1,421 research outputs found
Nanoscale buckling deformation in layered copolymer materials
In layered materials, a common mode of deformation involves buckling of the
layers under tensile deformation in the direction perpendicular to the layers.
The instability mechanism, which operates in elastic materials from geological
to nanometer scales, involves the elastic contrast between different layers. In
a regular stacking of "hard" and "soft" layers, the tensile stress is first
accommodated by a large deformation of the soft layers. The inhibited Poisson
contraction results in a compressive stress in the direction transverse to the
tensile deformation axis. The hard layers sustain this transverse compression
until buckling takes place and results in an undulated structure. Using
molecular simulations, we demonstrate this scenario for a material made of
triblock copolymers. The buckling deformation is observed to take place at the
nanoscale, at a wavelength that depends on strain rate. In contrast to what is
commonly assumed, the wavelength of the undulation is not determined by defects
in the microstructure. Rather, it results from kinetic effects, with a
competition between the rate of strain and the growth rate of the instability.
http://www.pnas.org/content/early/2011/12/23/1111367109.abstrac
Isogeometric analysis of fluid-saturated porous media including flow in the cracks
An isogeometric model is developed for the analysis of fluid transport in pre-existing faults or cracks that are embedded in a fluid-saturated deformable porous medium. Flow of the interstitial fluid in the porous medium and fluid transport in the discontinuities are accounted for and are coupled. The modelling of a fluid-saturated porous medium in general requires the interpolation of the displacements of the solid to be one order higher than that of the pressure of the interstitial fluid. Using order elevation and Bézier projection, a consistent procedure has been developed to accomplish this in an isogeometric framework. Particular attention has also been given to the spatial integration along the isogeometric interface element in order to suppress traction oscillations that can arise for certain integration rules when a relatively high dummy stiffness is used in a poromechanical model. © 2016 The Authors. International Journal for Numerical Methods in Engineering Published by John Wiley & Sons Ltd
Enhacement in the dymanic response of a viscoelastic fluid flowing through a longitudinally vibrating tube
We analyzed effects of elasticity on the dynamics of fluids in porous media
by studying a flow of a Maxwell fluid in a tube, which oscillates
longitudinally and is subject to oscillatory pressure gradient. The present
study investigates novelties brought about into the classic Biot's theory of
propagation of elastic waves in a fluid-saturated porous solid by inclusion of
non-Newtonian effects that are important, for example, for hydrocarbons. Using
the time Fourier transform and transforming the problem into the frequency
domain, we calculated: (A) the dynamic permeability and (B) the function
that measures the deviation from Poiseuille flow friction as a
function of frequency parameter . This provides a more complete theory
of flow of Maxwell fluid through the longitudinally oscillating cylindrical
tube with the oscillating pressure gradient, which has important practical
applications. This study has clearly shown transition from dissipative to
elastic regime in which sharp enhancements (resonances) of the flow are found
Low level of virological failure and drug resistance among patients receiving antiretroviral treatment under programme conditions in Maputo, Mozambique
Mexico AIDS Conference 200
The Biot-Savart operator and electrodynamics on subdomains of the three-sphere
We study steady-state magnetic fields in the geometric setting of positive
curvature on subdomains of the three-dimensional sphere. By generalizing the
Biot-Savart law to an integral operator BS acting on all vector fields, we show
that electrodynamics in such a setting behaves rather similarly to Euclidean
electrodynamics. For instance, for current J and magnetic field BS(J), we show
that Maxwell's equations naturally hold. In all instances, the formulas we give
are geometrically meaningful: they are preserved by orientation-preserving
isometries of the three-sphere.
This article describes several properties of BS: we show it is self-adjoint,
bounded, and extends to a compact operator on a Hilbert space. For vector
fields that act like currents, we prove the curl operator is a left inverse to
BS; thus the Biot-Savart operator is important in the study of curl
eigenvalues, with applications to energy-minimization problems in geometry and
physics. We conclude with two examples, which indicate our bounds are typically
within an order of magnitude of being sharp.Comment: 24 pages (was 28 pages) Revised to include a new introduction, a
detailed example, and results about helicity; other changes for readabilit
A simple derivation of the effective stress coefficient for seismic velocities in porous rocks
Analysis of coupled heat and moisture transfer in masonry structures
Evaluation of effective or macroscopic coefficients of thermal conductivity
under coupled heat and moisture transfer is presented. The paper first gives a
detailed summary on the solution of a simple steady state heat conduction
problem with an emphasis on various types of boundary conditions applied to the
representative volume element -- a periodic unit cell. Since the results
essentially suggest no superiority of any type of boundary conditions, the
paper proceeds with the coupled nonlinear heat and moisture problem subjecting
the selected representative volume element to the prescribed macroscopically
uniform heat flux. This allows for a direct use of the academic or commercially
available codes. Here, the presented results are derived with the help of the
SIFEL (SIimple Finite Elements) system.Comment: 23 pages, 11 figure
Shakedown for slab track substructures with stiffness variation
In this paper, shakedown analyses are carried out to predict the long-term response of slab track substructures under repeated moving train loads. The train loads are converted into a distributed moving load on the substructure surface by using a simplified track analysis. Based on Melan’s static shakedown theorem, a well-established shakedown analysis method is extended to determine shakedown limits of the slab track substructures. The influence of a linearly increasing stiffness modulus on the shakedown limits is considered by conducting finite- element analysis with a user-defined material. It is found that a rise in stiffness modulus or stiffness variation ratio can either increase or decrease the shakedown limit, depending on the competitive effects of the two mechanisms. Furthermore, the subgrade thickness determines the dominant mechanism
On the fixed-stress split scheme as smoother in multigrid methods for coupling flow and geomechanics
The fixed-stress split method has been widely used as solution method in the coupling of flow and geomechanics. In this work, we analyze the behavior of an inexact version of this algorithm as smoother within a geometric multigrid method, in order to obtain an efficient monolithic solver for Biot's problem. This solver combines the advantages of being a fully coupled method, with the benefit of decoupling the flow and the mechanics part in the smoothing algorithm. Moreover, the fixed-stress split smoother is based on the physics of the problem, and therefore all parameters involved in the relaxation are based on the physical properties of the medium and are given a priori. A local Fourier analysis is applied to study the convergence of the multigrid method and to support the good convergence results obtained. The proposed geometric multigrid algorithm is used to solve several tests in semi-structured triangular grids, in order to show the good behavior of the method and its practical utility
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