1,720,961 research outputs found
Corrigendum to “Periodic rhomboidal cells for symmetry-preserving homogenization and isotropic metamaterials” [Mech. Res. Commun. 126 (2022) 104001] (Mechanics Research Communications (2022) 126, (S0093641322001331), (10.1016/j.mechrescom.2022.104001))
No full textWe correct a mistake in the coefficients of a transformation matrix and accordingly update the subsequent calculations and the conclusions of our paper (Giusteri and Penta, 2022). We conclude that arrangements of spherical inclusions of isotropic materials in an isotropic matrix based on a rhomboidal cell that generates the Face-Centered Cubic lattice produce effectively isotropic composites if and only if an additional condition is satisfied. This condition entails the vanishing of a single component of the effective elasticity matrix. In spite of numerical evidence, we could not prove that this condition is always satisfied
A multi-scale method for complex flows of non-Newtonian fluids
Full text linkWe introduce a new heterogeneous multi-scale method for the simulation of flows of non-Newtonian fluids in general geometries and present its application to paradigmatic two-dimensional flows of polymeric fluids. Our method combines micro-scale data from non-equilibrium molecular dynamics (NEMD) with macro-scale continuum equations to achieve a data-driven prediction of complex flows. At the continuum level, the method is model-free, since the Cauchy stress tensor is determined locally in space and time from NEMD data. The modelling effort is thus limited to the identification of suitable interaction potentials at the micro-scale. Compared to previous proposals, our approach takes into account the fact that the material response can depend strongly on the local flow type and we show that this is a necessary feature to correctly capture the macroscopic dynamics. In particular, we highlight the importance of extensional rheology in simulating generic flows of polymeric fluids
Viscoelasticity, logarithmic stresses, and tensorial transport equations
Full text linkWe introduce models for viscoelastic materials, both solids and fluids, based on
logarithmic stresses to capture the elastic contribution to the material response.
The matrix logarithm allows to link the measures of strain, that naturally belong
to a multiplicative group of linear transformations, to stresses, that are additive
elements of a linear space of tensors. As regards the viscous stresses, we simply
assume a Newtonian constitutive law, but the presence of elasticity and plastic relaxation makes the materials non-Newtonian. Our aim is to discuss the
existence of weak solutions for the corresponding systems of partial differential
equations in the nonlinear large-deformation regime. The main difficulties arise
in the analysis of the transport equations necessary to describe the evolution
of tensorial measures of strain. For the solid model, we only need to consider
the equation for the left Cauchy–Green tensor, while for the fluid model, we
add an evolution equation for the elastically-relaxed strain. Due to the tensorial
nature of the fields, available techniques cannot be applied to the analysis of such
transport equations. To cope with this, we introduce the notion of charted weak
solution, based on non-standard a priori estimates, that lead to a global-in-time
existence of solutions for the viscoelastic models in the natural functional setting
associated with the energy inequality
Shape instabilities driven by topological defects in nematic polymer networks
Full text linkLiquid crystalline networks (LCNs) are stimuli-responsive materials formed from polymeric chains cross-linked with rod-like mesogenic segments, which, in the nematic phase, align along a non-polar director. A key characteristic of these nematic systems is the existence of singularities in the director field, known as topological defects or disclinations, and classified by their topological charge. In this study, we address the open question of modeling theoretically the coupling between mesogens disclination and polymeric network by providing a mathematical framework describing the out-of-plane shape changes of initially flat LCN sheets containing a central topological defect. Adopting a variational approach, we define an energy associated with the deformations consisting of two contributions: an elastic energy term accounting for spatial director variations, and a strain-energy function describing the elastic response of the polymer network. The interplay between nematic elasticity, which seeks to minimize distortions in the director field, variations in the degree of order, with the consequent tendency of monomers in the polymer chains to distribute anisotropically in response to an external stimulus, and mechanical stiffness, which resists deformation, determines the resulting morphology. We analyze the transition to instability of the ground-state flat configuration and characterize the corresponding buckling modes
Simulation of viscoelastic Cosserat rods based on the geometrically exact dynamics of special Euclidean strands
Full text linkWe propose a method for the description and simulation of the nonlinear dynamics of slender structures modeled as Cosserat rods. It is based on interpreting the strains and the generalized velocities of the cross sections as basic variables and elements of the special Euclidean algebra. This perspective emerges naturally from the evolution equations for strands, that are one-dimensional submanifolds, of the special Euclidean group. The discretization of the corresponding equations for the three-dimensional motion of a Cosserat rod is performed, in space, by using a staggered grid. The time evolution is then approximated with a semi-implicit method. Within this approach, we can easily include dissipative effects due to both the action of external forces and the presence of internal mechanical dissipation. The comparison with results obtained with different schemes shows the effectiveness of the proposed method, which is able to provide very good predictions of nonlinear dynamical effects and shows competitive computation times also as an energy-minimizing method to treat static problems
Three-dimensional nonsimple viscous liquids dragged by one-dimensional immersed bodies
No full textWe model the interaction of one-dimensional moving structures with a surrounding three-dimensional
fluid, physically close to a Newtonian liquid. The interaction is the adherence of the fluid to the immersed
structures, which drag it while moving as rigid bodies. To get solutions of the dynamical problem, we need
a model of viscous fluid slightly more general than the Newtonian one, in which the Cauchy stress tensor
depends upon higher-order derivatives of the velocity field. Assuming reasonable hypotheses on the
motion of the one-dimensional rigid bodies, existence and uniqueness of the solution for the dynamical
problem can be proved
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Efficient light harvesting and photon sensing via engineered cooperative effects
Full text linkEfficient devices for light harvesting and photon sensing are fundamental building blocks of basic energy science and many essential technologies. Recent efforts have turned to biomimicry to design the next generation of light-capturing devices, partially fueled by an appreciation of the fantastic efficiency of the initial stages of natural photosynthetic systems at capturing photons. In such systems extended excitonic states are thought to play a fundamental functional role, inducing cooperative coherent effects, such as superabsorption of light and supertransfer of photoexcitations. Inspired by this observation, we design an artificial light-harvesting and photodetection device that maximally harnesses cooperative effects to enhance efficiency. The design relies on separating absorption and transfer processes (energetically and spatially) in order to overcome the fundamental obstacle to exploiting cooperative effects to enhance light capture: the enhanced emission processes that accompany superabsorption. This engineered separation of processes greatly improves the efficiency and the scalability of the system
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
- …
