971 research outputs found
Effects of Force Distribution and Rebound on Electromagnetically Formed Sheet Metal
Electromagnetic forming (EMF) is a high speed forming process that has been shown to increase the formability of aluminum alloys under certain conditions. Many authors have reported significant increases in formability; however, there is as of yet no complete understanding of the process. Obtaining a gain in formability is not the only factor that must be considered when studying EMF. The process rapidly generates significant forces which lead to the deformation of the material at very high rates. The applied forces depend on the shape of the electromagnetic coil used, which leads to force distributions that may not be ideal for forming a particular part. Once the sheet is accelerated it will travel at high speeds until it impacts the die. This high speed impact results in the sheet rebounding from the die. Both the force distribution and the rebound affect the final shape of the part. This paper presents the results of experimental and numerical study carried out to determine the effect of the force distribution and the rebound on samples of conical and "v-channel" geometry. It was found that both sample geometries are affected by the force distribution and the rebound, with the v-channel samples being considerably more affected. The results indicate that these effects must be carefully considered when EMF processes are designed
On the Numerical Approximations of an Optimal Correction Problem
The numerical solution of an optimal correction problem for a damped random linear oscillator is studied. A numerical algorithm for the discretized system of the associated dynamic programming equation is given. To initiate the computation, we adopt a numerical scheme derived from the deterministic version of the problem. Next, a correction-type algorithm based on a discrete maximum principle is introduced to ensure the convergence of the iteration procedure
Role of beam propagation in Goos-H\"{a}nchen and Imbert-Fedorov shifts
We derive the polarization-dependent displacements parallel and perpendicular
to the plane of incidence, for a Gaussian light beam reflected from a planar
interface, taking into account the propagation of the beam. Using a
classical-optics formalism we show that beam propagation may greatly affect
both Goos-H\"{a}nchen and Imbert-Fedorov shifts when the incident beam is
focussed.Comment: 3 pages, 1 figure, submitted to Opt. Let
Repeated games for eikonal equations, integral curvature flows and non-linear parabolic integro-differential equations
The main purpose of this paper is to approximate several non-local evolution
equations by zero-sum repeated games in the spirit of the previous works of
Kohn and the second author (2006 and 2009): general fully non-linear parabolic
integro-differential equations on the one hand, and the integral curvature flow
of an interface (Imbert, 2008) on the other hand. In order to do so, we start
by constructing such a game for eikonal equations whose speed has a
non-constant sign. This provides a (discrete) deterministic control
interpretation of these evolution equations. In all our games, two players
choose positions successively, and their final payoff is determined by their
positions and additional parameters of choice. Because of the non-locality of
the problems approximated, by contrast with local problems, their choices have
to "collect" information far from their current position. For integral
curvature flows, players choose hypersurfaces in the whole space and positions
on these hypersurfaces. For parabolic integro-differential equations, players
choose smooth functions on the whole space
Homogenization and enhancement for the G-equation
We consider the so-called G-equation, a level set Hamilton-Jacobi equation,
used as a sharp interface model for flame propagation, perturbed by an
oscillatory advection in a spatio-temporal periodic environment. Assuming that
the advection has suitably small spatial divergence, we prove that, as the size
of the oscillations diminishes, the solutions homogenize (average out) and
converge to the solution of an effective anisotropic first-order
(spatio-temporal homogeneous) level set equation. Moreover we obtain a rate of
convergence and show that, under certain conditions, the averaging enhances the
velocity of the underlying front. We also prove that, at scale one, the level
sets of the solutions of the oscillatory problem converge, at long times, to
the Wulff shape associated with the effective Hamiltonian. Finally we also
consider advection depending on position at the integral scale
Existence of solutions for a higher order non-local equation appearing in crack dynamics
In this paper, we prove the existence of non-negative solutions for a
non-local higher order degenerate parabolic equation arising in the modeling of
hydraulic fractures. The equation is similar to the well-known thin film
equation, but the Laplace operator is replaced by a Dirichlet-to-Neumann
operator, corresponding to the square root of the Laplace operator on a bounded
domain with Neumann boundary conditions (which can also be defined using the
periodic Hilbert transform). In our study, we have to deal with the usual
difficulty associated to higher order equations (e.g. lack of maximum
principle). However, there are important differences with, for instance, the
thin film equation: First, our equation is nonlocal; Also the natural energy
estimate is not as good as in the case of the thin film equation, and does not
yields, for instance, boundedness and continuity of the solutions (our case is
critical in dimension in that respect)
Formation of linear planform chimneys controlled by preferential hydrocarbon leakage and anisotropic stresses in faulted fine-grained sediments, offshore Angola
A new type of gas chimney exhibiting an unconventional linear
planform is found. These chimneys are termed Linear Chimneys, which have been observed in 3-D
seismic data offshore of Angola. Linear Chimneys occur parallel to
adjacent faults, often within preferentially oriented tier-bound fault
networks of diagenetic origin (also known as anisotropic polygonal faults,
PFs), in salt-deformational domains. These anisotropic PFs are parallel to
salt-tectonic-related structures, indicating their submission to horizontal
stress perturbations generated by the latter. Only in areas with these
