4,903 research outputs found
Electrodynamics of Black Holes in STU Supergravity
External magnetic fields can probe the composite structure of black holes in
string theory. With this motivation we study magnetised four-charge black holes
in the STU model, a consistent truncation of maximally supersymmetric
supergravity with four types of electromagnetic fields. We employ solution
generating techniques to obtain Melvin backgrounds, and black holes in these
backgrounds. For an initially electrically charged static black hole immersed
in magnetic fields, we calculate the resultant angular momenta and analyse
their global structure. Examples are given for which the ergoregion does not
extend to infinity. We calculate magnetic moments and gyromagnetic ratios via
Larmor's formula. Our results are consistent with earlier special cases. A
scaling limit and associated subtracted geometry in a single surviving magnetic
field is shown to lift to . Magnetizing magnetically charged
black holes give static solutions with conical singularities representing
strings or struts holding the black holes against magnetic forces. In some
cases it is possible to balance these magnetic forces.Comment: 31 page
Statistical characteristics of the envelope in diversity combining of two correlated Rayleigh fading channels
Performance of diversity systems is often evaluated under the assumption of perfect interleaving and characterised in terms of long-term parameters such as the average bit-error rate, which does not capture the dynamics of fading channels. Statistical characteristics (static and dynamic) of the envelope of two correlated Rayleigh fading channels are explored using a physical model. For two popular diversity-combining schemes, maximal ratio combining and selection combining, both static and dynamic (level-crossing rate) properties of correlated fading channels are derived. These results are very useful for performance evaluation of diversity systems without bit-level simulations. The results can also provide very useful characteristics such as average duration of fades, fading rate and outage probability for two-channel diversity systems and can be extended to multiple fading channels
Defending Against Firmware Cyber Attacks on Safety-Critical Systems
In the past, it was not possible to update the underlying software in many industrial control devices. Engineering
teams had to ‘rip and replace’ obsolete components. However, the ability to make firmware updates has provided
significant benefits to the companies who use Programmable Logic Controllers (PLCs), switches, gateways and
bridges as well as an array of smart sensor/actuators. These updates include security patches when vulnerabilities are
identified in existing devices; they can be distributed by physical media but are increasingly downloaded over
Internet connections. These mechanisms pose a growing threat to the cyber security of safety-critical applications,
which are illustrated by recent attacks on safety-related infrastructures across the Ukraine. Subsequent sections
explain how malware can be distributed within firmware updates. Even when attackers cannot reverse engineer the
code necessary to disguise their attack, they can undermine a device by forcing it into a constant upload cycle where
the firmware installation never terminates. In this paper, we present means of mitigating the risks of firmware attack
on safety-critical systems as part of wider initiatives to secure national critical infrastructures. Technical solutions,
including firmware hashing, must be augmented by organizational measures to secure the supply chain within
individual plants, across companies and throughout safety-related industries
Shape Complexity from Image Similarity
We present an approach to automatically compute the complexity of a given 3D shape. Previous approaches have made use of geometric and/or topological properties of the 3D shape to compute complexity. Our approach is based on shape appearance and estimates the complexity of a given 3D shape according to how 2D views of the shape diverge from each other. We use similarity among views of the 3D shape as the basis for our complexity computation. Hence our approach uses claims from psychology that humans mentally represent 3D shapes as organizations of 2D views and, therefore, mimics how humans gauge shape complexity. Experimental results show that our approach produces results that are more in agreement with the human notion of shape complexity than those obtained using previous approaches
Thermodynamic properties of an interacting hard-sphere Bose gas in a trap using the static fluctuation approximation
A hard-sphere (HS) Bose gas in a trap is investigated at finite temperatures
in the weakly-interacting regime and its thermodynamic properties are evaluated
using the static fluctuation approximation (SFA). The energies are calculated
with a second-quantized many-body Hamiltonian and a harmonic oscillator wave
function. The specific heat capacity, internal energy, pressure, entropy and
the Bose-Einstein (BE) occupation number of the system are determined as
functions of temperature and for various values of interaction strength and
number of particles. It is found that the number of particles plays a more
profound role in the determination of the thermodynamic properties of the
system than the HS diameter characterizing the interaction, that the critical
temperature drops with the increase of the repulsion between the bosons, and
that the fluctuations in the energy are much smaller than the energy itself in
the weakly-interacting regime.Comment: 34 pages, 24 Figures. To appear in the International Journal of
Modern Physics
Vacuum Polarization of STU Black Holes and their Subtracted Geometry Limit
We study the vacuum polarization of a massless minimally coupled scalar field
at the horizon of four-charge STU black holes. We compare the results for the
standard asymptotically flat black holes and for the black holes obtained in
the "subtracted limit", both in the general static case and at the horizon pole
for the general rotating case. The original and the subtracted results are
identical only in the BPS limit, and have opposite sign in the extremal Kerr
limit. We also compute the vacuum polarization on the static solutions that
interpolate between both the original and the subtracted case through a
solution-generating transformation and show that the vacuum polarization stays
positive throughout the interpolating solution. In the Appendix we provide a
closed-form solution for the Green's function on general (static or rotating)
subtracted black hole geometries.Comment: 22 pages, 2 figure
Numerical modeling and analysis of Ti6Al4V alloy chip for biomedical applications
The influence of cutting forces during the machining of titanium alloys has attained prime attention in selecting the optimal cutting conditions to improve the surface integrity of medical implants and biomedical devices. So far, it has not been easy to explain the chip morphology of Ti6Al4V and the thermo-mechanical interactions involved during the cutting process. This paper investigates the chip configuration of the Ti6Al4V alloy under dry milling conditions at a macro and micro scale by employing the Johnson-Cook material damage model. 2D modeling, numerical milling simulations, and post-processing were conducted using the Abaqus/Explicit commercial software. The uncut chip geometry was modeled with variable thicknesses to accomplish the macro to micro-scale cutting by adapting a trochoidal path. Numerical results, predicted for the cutting reaction forces and shearing zone temperatures, were found in close approximation to experimental ones with minor deviations. Further analyses evaluated the influence of cutting speeds and contact friction coefficients over the chip flow stress, equivalent plastic strain, and chip morphology. The methodology developed can be implemented in resolving the industrial problems in the biomedical sector for predicting the chip morphology of the Ti6Al4V alloy, fracture mechanisms of hard-to-cut materials, and the effects of different cutting parameters on workpiece integrity
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