Seminars 2011
Seminario
Lunes, 5 de diciembre de 2011
Higherorder averaging of linear FokkerPlanck
equations
Universidad de Kaiserslautern
Abstract
Energy dynamics of stochastic Hamiltonian systems can
be investigated by the method of stochastic averaging.
If the mechanical system is under additional periodic
forcing, say friction, the energy dynamics and
equilibria will change. These small deviations in
energy cannot be captured by classical stochastic
averaging. We present a method of higherorder
averaging based on an asymptotic expansion of
equilibria, which is able to (numerically) generate
averages in the subtle limit of small noise and small
friction. The influence of periodic forcing will become
visible only to second order. We present our method and
its results for standard oscillators as well as an
application to fiber dynamics.
El Seminario tendrá lugar a las 12:30 horas en la Sala
2.1.C17 (Edificio Sabatini) Universidad Carlos III
Seminario
Jueves 1 de diciembre de 2011
Carbon nanotubebased motor driven by a thermal
gradient
Universidad de Barcelona
Abstract
We present a model able to reproduce experimental
observations and computer simulation results of the
movement of two coaxial carbon nanotubes induced by a
thermal gradient. The model is formulated in terms of a
Langevin equation which includes the friction force,
the van der Waals forces between both nanotubes, that
depend on their chiralities, and the inhomogeneous
temperature distribution which give rise to an
inhomogeneous phonon distribution. The random force
term is assumed to be related to the fluctuations of
the heat current along the inner nanotube and therefore
its intensity is proportional to the heat conductivity.
The model reproduces the rich variety of possible
dynamic behaviors and proves the conjecture that the
driving force is the phononic current induced by the
thermal gradient. Applications to other
nanoelectromechanical devices are also analyzed.
El Seminario tendrá lugar a las 12:30 horas en la Sala
2.1.C17 (Edificio Sabatini) Universidad Carlos III
Seminario
Martes 22 de noviembre de 2011
Homogeneous vapor condensation in boundary layer flows
Universidad Carlos III de Madrid
Abstract
Condensation of vapors containing combustion products
occurs very often in combustion chambers. These
products are typically highly agressive, so subsequent
deposition of droplets containing them on chamber walls
may result in serious damages. There are other examples
where condensation and deposition on a surface is
desirable, as in the case of technological processes
that produce materials from vaporized substances.
Any case, calculation of deposition rates is very important for regulating its effects, beneficial or not, in many situations in industry. Condensation of vapor molecules may occur on particles already present inside the chamber or, on the contrary, on clusters produced simultaneously by nucleation of the same molecules. In the first case we talk about heterogeneous condensation and in the last one about homogeneous condensation. Heterogenous condensation in boundary layer flows has been treated extensively on literature, but much less in known in the case of homogeneous condensation. We propose a thermophysical model for describing
homogeneous condensation in laminar boundary layer
flows focusing on a stagnationpoint flow. The
numerical solution of the model provides the spatial
profiles of vapor density, droplet density and number
of condensate molecules per droplet. In addition we
have obtained approximate solutions by using
Perturbation Theory techniques which shed new light on
the numerical results.
El Seminario tendrá lugar a las 12:30 horas en la Sala
2.1.C17 (Edificio Sabatini) Universidad Carlos III
Seminario
Martes 15 de noviembre de 2011
Graphene as a bridge between high and low energy
physics
Instituto de Ciencia de Materiales de Madrid, C.S.I.C.
Abstract
The main conceptual advances in Physics have usually
been prompted by almost simultaneous discoveries in
different branches. In the past century, Statistical
physics, Quantum Field Theory and Condensed Matter have
had their main developments in parallel with the best
physicists (Feynman, Landau, Wigner) contributing to
them all. Cosmology and astrophysics developed with
inputs from Particle Physics. The experimental
realization of graphene, a twodimensional crystal made
of carbon atoms in 2004 provides a new and unexpected
bridge between Condensed Matter and HighEnergy
Physics. The low energy excitations of the graphene
system are massless Dirac fermions in (2+1) dimensions
showing amazing parallelisms with Quantum
Electrodynamics. The graphene samples show corrugations
that can be modeled with General Relativity techniques.
In this talk we will review the main properties of the
graphene system using QFT concepts and will show the
analogy with General Relativity at work in a few
examples. We will also review some on going
applications.
