On three-dimensional SPH modelling of large-scale landslides

Lagrangian particle-based smoothed particle hydrodynamics (SPH) is increasingly widely used in landslide modelling. This paper investigates four important issues not addressed by previous studies on SPH modelling of large-scale landslides, i.e., convergence property, influence of constitutive parameters, scale effect and friction reduction, and influence of different treatments of the viscous effect. The GPU-acceleration technique is employed to achieve high-resolution three-dimensional (3D) modelling. The Baige landslide is investigated by comparing numerical results with field data, and detailed analyses on the four issues are provided. Suggestions on particle resolution, constitutive parameter, and formulations of viscous discretization are also presented for future SPH modelling of large-scale landslides.

Unsteady shock wave dynamics in accelerating and decelerating flight

Increasingly agile manoeuvre is an advantage in the flight of aircraft, missiles and aerial vehicles, but the principles of accelerating aerodynamics in the transonic regime are only now being fully investigated. This study contributes to the understanding of shock and separation effects on drag during axial acceleration, using a simple geometric configuration. Unsteady shock wave behavior was numerically investigated for an axisymmetric cone-cylinder using a commercial solver and the Moving Reference Frame acceleration technique. This acceleration technique was validated using unsteady numerical and experimental methods. The cone-cylinder was accelerated from Mach number 0.6 to Mach number 1.2 at 100g constant and deceleration was from Mach number 1.2 until Mach number 0.6 at –100g constant. Three cone angles were tested for the cone-cylinder with uniform cylinder diameter. Acceleration through the transonic Mach regime was characterised by a delayed and gradual shock wave development when compared to steady state, demonstrating a clear flow history effect. Deceleration through the transonic Mach regime was characterised by shock wave propagation from the base to the nose. New flow structures appeared during deceleration that do not have counterparts in the steady state, including shock interactions and propagating expansion-compression features. Gross changes in the unsteady drag coefficient curves for each cone-angle are explained with reference to unsteady shock wave behaviour for accelerating and decelerating motion.

Fast Matching Pursuit with Multi-Gabor Dictionaries

Finding the best K -sparse approximation of a signal in a redundant dictionary is an NP-hard problem. Suboptimal greedy matching pursuit algorithms are generally used for this task. In this work, we present an acceleration technique and an implementation of the matching pursuit algorithm acting on a multi-Gabor dictionary, i.e., a concatenation of several Gabor-type time-frequency dictionaries, each of which consists of translations and modulations of a possibly different window and time and frequency shift parameters. The technique is based on pre-computing and thresholding inner products between atoms and on updating the residual directly in the coefficient domain, i.e., without the round-trip to the signal domain. Since the proposed acceleration technique involves an approximate update step, we provide theoretical and experimental results illustrating the convergence of the resulting algorithm. The implementation is written in C (compatible with C99 and C++11), and we also provide Matlab and GNU Octave interfaces. For some settings, the implementation is up to 70 times faster than the standard Matching Pursuit Toolkit.

Plateformes digitales et concurrence par la rapidité. Le cas des vêtements d’occasion

Des recherches récentes explorent ce que les plateformes digitales font et font faire aux utilisateurs. Dans leur prolongement, cet article étudie comment les plateformes digitales P2P accélèrent les activités marchandes, et comment cette accélération modèle les relations entre utilisateurs. Une étude qualitative à caractère ethnographique a été menée au sein du marché des vêtements d’occasion de 2013 à 2020. En mobilisant la théorie de l’accélération sociale, nous montrons que les affordances des plateformes digitales encouragent une accélération du rythme de vie, une accélération du renouvellement des objets possédés, et une accélération technique multi-domaines. Ces formes d’accélération aboutissent à une concurrence par la rapidité entre utilisateurs : chacun d’eux se doit d’être plus réactif, plus à la pointe, et plus averti que les autres. De plus, les plateformes constituent un espace digital particulier qui invite à repenser l’aliénation sous plusieurs formes, et notamment l’aliénation par rapport à l’espace et par rapport au temps. Outre ses contributions théoriques, cette recherche offre des contributions sociétales visant à faire prendre conscience aux utilisateurs de l’ampleur de leurs activités marchandes accélérées sur les plateformes digitales.