2,355 research outputs found
Effects of constraints in general branched molecules: A quantitative ab initio study in HCO-L-Ala-NH2
A general approach to the design of accurate classical potentials for protein
folding is described. It includes the introduction of a meaningful statistical
measure of the differences between approximations of the same potential energy,
the definition of a set of Systematic and Approximately Separable and Modular
Internal Coordinates (SASMIC), much convenient for the simulation of general
branched molecules, and the imposition of constraints on the most rapidly
oscillating degrees of freedom. All these tools are used to study the effects
of constraints in the Conformational Equilibrium Distribution (CED) of the
model dipeptide HCO-L-Ala-NH2. We use ab initio Quantum Mechanics calculations
including electron correlation at the MP2 level to describe the system, and we
measure the conformational dependence of the correcting terms to the naive CED
based in the Potential Energy Surface (PES) without any simplifying assumption.
These terms are related to mass-metric tensors determinants and also occur in
the Fixman's compensating potential. We show that some of the corrections are
non-negligible if one is interested in the whole Ramachandran space. On the
other hand, if only the energetically lower region, containing the principal
secondary structure elements, is assumed to be relevant, then, all correcting
terms may be neglected up to peptides of considerable length. This is the first
time, as far as we know, that the analysis of the conformational dependence of
these correcting terms is performed in a relevant biomolecule with a realistic
potential energy function.Comment: 8 pages, 1 figure, LaTeX, aipproc style (included
Necesidad de la Terapia Miofuncional en el tratamiento de la Deglución Atípica
la importancia de la deglución atípica se ha visto incrementada en los últimos años debido a sus posibles consecuencias negativas, y a la que se le presta mayor atención desde el ámbito logopédico y ortodóncico. La deglución atípica es un movimiento inadecuado y una posición incorrecta de la lengua en relación al resto de las estructuras orales en la primera fase del proceso deglutorio. De etiología diversa, oscilando desde un desequilibrio del control nervioso hasta malos hábitos como la respiración bucal, provoca alteraciones a nivel anatómico y funcional con consecuencias perjudiciales para el correcto desarrollo anatomofisiológico del individuo. En la mayoría de las ocasiones la deglución atípica aparece vinculada a problemas de oclusiones, actuando una de ellas como causa y la otra como consecuencia, por lo que cobra especial importancia la puesta en práctica de un tratamiento multidisciplinar, el cual radicará en una intervención logopédica utilizando la terapia miofuncional como metodología de intervención junto con una intervención odontológica haciendo uso de la ortodoncia como mecanismo rehabilitador. El presente documento incluye una revisión sistemática tanto de la deglución atípica partiendo del proceso anatomofisiológico adecuado de la deglución como de la terapia miofuncional como mecanismo de intervención más efectivo desde el ámbito logopédico para corregir el mal posicionamiento lingual. También se hace referencia a los hábitos miofuncionales, Además, se incluye un programa de intervención logopédica diseñado para un caso prototípico de deglución atípica.Grado en Logopedi
When omnigeneity fails
A generic non-symmetric magnetic field does not confine magnetized charged
particles for long times due to secular magnetic drifts. Stellarator magnetic
fields should be omnigeneous (that is, designed such that the secular drifts
vanish), but perfect omnigeneity is technically impossible. There always are
small deviations from omnigeneity that necessarily have large gradients. The
amplification of the energy flux caused by a deviation of size is
calculated and it is shown that the scaling with of the
amplification factor can be as large as linear. In opposition to common wisdom,
most of the transport is not due to particles trapped in ripple wells, but to
the perturbed motion of particles trapped in the omnigeneous magnetic wells
around their bounce points.Comment: 6 pages, 2 figure
Flow damping in stellarators close to quasisymmetry
Quasisymmetric stellarators are a type of optimized stellarators for which
flows are undamped to lowest order in an expansion in the normalized Larmor
radius. However, perfect quasisymmetry is impossible. Since large flows may be
desirable as a means to reduce turbulent transport, it is important to know
when a stellarator can be considered to be sufficiently close to quasisymmetry.
The answer to this question depends strongly on the size of the spatial
gradients of the deviation from quasisymmetry and on the collisionality regime.
