14,102 research outputs found
Modelling plasticity of unsaturated soils in a thermodynamically consistent framework
Constitutive equations of unsaturated soils are often derived in a
thermodynamically consistent framework through the use a unique 'effective'
interstitial pressure. This later is naturally chosen as the space averaged
interstitial pressure. However, experimental observations have revealed that
two stress state variables were needed to describe the stress-strain-strength
behaviour of unsaturated soils. The thermodynamics analysis presented here
shows that the most general approach to the behaviour of unsaturated soils
actually requires three stress state variables: the suction, which is required
to describe the retention properties of the soil and two effective stresses,
which are required to describe the soil deformation at water saturation held
constant. Actually, it is shown that a simple assumption related to internal
deformation leads to the need of a unique effective stress to formulate the
stress-strain constitutive equation describing the soil deformation. An
elastoplastic framework is then presented and it is finally shown that the
Barcelona Basic Model, a commonly accepted model for unsaturated soils, as well
as all models deriving from it, appear as special extreme cases of the
thermodynamic framework proposed here
Time resolved tracking of a sound scatterer in a turbulent flow: non-stationary signal analysis and applications
It is known that ultrasound techniques yield non-intrusive measurements of
hydrodynamic flows. For example, the study of the echoes produced by a large
number of particle insonified by pulsed wavetrains has led to a now standard
velocimetry technique. In this paper, we propose to extend the method to the
continuous tracking of one single particle embedded in a complex flow. This
gives a Lagrangian measurement of the fluid motion, which is of importance in
mixing and turbulence studies. The method relies on the ability to resolve in
time the Doppler shift of the sound scattered by the continuously insonfied
particle.
For this signal processing problem two classes of approaches are used:
time-frequency analysis and parametric high resolution methods. In the first
class we consider the spectrogram and reassigned spectrogram, and we apply it
to detect the motion of a small bead settling in a fluid at rest. In more
non-stationary turbulent flows where methods in the second class are more
robust, we have adapted an Approximated Maximum Likelihood technique coupled
with a generalized Kalman filter.Comment: 16 pages 9 figure
Fourier analysis of finite element preconditioned collocation schemes
The spectrum of the iteration operator of some finite element preconditioned Fourier collocation schemes is investigated. The first part of the paper analyses one-dimensional elliptic and hyperbolic model problems and the advection-diffusion equation. Analytical expressions of the eigenvalues are obtained with use of symbolic computation. The second part of the paper considers the set of one-dimensional differential equations resulting from Fourier analysis (in the tranverse direction) of the 2-D Stokes problem. All results agree with previous conclusions on the numerical efficiency of finite element preconditioning schemes
Clumpy Disc and Bulge Formation
We present a set of hydrodynamical/Nbody controlled simulations of isolated
gas rich galaxies that self-consistently include SN feedback and a detailed
chemical evolution model, both tested in cosmological simulations. The initial
conditions are motivated by the observed star forming galaxies at z ~ 2-3. We
find that the presence of a multiphase interstellar media in our models
promotes the growth of disc instability favouring the formation of clumps which
in general, are not easily disrupted on timescales compared to the migration
time. We show that stellar clumps migrate towards the central region and
contribute to form a classical-like bulge with a Sersic index, n > 2. Our
physically-motivated Supernova feedback has a mild influence on clump survival
and evolution, partially limiting the mass growth of clumps as the energy
released per Supernova event is increased, with the consequent flattening of
the bulge profile. This regulation does not prevent the building of a
classical-like bulge even for the most energetic feedback tested. Our Supernova
feedback model is able to establish a self-regulated star formation, producing
mass-loaded outflows and stellar age spreads comparable to observations. We
find that the bulge formation by clumps may coexit with other channels of bulge
assembly such as bar and mergers. Our results suggest that galactic bulges
could be interpreted as composite systems with structural components and
stellar populations storing archaeological information of the dynamical history
of their galaxy.Comment: Accepted for publication in MNRAS - Aug. 20, 201
Climate Change, Insurability of Large-scale Disasters and the Emerging Liability Challenge
