Dense granular flow around a penetrating object: Experiments and hydrodynamic model

We present in this Letter experimental results on the bidimensional flow field around a cylinder penetrating into dense granular matter together with drag force measurements. A hydrodynamic model based on extended kinetic theory for dense granular flow reproduces well the flow localization close to the cylinder and the corresponding scalings of the drag force, which is found to not depend on velocity, but linearly on the pressure and on the cylinder diameter and weakly on the grain size. Such a regime is found to be valid at a low enough "granular" Reynolds number.Comment: 5 pages, 4 figure

Semiclassical Vlasov and fluid models for an electron gas with spin effects

We derive a four-component Vlasov equation for a system composed of spin-1/2 fermions (typically electrons). The orbital part of the motion is classical, whereas the spin degrees of freedom are treated in a completely quantum-mechanical way. The corresponding hydrodynamic equations are derived by taking velocity moments of the phase-space distribution function. This hydrodynamic model is closed using a maximum entropy principle in the case of three or four constraints on the fluid moments, both for Maxwell-Boltzmann and Fermi-Dirac statistics.Comment: To appear in the European Physical Journal D, Topical Issue "Theory and Applications of the Vlasov Equation

Magnetic moment generation in small gold nanoparticles via the plasmonic inverse Faraday effect

We theoretically investigate the creation of a magnetic moment in gold nanoparticles by circularly polarized laser light. To this end, we describe the collective electron dynamics in gold nanoparticles using a semiclassical approach based on a quantum hydrodynamic model that incorporates the prin- cipal quantum many-body and nonlocal effects, such as the electron spill-out, the Hartree potential, and the exchange and correlation effects. We use a variational approach to investigate the breathing and the dipole dynamics induced by an external electric field. We show that gold nanoparticles can build up a static magnetic moment through the interaction with a circularly polarized laser field. We analyze that the responsible physical mechanism is a plasmonic, orbital inverse Faraday effect, which can be understood from the time-averaged electron current that contains currents rotating on the nanoparticles surface. The computed laser-induced magnetic moments are sizeable, of about 0.35 muB/atom for a laser intensity of 450 GW/cm2 at plasmon resonance

Spin current generation by ultrafast laser pulses in ferromagnetic nickel films

International audienceA semiclassical phase-space model is used to study the ultrafast charge and spin dynamics in thin ferromagnetic films. Both itinerant and localized magnetism are taken into account. It is shown that an oscillating spin current can be generated in the film via the application of a femtosecond laser pulse in the visible range

Interaction avec un picoprojecteur : État de l'art et analyse des attentes des utilisateurs

4 pagesNational audienceUn picoprojecteur est un vidéoprojecteur portatif aux dimensions réduites. On appelle également picophone, un smartphone qui intègre un tel dispositif. Encore très peu diffusé, ce nouveau dispositif interactif mobile est commercialisé depuis 2 ans. Cet article dresse un état de l'art des recherches actuellement menées sur le sujet, puis présente les attentes d'un panel de 50 utilisateurs potentiels

Dispersion Estimation From Linear Array Data in the Time-Frequency Plane

International audienceWe consider the problem of estimating the dispersion of a wave field from data recorded by a linear array of geophones. The fact that the data we are looking at may contain several propagating waves make this even more challenging. In this paper, a new algorithm is proposed to solve this issue. Currently, there are two methods for estimating wave dispersion described in the literature. The first method estimates the group delay function from the time-frequency representation (TFR) of each sensor separately. It is efficient as long as the patterns of the different waves do not overlap in the time-frequency plane. The second method estimates the dispersion from the two-dimensional (2-D) Fourier transform of the profile (or more generally from a velocity-frequency representation). This assumes that the dispersion is constant along the entire sensor array. It is efficient as long as the patterns of the waves do not overlap in the frequency domain. Our method can be thought of as a hybrid of the above two methods as it is based on the construction of a TFR where the energy of waves that propagate at a selected velocity are amplified. The primary advantage of our algorithm is the use of the velocity variable to separate the patterns of the propagating waves in the time-frequency plane. When applied to both synthetic and real data, this new algorithm gives much improved results when compared with other standard methods

Word Order Phenomena in Spoken French : a Study on Four Corpora of Task-Oriented Dialogue and its Consequences on Language Processing

International audienceThis paper presents a corpus study that investigates the question of word order variations (WOV) in spontaneous spoken French and its consequences on the parsing techniques that are used in Natural Language Processing. We have studied four taskoriented spoken dialogue corpora which concern different application tasks (air transport or tourism information, switchboard calls). Two corpora concern phone conversations while the other two correspond to direct interaction. Every word order variation has been manually annotated by 3 experts, following a cross-validation procedure. Our results show that, while conversational spoken French should be highly affected by WOVs, it should also still be considered as a rigid order language: WOVs follow some impressive structural regularity and they result very rarely in discontinuous syntactic structures. As a result, non-projective parsers remain well adapted to conversational spoken French

Robust Optimal Control for Nonlinear Systems with Parametric Uncertainties via System Level Synthesis

This paper addresses the problem of optimally controlling nonlinear systems with norm-bounded disturbances and parametric uncertainties while robustly satisfying constraints. The proposed approach jointly optimizes a nominal nonlinear trajectory and an error feedback, requiring minimal offline design effort and offering low conservatism. This is achieved by decomposing the affine-in-the-parameter uncertain nonlinear system into a nominal $\textit{nonlinear}$ system and an uncertain linear time-varying system. Using this decomposition, we can apply established tools from system level synthesis to $\textit{convexly}$ over-bound all uncertainties in the nonlinear optimization problem. Moreover, it enables tight joint optimization of the linearization error bounds, parametric uncertainties bounds, nonlinear trajectory, and error feedback. With this novel controller parameterization, we can formulate a convex constraint to ensure robust performance guarantees for the nonlinear system. The presented method is relevant for numerous applications related to trajectory optimization, e.g., in robotics and aerospace engineering. We demonstrate the performance of the approach and its low conservatism through the simulation example of a post-capture satellite stabilization.Comment: Accepted for CDC (Singapore, 13-15 December 2023). Code: https://gitlab.ethz.ch/ics/nonlinear-parametric-SL

Socioeconomic agents as active matter in nonequilibrium Sakoda-Schelling models

How robust are socioeconomic agent-based models with respect to the details of the agents' decision rule? We tackle this question by considering an occupation model in the spirit of the Sakoda-Schelling model, historically introduced to shed light on segregation dynamics among human groups. For a large class of utility functions and decision rules, we pinpoint the nonequilibrium nature of the agent dynamics, while recovering the equilibrium-like phase separation phenomenology. Within the mean field approximation we show how the model can be mapped, to some extent, onto an active matter field description (Active Model B). Finally, we consider non-reciprocal interactions between two populations, and show how they can lead to non-steady macroscopic behavior. We believe our approach provides a unifying framework to further study geography-dependent agent-based models, notably paving the way for joint consideration of population and price dynamics within a field theoretic approach.Comment: 12 pages, 7 figure