El Seminario tendrá lugar a las 12:30 horas en la Sala
2.3.B02 (Edificio Sabatini) Universidad Carlos III
Seminario
Martes 8 de noviembre de 2011
Coupled phenomena and quantumcontinuum coupling in
modeling low dimensional nanostructures
Wilfrid Laurier University
Abstract
Lowdimensional semiconductor nanostructures (LDSNs)
are challenging objects to study from both, fundamental
physics and mathematical points of view. These objects
are receiving increasing attention as key components of
many optoelectronic devices. Quantum dots (QDs), LDSNs
in which the motion of electrons is confined from all
three spatial dimensions, can also be used as
biological tags in DNA analysis, as well as in other
biotechnological applications, while the idea of using
a spin confined to a QD as a qubit promises imminent
breakthrough in quantum information processing. The
number of practical applications of LDSNs continues to
grow which requires the development of adequate
mathematical models for their description and efficient
numerical approximations.
Despite a wide range of current and potential applications, properties of LDSNs, and QDs in particular, are still frequently analyzed with simplified mathematical models, incapable to account correctly for many effects that are coming from other than quantum mechanical scales (e.g., strain, piezoelectric, thermal and other important effects). In this talk, our main emphasis will be on the mathematical models where the coupling between quantum and continuum mechanics parts is essential. A number of numerical examples will be given to illustrate the theory. If time permits, we will also discuss new analytical
and numerical modelling techniques to control single
electron spin states adiabatically through the
application of the geometric Berry phase.
El Seminario tendrá lugar a las 12:30 horas en la Sala
2.3.B02 (Edificio Sabatini) Universidad Carlos III
Seminario
Jueves 3 de noviembre de 2011
Diffusion front capturing schemes for a class of
FokkerPlanck equations: Application to the
relativistic heat equation
Universidad de Valencia
Abstract
In this research work we introduce and analyze an
explicit conservative finite difference scheme to
approximate the solution of initialboundary value
problems for a class of limited diffusion FokkerPlanck
equations under homogeneous Neumann boundary
conditions. We show stability and positivity preserving
property under a CourantFriedrichsLewy parabolic time
step restriction. We focus on the relativistic heat
equation as a model problem of the mentioned limited
diffusion FokkerPlanck equations. We analyze its
dynamics and observe the presence of a singular flux
and an implicit combination of nonlinear effects that
include anisotropic diffusion and hyperbolic transport.
We present numerical approximations of the solution of
the relativistic heat equation for a set of examples in
one and two dimensions including continuous initial
data that develops jump discontinuities in finite time.
We perform the numerical experiments through a class of
explicit high order accurate conservative and stable
numerical schemes and a semiimplicit nonlinear
CrankNicolson type scheme.
El Seminario tendrá lugar a las 12:30 horas en la Sala
2.1.D03 (Edificio Sabatini) Universidad Carlos III
Seminario
Viernes 17 de junio de 2011
Unraveling molecular complexity by manipulating single
molecules one at a time
Universidad de Barcelona
Abstract
Recent developments in micro and nano technologies
allow for the controlled manipulation of individual
molecules by exerting and detecting forces in the
piconewton range. The possibility to detect such tiny
forces together with the ability of measuring
extensions with nanometer resolution allows scientists
to monitor molecular reactions in real time (e.g.
folding) and characterize thermodynamics and kinetics
of individual molecules (e.g. nucleic acids and
proteins) within unprecedented accuracy.
In this talk I will review some applications of
single molecule experiments to molecular biophysics
putting particular emphasis on the use optical tweezers
technology to mechanically manipulate single molecules
(nucleic acids and proteins). I will also present a few
experimental results obtained in our lab on mechanical
unzipping of nucleic acids aiming to characterize the
energetics of the double helix and the translocation
motion of enzymes involved in DNA replication.
El Seminario tendrá lugar a las 12:30 horas en la Sala
2.1.C19 (Edificio Sabatini) Universidad Carlos III
Seminario
Viernes 3 de junio de 2011
Robust statistical techniques for the analysis of high
dimensional gene expression data
UC3M
Abstract
Microarray experiments provide data on the expression
level of thousands of genes simultaneously, suggesting
several considerable challenges in its preprocessing,
management and analysis, that are becoming more and
more serious as new high throughput sequencing
technologies are being developed. We will describe some
robust statistical tools, based on the concept of
functional data depth, which measures the centrality of
a function within a sample. These techniques include
the scale curve, as a way to visualise the dispersion
of a set of multivariate observations, and supervised
classifications techniques, to assign new observations
to one of a given set of (known) groups. These tools
are very appropriate for the analysis of high
dimensional data, and in particular of microarray data,
as its application to real examples has shown. We will
also discuss some technical issues that have been taken
into account to improve its implementation as a
contributed package to the R project, offering the
possibilities of using either commandline code or a
userfriendly interface.