This paper focuses on the interaction between uncertainty and insurability in the context of some of the risks associated with climate change. It discusses the evolution of insured losses due to weather-related disasters over the past decade, and the key drivers of the sharp increases in both economic and insured catastrophe losses over the past 20 years. In particular we examine the impact of development in hazard-prone areas and of global warming on the potential for catastrophic losses in the future. In this context we discuss the implications for insurance risk capital and the capacity of the insurance industry to handle large-scale events. A key question that needs to be addressed is the factors that determine the insurability of a risk and the extent of coverage offered by the private sector to provide protection against extreme events where there is significant uncertainty surrounding the probability and consequences of a catastrophic loss. We discuss the concepts of insurability by focusing on coverage for natural hazards, such as earthquakes, hurricanes and floods. The paper also focuses on the liability issues associated with global climate change, and possible implications for insurers (including D&O), given the difficulty in identifying potential defendants, tracing harm to their actions and apportioning damages among them. The paper concludes by suggesting ways that insurers can help mitigate future damages from global climate change by providing premium reductions and rate credits to companies investing in risk-reducing measures.
Evaluating The Effectiveness of Terrorism Risk Financing Solutions
The 9/11 attacks in the United States, as well as other attacks in different parts of the world, raise important questions related to the economic impact of terrorism. What are the most effective ways for a country to recover from these economic losses? Who should pay for the costs of future large-scale attacks? To address these two questions, we propose five principles to evaluate alternative programs. We first discuss how a federal insurance program with mandatory coverage and a laissez faire free-market approach for providing private insurance will fare relative to these principles. We conclude that neither solution is likely to be feasible here in the United States given the millions of firms at risk and the current structure of insurance regulation. We then evaluate how well the U.S. Terrorism Risk Insurance Act (TRIA), a public-private program to cover commercial enterprises against foreign terrorism on U.S. soil, meets the five principles. In particular, we show that TRIA has had a positive effect on availability of terrorism coverage and also has significantly contributed to reducing insurance premiums. TRIA is scheduled to terminate at the end of the year, but pending legislation would extend the program for fifteen years after December 31 (HR. 2761). In this paper, we show that such a long-term extension might have important impacts on the market. This could increase the take-up rate, as prices might be even lower than they are today. We show also, however, that if TRIA were extended for a long period of time in its current form, some insurers could "game" the program by collecting ex ante a large amount of premiums for terrorism insurance, while being financially responsible for only a small portion of the claims ex post. The general taxpayer and the general commercial policyholder (whether or not covered against terrorism) would absorb the residual insured losses. This raises major equity issues inherent in the design of the program.
A coupled approximate deconvolution and dynamic mixed scale model for large-eddy simulation
Large-eddy simulations of incompressible Newtonian fluid flows with
approximate deconvolution models based on the van Cittert method are reported.
The Legendre spectral element method is used for the spatial discretization to
solve the filtered Navier--Stokes equations. A novel variant of approximate
deconvolution models blended with a mixed scale model using a dynamic
evaluation of the subgrid-viscosity constant is proposed. This model is
validated by comparing the large-eddy simulation with the direct numerical
simulation of the flow in a lid-driven cubical cavity, performed at a Reynolds
number of 12'000. Subgrid modeling in the case of a flow with coexisting
laminar, transitional and turbulent zones such as the lid-driven cubical cavity
flow represents a challenging problem. Moreover, the coupling with the spectral
element method having very low numerical dissipation and dispersion builds a
well suited framework to analyze the efficiency of a subgrid model. First- and
second-order statistics obtained using this new model are showing very good
agreement with the direct numerical simulation. Filtering operations rely on an
invertible filter applied in a modal basis and preserving the C0-continuity
across elements. No clipping on dynamic parameters was needed to preserve
numerical stability
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