252 research outputs found
Propositions de méthodes et modèles eulériens pour les problèmes à interfaces entre fluides compressibles en présence de transfert de chaleur
On évalue différentes formulations euleriennes aptes au traitement de problèmes à interfaces entre fluides compressibles. La difficulté dans ce type de problème réside dans le calcul des variables thermodynamiques dans les zones de diffusion numériques produites aux interfaces. En effet, tout schéma eulérien diffuse artificiellement les discontinuités de contact (ou interfaces) et produit donc un mélange artificiel pour lequel la détermina- tion de l'état thermodynamique est difficile. De plus, lorsque l'état thermodynamique est mal déterminé, les méthodes échouent très rapidement en raison de pressions négatives ou d'arguments négatifs dans le calcul de la vitesse du son. Les modèles et les méthodes de résolution qui sont évaluées n'ont jamais été examinées pour le calcul de la température aux interfaces. L'examen des défauts et avantages de ces formulations nous conduit à en rejeter certaines et à en proposer une nouvelle, très efficace. Ce nouveau modèle est accompagné de son schéma numérique. On présente ensuite le traitement des transferts diffusifs aux interfaces, puis un exemple de résolution en deux dimensions d'espace. L'évaluation est effectuée sur une série de problèmes possédant des solutions exactes
Virtual community based secure service discovery and access for 3D video steaming applications
The Freehand I-Share project aims to define the mechanisms for trust, willingness, resource discovery and sharing mechanisms in virtual communities. To improve the secure and performance of a 3D video streaming application, which is a research vehicle of the I-Share project, we propose a virtual community based access control approach for secure service discovery and access (VICSDA) which groups services in virtual communities and only grants authenticated community members to discover and access these community services. There are two main contributions associated with this approach. First, different from most of the other access control approaches it adopts a dual access control mechanism which allows community services to define their local access control policy besides following the community membership policy. Second, behavior of these community services is monitored in order to guarantee a better QoS provision. Using this approach, the 3D video streaming application can be guaranteed with authentication and message confidentiality through the dual secure service discovery and access mechanism. Better application performance can also be achieved through the community member behavior audit.</p
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Climate risk and food security in Mali: A historical perspective on adaptation
We combine socioeconomic data from a large‐scale household survey with historical climate data to map the climate sensitivity of availability and access dimensions of food security in Mali, and infer the ways in which at‐risk communities may have been impacted by persistent climatic shift. Thirty years after 1982–1984, the period of most intense drought during the protracted late 20th century drying of the Sahel, the impact of drought on livelihoods and food security is still recognizable in the Sahelian center of Mali. This impact is expressed in the larger fraction of households in this Sahelian center of the country—the agro‐ecological transition between pastoralism in the north, and sedentary agriculture in the south—who practice agriculture but not livestock raising, despite environmental conditions that are suitable to their combination. These households have lower food security and rely more frequently on detrimental nutrition‐based coping strategies, such as reducing the quantity or quality of meals. In contrast, the more food secure households show a clear tendency toward livelihood diversification away from subsistence agriculture. These households produce less of what they consume, yet spend less on food in proportion. The analysis points to the value of interdisciplinary research—in this case bridging climate science and vulnerability analysis—to gain a dynamical understanding of complex systems, understanding which may be exploited to address real‐world challenges, offering lessons about food security and local adaptation strategies in places among the most vulnerable to climate
Electric Field Conjugation with the Project 1640 coronagraph
The Project 1640 instrument on the 200-inch Hale telescope at Palomar
Observatory is a coronagraphic instrument with an integral field spectrograph
at the back end, designed to find young, self-luminous planets around nearby
stars. To reach the necessary contrast for this, the PALM-3000 adaptive optics
system corrects for fast atmospheric speckles, while CAL, a phase-shifting
interferometer in a Mach-Zehnder configuration, measures the quasistatic
components of the complex electric field in the pupil plane following the
coronagraphic stop. Two additional sensors measure and control low-order modes.
These field measurements may then be combined with a system model and data
taken separately using a white-light source internal to the AO system to
correct for both phase and amplitude aberrations. Here, we discuss and
demonstrate the procedure to maintain a half-plane dark hole in the image plane
while the spectrograph is taking data, including initial on-sky performance.Comment: 9 pages, 7 figures, in Proceedings of SPIE, 8864-19 (2013
Atomistic insights into ultrafast SiGe nanoprocessing
Controlling ultrafast material transformations with atomic precision is
essential for future nanotechnology. Pulsed laser annealing (LA), inducing
extremely rapid and localized phase transitions, is a powerful way to achieve
this, but it requires careful optimization together with the appropriate system
design. We present a multiscale LA computational framework able to simulate
atom-by-atom the highly out-of-equilibrium kinetics of a material as it
interacts with the laser, including effects of structural disorder. By
seamlessly coupling a macroscale continuum solver to a nanoscale super-lattice
Kinetic Monte Carlo code, this method overcomes the limits of state-of-the-art
continuum-based tools. We exploit it to investigate nontrivial changes in
composition, morphology and quality of laser-annealed SiGe alloys. Validations
against experiments and phase-field simulations, as well as advanced
applications to strained, defected, nanostructured and confined SiGe are
presented, highlighting the importance of a multiscale atomistic-continuum
approach. Current applicability and potential generalization routes are finally
discussed
Impact of surface reflectivity on the ultra-fast laser melting of silicon-germanium alloys
Ultraviolet nanosecond laser annealing (LA) is a powerful tool where strongly
confined heating and melting are desirable. In semiconductor technologies the
importance of LA increases with the increasing complexity of the proposed
integration schemes. Optimizing the LA process along with the experimental
design is challenging, especially when complex 3D nanostructured systems with
various shapes and phases are involved. Within this context, reliable
simulations of laser melting are required for optimizing the process parameters
while reducing the number of experimental tests. This gives rise to a virtual
Design of Experiments (DoE). SiGe alloys are nowadays used for their
compatibility with silicon devices enabling to engineer properties such as
strain, carrier mobilities and bandgap. In this work, the laser melting process
of relaxed and strained SiGe is simulated with a finite element method / phase
field approach. Particularly, we calibrated the dielectric functions of the
alloy for its crystal and liquid phase using experimental data. We highlighted
the importance of reproducing the exact reflectivity of the material in its
different aggregation states, to correctly mimic the process
How to Balance Lives and Livelihoods in a Pandemic.
Control measures, such as “lockdowns”, have been widely used to suppress the COVID-19 pandemic. Under some conditions, they prevent illness and save lives. But they also exact an economic toll. How should we balance the impact of such policies on individual lives and livelihoods (and other dimensions of concern) to determine which is best? A widely used method of policy evaluation, benefit–cost analysis (BCA), answers these questions by converting all the effects of a policy into monetary equivalents and then summing them up. A different method, social welfare analysis, proceeds by determining the effects of a policy on individual wellbeing and then applying an aggregation formula to them to evaluate the overall effects of a policy. In this chapter, we survey these methods and argue that social welfare analysis has important advantages. One crucial advantage is that it enables ethical considerations relating to the impact of policies on individual wellbeing and its distribution to be incorporated into policy assessments in a transparent way. We illustrate this with a simple numerical model for evaluating pandemic policies that vary in terms of the stringency of the controls that they impose on individual behaviour, showing how the evaluation depends on the ethical significance accorded to their impact on the wellbeing of different age and income groups
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