Risk measurement: an introduction to value at risk

This paper is a self-contained introduction to the concept and methodology of "value at risk," which is a new tool for measuring an entity's exposure to market risk. We explain the concept of value at risk, and then describe in detail the three methods for computing it: historical simulation; the variance-covariance method; and Monte Carlo or stochastic simulation. We then discuss the advantages and disadvantages of the three methods for computing value at risk. Finally, we briefly describe some alternative measures of market risk.Risk and Uncertainty,

The effects of pressure, nozzle diameter and meteorological conditions on the performance of agricultural impact sprinklers

19 Pags. The definitive version, with Figs. y Tabls., is available at: http://www.sciencedirect.com/science/journal/03783774This study evaluates agricultural impact sprinklers under different combinations of pressure (p), nozzle diameter (D) and meteorological conditions. The radial curve (Rad) of an isolated sprinkler, i.e., the water distribution along the wetted radius, was evaluated through 25 tests. Christiansen's uniformity coefficient (CUC) and the wind drift and evaporation losses (WDEL) were evaluated for a solid-set system using 52 tests. The Rad constitutes the footprint of a sprinkler. The CUC is intimately connected to the Rad. The Rad must be characterized under calm conditions. Very low winds, especially prevailing winds, significantly distort the water distribution. The vector average of the wind velocity (V’) is recommended as a better explanatory variable than the more popular arithmetic average (V). We recommend characterizing the Rad under indoor conditions or under conditions that meet V’ < 0.6 m s−1 in open-air conditions. The Rad was mostly affected by the sprinkler model. V’ was the main explanatory variable for the CUC; p was significant as well. V was the main variable explaining the WDEL; the air temperature (T) was significant, too. Sprinkler irrigation simulators simplify the selection of a solid-set system for farmers, designers and advisors. However, the quality of the simulations greatly depends on the characterization of the Rad. This work provides useful recommendations in this area.This research was funded by the Government of Spain through grants AGL2004-06675-C03-03/AGR, AGL2007-66716-C03 and AGL2010-21681, by the Government of Aragón through grant PIP090/2005, and by the INIA and CITA through the PhD grants program.Peer reviewe

Disordered actomyosin networks are sufficient to produce cooperative and telescopic contractility

While the molecular interactions between individual myosin motors and F-actin are well established, the relationship between F-actin organization and actomyosin forces remains poorly understood. Here we explore the accumulation of myosin-induced stresses within a two-dimensional biomimetic model of the disordered actomyosin cytoskeleton, where myosin activity is controlled spatiotemporally using light. By controlling the geometry and the duration of myosin activation, we show that contraction of disordered actin networks is highly cooperative, telescopic with the activation size, and capable of generating non-uniform patterns of mechanical stress. We quantitatively reproduce these collective biomimetic properties using an isotropic active gel model of the actomyosin cytoskeleton, and explore the physical origins of telescopic contractility in disordered networks using agent-based simulations

Dissociation of hydrogen molecules on the clean and hydrogen-preadsorbed Be(0001) surface

Using first-principles calculations, we systematically study the potential energy surfaces and dissociation processes for hydrogen molecules on the clean and hydrogen-preadsorbed Be(0001) surfaces. It is found that the most energetically favored dissociation channel for H2 molecules on the clean Be surface is at the surface top site, with the minimum energy barrier of 0.75 eV. It is further found that after dissociation, hydrogen atoms do not like to cluster with each other, as well as to penetrate into subsurface sites. For the hydrogen-preadsorbed Be(0001) surface, the smallest dissociation energy barrier for H2 molecules is found to be 0.50 eV, which is smaller than the dissociation energy barrier on a clean Be(0001) surface. The critical dependence of the dissociation energy barriers for H2 molecules on their horizontal distances from the preadsorbed hydrogen atom is revealed. Our studies well describe the adsorption behaviors of hydrogen on the Be(0001) surface.Comment: 17 pages, 9 figure

