UPCommons - E-prints UPC Universitat Politècnica de Catalunya
(79,306 recursos)
E-prints UPC cobreix dues finalitats: per una banda, és el dipòsit institucional de la UPC que recull els articles de revista, les comunicacions de congrés i els reports de recerca generats en les activitats de recerca del personal docent i investigador de la universitat; per l'altra, és una eina que permet accelerar la producció científica, allotjant versions de documents prèvies a la publicació en una revista o a les actes d’un congrés.

Articles de revista

Mostrando recursos 41 - 60 de 153

41.
High-throughput microcapillary pump with efficient integrated low aspect ratio micropillars - Madadi, Hojjat; Casals Terré, Jasmina; Castilla López, Roberto; Sureda Anfres, Miquel
Prediction and reduction of pressure drop and resistance flow in micropillar arrays are important for the design of microfluidic circuits used in different lab-on-a-chip and biomedical applications. In this work, a diamond microchannel-integrated micropillar pump (dMIMP) with a resistance flow 35.5 % lower than a circular-based micropillar pump (cMIMP) has been developed via the optimization of the fluid dynamic behavior of different pillar shapes in a low aspect ratio (H/D ranged from 0.06 to 0.2) integrated pillar microchannel. The effect of different geometrical parameters (such as pillar shape and its distribution) has been considered to minimize the microchannel resistance flow....

42.
The mechanism why colliders could create quasi-stable black holes - Torres Herrera, Ramon; Fayos Vallés, Francisco; Lorente Espín, Oscar
It has been postulated that black holes could be created in particle collisions within the range of the available energies for nowadays colliders (LHC). In this paper we analyze the evaporation of a type of black holes that are candidates for this specific behaviour, namely, small black holes on a brane in a world with large extra-dimensions. We examine their evolution under the assumption that energy conservation is satisfied during the process and compare it with the standard evaporation approach. We claim that, rather than undergoing a quick total evaporation, black holes become quasi-stable. We comment on the (absence of)...

43.
On how energy conservation might prevent black hole explosions and violations of the third law - Torres Herrera, Ramon; Fayos Vallés, Francisco
We claim that imposing energy conservation to the emission of Hawking radiation and to the modeling of black hole (BH) evaporation might prevent BH explosions as well as violations of the third law of BH thermodynamics. This is specifically shown for the general class of spherically symmetric quantum BHs described by an effective quantum vacuum invariant under boosts in the radial direction. No assumptions are made with regard to the specific framework from which the quantum BHs are derived.

44.
Exponential versus IMEX high-order time integrators for thermal convection in rotating spherical shells - Garcia Gonzalez, Ferran; Bonaventura, Luca; Net Marcé, Marta; Sánchez Umbría, Juan
We assess the accuracy and efficiency of several exponential time integration methods coupled to a spectral discretization of the three-dimensional Boussinesq thermal convection equations in rotating spherical shells. Exponential methods are compared to implicit–explicit (IMEX) multi-step methods already studied previously in [1]. The results of a wide range of numerical simulations highlight the superior accuracy of exponential methods for a given time step, especially when employed with large time steps and at low Ekman number. However, presently available implementations of exponential methods appear to be in general computationally more expensive than those of IMEX methods and further research is needed...

45.
A comparison of high-order time integrators for highly supercritical thermal convection in rotating spherical shells - Garcia Gonzalez, Ferran; Net Marcé, Marta; Sánchez Umbría, Juan
The efficiency of implicit and semi-implicit time integration codes based on backward differentiation and extrapolation formulas for the solution of the three-dimensional Boussinesq thermal convection equations in rotating spherical shells was studied in [5] at weakly supercritical Rayleigh numbers R, moderate (10−3) and low (10−4) Ekman numbers, E, and Prandtl number = 1. The results presented here extend the previous study and focus on the effect of and R by analyzing the efficiency of the methods for obtaining solutions at E = 10−4, = 0.1 and low and high supercritical R. In the first case (quasiperiodic solutions) the decrease of...

