Fluid-solid transition in unsteady, homogeneous, granular shear flows

Discrete element numerical simulations of unsteady, homogeneous shear flows have been performed by instantly applying a constant shear rate to a random, static, isotropic assembly of identical, soft, frictional spheres at either zero or finite pressure by keeping constant the solid volume fraction until the steady state is reached. If the system is slowly sheared, or, equivalently, if the particles are sufficiently rigid, the granular material exhibits either large or small fluctuations in the evolving pressure, depending whether the average number of contacts per particle (coordination number) is less or larger than a critical value. The amplitude of the pressure fluctuations is rate-dependent when the coordination number is less than the critical and rate-independent otherwise, signatures of fluid-like and solid-like behaviour, respectively. The same critical coordination number has been previously found to represent the minimum value at which rate-independent components of the stresses develop in steady, simple shearing and the jamming transition in isotropic random packings. The observed complex behaviour of the measured pressure in the fluid-solid transition clearly suggests the need for incorporating in a nontrivial way the coordination number, the solid volume fraction, the particle stiffness and the intensity of the particle agitation in constitutive models for the onset and the arrest of granular flows.Comment: 20 pages, 14 figures, submitted to Granular Matte

Adorno et le projet d'une théorie dialectique de l'oeuvre d'art

Le projet d’une théorie dialectique de l’œuvre d’art est sans doute un des plus complexes que Theodor Adorno ait tenu en haleine, projet qui table sur les horizons théoriques les plus diversifiés du philosophe de Francfort. Il ne s’agit donc pas ici d’en résumer la pertinence et les prémisses, de rendre justice à sa pleine complexité. Ce que je propose bien plutôt de faire c’est de développer trois façons inter-reliées de comprendre ce projet. En partant d’abord de la situation de l’esthétique contemporaine à Adorno, nous voudrions regarder en face la solution qu’offre la Théorie esthétique pour revitaliser l’esthétique philosophique. Il s’agit donc d’abord de questionner les catégories traditionnelles de l’esthétique qui semblent inadéquates à la réalité de l’art moderne « à l’époque où il devient objet de réflexion ». Le second aspect concerne l’interprétation et la méthode de présentation. « L’essai comme forme » qu’Adorno exécute et développe de pair avec la tendance parataxique de la poésie d’Hölderlin est l’objet des Notes sur la littérature. Ces travaux font écho à des notions clefs de la Théorie esthétique, déjà en germes dans « L’actualité de la philosophie » : l’interprétation, le contenu de vérité et le caractère énigmatique. Je propose finalement de restituer une certaine évolution du matériau-dissonance, ce qui recoupe et « matérialise » les deux premières perspectives.The project of a dialectical theory of the artwork is probably one of the most complex aspects of Theodor Adorno’s thought. The aim of this thesis is neither to summarize this project nor to do justice to its full complexity. What I propose, more specifically, is to present three interrelated ways of understanding Adorno’s project. The first concerns the aim and relevance of contemporary philosophical aesthetics. The question here is how Adorno’s Aesthetic Theory contributes to a revitalization of philosophical aesthetics. By juxtaposing traditional categories, which seem inappropriate to the experience of modern art, and contemporary aesthetic experience, I show how Adorno’s negative dialectical philosophy provides a relay of sorts between the traditional and the modern, made necessary by modern art itself "at a time when it is becoming an object of reflection". The second perspective concerns questions of interpretation and presentation, centered on Adorno’s explorations of the essay as “form” and the concept of parataxis, as discussed in Notes to Literature. Here I focus on how these explorations intersect with key concepts in his Aesthetic Theory and his early essay on “The Actuality of Philosophy”: interpretation, truth content, and enigmaticity. Finally, I propose a more specific reflection on dissonance that illustrates and “materializes” aspects of the first two perspectives

Plane shear flows of frictionless spheres: Kinetic theory and 3D soft-sphere discrete element method simulations

We use existing 3D Discrete Element simulations of simple shear flows of spheres to evaluate the radial distribution function at contact that enables kinetic theory to correctly predict the pressure and the shear stress, for different values of the collisional coefficient of restitution. Then, we perform 3D Discrete Element simulations of plane flows of frictionless, inelastic spheres, sheared between walls made bumpy by gluing particles in a regular array, at fixed average volume fraction and distance between the walls. The results of the numerical simulations are used to derive boundary conditions appropriated in the cases of large and small bumpiness. Those boundary conditions are, then, employed to numerically integrate the differential equations of Extended Kinetic Theory, where the breaking of the molecular chaos assumption at volume fraction larger than 0.49 is taken into account in the expression of the dissipation rate. We show that the Extended Kinetic Theory is in very good agreement with the numerical simulations, even for coefficients of restitution as low as 0.50. When the bumpiness is increased, we observe that some of the flowing particles are stuck in the gaps between the wall spheres. As a consequence, the walls are more dissipative than expected, and the flows resemble simple shear flows, i.e., flows of rather constant volume fraction and granular temperature

Local cross-border cooperation as a territorial strategy for peripheral borderlands? The analysis of two study cases along the Eastern French-Spanish border using the territorialist approach