Chemical vapor deposition and infiltration for the production of tungsten fiber reinforced tungsten composite material

Contribution submission to the conference Regensburg 2016Chemical vapor deposition and infiltration for the productionof tungsten fiber reinforced tungsten composite material —∙Martin Aumann1, Jan Willem Coenen1, Hanns Gietl2, TillHoeschen2, Johann Riesch2, Klaus Schmid2, Rudolf Neu2, andChristian Linsmeier1 — 1Forschungszentrum Juelich GmbH, Institutfür Energie- und Klimaforschung, 52425 Juelich — 2Max-Planck-Institut für Plasmaphysik, 85748 GarchingDue to its high melting point, high corrosion resistance and its preferableproperties in terms of hydrogen retention, tungsten is a promisingcandidate in future nuclear fusion devices. However, the mechanicalbehavior of tungsten is crucial, as it is inherently brittle at room temperature.As possibility to overcome this brittleness, a composite materialcan be formed, which shows pseudo-ductility and therefore avoidscatastrophic failure of the material. A possibility to produce such aWf/W-composite is chemical vapor deposition and chemical vapor infiltration,where tungsten is deposited on small tungsten wires throughthe reaction of WF6 and H2. With ongoing infiltration time, pores areformed between the fibers, which decrease in size through the chemicalreaction. For better process understanding, a pore model was established,which solves the mass balance inside the pore and the resultingpore diameter simultaneously. It shows a significant difference in diameterfor longer infiltration times. This behavior shall be proved inexperiments with an experimental pore, which is similar to the simulatedone. Furthermore also kinetic investigations on the chemicalsurface reaction are carried out to increase the process understanding.Part: MMType: Vortrag;TalkTopic: Transport (Diffusion, Leitfähigkeit,Wärme)/ Transport (Diffusion,conductivity, heat)Email: [email protected]

Surface chemistry of fusion first wall materials

Als erste Wand in einem Fusionsreaktor können, entsprechend den lokalen Wandbelastungen, verschiedene Materialien eingesetzt werden. Für das im Bau befindliche Experiment ITER sind Beryllium im Hauptraum sowie Wolfium und Kohlenstoff im Divertor geplant. Im Betrieb werden diese Elemente erodiert, transportiert und in Bereichen deponiert, in denen Temperatur- und Teilchenbelastungen zu chemischen Reaktionen, Diffusion sowie Reerosionsprozessen führen. Neben den Wandelementen sind Wasserstoffisotope sowie Plasmaverunreinigungen wie Sauerstoff weitere Reaktionspartner. Die möglichen Verbindungsbildungen (Oxide, Carbide, Legierungen) sowie deren Einfluss auf die Wasserstoffrückhaltung in der ersten Wand sind kritisch für den Betrieb eines Fusionsreaktors. Die Arbeit umfasst die qualitative und quantitative Beschreibung der Oberflächenreaktionen zwischen den beteiligten Elementen zu binären und ternären Verbindungen sowie der Mechanismen der Rückhaltung von Wasserstoffisotopen im Wandmaterial Beryllium. Sowohl thermisch als auch durch Ionen getriebene Reaktionen werden untersucht. Die dabei bestimmten Reaktionsraten und Aktivierungsbarrieren erlauben zukünftig eine Modellierung der chemischen und physikalischen Oberflächenprozesse

Improving K-12 ELL Students\u27 Comprehension of Academic Text

Successful acquisition of academic English is dependent upon a multitude of cognitive, linguistic, and educational variables, all based on the socio-cultural perspectives and powers that regulate what occurs in the classroom. The function and meaning of academic texts may be lost on the student with limited knowledge and experience with academic social contexts. The teacher must work within the confines of societal and educational boundaries. The provision of experience, motivation, and opportunities for interaction is highly beneficial, in addition to explicitly teaching vocabulary and learning strategies. A functional linguistics approach, delivered through Readers\u27 Workshop, based on the work of Aguirre-Munoz et al., 2006, is described