El Seminario tendrá lugar a las 12:30 horas en la Sala
2.3.A02 (Edificio Sabatini) Universidad Carlos III
Seminario
Viernes 3 de junio de 2011
On the Piloted Ignition of Solid Fuels in Spacecraft
Environments
University of California, Berkeley
Abstract
The effect of environmental variables on the ignition
of solid combustible materials is explored through a
combination of experimental, analytical and numerical
analyses. This research stems from NASA's design
requirement to reduce the cabin internal pressure and
increase the oxygen concentration in future human space
exploration vehicles. These new environmental
conditions may result in an increased fire risk of
combustible solid materials due to higher flame
temperatures (attributed to enhanced oxygen), reduced
convective heat losses from heated surfaces and reduced
pyrolyzate needed to reach a flammable mixture near the
pilot (both attributed to lower pressure). In
particular, the influence of low pressure on ignition
is emphasized because little is known concerning this
topic. A series of experiments conducted in a
laboratoryscale combustion wind tunnel with externally
irradiated samples of PMMA (polymethylmethacrylate)
showed that both the ignition delay time and the fuel
mass flux at ignition decrease when the ambient
pressure is lowered. An analytical model is used to
identify the governing processes that lead to these
results and then a numerical model is applied to
quantify the influence of ambient variables
(particularly pressure) on the piloted ignition of
PMMA. The results that will be presented indicate that
the flammability of combustible materials is enhanced
at low ambient pressures and elevated oxygen
concentrations. This may have significant consequences
in the assessment of material fire risk in spacecraft
and other environments where these conditions are
encountered, such as aircrafts and high altitude
cities.
El Seminario tendrá lugar a las 16:00 horas en la Sala
1.1.F02 (Edificio Betancourt) Universidad Carlos III
Seminario
Jueves 2 de junio de 2011
Effects of stable stratification on wallbounded
turbulent flows
UC3M
Abstract
Stablystratified turbulent wallbounded flows are
relevant to many applications in engineering. These
flows are characterized by a variation of fluid density
in the vertical direction, that often results in strong
modifications of the flow patterns by buoyancy. The
dynamics of stablystratified wall turbulence is driven
by two competing mechanisms. Vertical motions can
extract turbulent kinetic energy from the mean shear
but stratification requires them to pay a potential
energy toll. Due to the statistical inhomogeneity of
wall turbulence, the interplay between these mechanisms
can vary significantly with the wall distance in the
same flow, specially as the Reynolds number increases.
In some cases, the flow even segregates into regions
with disparate features, ranging from long velocity
streaks in sheardominated regions to internal gravity
waves in buoyancydominated regions. In this talk we
are going to present results from direct numerical
simulations of stablystratified wallbounded turbulent
flows. Various cases will be presented and discussed,
showing the richness and complexity of these flows.
El Seminario tendrá lugar a las 12:00 horas en la Sala
2.3.A05 (Edificio Sabatini) Universidad Carlos III
Seminario
Jueves 19 de mayo de 2011
Meshfree fiite dirence methods with minimal positive
stencils
Temple University
Abstract
The finite dirence method is one approach to
approximate a partial dirential equation byi a finite
dimension problem. An important example is the
approximation of the Poisson equation by a large,
sparse linear system. While finite dirences are well
established for friendly geometries with regular grids,
the approach can also be generalized to fully
unstructured geometries, and even to disconnected
"clouds" of points, which frequently arise in particle
methods. Two new aspects come into the game here:
first, there are many possible consistent finite
diference approximations, and there is no simple
principle that defines a "best" one; second, with many
classical meshfree approaches, the arising linear
systems are in general nonsymmetric, do not have an
Mmatrix structure, and are significantly less sparse
than fiite dirence matrices on regular geometries. All
of the latter aspects are detrimental for the
performance of linear solvers. I will present an
approach, based on $\ell^1$ optimization, that selects
minimal positive stencils, and thus generates optimally
sparse Mmatrices.