anisotropic PF arrangements do chimneys and their associated gas-related
structures, such as methane-derived authigenic carbonates and pockmarks, have
linear planforms. In areas with the classic isotropic polygonal fault
arrangements, the stress state is isotropic, and gas expulsion structures of the
same range of sizes exhibit circular geometry. These events indicate that
chimney's linear planform is heavily influenced by stress anisotropy around
faults. The initiation of polygonal faulting occurred 40 to 80 m below the
present day seafloor and predates Linear Chimney formation. The majority of
Linear Chimneys nucleated in the lower part of the PF tier below the
impermeable portion of fault planes and a regional impermeable barrier within
the PF tier. The existence of polygonal fault-bound traps in the lower part
of the PF tier is evidenced by PF cells filled with gas. These PF gas traps
restricted the leakage points of overpressured gas-charged fluids
along the lower portion of PFs, hence controlling the nucleation sites of
chimneys. Gas expulsion along the lower portion of PFs preconfigured the
spatial organisation of chimneys. Anisotropic stress conditions surrounding
tectonic and anisotropic polygonal faults coupled with the impermeability of PFs
determined the directions of long-term gas migration and linear geometries of
chimneys. Methane-related carbonates that precipitated above Linear Chimneys
inherited the same linear planform geometry, and both structures record the
timing of gas leakage and palaeo-stress state; thus, they can be used as a tool
to reconstruct orientations of stress in sedimentary successions. This study
demonstrates that overpressure hydrocarbon migration via hydrofracturing may
be energetically more favourable than migration along pre-existing faults.</p
Absence of zero field muon spin relaxation induced by superconductivity in the B phase of UPt
We present muon spin relaxation measurements performed on crystals of the
heavy fermion superconductor UPt. In zero applied field, contrary to a
previous report, we do not observe an increase of the internal magnetic field
in the lower superconducting phase (the B phase). Our result gives an
experimental upper bound of the magnetic field that could be associated with
the superconducting state.Comment: 4 pages, REVTeX 3.0, 2 PostScript figure
Neonatal infections with multidrug-resistant ESBL-producing E. cloacae and K. pneumoniae in Neonatal Units of two different Hospitals in Antananarivo, Madagascar
Background: We investigated the molecular mechanism of ß-lactam resistance in extended-spectrum ß-lactamase (ESBL)-producing Enterobacterial strains isolated in neonatal units of different hospitals in Anatnanarivo, Madagascar.Methods: Bacteria were identified by standard biochemical methods, disc diffusion antibiograms and Etest. Resistance genes were sought by PCR. Strains were characterized by Rep- PCR (Diversilab), plasmid analysis and rep-typing.Results: From April 2012 to March 2013, 29 ESBL-producing E. cloacae and 15 K. pneumoniae were isolated from blood culture (n = 32) or gastric samples (n = 12) performed at day 0 or 2 from 39/303 newborns suspected of early neonatal infection. These infants were treated with expanded spectrum cephalosporins, due to lack of carbapenems, leading to a high mortality rate (45 %). Isolates recovered were all, but 4, multidrug resistant, particularly to fluoroquinolones (FQ) except for 21 E. cloacae isolates. Isolates produced TEM-1 and CTX-M-15 ß-lactamases and their genes were located on several self- transferable plasmids of variable sizes sizes that could not be linked to a major plasmid incompatibility group. E. cloacae isolates belonged to 6 Rep-types among which two counted for 11 isolates each. The FQ resistant E. cloacae isolates belonged to one clone, whereas the FQ susceptible E. cloacae isolates belonged to four clones. The K. pneumoniae isolates belonged to 9 Rep-types among which one included five isolates.Conclusion: This study is the first molecular characterization of ESBL- producing isolates from neonatology units in Madagascar, a country with limited epidemiological data. It revealed an important multi-clonal dissemination of CTX-M-15- producing isolates reflecting both the high community carriage and the very early nosocomial contamination of the neonates
Neonatal infections with multidrug-resistant ESBL-producing E. cloacae and K. pneumoniae in Neonatal Units of two different Hospitals in Antananarivo, Madagascar
Background: We investigated the molecular mechanism of ß-lactam resistance in extended-spectrum ß-lactamase (ESBL)-producing Enterobacterial strains isolated in neonatal units of different hospitals in Anatnanarivo, Madagascar.Methods: Bacteria were identified by standard biochemical methods, disc diffusion antibiograms and Etest. Resistance genes were sought by PCR. Strains were characterized by Rep- PCR (Diversilab), plasmid analysis and rep-typing.Results: From April 2012 to March 2013, 29 ESBL-producing E. cloacae and 15 K. pneumoniae were isolated from blood culture (n = 32) or gastric samples (n = 12) performed at day 0 or 2 from 39/303 newborns suspected of early neonatal infection. These infants were treated with expanded spectrum cephalosporins, due to lack of carbapenems, leading to a high mortality rate (45 %). Isolates recovered were all, but 4, multidrug resistant, particularly to fluoroquinolones (FQ) except for 21 E. cloacae isolates. Isolates produced TEM-1 and CTX-M-15 ß-lactamases and their genes were located on several self- transferable plasmids of variable sizes sizes that could not be linked to a major plasmid incompatibility group. E. cloacae isolates belonged to 6 Rep-types among which two counted for 11 isolates each. The FQ resistant E. cloacae isolates belonged to one clone, whereas the FQ susceptible E. cloacae isolates belonged to four clones. The K. pneumoniae isolates belonged to 9 Rep-types among which one included five isolates.Conclusion: This study is the first molecular characterization of ESBL- producing isolates from neonatology units in Madagascar, a country with limited epidemiological data. It revealed an important multi-clonal dissemination of CTX-M-15- producing isolates reflecting both the high community carriage and the very early nosocomial contamination of the neonates
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