On the dual effect of obstacles in preventing silo clogging in 2D

Placing an obstacle in front of a bottleneck has been proposed as a sound alternative to improve the flow of discrete materials in a wide variety of scenarios. Nevertheless, the physical reasons behind this behavior are not fully understood and the suitability of this practice has been recently challenged for pedestrian evacuations. In this work, we experimentally demonstrate that for the case of inert grains discharging from a silo, an obstacle above the exit leads to a reduction of clog formation via two different mechanisms: i) an alteration of the kinematic properties in the outlet proximities that prevents the stabilization of arches; and ii) an introduction of a clear anisotropy in the contact fabric tensor that becomes relevant when working at a quasi-static regime. Then, both mechanisms are encompassed using a single formulation that could be inspiring for other, more complex, systems.

Destabilization of Immersed Dense Granular Material Submitted to Localized Fluidization: An Experimental and Numerical Study

An alternative experimental approach and a numerical analysis for the study of destabilization by localized fluidization of an immersed dense granular material are presented. To visualize the evolutions of the internal structure of the granular medium, the hydrogel beads, composed of about 99% of water and having substantially the same refraction indexes, are used as solid phase. A LED lighting system is used in place of a laser lighting system. As a result, the optical access restriction of porous structure is removed. A real economic alternative for the experimental study of fluid-grain coupling during destabilization by localized fluidization of a granular material is created. The experimental phenomenology presented in the literature is verified: the system passes successively through three different stationary regimes: static regime, fluidized cavity regime, and fluidized chimney regime. Some restrictions of using hydrogel beads as particles in the study of liquid-solid interaction are also discussed.

A rational design of multimodal asymmetric nanoshells as efficient tunable absorbers within the biological optical window

In this work, the optical properties of asymmetric nanoshells with different geometries are comprehensively investigated in the quasi-static regime by applying the dipolar model and effective medium theory. The plasmonic behaviors of these nanostructures are explained by the plasmon hybridization model. Asymmetric hybrid nanoshells, composed of off-center core or nanorod core surrounded by a spherical metallic shell layer possess highly geometrically tunable optical resonances in the near-infrared regime. The plasmon modes of this nanostructures arise from the hybridization of the cavity and solid plasmon modes at the inner and outer surfaces of the shell. The results reveal that the symmetry breaking drastically affects the strength of hybridization between plasmon modes, which ultimately affects the absorption spectrum by altering the number of resonance modes, their wavelengths and absorption efficiencies. Therefore, offsetting the spherical core as well as changing the internal geometry of the nanoparticle to nanorod not only shift the resonance frequencies but can also strongly modify the relative magnitudes of the absorption efficiencies. Furthermore, higher order multipolar plasmon modes can appear in the spectrum of asymmetric nanoshell, especially in nanoegg configuration. The results also indicate that the strength of hybridization strongly depends on the metal of shell, material of core and the filling factor. Using Au-Ag alloy as a material of the shell can provide red-shifted narrow resonance peak in the near-infrared regime by combining the specific features of gold and silver. Moreover, inserting a high permittivity core in a nanoshell corresponds to a red-shift, while a core with small dielectric constant results in a blue-shift of spectrum. We envision that this research offers a novel perspective and provides a practical guideline in the fabrication of efficient tunable absorbers in the nanoscale regime.

On the dual effect of obstacles in preventing silo clogging

We experimentally demonstrate that an obstacle, when suitably placed above a bottleneck, leads to a reduction of clog formation in a silo via two different mechanisms. The first one, already suggested in previous works, is related to an alteration of the kinematic properties in the outlet proximities that prevents the stabilization of arches. The second, that is discovered when working at a quasi-static regime, appears because the obstacle induces a clear anisotropy in the contact fabric tensor. Then, both mechanisms are encompassed using a single equation in which two parameters -one related with the geometrical effects and the other to the kinematic ones- are enough to reproduce all the experimental results

Design Criteria for Scantling of Longitudinal and Transverse Connections in the Torsion Box Under Fatigue Loading

Fatigue is one of the main failure modes in marine structures, and it is caused by the strong cyclic characteristics of the loads they support. This failure mode is amplified in areas of high stress concentration, such as at the intersection of primary and secondary elements. In this paper, a two-phase study is proposed that compares numerical and experimental results using a digital image correlation technique. The described procedure establishes selection, design, and scantling criteria and provides recommendations for the design of the transverse structure using specimens with different geometries. These geometries correspond to different designs for the transverse primary structure that use a longitudinal secondary stiffener with variable thickness and longitudinal spacing to transverse in a dynamic and quasi-static regime. The stress state for this regime is calculated based on the biaxiality indication concept, which uses the fatigue phenomenon (safety factor and sensitivity curves) and fracture mechanics (parameters of the Paris crack propagation law, correlation value, and law of variation of the stress intensity factor).

