Identification of rolling resistance as a shape parameter in sheared granular media

Using contact dynamics simulations, we compare the effect of rolling resistance at the contacts in granular systems composed of disks with the effect of angularity in granular systems composed of regular polygonal particles. In simple shear conditions, we consider four aspects of the mechanical behavior of these systems in the steady state: shear strength, solid fraction, force and fabric anisotropies, and probability distribution of contact forces. Our main finding is that, based on the energy dissipation associated with relative rotation between two particles in contact, the effect of rolling resistance can explicitly be identified with that of the number of sides in a regular polygonal particle. This finding supports the use of rolling resistance as a shape parameter accounting for particle angularity and shows unambiguously that one of the main influencing factors behind the mechanical behavior of granular systems composed of noncircular particles is the partial hindrance of rotations as a result of angular particle shape.Comment: Soumis a Physical Review E; Statistical, Nonlinear, and Soft Matter Physics http://link.aps.org/doi/10.1103/PhysRevE.84.01130

Optimal packing of poly-disperse spheres in 3D: effect of the grain size span and shape

Abstract. In this work, we explore the effect of the grain size distribution (gsd) on the packing of 3D granular materials composed of spheres, and find the optimal packing with the highest density as a function of the gsd parameters

Modelling the spread of Covid19 in Italy using a revised version of the SIR model

In this paper, we present a model to predict the spread of the Covid-19 epidemic and apply it to the specific case of Italy. We started from a simple Susceptible, Infected, Recovered (SIR) model and we added the condition that, after a certain time, the basic reproduction number $R_0$ exponentially decays in time, as empirically suggested by world data. Using this model, we were able to reproduce the real behavior of the epidemic with an average error of 5\%. Moreover, we illustrate possible future scenarios, associated to different intervals of $R_0$. This model has been used since the beginning of March 2020, predicting the Italian peak of the epidemic in April 2020 with about 100.000 detected active cases. The real peak of the epidemic happened on the 20th of April 2020, with 108.000 active cases. This result shows that the model had predictive power for the italian case.Comment: The model presented in this paper has been adopted on Covstat.it. Errata corrige in the abstrac

Comparison of the effects of rolling resistance and angularity in sheared granular media

International audienceIn this paper, we compare the effect of rolling resistance at the contacts in granular systems composed of disks with the effect of angularity in granular systems composed of regular polygonal particles. For this purpose, we use contact dynamics simulations. By means of a simple shear numerical device, we investigate the mechanical behavior of these materials in the steady state in terms of shear strength, solid fraction, force and fabric anisotropies, and probability distribution of contact forces. We find that, based on the energy dissipation associated with relative rotation between two particles in contact, the effect of rolling resistance can explicitly be identified with that of the number of sides in a regular polygonal particle. This finding supports the use of rolling resistance as a shape parameter accounting for particle angularity and shows unambiguously that one of the main influencing factors behind the mechanical behavior of granular systems composed of noncircular particles is the partial hindrance of rotations as a result of angular particle shape

Impact of grading on steady-state strength

International audienceIn the mining industry, waste dumps are earthen structures typically built by loose waste tipping. They may reach heights of hundreds of metres and undergo large deformations. For this reason, their stability design is based on the steady-state shear strength of the waste material. Waste materials are widely graded and may contain particles of up to metric order. Particle shape depends on the pattern of dissecting discontinuities at the source rock mass and the relation between the size of the fragments and discontinuity spacing. The shear strength of this material is determined in the laboratory using scaled samples with altered particle-size distribution (PSD). However, altering the PSD is known to impact shear strength, and this impact is poorly studied. The representativeness of laboratory parameters obtained from scaled samples is thus arguable. Discrete-element simulations are used here to investigate steady-state shear strength changes with the alteration of the PSD when particle size and shape are correlated. It is observed that shear strength changes result from the variation of the particle shapes induced by the alteration of the PSD. Consequently, identifying size−shape correlations and their potential impact on shear strength is of paramount importance when scaling materials for laboratory testing

Micrometeoroid infall onto Saturn’s rings constrains their age to no more than a few hundred million years

There is ongoing debate as to whether Saturn’s main rings are relatively young or ancient— having been formed shortly after Saturn or during the Late Heavy Bombardment. The rings are mostly water-ice but are polluted by non-icy material with a volume fraction ranging from ∼0.1 to 2%. Continuous bombardment by micrometeoroids exogenic to the Saturnian system is a source of this non-icy material. Knowledge of the incoming mass flux of these pollutants allows estimation of the rings’ exposure time, providing a limit on their age. Here we report the final measurements by Cassini’s Cosmic Dust Analyzer of the micrometeoroid flux into the Saturnian system. Several populations are present, but the flux is dominated by low-relative velocity objects such as from the Kuiper belt. We find a mass flux between 6.9 · 10−17 and 2.7 · 10−16 kg m−2s−1 from which we infer a ring exposure time ≲100 to 400 million years in support of recent ring formation scenarios