Recently, formal criteria for closeness to quasisymmetry have been derived in a
variety of situations. In particular, the case of deviations with large
gradients was solved in the regime. Denoting by a parameter
that gives the size of the deviation from quasisymmetry, it was proven that
particle fluxes do not scale with , as typically claimed, but
with . It was also shown that ripple wells are not necessarily the main
cause of transport. This paper reviews those works and presents a new result in
another collisionality regime, in which particles trapped in ripple wells are
collisional and the rest are collisionless.Comment: 14 pages, 2 figures. To appear in Plasma Physics and Controlled
Fusio
Optimizing stellarators for large flows
Plasma flow is damped in stellarators because they are not intrinsically
ambipolar, unlike tokamaks, in which the flux-surface averaged radial electric
current vanishes for any value of the radial electric field. Only
quasisymmetric stellarators are intrinsically ambipolar, but exact
quasisymmetry is impossible to achieve in non-axisymmetric toroidal
configurations. By calculating the violation of intrinsic ambipolarity due to
deviations from quasisymmetry, one can derive criteria to assess when a
stellarator can be considered quasisymmetric in practice, i.e. when the flow
damping is weak enough. Let us denote by a small parameter that
controls the size of a perturbation to an exactly quasisymmetric magnetic
field. Recently, it has been shown that if the gradient of the perturbation is
sufficiently small, the flux-surface averaged radial electric current scales as
for any value of the collisionality. It was also argued that when
the gradient of the perturbation is large, the quadratic scaling is replaced by
a more unfavorable one. In this paper, perturbations with large gradients are
rigorously treated. In particular, it is proven that for low collisionality a
perturbation with large gradient yields, at best, an deviation
from quasisymmetry. Heuristic estimations in the literature incorrectly
predicted an deviation.Comment: 24 pages, 2 figures. To appear in Plasma Physics and Controlled
Fusio
The effect of tangential drifts on neoclassical transport in stellarators close to omnigeneity
In general, the orbit-averaged radial magnetic drift of trapped particles in
stellarators is non-zero due to the three-dimensional nature of the magnetic
field. Stellarators in which the orbit-averaged radial magnetic drift vanishes
are called omnigeneous, and they exhibit neoclassical transport levels
comparable to those of axisymmetric tokamaks. However, the effect of deviations
from omnigeneity cannot be neglected in practice. For sufficiently low
collision frequencies (below the values that define the regime), the
components of the drifts tangential to the flux surface become relevant. This
article focuses on the study of such collisionality regimes in stellarators
close to omnigeneity when the gradient of the non-omnigeneous perturbation is
small. First, it is proven that closeness to omnigeneity is required to
preserve radial locality in the drift-kinetic equation for collisionalities
below the regime. Then, it is shown that neoclassical transport is
determined by two layers in phase space. One of the layers corresponds to the
regime and the other to the superbanana-plateau regime. The
importance of the superbanana-plateau layer for the calculation of the
tangential electric field is emphasized, as well as the relevance of the latter
for neoclassical transport in the collisionality regimes considered in this
paper. In particular, the tangential electric field is essential for the
emergence of a new subregime of superbanana-plateau transport when the radial
electric field is small. A formula for the ion energy flux that includes the
regime and the superbanana-plateau regime is given. The energy
flux scales with the square of the size of the deviation from omnigeneity.
Finally, it is explained why below a certain collisionality value the
formulation presented in this article ceases to be valid.Comment: 36 pages. Version to be published in Plasma Physics and Controlled
Fusio
Time-dependent patterns in quasivertical cylindrical binary convection
This paper reports on numerical investigations of the effect of a slight inclination a on pattern formation in a shallow vertical cylindrical cell heated from below for binary mixtures with a positive value of the Soret coefficient. By using direct numerical simulation of the three-dimensional Boussinesq equations with Soret effect in cylindrical geometry, we show that a slight inclination of the cell in the range a˜0.036rad=2° strongly influences pattern selection. The large-scale shear flow (LSSF) induced by the small tilt of gravity overcomes the squarelike arrangements observed in noninclined cylinders in the Soret regime, stratifies the fluid along the direction of inclination, and produces an enhanced separation of the two components of the mixture. The competition between shear effects and horizontal and vertical buoyancy alters significantly the dynamics observed in noninclined convection. Additional unexpected time-dependent patterns coexist with the basic LSSF. We focus on an unsual periodic state recently discovered in an experiment, the so-called superhighway convection state (SHC), in which ascending and descending regions of fluid move in opposite directions. We provide numerical confirmation that Boussinesq Navier-Stokes equations with standard boundary conditions contain the essential ingredients that allow for the existence of such a state. Also, we obtain a persistent heteroclinic structure where regular oscillations between a SHC pattern and a state of nearly stationary longitudinal rolls take place. We characterize numerically these time-dependent patterns and investigate the dynamics around the threshold of convection.Postprint (author's final draft
Generación de cuestionarios Moodle con R + exams + Sweave
El paquete exams de R tiene como objetivo la generación automática de cuestionarios. Cada cuestionario se caracteriza por tener variantes –tantas como elija el equipo docente- que comparten un enunciado común pero disponen de diferentes datos. El código fuente se redacta en formato Sweave, es decir, una combinación de instrucciones LaTeX y código R mientras que el formato de salida incluye, entre otros, PDF, HTML y XML. El taller se centra en este último formato que permite de forma relativamente sencilla generar actividades para la plataforma de aprendizaje Moodle. En la sesión se practican los tres pasos necesarios para generar un quiz: 1) elaboración del código fuente, 2) obtención del fichero XML, 3) importación a Moodle y configuración del cuestionario. A partir de ejemplos de complejidad creciente se muestra la potencialidad de esta herramienta para implementar actividades on-line en un rango de asignaturas de estadística que abarca desde primer ciclo hasta máster
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