled the
phase-dependent H-alpha line profiles of this system. To this end, we developed
a multi-purpose binary-object spectral-synthesis code in 3D (BOSS-3D). Methods:
BOSS-3D calculates synthetic line profiles for a given state of the
circumstellar material. The standard pz-geometry commonly used for single stars
is extended by defining individual coordinate systems for each involved object
and by accounting for the appropriate coordinate transformations. The code is
then applied to the LB-1 system, considering two main hypotheses, a binary
containing a stripped star and Be star, or a B star and a black hole with a
disc. Results: Comparing these two scenarios, neither model can reproduce the
detailed phase-dependent shape of the H-alpha line profiles. A satisfactory
match with the observations, however, is obtained by invoking a disc around the
primary object in addition to the Be-star disc or the black-hole accretion
disc. Conclusions: The developed code can be used to model synthetic line
profiles for a wide variety of binary systems, ranging from transit spectra of
planetary atmospheres, to post-asymptotic giant branch binaries including
circumstellar and circumbinary discs and massive-star binaries with stellar
winds and disc systems. For the LB-1 system, our modelling provides strong
evidence that each object in the system contains a disc-like structure
Port Protocols for Deadlock-Freedom of Component Systems
In component-based development, approaches for property verification exist
that avoid building the global system behavior of the component model.
Typically, these approaches rely on the analysis of the local behavior of fixed
sized subsystems of components. In our approach, we want to avoid not only the
analysis of the global behavior but also of the local behaviors of the
components. Instead, we consider very small parts of the local behaviors called
port protocols that suffice to verify properties.Comment: In Proceedings ICE 2010, arXiv:1010.530
Ultraviolet Line Profiles of Slowly Rotating Massive Star Winds Using the "Analytic Dynamical Magnetosphere" Formalism
Recent large-scale spectropolarimetric surveys have established that a small
but significant percentage of massive stars host stable, surface dipolar
magnetic fields with strengths on the order of kG. These fields channel the
dense, radiatively driven stellar wind into circumstellar magnetospheres, whose
density and velocity structure can be probed using ultraviolet (UV)
spectroscopy of wind-sensitive resonance lines. Coupled with appropriate
magnetosphere models, UV spectroscopy provides a valuable way to investigate
the wind-field interaction, and can yield quantitative estimates of the wind
parameters of magnetic massive stars. We report a systematic investigation of
the formation of UV resonance lines in slowly rotating magnetic massive stars
with dynamical magnetospheres. We pair the Analytic Dynamical Magnetosphere
(ADM) formalism with a simplified radiative transfer technique to produce
synthetic UV line profiles. Using a grid of models, we examine the effect of
magnetosphere size, the line strength parameter, and the cooling parameter on
the structure and modulation of the line profile. We find that magnetic massive
stars uniquely exhibit redshifted absorption at most viewing angles and
magnetosphere sizes, and that significant changes to the shape and variation of
the line profile with varying line strengths can be explained by examining the
individual wind components described in the ADM formalism. Finally, we show
that the cooling parameter has a negligible effect on the line profiles.Comment: 16 pages, 15 figures, accepted to MNRA
Extended ML: Past, present and future
An overview of past, present and future work on the Extended ML formal program development framework is given, with emphasis on two topics of current active research: the semantics of the Extended ML specification language, and tools to support formal program development
Verification for Everyone? An Overview of Dynamic Logic
This note, reporting the homonym keynote presented in the International Symposium on Molecular Logic and Computational Synthetic Biology 2018, traces an informal roadmap on Dynamic Logic (DL) field, focusing on its versatility and resilience to be adjusted and adopted in a wide class of application domains and computational paradigms. The exposition argues the room for developments on tagging DL to the analysis of synthetic biologic domain.publishe
Ultraviolet line profiles of slowly rotating massive star winds using the 'analytic dynamical magnetosphere' formalism
peer reviewedRecent large-scale spectropolarimetric surveys have established that a small but significant percentage of massive stars host stable, surface dipolar magnetic fields with strengths on the order of kG. These fields channel the dense, radiatively driven stellar wind into circumstellar magnetospheres, whose density and velocity structure can be probed using ultraviolet (UV) spectroscopy of wind-sensitive resonance lines. Coupled with appropriate magnetosphere models, UV spectroscopy provides a valuable way to investigate the wind-field interaction, and can yield quantitative estimates of the wind parameters of magnetic massive stars. We report a systematic investigation of the formation of UV resonance lines in slowly rotating magnetic massive stars with dynamical magnetospheres. We pair the analytic dynamical magnetosphere (ADM) formalism with a simplified radiative transfer technique to produce synthetic UV line profiles. Using a grid of models, we examine the effect of magnetosphere size, the line strength parameter, and the cooling parameter on the structure and modulation of the line profile. We find that magnetic massive stars uniquely exhibit redshifted absorption at most viewing angles and magnetosphere sizes, and that significant changes to the shape and variation of the line profile with varying line strengths can be explained by examining the individual wind components described in the ADM formalism. Finally, we show that the cooling parameter has a negligible effect on the line profiles
Testing data types implementations from algebraic specifications
Algebraic specifications of data types provide a natural basis for testing
data types implementations. In this framework, the conformance relation is
based on the satisfaction of axioms. This makes it possible to formally state
the fundamental concepts of testing: exhaustive test set, testability
hypotheses, oracle. Various criteria for selecting finite test sets have been
proposed. They depend on the form of the axioms, and on the possibilities of
observation of the implementation under test. This last point is related to the
well-known oracle problem. As the main interest of algebraic specifications is
data type abstraction, testing a concrete implementation raises the issue of
the gap between the abstract description and the concrete representation. The
observational semantics of algebraic specifications bring solutions on the
basis of the so-called observable contexts. After a description of testing
methods based on algebraic specifications, the chapter gives a brief
presentation of some tools and case studies, and presents some applications to
other formal methods involving datatypes
- …