Geometrical properties of rigid frictionless granular packings as a function of particle size and shape

Three-dimensional discrete numerical simulation is used to investigate the properties of close-packed frictionless granular assemblies as a function of particle polydispersity and shape. Unlike some experimental results, simulations show that disordered packings of pinacoids (eight-face convex polyhedron) achieve higher solid fraction values than amorphous packings of spherical or rounded particles, thus fulfilling the analogue of Ulam's conjecture stated by Jiao and co-workers for random packings [Y. Jiao and S. Torquato, Phys. Rev. E $\textbf{84}$, $041309$ ($2011$)]. This seeming discrepancy between experimental and numerical results is believed to lie with difficulties in overcoming interparticle friction through experimental densification processes. Moreover, solid fraction is shown to increase further with bidispersity and peak when the volume proportion of small particles reaches $30\%$. Contrarywise, substituting up to $50\%$ of flat pinacoids for isometric ones yields solid fraction decrease, especially when flat particles are also elongated. Nevertheless, particle shape seems to play a minor role on packing solid fraction compared to polydispersity. Additional investigations focused on the packing microstructure confirm that pinacoid packings fulfill the isostatic conjecture and that they are free of order except beyond $30$ to $50\%$ of flat or flat \& elongated polyhedra in the packing. This order increase progressively takes the form of a nematic phase caused by the reorientation of flat or flat \& elongated particles to minimize the packing potential energy. Simultaneously, this reorientation seems to increase the solid fraction value slightly above the maximum achieved by monodisperse isometric pinacoids, as well as the coordination number. Finally, partial substitution of elongated pinacoids for isometric ones has limited effect on packing solid fraction or order.Comment: 12 figures, 12 page

Continuous collision detection for ellipsoids

We present an accurate and efficient algorithm for continuous collision detection between two moving ellipsoids. We start with a highly optimized implementation of interference testing between two stationary ellipsoids based on an algebraic condition described in terms of the signs of roots of the characteristic equation of two ellipsoids. Then we derive a time-dependent characteristic equation for two moving ellipsoids, which enables us to develop a real-time algorithm for computing the time intervals in which two moving ellipsoids collide. The effectiveness of our approach is demonstrated with several practical examples. © 2006 IEEE.published_or_final_versio

3D morphological modeling of concrete using multiscale Poisson polyhedra

Supplementary file (library of Poisson polyhedra) available at: https://people.cmm.minesparis.psl.eu/users/willot/PoissonLibrary.tgzInternational audienceThis paper aims at developing a random morphological model for concrete mi-crostructures. A 3D image of concrete is obtained by micro-tomography and is used in conjunction with the concrete formulation to build and validate the model through morphological measurements. The morphological model is made up of two phases, cor-responding to the matrix, or cement paste and to the aggregates. The set of aggregates in the sample is modeled as a combination of Poisson polyhedra of different scales. An algorithm is introduced to generate polyhedra packings in the continuum space. The latter is validated with morphological measurements

The topology of fullerenes

Fullerenes are carbon molecules that form polyhedral cages. Their bond structures are exactly the planar cubic graphs that have only pentagon and hexagon faces. Strikingly, a number of chemical properties of a fullerene can be derived from its graph structure. A rich mathematics of cubic planar graphs and fullerene graphs has grown since they were studied by Goldberg, Coxeter, and others in the early 20th century, and many mathematical properties of fullerenes have found simple and beautiful solutions. Yet many interesting chemical and mathematical problems in the field remain open. In this paper, we present a general overview of recent topological and graph theoretical developments in fullerene research over the past two decades, describing both solved and open problems. WIREs Comput Mol Sci 2015, 5:96–145. doi: 10.1002/wcms.1207 Conflict of interest: The authors have declared no conflicts of interest for this article. For further resources related to this article, please visit the WIREs website

The Minimal Volume of Simplices Containing a Convex Body

Let K⊂ Rn be a convex body with barycenter at the origin. We show there is a simplex S⊂ K having also barycenter at the origin such that (vol(S)vol(K))1/n≥cn, where c> 0 is an absolute constant. This is achieved using stochastic geometric techniques. Precisely, if K is in isotropic position, we present a method to find centered simplices verifying the above bound that works with extremely high probability. By duality, given a convex body K⊂ Rn we show there is a simplex S enclosing Kwith the same barycenter such that(vol(S)vol(K))1/n≤dn,for some absolute constant d> 0. Up to the constant, the estimate cannot be lessened.Fil: Galicer, Daniel Eric. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Investigaciones Matemáticas "Luis A. Santaló". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Investigaciones Matemáticas "Luis A. Santaló"; ArgentinaFil: Merzbacher, Diego Mariano. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Investigaciones Matemáticas "Luis A. Santaló". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Investigaciones Matemáticas "Luis A. Santaló"; ArgentinaFil: Pinasco, Damian. Universidad Torcuato Di Tella; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

