11,930 research outputs found
Elastic shocks in relativistic rigid rods and balls
We study the free boundary problem for the "hard phase" material introduced
by Christodoulou, both for rods in (1+1)-dimensional Minkowski spacetime and
for spherically symmetric balls in (3+1)-dimensional Minkowski spacetime.
Unlike Christodoulou, we do not consider a "soft phase", and so we regard this
material as an elastic medium, capable of both compression and stretching. We
prove that shocks must be null hypersurfaces, and derive the conditions to be
satisfied at a free boundary. We solve the equations of motion of the rods
explicitly, and we prove existence of solutions to the equations of motion of
the spherically symmetric balls for an arbitrarily long (but finite) time,
given initial conditions sufficiently close to those for the relaxed ball at
rest. In both cases we find that the solutions contain shocks if and only if
the pressure or its time derivative do not vanish at the free boundary
initially. These shocks interact with the free boundary, causing it to lose
regularity.Comment: 22 pages, 3 figures; v2: small changes, matches final published
version; v3: typos in the references fixe
Exploring the Venus global super-rotation using a comprehensive General Circulation Model
The atmospheric circulation in Venus is well known to exhibit strong
super-rotation. However, the atmospheric mechanisms responsible for the
formation of this super-rotation are still not fully understood. In this work,
we developed a new Venus general circulation model to study the most likely
mechanisms driving the atmosphere to the current observed circulation. Our
model includes a new radiative transfer, convection and suitably adapted
boundary layer schemes and a dynamical core that takes into account the
dependence of the heat capacity at constant pressure with temperature.
The new Venus model is able to simulate a super-rotation phenomenon in the
cloud region quantitatively similar to the one observed. The mechanisms
maintaining the strong winds in the cloud region were found in the model
results to be a combination of zonal mean circulation, thermal tides and
transient waves. In this process, the semi-diurnal tide excited in the upper
clouds has a key contribution in transporting axial angular momentum mainly
from the upper atmosphere towards the cloud region. The magnitude of the
super-rotation in the cloud region is sensitive to various radiative parameters
such as the amount of solar radiative energy absorbed by the surface, which
controls the static stability near the surface. In this work, we also discuss
the main difficulties in representing the flow below the cloud base in Venus
atmospheric models.
Our new radiative scheme is more suitable for 3D Venus climate models than
those used in previous work due to its easy adaptability to different
atmospheric conditions. This flexibility of the model was crucial to explore
the uncertainties in the lower atmospheric conditions and may also be used in
the future to explore, for example, dynamical-radiative-microphysical
feedbacks.Comment: Accepted for publication in Planet. Space Sc
Spherical linear waves in de Sitter spacetime
We apply Christodoulou's framework, developed to study the Einstein-scalar
field equations in spherical symmetry, to the linear wave equation in de Sitter
spacetime, as a first step towards the Einstein-scalar field equations with
positive cosmological constant. We obtain an integro-differential evolution
equation which we solve by taking initial data on a null cone. As a corollary
we obtain elementary derivations of expected properties of linear waves in de
Sitter spacetime: boundedness in terms of (characteristic) initial data, and a
Price law establishing uniform exponential decay, in Bondi time, to a constant.Comment: 9 pages, 1 figure; v2: minor changes, references added, matches final
published versio
Automated Identification and Classification of Stereochemistry: Chirality and Double Bond Stereoisomerism
Stereoisomers have the same molecular formula and the same atom connectivity
and their existence can be related to the presence of different
three-dimensional arrangements. Stereoisomerism is of great importance in many
different fields since the molecular properties and biological effects of the
stereoisomers are often significantly different. Most drugs for example, are
often composed of a single stereoisomer of a compound, and while one of them
may have therapeutic effects on the body, another may be toxic. A challenging
task is the automatic detection of stereoisomers using line input
specifications such as SMILES or InChI since it requires information about
group theory (to distinguish stereoisomers using mathematical information about
its symmetry), topology and geometry of the molecule. There are several
software packages that include modules to handle stereochemistry, especially
the ones to name a chemical structure and/or view, edit and generate chemical
structure diagrams. However, there is a lack of software capable of
automatically analyzing a molecule represented as a graph and generate a
classification of the type of isomerism present in a given atom or bond.
Considering the importance of stereoisomerism when comparing chemical
structures, this report describes a computer program for analyzing and
processing steric information contained in a chemical structure represented as
a molecular graph and providing as output a binary classification of the isomer
type based on the recommended conventions. Due to the complexity of the
underlying issue, specification of stereochemical information is currently
limited to explicit stereochemistry and to the two most common types of
stereochemistry caused by asymmetry around carbon atoms: chiral atom and double
bond. A Webtool to automatically identify and classify stereochemistry is
available at http://nams.lasige.di.fc.ul.pt/tools.ph
Essence and Cause: Making Something Be What It Is
Aristotle frequently describes essence as a “cause” or “explanation”, thus ascribing to essence some sort of causal or explanatory role. This explanatory role is often explicated by scholars in terms of essence “making the thing be what it is” or “making it the very thing that it is”. I argue that this is problematic, at least on the assumption that “making” expresses an explanatory relation, since it violates certain formal features of explanation. I then consider whether Aristotle is vulnerable to this problem by examining the explanatory role of essence in Posterior Analytics and Metaphysics Z 17
Towards a collective knowledge for a smart electric vehicle charging strategy
In this work is proposed the design of a system to
create and handle Electric Vehicles (EV) charging procedures,
based on intelligent process. Due to the electrical power
distribution network limitation and absence of smart meter
devices, Electric Vehicles charging should be performed in a
balanced way, taking into account past experience (spread in a
social network). In order to allow information exchange and to
help user mobility, it was also created a mobile application to
assist the EV driver on these processes. This proposed Smart
Electric Vehicle Charging System uses Vehicle-to-Grid (V2G)
technology, in order to connect Electric Vehicles and also
renewable energy sources to Smart Grids (SG). This system
also explores the new paradigm of Electrical Markets (EM),
with deregulation of electricity production and use, in order to
obtain the best conditions for commercializing electrical
energy.The authors are grateful to the FCT (Fundação para a
Ciência e a Tecnologia) and to the MIT-Portugal Program,
for funding the Project MIT-PTIEDAM-SMS/00301200
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