1,212 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
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
Dissecting the Extended X-Ray Emission in the Merging Pair NGC 6240: Photoionization and Winds
A new flaring high energy gamma-ray source
We report the detection of a new gamma-ray source in the Fermi-LAT sky using
a source detection tool based on the Minimal Spanning Tree algorithm. The
source, not reported in previous LAT catalogues but very recently observed in
the X-rays and optical bands, is characterized by an increasing gamma-ray
activity in 2012 June-September, reaching a weekly peak flux of
(3.3+-0.6)*10^-7 photons cm^-2 s^-1. A search for a possible counterpart
provides indication that it can be associated with the radio source NVSS
J141828+354250 whose optical SDSS colours are typical of a blazar.Comment: 4 pages, 3 figures. Accepted for publication in Astronomy &
Astrophysic
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