46.
Numerical bifurcation methods and their application to fluid dynamics: analysis beyond simulation - Dijkstra, Hendrik; Wubs, Fred W.; Cliffe, Andrew K.; Doedel, Eusebius J.; Dragomirescu, Ioana Florica; Eckhardt, Bruno; Gelfgat, Alexander Yu; Hazel, Andrew L.; Lucarini, Valerio; Salinger, Andrew G.; Phipps, Erik T.; Sánchez Umbría, Juan; Schuttelaars, Henk M.; Tuckerman, Laurette S.; Thiele, Uwe
We provide an overview of current techniques and typical applications of numerical bifurcation analysis in fluid dynamical problems. Many of these problems are characterized by high-dimensional dynamical systems which undergo transitions as parameters are changed. The computation of the critical conditions associated with these transitions, popularly referred to as 'tipping points', is important for understanding the transition mechanisms. We describe the two basic classes of methods of numerical bifurcation analysis, which differ in the explicit or implicit use of the Jacobian matrix of the dynamical system. The numerical challenges involved in both methods are mentioned and possible solutions to current...

47.
Instability of plumes driven by localized heating - López Moscat, Juan Manuel; Marqués Truyol, Francisco
Plumes due to localized buoyancy sources are of wide interest owing to their prevalence in many situations. This study investigates the transition from laminar to turbulent dynamics. Several experiments have reported that this transition is sensitive to external perturbations. As such, a well-controlled set-up has been chosen for our numerical study, consisting of a localized heat source at the bottom of an enclosed cylinder whose walls are all maintained at a fixed uniform temperature, except for the localized heat source. At moderate Rayleigh numbers Ra, the flow consists of a steady, axisymmetric purely poloidal plume. On increasing Ra, the flow...

48.
Analysis of turbulent exchange and coherent structures in the stable atmospheric boundary layer based on tower observations - Ferreres Soler, Enriqueta; Soler Duffour, Maria Rosa; Terradellas, Enric
Using data collected at the Spanish low troposphere research centre CIBA
(Centro de Investigación de la Baja Atmósfera) and at the Cabauw Experimental Site for
Atmospheric Research (CESAR) in the Netherlands, we analysed the most significant
features of different coherent structures occurring in the stable atmospheric boundary layer.
In particular, we used both the Reynolds and wavelet methods to analyse a solitary wave, a
gravity wave, a density current and a low-level jet. For each of these structures, we found
that wavelet analysis had the capacity to distinguish the different scales involved in these
events due to the different timing and heights of the thermal instabilities and...

49.
A novel fabrication technique to minimize poly(dimethylsiloxane)-microchannels deformation under high-pressure operation - Madadi, Hojjat; Mohammadi, Mahdi; Casals Terré, Jasmina; Castilla López, Roberto
PDMS is one of the most common materials used for the flow delivery in the microfluidics chips, since it is clear, inert, nontoxic, and nonflammable. Its inexpensiveness, straightforward fabrication, and biological compatibility have made it a favorite material in the exploratory stages of the bio-microfluidic devices. If small footprint assays want to be performed while keeping the throughput, high pressure-rated channels should be used, but PDMS flexibility causes an important issue since it can generate a large variation of microchannel geometry. In this work, a novel fabrication technique based on the prevention of PDMS deformation is developed. A photo-sensible thiolene...

50.
Una experiencia de unificación de asignaturas para desplegar PBL (y las quejas que originó) - Barrado Muxí, Cristina; Cuadrado Santolaria, Raúl; Delgado Muñoz, Luis; Mellibovsky Elstein, Fernando; Pastor Llorens, Enric; Pérez Batlle, Marcos; Prats Menéndez, Xavier; Rojas Gregorio, José Ignacio; Royo Chic, Pablo; Valero García, Miguel
En este artículo se describen los aspectos esenciales de una experiencia de unificación de parejas de asignaturas con el objetivo de crear un escenario más adecuado para el despliegue de Aprendizaje Basado en Proyectos. Como guía para el repaso de esos aspectos esenciales se utiliza una carta que elaboraron los estudiantes de la primera edición para protestar por el funcionamiento de las asignaturas. El análisis de las quejas de los estudiantes puede ser de mucha utilidad para otros que se planeen retos similares.