Unified constitutive model for granular-fluid mixture in quasi-static and dense flow regimes

Most granular materials encountered in nature and industry lie either in the quasi-static regime or the intermediate dense flow regime. Debris materials are a typical granular material with viscous interstitial fluid, and shows solid-like behaviors before failure and fluid-like behaviors after failure. Based on Bagnold’s pioneering work on granular-fluid flows, we propose a framework for constitutive model development, which has an additive form. Based on this framework, a unified constitutive model for granular-fluid material in the quasi-static and dense flow regimes is developed. The main intergranular interactions and granular-fluid interactions controlling the mechanical behaviors are taken into account using the Mohr-Coulomb model and a Bagnold-type relation. Dry granular flows in three simple configurations, i.e., plain shear, vertical chute flow and flow on an inclined plane, are studied. Analytical solutions based on the presented unified model are obtained. Comparisons between results from the presented model and the mu(I) model indicate that the explicit partition of frictional and collisional stress components provides insights in dense granular flows. In addition, the new model is used to predict the stress-strain relations in two annular shear tests. The applicability and advantages of the unified model are discussed.

Theoretical Study of a Bifacial Silicon Solar Cell Front Side Illuminated: Magnetic Field Effect on the Recombination Velocities Inducing the Short Circuit and Limiting the Open Circuit

The aim of this work is to present a study of the recombination velocities at the junction initiating the shortcircuit (Sfsc) and limiting the open circuit (Sfoc) of a silicon solar cell under magnetic field in the static regime. From the continuity equation, the density of minority charge carriers in the base, the photocurrent density, and the phototension are determined. The study of the photocurrent density and the phototension, as a function of the junction recombination velocity, makes it possible to determine the recombination velocities at the junction initiating the short-circuit and limiting the open circuit respectively. From the profile of the variation of the photocurrent density and of the phototension as a function of the junction recombination velocity, a technique for determining the junction recombination velocities initiating the short circuit situation and limiting the open circuit is presented.

A perturbative approach to the time-dependent Karmarkar condition

In this work we employ a perturbative approach to study the gravitational collapse of a shear-free radiating star. The collapse proceeds from an initial static core satisfying the time-independent Karmarkar condition and degenerates into a quasi-static regime with the generation of energy in the form of a radial heat flux. The time-dependent Karmarkar condition is solved together with the boundary condition to yield the full gravitational behaviour of the star. Our model is subjected to rigorous regularity, causality and stability tests.

ОБ УПРАВЛЕНИИ ИНТЕНСИВНОСТЬЮ КОНВЕКЦИИ ХИМИЧЕСКИ РЕАГИРУЮЩЕГО ГАЗА

A linear analysis of the stability of Rayleigh-Benard static regime convection is performed in a chemically reacting equilibrium hydrogen-oxygen gas mixture with added chemically inert microparticles of aluminum oxide. It is shown in the Boussinesq approach that a suitable choice of temperature can intensify convection and adding chemically inert microparticles can suppress convective motion. It is established that the isobaric convection regime is realized when the height of the region is less than the critical value, and when the height of the region exceeds this critical value, the convection regime is superadiabatic. During convection in the superadiabatic regime, its adiabatic suppression is observed. Within the framework of the isobaric convection regime, the limits of applicability of the Boussinesq approach are determined.

On the Tidal Prism: The Roles of Basin Extension, Bottom Friction and Inlet Cross-Section

The prism of the Lignano tidal inlet was approximately constant over the last forty years, although the section width has halved. This has led to questions concerning the factors that most influence the tidal prism, and on the applicability of the well-known A–P relationship. A conceptual scheme of the sea–channel–lagoon system has been used to perform a sensitivity analysis of different parameters that characterize both the basin and the inlet cross-section. A 2D hydrodynamic model has been applied to evaluate the prism and compare it to the one derived by a static method, which is the basis of the analytical derivation of the A–P linkage. Three regimes have been found in the prism variability as a function of the basin extension: a linear static regime between prism and basin area; an asymptotic regime in which the prism depends only on the basin bottom friction; and an intermediate one. In addition, the roles of the inlet and channel sizes on the prism value have been investigated. The results, compared to the empirical relationships between the prism and the inlet cross-section, show that a variation in the cross-sectional area does not always corresponds to a change in tidal prism.