Der Beitrag befasst sich mit dem Territorialansatz in der Raumentwicklung zur Untersuchung der grenzübergreifenden Zusammenarbeit in zwei Gebieten an der östlichen französisch-spanischen Grenze. Es ist ein neuer, fachübergreifender und kulturorientierter Ansatz für Studien zur Raumentwicklung, der seine Wurzeln in der italienischen Geographieschule hat. Danach ist "Territorium" das Ergebnis eines lange währenden zyklischen Prozesses des Zusammenspiels zwischen menschlichen Siedlungen und der Umgebung, wobei jede Zivilisation eine Reihe von materiellen (Artefakte) und immateriellen (Bedeutungen, Werte und Symbole) Werten bewahrt, die die lokalen Beziehungen fördert und die lokale Identität (das Milieu) formt. Deshalb sehen wir Grenzregionen als komplizierte räumliche Einheiten, in denen geographische, sozio-ökonomische und kulturelle Verhaltensmuster durch die Entwicklung von Grenzfunktionen und Veränderungen der nationalen, internationalen und regionalen Politik beeinflusst worden sind. Sowohl formelle als auch informelle grenzüberschreitende Netzwerke tragen zur Erhaltung eines gemeinsamen lokalen Milieus bei. In Europa wird die grenzüberschreitende Zusammenarbeit durch politische, finanzielle und rechtliche Fördermaßnahmen gestärkt. Sie bietet für lokale Vermittler eine Möglichkeit zum Ausbau gemeinsamer Maßnahmen zur endogenen Stärkung des Grenzgebiets. Die Untersuchung zu Cerdanya zeigt, wie von der Gemeinde geleitete Initiativen, die auf lokalen Bedürfnissen und dem gemeinsamen räumlichen Kapital beruhen, eine machbare Alternative zur lokalen Entwicklung darstellen. Die Küste von Albera Marítima (CAM) dagegen liegt geographisch eher abseits, wobei das Fehlen grenzübergreifender lokaler Netzwerke einerseits und machbarer Projekte der grenzübergreifenden Zusammenarbeit andererseits seine geographische Abseitslage und den anthropischen Druck auf die sensible Küstenregion verschärfen.This contribution applies the Territorialist Approach (TA) in order to analyse cross-border cooperation in two areas located along the Eastern French-Spanish border. TA is an innovative, multi-disciplinary and culturally oriented approach for territorial studies, which is rooted in the Italian school of geography. According to this approach, "territory" is perceived as the result of a longstanding cyclical process of interaction between human settlements and the environment, whereby each civilization 'stores' a set of tangible (artefacts) as well as intangible (meanings, values and symbols) assets that drive local relations and shape the local identity (the milieu). Therefore, we regard cross-border territories as complex territorial units, wherein geographical, socioeconomic and cultural patterns have been affected by the evolution of border functions and by the changes in national, international and sub-national policies. In this sense, local cross-border networks, both formal and informal, contribute in keeping a shared local milieu. At the same time, in Europe, local Cross-Border Cooperation (CBC) is boosted by European integration processes by means of political, financial and legal support. It represents an opportunity for local agents to develop joint actions for the endogenous enhancement of the borderland. In this sense, the local milieu represents a very important factor for generating local development. The two case studies were conducted in Cerdanya and the Coast of Albera Marítima (CAM). Our analysis on Cerdanya demonstrates how community-led initiatives, which are based on local needs and the shared territorial capital, represent a feasible alternative for local development. The case of the Albera is geographically even more peripheral, wherein the absence of stable cross-border local networks, on the one hand, and of feasible CBC projects, on the other, exacerbates its geographic remoteness and the anthropic pressures on the fragile coastal environment

The relativistic Doppler effect: when a zero frequency shift or a red shift exists for sources approaching the observer

It is shown without making use of Lorentz transformation that there exists a phenomenon of relativistic zero-frequency shift in Doppler effect for a plane wave in free space, observed in two inertial frames of relative motion, and the zero shift takes place at a maximum aberration of light. When it is applied to analysis of a moving point light source, two unconventional physical implications result: (1) a light source, when it is approaching (moving closer to) the observer, may cause a red shift; (2) a zero-frequency-shift observation does not necessarily mean that the light source is not moving closer, and in contrast, the light source may be moving closer to the observer at a high speed. This fundamental result of special relativity may provide an alternative way to experimentally examine the principle of relativity, and might have a significant application in astrophysics.Comment: (1) The words:"...for a plane wave in free space, observed in two inertial frames of relative motion,...the definitions of 'approaching' and 'receding' do not apply. When the zero-shift effect is applied to approximate analysis..." are added on p. 7. (2) "Reply to Comment by Sfarti" is added on p. 9. 7 figures, 9 page

Dense, collisional, shearing flows of compliant spheres

We outline the development of theory to describe, dense, collisional shearing flows of identical compliant spheres. We begin with two simple theories: one for rigid, nearly elastic spheres that interact through instantaneous, binary collisions; the other for compliant spheres that interact through multiple, enduring contacts. We then join the two extremes by adding compliance to the collisions and collisions to the spheres in enduring contact. Finally, we compare the predictions of the resulting theory with the results of discrete numerical simulations of steady, homogeneous shearing of compliant frictional spheres

Dense, inhomogeneous shearing flows of spheres

We make use of recent extensions of kinetic theory of granular gases to include the role of particle stiffness in collisions to deal with pressure-imposed shearing flows between bumpy planes in relative motion, in which the solid volume fraction and the intensity of the velocity fluctuations are not uniformly distributed in the domain. As in previous numerical simulations on the flow of disks in an annular shear cell, we obtain an exponential velocity profile in the region where the volume fraction exceeds the critical value at which a rate-independent contribution to the stresses arises. We also show that the thickness of the inertial region, where the solid volume fraction is less than the critical value, and the shear stress at the moving boundary are determined functions of the relative velocity of the boundaries