Rôle des paramètres locaux de cohésion dans la résistance mécanique d'un milieu granulaire cohésif

Nous présentons une étude paramétrique de la résistance au cisaillement d'un milieu granulaire cohésif simulé par la méthode de dynamique des contacts. Cette étude est basée sur un modèle de cohésion qui tient compte de la résistance à la traction, au glissement, et au roulement dans les contacts entre les particules. La résistance macroscopique est évaluée par la compression biaxiale des échantillons numériques comportant plusieurs milliers de particules en deux dimensions. Nos résultats suggèrent que : 1) l'angle de frottement interne peut être vu comme la somme de trois contributions : une géométrique, une frictionnelle, et une associée à la dilatance ; 2) les coefficients de frottement sont les seuls paramètres de résistance microscopiques à l'origine de l'angle de frottement interne et de l'angle de dilatance ; et 3) l'adhésion entre particules est le seul paramètre de résistance microscopique à l'origine de la résistance à la traction

Opciones reales en el sector de la construcción

Las opciones reales se han convertido en un elemento fundamental a la hora de valorar proyectos de inversión, especialmente cuando estos poseen dentro de sus características principales, la opción de diferir la inversión. Este trabajo valora un proyecto inmobiliario, incorporando la opción de diferir por un período de tiempo de cinco años, el inicio de su construcción. La metodología utilizada para valorar la opción fue árboles binomiales, donde la volatilidad fue calculada con base en los flujos de caja del proyecto. Los resultados aportan evidencia que confirma que el valor de la opción incrementa el valor total del proyecto en un 58%.Real options have become a key element when it comes to valuing investment projects, especially when they have the option of deferring the investment. This work values a real estate project incorporating the option of deferring the beginning of its constructionfor for a period of five years, the methodology used to calculate the option value was binomial trees, where volatility was calculated based on the project cash flows. The results provide evidence confirming that the option value increases the total project value by 58%

Sistema web para el control de citas médicas del Consultorio odontológico Kitty Dent, Lima, 2023

La situación actual del consultorio odontológico Kitty Dent, carece de un control en los procesos de agendamiento de citas, administración de usuarios, gestión de pacientes, y almacenamiento de historiales odontológicos de cada tratamiento realizado. El objetivo del estudio es determinar en qué medida un sistema web mejora el control de citas médicas del consultorio odontológico Kitty Dent, Lima-2023. Se empleó la investigación de tipo aplicada, como diseño de investigación se escogió el pre-experimental y el enfoque es cuantitativo. El tamaño de la muestra estuvo conformado por 50 citas médicas. La técnica de recolección de datos fue el fichaje y el instrumento fue la ficha de registro, los cuales fueron validados por expertos, y procesados mediante el software SPSS Statistics V.25. Los resultados de la implementación del sistema web permitió incrementar el Nivel de citas atendidas en un 36%; así mismo, se disminuyó el Porcentaje de citas médicas incumplidas en un 29% y, por último, se redujo el Tiempo Promedio de Registro de Citas en un 11%. Se concluye que el sistema web mejora el control de citas médicas en la clínica Kitty Dent. En el sistema web se empleó la metodología ágil SCRUM, y en el desarrollo de software se utilizó la arquitectura web MVC, el lenguaje de programación PHP y el motor de base de datos MYSQL

Ama Llulla: análisis del medio de fact-checking y contenido de sus mensajes de Instagram

A partir de la pandemia del Covid-19, la proliferación de noticias falsas ha sido un problema recurrente. En este contexto, el fact checking o periodismo de verificación de datos surge como la alternativa idónea para dar frente a esta problemática. En Perú, son pocos los medios especializados en esta vertiente periodística, siendo la red Ama Llulla una de las propuestas más recientes. El presente trabajo de investigación tiene como objetivo principal describir la metodología de verificación de datos y la organización interna de Ama Llulla y cómo se articulan los mensajes a través de su cuenta de Instagram. Para ello, se optó por una metodología mixta conformada por entrevistas semi estructuradas a los periodistas e integrantes de la red, y el análisis de contenido con enfoque cuantitativo para el estudio de las publicaciones de su cuenta de Instagram.Since the Covid-19 pandemic, the proliferation of fake news has been a recurring issue. In this context, fact checking journalism has emerged as the ideal alternative to deal with this problem. In Peru, there are few media specialized in this type of journalism; however, one of the most recent proposals is the Ama Llulla network. The main objective of this research is to describe the methodology of data verification and the internal organization of Ama Llulla and how messages are articulated through its Instagram account. For this purpose, a mixed methodology was chosen, consisting of semi-structured interviews with journalists, members of the network, and content analysis with a quantitative approach for the study of the publications of its Instagram account