2,056 research outputs found
Decomposition driven interface evolution for layers of binary mixtures: {II}. Influence of convective transport on linear stability
We study the linear stability with respect to lateral perturbations of free
surface films of polymer mixtures on solid substrates. The study focuses on the
stability properties of the stratified and homogeneous steady film states
studied in Part I [U. Thiele, S. Madruga and L. Frastia, Phys. Fluids 19,
122106 (2007)]. To this aim, the linearized bulk equations and boundary
equations are solved using continuation techniques for several different cases
of energetic bias at the surfaces, corresponding to linear and quadratic
solutal Marangoni effects.
For purely diffusive transport, an increase in film thickness either
exponentially decreases the lateral instability or entirely stabilizes the
film. Including convective transport leads to a further destabilization as
compared to the purely diffusive case. In some cases the inclusion of
convective transport and the related widening of the range of available film
configurations (it is then able to change its surface profile) change the
stability behavior qualitatively.
We furthermore present results regarding the dependence of the instability on
several other parameters, namely, the Reynolds number, the Surface tension
number and the ratio of the typical velocities of convective and diffusive
transport.Comment: Published in Physics of Fluic
Dirac constraints in field theory and exterior differential systems
The usual treatment of a (first order) classical field theory such as
electromagnetism has a little drawback: It has a primary constraint submanifold
that arise from the fact that the dynamics is governed by the antisymmetric
part of the jet variables. So it is natural to ask if there exists a
formulation of this kind of field theories which avoids this problem, retaining
the versatility of the known approach. The following paper deals with a family
of variational problems, namely, the so called non standard variational
problems, which intends to capture the data necessary to set up such a
formulation for field theories; moreover, we will formulate a multisymplectic
structure for the family of non standard variational problems, and we will
relate this with the (pre)symplectic structure arising on the space of sections
of the bundle of fields. In this setting the Dirac theory of constraints will
be studied, obtaining among other things a novel characterization of the
constraint manifold which arises in this theory, as generators of an exterior
differential system associated to the equations of motion and the chosen
slicing. Several examples of application of this formalism are discussed: Two
of them motivated from the physical point of view, that is, electromagnetism
and Poisson sigma models, and two examples of mathematical application. In the
case of electromagnetism, it is shown that this formulation avoids the problems
arising in the usual approach.Comment: 51 pages, aims; added examples and references, typos added, improved
some content, to appear in "Journal of Geometric Mechanics"
La expresión de la finalidad en la Edad Media
This article tries to investigate the different ways of expressing finality and its evolution in medieval texts (from 13th, 14th and 15th centuries). All those syntactica forms that answer to the ‘human’ finality in the strict sense are compiled: that finality which starts from a human subject’s intention. Moreover, it is observed which contents can be conveyed with the formal models that are used for the intentional finality.Este artÃculo pretende determinar los distintos modos de expresar la finalidad y su evolución en textos medievales (de los siglos XIII, XIV y XV). Se recopilan todas aquellas formas sintácticas que responden a la finalidad propiamente ‘humana’: aquella que parte de la intención de un ‘sujeto humano’. Además, se observan qué contenidos se pueden recubrir con los modelos formales que se emplean para la finalidad intencional
The Inter-Mammary Sticky Roll: A Novel Technique for Securing a Doppler Ultrasonic Probe to the Precordium for Venous Air Embolism Detection.