51.
An approach to evaporating stars and black holes - Fayos Vallés, Francisco; Torres Herrera, Ramon
Many models in which the object under study loses all its mass have appeared in
the literature. No doubt, the most eminent examples are the models for evaporating black holes
and for evaporating stars. Here we describe a semiclassical study of these evaporating models
centered on the evaporating event itself. We pay special attention to the evaporating models as
a means of avoiding singularities during the collapse. In case of any pre-existing non-spacelike
curvature singularity, we show that these models tend to evaporate it.

52.
Mechanism of quasistabilization of primordial black holes - Torres Herrera, Ramon
It is argued that primordial black holes with initial masses satisfying M<1015 g, instead of having
explode, might currently be in a quasistable phase contributing to a tiny fraction of the measured dark
matter. This statement is based on a computation of black hole evaporation in which energy conservation
is taken into account that shows that the backreaction to Hawking radiation favors the quasistabilization of
the black hole. The result is specifically shown for general spherically symmetric quantum black holes
described by an effective metric independently of the specific framework from which it is derived. The
quintessential primordial black hole is fully analyzed as an example.

53.
On the interior of (quantum) black holes - Torres Herrera, Ramon
Different approaches to quantum gravity conclude that black holes may possess an
inner horizon, in addition to the (quantum corrected) outer ‘Schwarzschild’ horizon.
In this paper we assume the existence of this inner horizon and explain the physical
process that might lead to the tunneling of particles through it. It is shown that the
tunneling would produce a flux of particles with a spectrum that deviates from the pure
thermal one. Under the appropriate approximation the extremely high temperature of
this horizon is calculated for an improved quantum black hole. It is argued that the
flux of particles tunneled through the horizons affects the dynamics of the...

54.
Fold-pitchfork bifurcation for maps with Z(2) symmetry in pipe flow - Marqués Truyol, Francisco; Mellibovsky Elstein, Fernando; Meseguer Serrano, Álvaro
This study aims to provide a better understanding of recently identified transition scenarios exhibited by traveling wave solutions in pipe flow. This particular family of solutions are invariant under certain reflectional symmetry transformations and they emerge from saddle-node bifurcations within a two-dimensional parameter space characterized by the length of the pipe and the Reynolds number. The present work precisely provides a detailed analysis of a codimension-two saddle-node bifurcation arising in discrete dynamical systems (maps) with Z2 symmetry. Normal form standard techniques are applied in order to obtain the reduced map up to cubic order. All possible bifurcation scenarios exhibited by...

55.
A mechanism for the generation of wave-driven rhythmic patterns in the surf zone - Falqués Serra, Albert; Coco, G; Huntley, D A
The coupling between topographic irregularities and wave-driven mean water motion in the surf zone is examined. This coupling occurs because the topographic perturbations produce excess gradients in the wave radiation stress that cause a steady circulation. This circulation, in turn, creates a sediment transport pattern that can reinforce the bottom disturbance and may thereby lead to the growth of large-scale bed forms. To investigate this coupling mechanism, the linearized stability problem with an originally plane sloping beach and normal wave incidence is solved in two different cases. First, the breaking line is considered to be fixed, and second, the perturbations...

56.
A mechanism for sandbar straightening by oblique wave incidence - Garnier, Roland; Falqués Serra, Albert; Calvete Manrique, Daniel; Thiebot, Jerome; Ribas Prats, Francesca
Breaker bars in the surf zone of sandy beaches generally evolve between straight bars parallel to the shore and meandering crescentic bars associated with intense (dangerous) currents flowing seaward through rip channels. Understanding the behavior of such systems is fundamental as they control the entire surf zone dynamics, the shape of the coastline, and the exchange of floating material with the shoreface. Although the mechanisms behind the meandering of an originally straight bar have been studied extensively, a clear physical explanation on the crescentic bar straightening was missing. Recent field observations have highlighted that this morphological reset can be due...

