Force chains and contact network topology in packings of elongated particles

By means of contact dynamic simulations, we investigate the contact network topology and force chains in two-dimensional packings of elongated particles modeled by rounded-cap rectangles. The morphology of large packings of elongated particles in quasistatic equilibrium is complex due to the combined effects of local nematic ordering of the particles and orientations of contacts between particles. We show that particle elongation affects force distributions and force/fabric anisotropy via various local structures allowed by steric exclusions and the requirement of force balance. As a result, the force distributions become increasingly broader as particles become more elongated. Interestingly, the weak force network transforms from a passive stabilizing agent with respect to strong force chains to an active force-transmitting network for the whole system. The strongest force chains are carried by side/side contacts oriented along the principal stress direction.Comment: Soumis a Physical Review

Stress-strain behavior and geometrical properties of packings of elongated particles

We present a numerical analysis of the effect of particle elongation on the quasistatic behavior of sheared granular media by means of the Contact Dynamics method. The particle shapes are rounded-cap rectangles characterized by their elongation. The macroscopic and microstructural properties of several packings subjected to biaxial compression are analyzed as a function of particle elongation. We find that the shear strength is an increasing linear function of elongation. Performing an additive decomposition of the stress tensor based on a harmonic approximation of the angular dependence of branch vectors, contact normals and forces, we show that the increasing mobilization of friction force and the associated anisotropy are key effects of particle elongation. These effects are correlated with partial nematic ordering of the particles which tend to be oriented perpendicular to the major principal stress direction and form side-to-side contacts. However, the force transmission is found to be mainly guided by cap-to-side contacts, which represent the largest fraction of contacts for the most elongated particles. Another interesting finding is that, in contrast to shear strength, the solid fraction first increases with particle elongation, but declines as the particles become more elongated. It is also remarkable that the coordination number does not follow this trend so that the packings of more elongated particles are looser but more strongly connected.Comment: Submited to Physical Review

Taphonomical and chronological studies of a concentration of European glass trade beads from Ashuapmushuan, Central Quebec (Canada)

As early as the sixteenth century, glass beads were among the several categories of goods brought from Europe to be traded to Northeastern North American Indians. Attempts to set up a chronological framework on the basis of the chemistry of these beads has been recently enhanced by the application of neutron activation analysis of turquoise blue glass beads from many sites in Southwestern Ontario and Central Quebec. Subsequently, in the Quebec site of Ashuapmushuan was found a collection of 515 beads excavated from a very restricted area, indicating that they pertain to a single depositional event. This suggestion is tested through taphonomical studies , in particular on the basis of the spatial distributions of the beads. Chronological placement is also assessed on the basis of neutron activation of 122 turquoise blue glass beads. These taphonomical and chronological observations lead to some challenging cultural interpretations

Force transmission in a packing of pentagonal particles

We perform a detailed analysis of the contact force network in a dense confined packing of pentagonal particles simulated by means of the contact dynamics method. The effect of particle shape is evidenced by comparing the data from pentagon packing and from a packing with identical characteristics except for the circular shape of the particles. A counterintuitive finding of this work is that, under steady shearing, the pentagon packing develops a lower structural anisotropy than the disk packing. We show that this weakness is compensated by a higher force anisotropy, leading to enhanced shear strength of the pentagon packing. We revisit "strong" and "weak" force networks in the pentagon packing, but our simulation data provide also evidence for a large class of "very weak" forces carried mainly by vertex-to-edge contacts. The strong force chains are mostly composed of edge-to-edge contacts with a marked zig-zag aspect and a decreasing exponential probability distribution as in a disk packing

Mimicking state diplomacy: The legitimizing strategies of unofficial diplomacies

Diplomacy and recognition play central roles in the conventional conferral of state legitimacy and functioning of the inter-state system. In broadening the diplomatic frame by stepping outside the conventional state-system, this paper brings a poststructuralist and performative toolkit to mimetic diplomatic practices. Adapting Bhabha’s notion of mimicry to diplomatic discourse, it demonstrates how non-state diplomacies draw on, mimic and intervene in the realm of formal political action in ways which both promote ‘official’ state diplomacy as an ideal and dilute its distinction from other, ‘unofficial’ diplomacies. In thereby examining the enactment of international diplomacy in unexpected spaces, this paper brings together three empirical studies: a Government-in-Exile, a religious community and micropatrias (self-declared parodic nations). In each of these cases, attention focuses on: discourses of recognition; sovereignty and legitimacy; the diplomatic relationships fostered and institutions of diplomacy constructed; and the strategic position of such diplomacy vis-à-vis the conventional state-system. Unpacking the relationship between legitimacy, recognition and diplomacy and exploring the tension between state-centric and non-state diplomatic practices, this paper foregrounds the points of connection between the official and the unofficial. As a result, this paper expands the analytical gaze of diplomacy studies while incorporating lessons from the margins into our understandings of legitimacy, recognition, statecraft and sovereignty

On knotted streamtubes in incompressible hydrodynamical flow and a restricted conserved quantity

For certain families of fluid flow, a new conserved quantity -- stream-helicity -- has been established.Using examples of linked and knotted streamtubes, it has been shown that stream-helicity does, in certain cases, entertain itself with a very precise topological meaning viz, measure of the degree of knottedness or linkage of streamtubes.As a consequence, stream-helicity emerges as a robust topological invariant.Comment: This extended version is the basically a more clarified version of the previous submission physics/0611166v

An Implicit Tensor-Mass solver on the GPU for soft bodies simulation

International audienceThe realistic and interactive simulation of deformable objects has become a challenge in Computer Graphics. In this paper, we propose a GPU implementation of the resolution of the mechanical equations, using a semi-implicit as well as an implicit integration scheme. At the contrary of the classical FEM approach, forces are directly computed at each node of the discretized objects, using the evaluation of the strain energy density of the elements. This approach allows to mix several mechanical behaviors in the same object. Results show a notable speedup of 30, especially in the case of complex scenes. Running times shows that this efficient implementation may contribute to make this model more popular for soft bodies simulations

Elastic behavior in Contact Dynamics of rigid particles

The systematic errors due to the practical implementation of the Contact Dynamics method for simulation of dense granular media are examined. It is shown that, using the usual iterative solver to simulate a chain of rigid particles, effective elasticity and sound propagation with a finite velocity occur. The characteristics of these phenomena are investigated analytically and numerically in order to assess the limits of applicability of this simulation method and to compare it with soft particle molecular dynamics.Comment: submitted to PRE, 7 pages, 6 figure