Venous air embolism is a devastating and potentially life-threatening complication that can occur during neurosurgical procedures. We report the development and use of the "inter-mammary sticky roll,"Â a technique to reliably secure a precordial Doppler ultrasonic probe to the chest wall during neurosurgical cases that require lateral decubitus positioning. We have found that this noninvasive technique is safe, and effectively facilitates a constant Doppler signal with no additional risk to the patient
Effective geometry in Astrophysics
The effective metric is introduced by means of two examples (non-linear
electromagnetism and hydrodynamics),along with applications in Astrophysics. A
sketch of the generality of the effect is also given.Comment: 9 pages, contributions for the proceedings of the First International
Workshop on Astronomy and Relativistic Astrophysics (IWARA 2003), Olinda
(Brazil
Robot Formations Control Using Fast Marching
This paper presents the application of the Voronoi Fast Marching (V FM) method to the Control of Robot Formations. The V FM method uses the propagation of a wave (Fast Marching) operating on the world model to de- termine a motion plan over a viscosity map (similar to the refraction index in optics) extracted from the updated map model. The computational effciency of the method allows the planner to operate at high rate sensor frequencies. This method allows us to maintain good response time and smooth and safe planned trajectories. The navigation function can be classiffed as a type of potential field, but it has no local minima, it is complete (it finds the solu- tion path if it exists) and it has a complexity of order n (O(n)), where n is the number of cells in the environment map. The results presented in this paper show how the proposed method behaves with mobile robot formations and generates trajectories of good quality without problems of local minima when the formation encounters non-convex obstacles
High-pressure, low-abundance water in bipolar outflows. Results from a Herschel-WISH survey
(Abridged) We present a survey of the water emission in a sample of more than
20 outflows from low mass young stellar objects with the goal of characterizing
the physical and chemical conditions of the emitting gas. We have used the HIFI
and PACS instruments on board the Herschel Space Observatory to observe the two
fundamental lines of ortho-water at 557 and 1670 GHz. These observations were
part of the "Water In Star-forming regions with Herschel" (WISH) key program,
and have been complemented with CO and H2 data. We find that the emission from
water has a different spatial and velocity distribution from that of the J=1-0
and 2-1 transitions of CO, but it has a similar spatial distribution to H2, and
its intensity follows the H2 intensity derived from IRAC images. This suggests
that water traces the outflow gas at hundreds of kelvins responsible for the H2
emission, and not the component at tens of kelvins typical of low-J CO
emission. A warm origin of the water emission is confirmed by a remarkable
correlation between the intensities of the 557 and 1670 GHz lines, which also
indicates the emitting gas has a narrow range of excitations. A non-LTE
radiative transfer analysis shows that while there is some ambiguity on the
exact combination of density and temperature values, the gas thermal pressure
nT is constrained within less than a factor of 2. The typical nT over the
sample is 4 10^{9} cm^{-3}K, which represents an increase of 10^4 with respect
to the ambient value. The data also constrain within a factor of 2 the water
column density. When this quantity is combined with H2 column densities, the
typical water abundance is only 3 10^{-7}, with an uncertainty of a factor of
3. Our data challenge current C-shock models of water production due to a
combination of wing-line profiles, high gas compressions, and low abundances.Comment: 21 pages, 13 figures. Accepted for publication in A&
Robot formation motion planning using Fast Marching
This paper presents the application of the Voronoi Fast Marching (VFM) method to path planning of mobile formation robots. The VFM method uses the propagation of a wave (Fast Marching) operating on the world model to determine a motion plan over a viscosity map (similar to the refraction index in optics) extracted from the updated map model. The computational efficiency of the method allows the planner to operate at high rate sensor frequencies. This method allows us to maintain good response time and smooth and safe planned trajectories. The navigation function can be classified as a type of potential field, but it has no local minima, it is complete (it finds the solution path if it exists) and it has a complexity of order n(O(n)), where n is the number of cells in the environment map. The results presented in this paper show how the proposed method behaves with mobile robot formations and generates trajectories of good quality without problems of local minima when the formation encounters non-convex obstacles.This work has been supported by the CAM Project S2009/DPI-1559/ROBOCITY2030 II,
developed by the research team RoboticsLab at the University Carlos III of Madrid.Publicad
Lipid membranes with an edge
Consider a lipid membrane with a free exposed edge. The energy describing
this membrane is quadratic in the extrinsic curvature of its geometry; that
describing the edge is proportional to its length. In this note we determine
the boundary conditions satisfied by the equilibria of the membrane on this
edge, exploiting variational principles. The derivation is free of any
assumptions on the symmetry of the membrane geometry. With respect to earlier
work for axially symmetric configurations, we discover the existence of an
additional boundary condition which is identically satisfied in that limit. By
considering the balance of the forces operating at the edge, we provide a
physical interpretation for the boundary conditions. We end with a discussion
of the effect of the addition of a Gaussian rigidity term for the membrane.Comment: 8 page
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