828 research outputs found
The dynamics of a shear band
A shear band of finite length, formed inside a ductile material at a certain
stage of a con- tinued homogeneous strain, provides a dynamic perturbation to
an incident wave field, which strongly influences the dynamics of the material
and affects its path to failure. The investigation of this perturbation is
presented for a ductile metal, with reference to the incremental mechanics of a
material obeying the J 2-deformation theory of plasticity (a special form of
prestressed, elastic, anisotropic, and incompressible solid). The treatment
originates from the derivation of integral representations relating the
incremental mechan- ical fields at every point of the medium to the incremental
displacement jump across the shear band faces, generated by an impinging wave.
The boundary integral equations (under the plane strain assumption) are
numerically approached through a collocation technique, which keeps into
account the singularity at the shear band tips and permits the analysis of an
incident wave impinging a shear band. It is shown that the presence of the
shear band induces a resonance, visible in the incremental displacement field
and in the stress intensity factor at the shear band tips, which promotes shear
band growth. Moreover, the waves scattered by the shear band are shown to
generate a fine texture of vibrations, par- allel to the shear band line and
propagating at a long distance from it, but leaving a sort of conical shadow
zone, which emanates from the tips of the shear band
Discrete solution of the electrokinetic equations
We present a robust scheme for solving the electrokinetic equations. This
goal is achieved by combining the lattice-Boltzmann method (LB) with a discrete
solution of the convection-diffusion equation for the different charged and
neutral species that compose the fluid. The method is based on identifying the
elementary fluxes between nodes, which ensures the absence of spurious fluxes
in equilibrium. We show how the model is suitable to study electro-osmotic
flows. As an illustration, we show that, by introducing appropriate dynamic
rules in the presence of solid interfaces, we can compute the sedimentation
velocity (and hence the sedimentation potential) of a charged sphere. Our
approach does not assume linearization of the Poisson-Boltzmann equation and
allows us for a wide variation of the Peclet number.Comment: 24 pages, 7 figure
Quantifying the entropic cost of cellular growth control
We quantify the amount of regulation required to control growth in living
cells by a Maximum Entropy approach to the space of underlying metabolic states
described by genome-scale models. Results obtained for E. coli and human cells
are consistent with experiments and point to different regulatory strategies by
which growth can be fostered or repressed. Moreover we explicitly connect the
`inverse temperature' that controls MaxEnt distributions to the growth
dynamics, showing that the initial size of a colony may be crucial in
determining how an exponentially growing population organizes the phenotypic
space.Comment: 3 page
Quantitative constraint-based computational model of tumor-to-stroma coupling via lactate shuttle
Cancer cells utilize large amounts of ATP to sustain growth, relying primarily on non-oxidative,
fermentative pathways for its production. In many types of cancers this leads, even in the presence
of oxygen, to the secretion of carbon equivalents (usually in the form of lactate) in the cell’s
surroundings, a feature known as the Warburg effect. While the molecular basis of this phenomenon
are still to be elucidated, it is clear that the spilling of energy resources contributes to creating a
peculiar microenvironment for tumors, possibly characterized by a degree of toxicity. This suggests
that mechanisms for recycling the fermentation products (e.g. a lactate shuttle) may be active,
effectively inducing a mutually beneficial metabolic coupling between aberrant and non-aberrant
cells. Here we analyze this scenario through a large-scale in silico metabolic model of interacting
human cells. By going beyond the cell-autonomous description, we show that elementary physico-
chemical constraints indeed favor the establishment of such a coupling under very broad conditions.
The characterization we obtained by tuning the aberrant cell’s demand for ATP, amino-acids and
fatty acids and/or the imbalance in nutrient partitioning provides quantitative support to the idea
that synergistic multi-cell effects play a central role in cancer sustainmen
Plastically-driven variation of elastic stiffness in green bodies during powder compaction. Part II: Micromechanical modelling
A micromechanical approach is set-up to analyse the increase in elastic
stiffness related to development of plastic deformation (the elastoplastic
coupling concept) occurring during the compaction of a ceramic powder.
Numerical simulations on cubic (square for 2D) and hexagonal packings of
elastoplastic cylinders and spheres validate both the variation of the elastic
modulus with the forming pressure and the linear dependence of it on the
relative density as experimentally found in Part~I of this study, while the
dependence of the Poisson's ratio on the green's density is only qualitatively
explained
Constrained Mean Field Games Equilibria as Fixed Point of Random Lifting of Set-Valued Maps
We introduce an abstract framework for the study of general mean field game and mean field control problems. Given a multiagent system, its macroscopic description is provided by a time-depending probability measure, where at every instant of time the measure of a set represents the fraction of (microscopic) agents contained in it. The trajectories available to each of the microscopic agents are affected also by the overall state of the system. By using a suitable concept of random lift of set-valued maps, together with fixed point arguments, we are able to derive properties of the macroscopic description of the system from properties of the set-valued map expressing the admissible trajectories for the microscopical agents. We apply the results in the case in which the admissible trajectories of the agents are the minimizers of a suitable integral functional depending also from the macroscopic evolution of the system. Copyright (C) 2022 The Authors
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