El Seminario tendrá lugar a las 12:30 horas en la Sala
2.3.A04 (Edificio Sabatini) Universidad Carlos III
Seminario
Jueves 5 de mayo de 2011
Transiciones de fase en sistemas de osciladores
acoplados con espines
U. Sevilla
Abstract
Consideramos un oscilador armónico acoplado linealmente
con una cadena lineal de espines. Para un cierto valor
de la temperatura, aparece una transición de fase de
segundo orden con la posición de equilibrio del
oscilador como parámetro de orden. Estudiaremos la
dinámica del sistema en dos limites de interés: espines
rápidos y oscilador rápido. Asimismo, exploraremos la
posibilidad de aplicar este modelo para analizar las
curvas fuerza extensión de moléculas biológicas.
El Seminario tendrá lugar a las 12:30 horas en la Sala
2.3.A04 (Edificio Sabatini) Universidad Carlos III
Seminario
Jueves 14 de abril de 2011
Spin models on random graphs with controlled topologies
U. Barcelona
Abstract
We study Ising spin models on finitely connected random
graphs drawn from an ensemble which allows for a
precise tuning of the topology. We discuss how to solve
these models using finite connectivity equilibrium
replica theory as well as possible applications of the
theory to realworld interacting particle systems.
El Seminario tendrá lugar a las 12:30 horas en la Sala
2.3.B04 (Edificio Sabatini) Universidad Carlos III
Seminario
Miércoles 2 de marzo de 2011
Transmission and Resonance in Photonic Crystal
Materials
Duke University
Abstract
In the last twenty years, there has been increasing
scientific and technological interest in manipulating
the propagation properties of light through its
interaction with new materials, such as photonic
crystals (PC). Typically, these are composite
materials, engineered so that their dielectric
properties depend periodically on the space variable.
They are known also as photonic bandgap materials, a
term that derives from gaps that may be present in
their continuous EM propagation spectrum.
Our interest lies in the transmission properties of
photonic crystal slabs, that display spatial
periodicity along the slab and are finite and are
possibly repetitive in the incidence direction. We
describe resonant behavior and anomalous transmission
and we connect this behavior with the existence of
guided or leaky EM modes along the PC slab. We derive a
generic formula for the description of the asymptotic
behavior of the transmission coefficient versus the
deviation of the incident frequency from the resonant
frequency and the angle of deviation from normal
incidence. To understand nonlinear behavior in a
rigorous way, we introduce a nonlinear model, retaining
critical features, but simplified enough that analytic
calculation is possible. We are finally in the process
of implementing a 3D boundary integral code for the
numerical study of such PC. The code implements the
fast calculation of the Green functions in a way that
displays superalgebraic convergence. We give most
descriptions in the talk, through fields obtained by a
2D code.
El Seminario tendrá lugar a las 12:30 horas en la Sala
2.3.B04 (Edificio Sabatini) Universidad Carlos III
Seminarios del Instituto Gregorio Millán
Viernes 28 de enero de 2011
Slow pulse due to calcium current induces phase2
reentry in heterogeneous tissue
UPC
Abstract
Phase2 reentry is a basic mechanism for the transition
to VT and VF in the heart. It is thought to underlie
many causes of idiopathic ventricular arrhythmias as,
for instance, those occurring in Brugada syndrome.
Reentry is usually linked to heterogeneity in tissue
repolarization. We study some circumstances under which
a region of depolarized tissue can reexcite adjacent
regions that exhibit shorter action potential duration
(APD), eventually inducing reentry. Simulations are
performed using a simplified ionic model that
reproduces well the ventricular action potential (AP).
We analyze first the conditions that lead to very short
action potentials (APs). Then, we show that
reexcitation takes place via a slow (calcium current
induced) pulse that propagates into the region of short
APs until it encounters excitable tissue. In two
dimensions, this may give rise to reentry with the
formation of counterrotating spiral waves.
El Seminario tendrá lugar a las 12:30 horas en la Sala
2.1.C19 (Edificio Sabatini) Universidad Carlos III
Seminarios del Instituto Gregorio Millán
Lunes 17 de enero de 2011
Graphene and its unique properties
CSIC
Abstract
Graphene, two dimensional membrane one atom thick is a
novel material which shows features not found
previously in other systems. Some of these properties,
along with the research erffort which is being carried
out in order to elucidate their origin and
consequences, will be reviewed.
El Seminario tendrá lugar a las 12:30 horas en la Sala
2.1.D04 (Edificio Sabatini) Universidad Carlos III


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