Hemihelical local minimizers in prestrained elastic bi-strips

We consider a double layered prestrained elastic rod in the limit of vanishing cross section. For the resulting limit Kirchoff-rod model with intrinsic curvature we prove a supercritical bifurcation result, rigorously showing the emergence of a branch of hemihelical local minimizers from the straight configuration, at a critical force and under clamping at both ends. As a consequence we obtain the existence of nontrivial local minimizers of the $3$-d system.Comment: 16 pages, 2 figure

Imprints of log-periodic self-similarity in the stock market

Detailed analysis of the log-periodic structures as precursors of the financial crashes is presented. The study is mainly based on the German Stock Index (DAX) variation over the 1998 period which includes both, a spectacular boom and a large decline, in magnitude only comparable to the so-called Black Monday of October 1987. The present example provides further arguments in favour of a discrete scale-invariance governing the dynamics of the stock market. A related clear log-periodic structure prior to the crash and consistent with its onset extends over the period of a few months. Furthermore, on smaller time-scales the data seems to indicate the appearance of analogous log-periodic oscillations as precursors of the smaller, intermediate decreases. Even the frequencies of such oscillations are similar on various levels of resolution. The related value $\lambda \approx 2$ of preferred scaling ratios is amazingly consistent with those found for a wide variety of other complex systems. Similar analysis of the major American indices between September 1998 and February 1999 also provides some evidence supporting this concept but, at the same time, illustrates a possible splitting of the dynamics that a large market may experience.Comment: 13 pages, LaTeX-REVTeX, 4 PS figures. Significantly extended version to appear in The European Physical Journal

Numerical studies of pair creation in counterpropagating laser fields

Pair creation from vacuum induced by electromagnetic fields is probably one of the most intriguing phenomena in physics. If the fields are sufficiently strong, the QED vacuum become unstable. Due to the remarkable progress inm laser technology during recent years an experimental investigation of pair creation by pure laser light is coming into reach. The focus of this thesis is on pair creation in counterpropagating laser beams. The pair creation probability is calculated employing the numerically obtained solutions of the Dirac equation. This numerical ansatz has the capability of calculating the momentum distribution of the created pairs in a single propagation, for pure time dependent field configurations. Furthermore, it allows to take the magnetic component of the laser fields into account, which is usually neglected. The latter strongly affects the creation process at high laser frequency. The involved numerical calculations are rather time consuming, therefore the second project of this thesis was to develop a highly efficient code for solving relativistic quantum mechanical problems. This is accomplished by adopting the split-operator method to the Klein-Gordon equation. Here the possibility arises to use parallel computing. However the corresponding spin-statistics becomes crucial in the case of pair creation, demonstrated in several examples

Coordinations hydrauliques et justices sociales

La gestion participative de l'irrigation est une démarche formulée par la Banque mondiale et imposée dans la plupart des projets de modernisation de l'irrigation dans le Monde. Cette communication montre comment cette politique s'applique sur des systèmes anciens de gestion de l'eau en Egypte et au Maroc. Après avoir comparé les histoires hydrauliques et politiques des deux pays, nous examinons les différences de prospérité entre les paysanneries pour croiser ensuite nos perceptions des coordinations hydrauliques et des institutions locales étudiées à l'occasion de formation commune d'agronomes français, égyptiens et marocains. Nous insistons sur la fragmentation et la complexité des territoires pour conclure comme injuste l'absence de considération des sociétés locales dans la coordination hydraulique des grands ouvrages d'irrigatio

Estimation of the material budget of the LHCb detector

The material budget of the LHCb detector at the time of the DC 06 data challenge is estimated

Calculating Nozzle Side Loads using Acceleration Measurements of Test-Based Models

As part of a NASA/MSFC research program to evaluate the effect of different nozzle contours on the well-known but poorly characterized "side load" phenomena, we attempt to back out the net force on a sub-scale nozzle during cold-flow testing using acceleration measurements. Because modeling the test facility dynamics is problematic, new techniques for creating a "pseudo-model" of the facility and nozzle directly from modal test results are applied. Extensive verification procedures were undertaken, resulting in a loading scale factor necessary for agreement between test and model based frequency response functions. Side loads are then obtained by applying a wide-band random load onto the system model, obtaining nozzle response PSD's, and iterating both the amplitude and frequency of the input until a good comparison of the response with the measured response PSD for a specific time point is obtained. The final calculated loading can be used to compare different nozzle profiles for assessment during rocket engine nozzle development and as a basis for accurate design of the nozzle and engine structure to withstand these loads. The techniques applied within this procedure have extensive applicability to timely and accurate characterization of all test fixtures used for modal test.A viewgraph presentation on a model-test based pseudo-model used to calculate side loads on rocket engine nozzles is included. The topics include: 1) Side Loads in Rocket Nozzles; 2) Present Side Loads Research at NASA/MSFC; 3) Structural Dynamic Model Generation; 4) Pseudo-Model Generation; 5) Implementation; 6) Calibration of Pseudo-Model Response; 7) Pseudo-Model Response Verification; 8) Inverse Force Determination; 9) Results; and 10) Recent Work

Domain formation in magnetic polymer composites: an approach via stochastic homogenization

We study the magnetic energy of magnetic polymer composite materials as the average distance between magnetic particles vanishes. We model the position of these particles in the polymeric matrix as a stochastic lattice scaled by a small parameter $\varepsilon$ and the magnets as classical $\pm 1$ spin variables interacting via an Ising type energy. Under surface scaling of the energy we prove, in terms of $\Gamma$-convergence that, up to subsequences, the (continuum) $\Gamma$-limit of these energies is finite on the set of Caccioppoli partitions representing the magnetic Weiss domains where it has a local integral structure. Assuming stationarity of the stochastic lattice, we can make use of ergodic theory to further show that the $\Gamma$-limit exists and that the integrand is given by an asymptotic homogenization formula which becomes deterministic if the lattice is ergodic.Comment: 31 page

Coordinations hydrauliques et justices

La région de Marrakech dispose d'une des plus anciennes cultures hydrauliques marocaines. Elle est décrite dans les années 1970 comme une société composite comprenant toutes sortes d'exploitations agricoles et de groupes sociaux liés aux différents dispositifs hydrauliques. La mise en place d'un projet de grande hydraulique prévoyant des transferts d'eau entre bassins et la modernisation des réseaux s'est déroulée dans les années 1980-2000. Les auteurs examinent en 2004 quel est le bilan des transformations. Ils montrent comment le territoire hydraulique reste un maillage complexe entre réseaux anciens et modernes, et introduisent l'idée de générations hydrauliques superposées mais non co-ordonnées. L'injustice foncière et économique se renforce d'une différenciation des accès aux différents réseaux et des risques d'individualisation. La gestion collective de l'eau est encore possible à condition de reconstruire une politique de reconnaissance des différentes générations hydrauliques associées à une action dans le domaine agrair