3,253 research outputs found
A prover for the CRL toolset with applications : version 0.1
This document describes an automated theorem prover, based on an extension of binary decision diagrams. The prover transforms quantifier-free formulae into equivalent BDD-forms, w.r.t.~to some algebraic data specification. The prover is used by four tools for the symbolic analysis of distributed systems specified in CRL (i.e.~process algebra plus algebraic data types). The main techniques are invariants and confluence. Two case studies are reported: the DKR leader election protocol [13], and SPLICE [15], a coordination architecture of industrial origin. In both cases using confluence information leads to a reduced state space
Calculating τ-confluence compositionally
τ-confluence is a reduction technique used in enumerative model-checking of labeled transition systems to avoid the state explosion problem. In this paper, we propose a new on-the-fly algorithm to calculate partial τ-confluence, and propose new techniques to do so on large systems in a compositional manner. Using information inherent in the way a large system is composed of smaller systems, we show how we can deduce partial τ-confluence in a computationally cheap manner. Finally, these techniques are applied to a number of case studies, including the rel/REL atomic multicast protocol.peer-reviewe
Pressure Bifurcation Phenomenon on Supersonic Blowing Trailing Edges
Turbine blades operating in transonic-supersonic regime develop a complex
shock wave system at the trailing edge, a phenomenon that leads to unfavorable
pressure perturbations downstream and can interact with other turbine stages.
Understanding the fluid behavior of the area adjacent to the trailing edge is
essential in order to determine the parameters that have influence on these
pressure fluctuations. Colder flow, bled from the high-pressure compressor, is
often purged at the trailing edge to cool the thin blade edges, affecting the
flow behavior and modulating the intensity and angle of the shock waves system.
However, this purge flow can sometimes generate non-symmetrical configurations
due to a pressure difference that is provoked by the injected flow. In this
work, a combination of RANS simulations and global stability analysis is
employed to explain the physical reasons of this flow bifurcation. Analyzing
the features that naturally appear in the flow and become dominant for some
value of the parameters involved in the problem, an anti-symmetrical global
mode, related to the sudden geometrical expansion of the trailing edge slot, is
identified as the main mechanism that forces the changes in the flow topology.Comment: Submitted to AIAA Journa
On the analysis of stochastic timed systems
The formal methods approach to develop reliable and efficient safety- or performance-critical systems is to construct mathematically precise models of such systems on which properties of interest, such as safety guarantees or performance requirements, can be verified automatically. In this thesis, we present techniques that extend the reach of exhaustive and statistical model checking to verify reachability and reward-based properties of compositional behavioural models that support quantitative aspects such as real time and randomised decisions.
We present two techniques that allow sound statistical model checking for the nondeterministic-randomised model of Markov decision processes. We investigate the relationship between two different definitions of the model of probabilistic timed automata, as well as potential ways to apply statistical model checking. Stochastic timed automata allow nondeterministic choices as well as nondeterministic and stochastic delays, and we present the first exhaustive model checking algorithm that allows their analysis. All the approaches introduced in this thesis are implemented as part of the Modest Toolset, which supports the construction and verification of models specified in the formal modelling language Modest. We conclude by applying this language and toolset to study novel distributed control strategies for photovoltaic microgenerators
A class of well-posed parabolic final value problems
This paper focuses on parabolic final value problems, and well-posedness is
proved for a large class of these. The clarification is obtained from Hilbert
spaces that characterise data that give existence, uniqueness and stability of
the solutions. The data space is the graph normed domain of an unbounded
operator that maps final states to the corresponding initial states. It induces
a new compatibility condition, depending crucially on the fact that analytic
semigroups always are invertible in the class of closed operators. Lax--Milgram
operators in vector distribution spaces constitute the main framework. The
final value heat conduction problem on a smooth open set is also proved to be
well posed, and non-zero Dirichlet data are shown to require an extended
compatibility condition obtained by adding an improper Bochner integral.Comment: 16 pages. To appear in "Applied and numerical harmonic analysis"; a
reference update. Conference contribution, based on arXiv:1707.02136, with
some further development
On-the-fly confluence detection for statistical model checking (extended version)
Statistical model checking is an analysis method that circumvents the state space explosion problem in model-based verification by combining probabilistic simulation with statistical methods that provide clear error bounds. As a simulation-based technique, it can only provide sound results if the underlying model is a stochastic process. In verification, however, models are usually variations of nondeterministic transition systems. The notion of confluence allows the reduction of such transition systems in classical model checking by removing spurious nondeterministic choices. In this paper, we show that confluence can be adapted to detect and discard such choices on-the-fly during simulation, thus extending the applicability of statistical model checking to a subclass of Markov decision processes. In contrast to previous approaches that use partial order reduction, the confluence-based technique can handle additional kinds of nondeterminism. In particular, it is not restricted to interleavings. We evaluate our approach, which is implemented as part of the modes simulator for the Modest modelling language, on a set of examples that highlight its strengths and limitations and show the improvements compared to the partial order-based method
Is FS Tau B Driving an Asymmetric Jet?
FS Tau B is one of the few T Tauri stars that possess a jet and a counterjet
as well as an optically-visible cavity wall. We obtained images and spectra of
its jet-cavity system in the near-infrared H and K bands using Subaru/IRCS and
detected the jet and the counterjet in the [Fe II] 1.644 \mu m line for the
first time. Within the inner 2" the blueshifted jet is brighter, whereas beyond
~ 5" the redshifted counterjet dominates the [Fe II] emission. The innermost
blueshifted knot is spectrally resolved to have a large line width of ~ 110
km/s, while the innermost redshifted knot appears spectrally unresolved. The
velocity ratio of the jet to the counterjet is ~ 1.34, which suggests that FS
Tau B is driving an asymmetric jet, similar to those found in several T Tauri
Stars. Combining with optical observations in the literature, we showed that
the blueshifted jet has lower density and higher excitation than the redshifted
counterjet. We suggest that the asymmetry in brightness and velocity is the
manifestation of a bipolar outflow driving at different mass-loss rates, while
maintaining balance of linear momentum. A full explanation to the asymmetry in
the FS Tau B system awaits detail modeling and further investigation of the
kinematic structure of the wind-associated cavity walls.Comment: 14 pages, 2 figures, 1 table; accepted for publication in ApJ. Aspect
ratio changes for Fig.1
- …