The normal parameterization and its application to collision detection

Collision detection is a central task in the simulation of multibody systems. Depending on the description of the geometry, there are many efficient algorithms to address this need. A widespread approach is the common normal concept: potential contact points on opposing surfaces have antiparallel normal vectors. However, this approach leads to implicit equations that require iterative solutions when the geometries are described by implicit functions or the common parameterizations. We introduce the normal parameterization to describe the boundary of a strictly convex object as a function of the orientation of its normal vector. This parameterization depends on a scalar function, the so-called generating potential from which all properties are derived: points on the boundary, continuity/differentiability of the boundary, curvature, offset curves or surfaces. An explicit solution for collisions with a planar counterpart is derived and four iterative algorithms for collision detection between two arbitrary objects with the normal parametrization are compared. The application of this approach for collision detection in multibody models is illustrated in a case study with two ellipsoids and several planes

Fully-Automated Verification of Linear Systems Using Inner- and Outer-Approximations of Reachable Sets

Reachability analysis is a formal method to guarantee safety of dynamical systems under the influence of uncertainties. A major bottleneck of all reachability algorithms is the requirement to adequately tune certain algorithm parameters such as the time step size, which requires expert knowledge. In this work, we solve this issue with a fully-automated reachability algorithm that tunes all algorithm parameters internally such that the reachable set enclosure satisfies a user-defined accuracy in terms of distance to the exact reachable set. Knowing the distance to the exact reachable set, an inner-approximation of the reachable set can be efficiently extracted from the outer-approximation using the Minkowski difference. Finally, we propose a novel verification algorithm that automatically refines the accuracy of the outer- and inner-approximation until specifications given by time-varying safe and unsafe sets can either be verified or falsified. The numerical evaluation demonstrates that our verification algorithm successfully verifies or falsifies benchmarks from different domains without any requirement for manual tuning.Comment: 16 page

Voids and the large-scale structure of the Universe

The Cosmic Web describes the distribution of matter on the largest scales of the Universe. It is composed of dense regions full of galaxies, long filamentary structures and low density voids. In this thesis we introduce the Cosmic Web and we focus on the description of voids, large underdense regions pratically devoid of galaxies occupying the major volume of the Universe. Voids are a key component of the Cosmic Web, since their pristine environment is an important testing ground for our understanding of the importance of environmental influences on the evolution of galaxies. Then we introduce Voronoi and Delaunay tessellations, two random tessellation methods. Tessellation methods are used to divide a d-dimensional space into polytopes covering the whole space without overlapping. Voronoi and Delaunay tessellations are the basis of the DTFE method, useful when we want to compute a continuous field from a large point sample. Finally, we briefly discuss and compare two void finders: ZOBOV and WVF, whose aim is to find density depressions in a set of points, without introducing any free parameters

Visual perception of solid shape from occluding contours

The relative motion of object and observer induces a motion field in the observer's visual image that is smooth everywhere except along the object's occluding contours. Thus, occluding contours and smooth motion fields can be viewed as complementary and as separate sources of information about an object's shape. I studied how the human visual system perceives solid shape from the occluding contours of rotating objects and from the smooth motion field induced by moving planar surface patches.I propose a three-stage model for the perception of solid shape from the occluding contours of a rotating object. First, the object's motion is determined. I argue that this is only possible using points of correspondence and only when the object's axis of rotation is frontoparallel. In the second stage, the motion field along the contour is used to compute relative depth and surface curvature along the rim, the contour's pre-image. Third, local shape descriptors are propagated inside the figure to yield a global percept of solid shape. To determine which shape descriptors are computed by human subjects, I used a novel task in which subjects have to discriminate between flat ellipses and solid ellipsoids with varying thickness. I found that discriminability is proportional to the inverse of radial curvature but is not proportional to Gaussian or mean curvature. Certain slants of the axis of rotation decrease discriminability. Subjects who could discriminate ellipsoids and ellipses perceived the ellipsoids' angular velocity more veridically than did subjects who could not discriminate the two.Any smooth motion field can locally be described by divergence, curl, and deformation. If the motion field is induced by a rotating plane, the amount of deformation is proportional to the plane's slant and its angular velocity. Similarly, for translating planes, deformation is proportional to slant and image motion. Slant judgments of human observers were to a first-order approximation proportional to deformation per se, that is, observers do not take object motion into account. Recent psychophysical evidence suggests that human subjects need motion discontinuities for this. Thus, contours might be necessary to correctly perceive slant from smooth motion fields