57.
Vulnerability of sandy coasts to climate variability - Idier, Deborah; Castelle, Bruno; Poumadère, Marc; Balouin, Yann; Bohn Bertoldo, Raquel; Bouchette, Fréderic; Boulahya, Faiza; Brivois, Olivier; Calvete Manrique, Daniel; Capo, Sylvain; Certain, Raphael; Charles, Elodie; Chateauminois, Eric; Delvallée, Etienne; Falqués Serra, Albert; Fattal, Paul; Garcin, Manuel; Garnier, Roland; Héquette, Arnaud; Larroudé, Philippe; Lecacheux, Sophie; Le Cozannet, Gonéri; Maanan, Mohamed; Mallet, Cyril; Maspataud, Aurélie; Oliveros, Carlos; Paillart, Martin; Parisot, Jean-Paul; Pedreros, Rodrigo; Robin, Nicolas; Robin, Marc; Romieu, Emmanuel; Ruz, Marie-Hélène; Thiebot, Jerome; Vinchon, Charlotte
The main objective of the VULSACO (VULnerability of SAndy COasts to climate change and anthropic pressure) project was to investigate present day and potential future vulnerability of sandy coasts at the 2030 horizon, i.e. on a time scale related to climate variability. The method, based on a multidisciplinary approach bringing together geologists, geographers, physicists, social psychologists, engineers and stakeholders, was structured around 4 axes: field data analysis; numerical modelling; analysis of governance and stakeholder perceptions; and development of vulnerability indexes. This approach was designed to investigate vulnerability at a local scale and was applied to 4 contrasting beaches located in...

58.
Bifurcations with imperfect SO(2) symmetry and pinning of rotating waves - Marqués Truyol, Francisco; Meseguer Serrano, Álvaro; Lopez, Juan M.; Pacheco, Jose Rafael; López Alonso, José Manuel
Rotating waves are periodic solutions in SO(2) equivariant dynamical systems. Their precession frequency changes with parameters and it may change sign, passing through zero. When this happens, the dynamical system is very sensitive to imperfections that break the SO(2) symmetry and the waves may become trapped by the imperfections, resulting in steady solutions that exist in a finite region in parameter space. This is the so-called pinning phenomenon. In this study, we analyse the breaking of the SO(2) symmetry in a dynamical system close to a Hopf bifurcation whose frequency changes sign along a curve in parameter space. The problem...

59.
From travelling waves to mild chaos: a supercritical bifurcation cascade in pipe flow - Mellibovsky Elstein, Fernando; Eckhardt, Bruno
We study numerically a succession of transitions in pipe Poiseuille flow that leads from simple travelling waves to waves with chaotic time-dependence. The waves at the origin of the bifurcation cascade possess a shift-reflect symmetry and are both axially and azimuthally periodic with wave numbers {\kappa} = 1.63 and n = 2, respectively. As the Reynolds number is increased, successive transitions result in a wide range of time dependent solutions that includes spiralling, modulated-travelling, modulated-spiralling, doubly-modulated-spiralling and mildly chaotic waves. We show that the latter spring from heteroclinic tangles of the stable and unstable invariant manifolds of two shift-reflect-symmetric modulated-travelling...

60.
Mode competition in cylindrical flows driven by sidewall oscillations - Panadès Guinart, Carles; Marqués Truyol, Francisco; Meseguer Serrano, Álvaro
The transition from a two-dimensional to three-dimensional flow in systems with spatial O(2) symmetry and spatiotemporal Z2 symmetry happens in many fluid systems, like wakes or periodically forced flows. In most of these systems, the dynamics after the first bifurcation is very complex and involves cascades of bifurcations in a very narrow parameter range. A numerical study of a flow in an enclosed cylindrical cavity driven by axial oscillations of the sidewall, which allows a detailed study of the secondary bifurcations and the corresponding mode interactions, is presented. The study focuses on a codimension-2 point that acts as the organizing...