22,146 research outputs found
A fractal model for the lightning induced current on a transmission line
This article presents a model of a tortuous lightning channel, and a study of the consequent electromagnetic coupling to a transmission line. We analyse the fractal dimension of the induced current on the line, compared with the fractal dimension of the lightning channel and the impinging electromagnetic field on the line. This comparison confirms our previous studies pointing out that the channel fractal dimension and the electromagnetic field fractal dimension are related (in particular, for typical lightning parameters they are the same). A comparison of simulated fields and line currents with experimental measurements is also attempte
Effective thermal dynamics following a quantum quench in a spin chain
We study the nonequilibrium dynamics of the Quantum Ising Model following an
abrupt quench of the transverse field. We focus on the on-site autocorrelation
function of the order parameter, and extract the phase coherence time
from its asymptotic behavior. We show that the initial state
determines only through an effective temperature set by its
energy and the final Hamiltonian. Moreover, we observe that the dependence of
on the effective temperature fairly agrees with that obtained
in thermal equilibrium as a function of the equilibrium temperature.Comment: 4 pages, 4 figures. Published versio
Smoothed Particle Hydrodynamic Simulations of Viscous Accretion Discs Around Black Holes
Viscous Keplerian discs become sub-Keplerian close to a black hole since they
pass through sonic points before entering into it. We study the time evolution
of polytropic viscous accretion discs (both in one and two dimensional flows)
using Smoothed Particle Hydrodynamics. We discover that for a large region of
the parameter space, when the flow viscosity parameter is less than a critical
value, standing shock waves are formed. If the viscosity is very high then the
shock disappears. In the intermediate viscosity the disc oscillates very
significantly in viscous time-scale. Our simulations indicate that these
centrifugally supported high density region close to a black hole plays an
active role in the flow dynamics, and consequently, the radiation dynamics.Comment: MNRAS style 6 pages of output, macros included. MNRAS (submitted
Chiral pions in a magnetic background
We investigate the modification of the pion self-energy at finite temperature
due to its interaction with a low-density, isospin-symmetric nuclear medium
embedded in a constant magnetic background. To one loop, for fixed temperature
and density, we find that the pion effective mass increases with the magnetic
field. For the , interestingly, this happens solely due to the trivial
Landau quantization shift , since the real part of the self-energy
is negative in this case. In a scenario in which other charged particle species
are present and undergo an analogous trivial shift, the relevant behavior of
the effective mass might be determined essentially by the real part of the
self-energy. In this case, we find that the pion mass decreases by
for a magnetic field , which favors pion condensation at high
density and low temperatures.Comment: 7 pages, 5 figure
A guaranteed-convergence framework for passivity enforcement of linear macromodels
Passivity enforcement is a key step in the extraction of linear macromodels of electrical interconnects and packages for Signal and Power Integrity applications. Most state-of-the-art techniques for passivity enforcement are based on suboptimal or approximate formulations that do not guarantee convergence. We introduce in this paper a new rigorous framework that casts passivity enforcement as a convex non-smooth optimization problem. Thanks to convexity, we are able to prove convergence to the optimal solution within a finite number of steps. The effectiveness of this approach is demonstrated through various numerical example
Subgradient Techniques for Passivity Enforcement of Linear Device and Interconnect Macromodels
This paper presents a class of nonsmooth convex optimization methods for the passivity enforcement of reduced-order macromodels of electrical interconnects, packages, and linear passive devices. Model passivity can be lost during model extraction or identification from numerical field solutions or direct measurements. Nonpassive models may cause instabilities in transient system-level simulation, therefore a suitable postprocessing is necessary in order to eliminate any passivity violations. Different from leading numerical schemes on the subject, passivity enforcement is formulated here as a direct frequency-domain norm minimization through perturbation of the model state-space parameters. Since the dependence of this norm on the parameters is nonsmooth, but continuous and convex, we resort to the use of subdifferentials and subgradients, which are used to devise two different algorithms. We provide a theoretical proof of the global optimality for the solution computed via both schemes. Numerical results confirm that these algorithms achieve the global optimum in a finite number of iterations within a prescribed accuracy leve
3D Laparoscopy. A potential cutting edge in minimal invasive digestive surgery
Laparoscopic surgery has changed surgical landscape, providing reduced surgical trauma, shorter hospital stays, less postoperative pain and
better outcomes than open surgery. Since its first development in the 90ās, 3D technology applied to laparoscopic surgery has had several technical
improvements and now it represents, together with high definition technology, the best option in minimal invasive digestive surgery, providing shorter
operative times and lower blood loss, making easier to perform surgical tasks both for trainees than for skilled surgeons. It remains a little bit more
expensive than standard 2D laparoscopic devices but even cheaper than robotic equipment
Merger Mechanisms
A firm can merge with one of n potential partners. The owner of each firm has private information about both his firm's stand-alone value and a component of the synergies that would be realized by the merger involving his firm. We characterize incentive-efficient mechanisms in two cases. First, we assume that the value of any newly formed partnership is verifiable, hence transfers can be made contingent on the new information accruing after the merger. Second, we study the case of uncontingent rules. In the first case, we show that it is not optimal, in general, to redistribute shares of non-merging firms, and identify necessary and sufficient conditions for the implementability of efficient merger rules. In the second case, we show that the first-best can be obtained i) always, if the synergy values are privately known but the firms' stand-alone values are observable; ii) only with sufficiently large synergies, if the firms' stand-alone are privately known; and iii) never, if the set of feasible mechanisms is restricted to "auctions in shares".
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