arXiv subjects

I. Goldhirsch

Publications and source records attributed to I. Goldhirsch.

8 recordsLinked to original sources

arXiv2013

Application of Bogolyubov's approach to the derivation of kinetic equations for dissipative systems

The main goal of the present article is to extend the Bogolyubov method for deriving kinetic equations to dissipative many-body systems. The basic conjecture underlying the Bogolyubov approach is the functional hypothesis, according to which, the many-particle distribution functions are assumed to be functionals of the one-particle distribution function on kinetic time scales. Another ingredient in the Bogolyubov approach is the principle of the spatial weakening of correlations, which reflects statistical independence of physical values at distant spatial points. One can consider it as a reasonable mixing property of many-particle distribution functions. The motivation behind the generalization of Bogolyubov's approach to (classical) many-body dissipative systems is the wish to describe the dynamics of granular systems, in particular granular fluids. To this end we first define a general dissipative fluid through a dissipation function, thereby generalizing the commonly employed models for granular fluids. Using the Bogolyubov functional hypothesis we show how a reduction of the pertinent BBGKY hierarchy can be achieved. The method is then employed to cases which can be treated perturbatively, such as those in which the interactions are weak or the dissipation is small or the particle density is small. Kinetic descriptions are obtained in all of these limiting cases. As a test case, we show that the Bogolyubov method begets the now standard inelastic Boltzmann equation for dilute monodisperse collections of spheres whose collisions are characterized by a fixed coefficient of normal restitution. Possible further applications and implications are discussed.

cond-mat.stat-mech↗

arXiv2007

Effects of Friction and Disorder on the Quasi-Static Response of Granular Solids to a Localized Force

The response to a localized force provides a sensitive test for different models of stress transmission in granular solids. The elasto-plastic models traditionally used by engineers have been challenged by theoretical and experimental results which suggest a wave-like (hyperbolic) propagation of the stress, as opposed to the elliptic equations of static elasticity. Numerical simulations of two-dimensional granular systems subject to a localized external force are employed to examine the nature of stress transmission in these systems as a function of the magnitude of the applied force, the frictional parameters and the disorder (polydispersity). The results indicate that in large systems (typically considered by engineers), the response is close to that predicted by isotropic elasticity whereas the response of small systems (or when sufficiently large forces are applied) is strongly anisotropic. In the latter case the applied force induces changes in the contact network accompanied by frictional sliding. The larger the coefficient of static friction, the more extended is the range of forces for which the response is elastic and the smaller the anisotropy. Increasing the degree of polydispersity (for the range studied, up to 25%) decreases the range of elastic response. This article is an extension of a previously published letter [1].

cond-mat.soft↗

arXiv2007

The classical granular temperature and slightly beyond

One goal of this paper is to discuss the classical definition of granular temperature as an extension of its thermodynamic equivalent and a useful concept which provides an important characterization of fluidized granular matter. Following a review of some basic concepts and techniques (with emphasis on fundamental issues) we present new results for a system that can exhibit strong violations of equipartition, yet is amenable to description by classical granular hydrodynamics, namely a binary granular gas mixture. A second goal of this article is to present a result that pertains to dense granular and molecular solids alike, namely the existence of a correction to the elastic energy which is related to the heat flux in the equations of continuum mechanics. The latter is of the same (second) order in the strain as the elastic energy. Although recent definitions of temperatures for granular matter, glasses and other disordered many-body systems are not within the scope of this article we do make several general comments on this subject in the closing section.

cond-mat.stat-mech↗

arXiv2006

Strongly inelastic granular gases

The expansion of the velocity distribution function for the homogeneous cooling state (HCS) in a Sonine polynomial series around a Maxwellian is shown to be divergent, though Borel resummable. A convergent expansion for the HCS has been devised and employed to obtain the HCS velocity distribution function and (using it) the linear transport coefficients for a three dimensional monodisperse granular gas of smooth inelastic spheres, for all physical values of the coefficient of normal restitution. The results are in very good agreement with findings of DSMC simulations.

cond-mat.soft↗

arXiv2006

Scale separation in granular packings: stress plateaus and fluctuations

It is demonstrated, by numerical simulations of a 2D assembly of polydisperse disks, that there exists a range (plateau) of coarse graining scales for which the stress tensor field in a granular solid is nearly resolution independent, thereby enabling an `objective' definition of this field. Expectedly, it is not the mere size of the the system but the (related) magnitudes of the gradients that determine the widths of the plateaus. Ensemble averaging (even over `small' ensembles) extends the widths of the plateaus to sub-particle scales. The fluctuations within the ensemble are studied as well. Both the response to homogeneous forcing and to an external compressive localized load (and gravity) are studied. Implications to small solid systems and constitutive relations are briefly discussed.

cond-mat.stat-mech↗

arXiv2002

On the Microscopic Foundations of Elasticity

The modeling of the elastic properties of disordered or nanoscale solids requires the foundations of the theory of elasticity to be revisited, as one explores scales at which this theory may no longer hold. The only cases for which microscopically based derivations of elasticity are documented are (nearly) uniformly strained lattices. A microscopic approach to elasticity is proposed. As a first step, microscopically exact expressions for the displacement, strain and stress fields are derived. Conditions under which linear elastic constitutive relations hold are studied theoretically and numerically. It turns out that standard continuum elasticity is not self-evident, and applies only above certain spatial scales, which depend on details of the considered system and boundary conditions. Possible relevance to granular materials is briefly discussed.

cond-mat.mtrl-sci↗

arXiv2002

Granular and Nano-Elasticity

The modeling of the elastic properties of granular or nanoscale systems requires the foundations of the theory of elasticity to be revisited, as one explores scales at which this theory may no longer hold. The only cases for which a microscopic justification of elasticity exists are (nearly) uniformly strained lattices. A microscopic theory of elasticity, as well as simulations, reveal that standard continuum elasticity applies only at sufficiently large scales (typically 100 particle diameters). Interestingly, force chains, which have been observed in experiments on granular systems, and attributed to non-elastic effects, are shown to exist in systems composed of harmonically interacting constituents. The corresponding stress field, which is a continuum mechanical (averaged) entity, exhibits no chain structures even at near-microscopic resolutions, but it does reflect macroscopic anisotropy, when present.

cond-mat.stat-mech↗

arXiv2001

Force Chains, Microelasticity and Macroelasticity

It has been claimed that quasistatic granular materials, as well as nanoscale materials, exhibit departures from elasticity even at small loadings. It is demonstrated, using 2D and 3D models with interparticle harmonic interactions, that such departures are expected at small scales [below O(100) particle diameters], at which continuum elasticity is invalid, and vanish at large scales. The models exhibit force chains on small scales, and force and stress distributions which agree with experimental findings. Effects of anisotropy, disorder and boundary conditions are discussed as well.

cond-mat.stat-mech↗