1,190 research outputs found
Optimization algorithms for the solution of the frictionless normal contact between rough surfaces
This paper revisits the fundamental equations for the solution of the
frictionless unilateral normal contact problem between a rough rigid surface
and a linear elastic half-plane using the boundary element method (BEM). After
recasting the resulting Linear Complementarity Problem (LCP) as a convex
quadratic program (QP) with nonnegative constraints, different optimization
algorithms are compared for its solution: (i) a Greedy method, based on
different solvers for the unconstrained linear system (Conjugate Gradient CG,
Gauss-Seidel, Cholesky factorization), (ii) a constrained CG algorithm, (iii)
the Alternating Direction Method of Multipliers (ADMM), and () the
Non-Negative Least Squares (NNLS) algorithm, possibly warm-started by
accelerated gradient projection steps or taking advantage of a loading history.
The latter method is two orders of magnitude faster than the Greedy CG method
and one order of magnitude faster than the constrained CG algorithm. Finally,
we propose another type of warm start based on a refined criterion for the
identification of the initial trial contact domain that can be used in
conjunction with all the previous optimization algorithms. This method, called
Cascade Multi-Resolution (CMR), takes advantage of physical considerations
regarding the scaling of the contact predictions by changing the surface
resolution. The method is very efficient and accurate when applied to real or
numerically generated rough surfaces, provided that their power spectral
density function is of power-law type, as in case of self-similar fractal
surfaces.Comment: 38 pages, 11 figure
Node-to-segment and node-to-surface interface finite elements for fracture mechanics
The topologies of existing interface elements used to discretize cohesive
cracks are such that they can be used to compute the relative displacements
(displacement discontinuities) of two opposing segments (in 2D) or of two
opposing facets (in 3D) belonging to the opposite crack faces and enforce the
cohesive traction-separation relation. In the present work we propose a novel
type of interface element for fracture mechanics sharing some analogies with
the node-to-segment (in 2D) and with the node-to-surface (in 3D) contact
elements. The displacement gap of a node belonging to the finite element
discretization of one crack face with respect to its projected point on the
opposite face is used to determine the cohesive tractions, the residual vector
and its consistent linearization for an implicit solution scheme. The following
advantages with respect to classical interface finite elements are
demonstrated: (i) non-matching finite element discretizations of the opposite
crack faces is possible; (ii) easy modelling of cohesive cracks with
non-propagating crack tips; (iii) the internal rotational equilibrium of the
interface element is assured. Detailed examples are provided to show the
usefulness of the proposed approach in nonlinear fracture mechanics problems.Comment: 37 pages, 17 figure
A consistent interface element formulation for geometrical and material nonlinearities
Decohesion undergoing large displacements takes place in a wide range of
applications. In these problems, interface element formulations for large
displacements should be used to accurately deal with coupled material and
geometrical nonlinearities. The present work proposes a consistent derivation
of a new interface element for large deformation analyses. The resulting
compact derivation leads to a operational formulation that enables the
accommodation of any order of kinematic interpolation and constitutive behavior
of the interface. The derived interface element has been implemented into the
finite element codes FEAP and ABAQUS by means of user-defined routines. The
interplay between geometrical and material nonlinearities is investigated by
considering two different constitutive models for the interface (tension
cut-off and polynomial cohesive zone models) and small or finite deformation
for the continuum. Numerical examples are proposed to assess the mesh
independency of the new interface element and to demonstrate the robustness of
the formulation. A comparison with experimental results for peeling confirms
the predictive capabilities of the formulation.Comment: 14 pages, 11 figure
X-ray Photons in the CO 2-1 'Lacuna' of NGC 2110
A recent ALMA study of the Seyfert 2 Active Galactic Nucleus (AGN) NGC 2110
by Rosario et al. (2019) has reported a remarkable lack of CO 2-1 emission from
the circumnuclear region, where optical lines and H2 emission are observed,
leading to the suggestion of excitation of the molecular clouds by the AGN.
Since interaction with X-ray photons could be the cause of this excitation, we
have searched the archival Chandra data for corroborating evidence. We report
an extra-nuclear ~1'' (~170 pc) feature found in the soft (<1.0 keV) Chandra
data of the Seyfert 2 Active Galactic Nucleus (AGN) NGC 2110. This feature is
elongated to the north of the nucleus and its shape matches well that of the
optical lines and H2 emission observed in this region, which is devoid of CO
2-1 emission. The Chandra image completes the emerging picture of a multi-phase
circumnuclear medium excited by the X-rays from the AGN, with dense warm
molecular clouds emitting in H2 but depleted of CO 2-1 emission.Comment: ApJ Letters - in pres
An Analysis of the Italian Lockdown in Retrospective Using Particle Swarm Optimization in Machine Learning Applied to an Epidemiological Model
A critical analysis of the open data provided by the Italian Civil Protection Centre during phase 1 of Covid-19 epidemic—the so-called Italian lockdown—is herein proposed in relation to four of the most affected Italian regions, namely Lombardy, Reggio Emilia, Valle d’Aosta, and Veneto. A possible bias in the data induced by the extent in the use of medical swabs is found in relation to Valle d’Aosta and Veneto. Observed data are then interpreted using a Susceptible-Infectious-Recovered (SIR) epidemiological model enhanced with asymptomatic (infected and recovered) compartments, including lockdown effects through time-dependent model parameters. The initial number of susceptible individuals for each region is also considered as a parameter to be identified. The issue of parameters identification is herein addressed by a robust machine learning approach based on particle swarm optimization. Model predictions provide relevant information for policymakers in terms of the effect of lockdown measures in the different regions. The number of susceptible individuals involved in the epidemic, important for a safe release of lockdown during the next phases, is predicted to be around 10% of the population for Lombardy, 16% for Reggio Emilia, 18% for Veneto, and 40% for Valle d’Aosta
Fatigue degradation and electric recovery in Silicon solar cells embedded in photovoltaic modules
Cracking in Silicon solar cells is an important factor for the electrical power-loss of photovoltaic modules. Simple geometrical criteria identifying the amount of inactive cell areas depending on the position of cracks with respect to the main electric conductors have been proposed in the literature to predict worst case scenarios. Here we present an experimental study based on the electroluminescence (EL) technique showing that crack propagation in monocrystalline Silicon cells embedded in photovoltaic (PV) modules is a much more complex phenomenon. In spite of the very brittle nature of Silicon, due to the action of the encapsulating polymer and residual thermo-elastic stresses, cracked regions can recover the electric conductivity during mechanical unloading due to crack closure. During cyclic bending, fatigue degradation is reported. This pinpoints the importance of reducing cyclic stresses caused by vibrations due to transportation and use, in order to limit the effect of cracking in Silicon cells
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