UPCommons - E-prints UPC Universitat Politècnica de Catalunya
(83,371 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 164

41.
Application of multifractal analysis to the study of SAR features and oil spills on the ocean surface - Tarquis Alfonso, Ana Maria; Platonov, A.; Matulka, Anna Magdalena; Grau, J.; Sekula, Emil; Diez, M.; Redondo Apraiz, José Manuel
The use of synthetic aperture radar (SAR) to investigate the ocean surface provides a wealth of useful information that is very seldom used to its full potential. Here we will discuss the application of multifractal techniques to detect oil spills and the dynamic state of the sea regarding turbulent diffusion. We present different techniques in order to relate the shape of the multifractal spectral functions and the maximum fractal dimension to the behaviour of the ocean surface. We compare eddy and sheared dominated flows with convective driven flows and discuss the different features and observation methods. We also compare the...

42.
Modeling the response of shoreface-connected sand ridges to sand extraction on an inner shelf - Nnafie, abdel; Swart, Huib E. de; Calvete Manrique, Daniel; Garnier, Roland
Shoreface-connected sand ridges are rhythmic bedforms that occur on many storm-dominated inner shelves. The ridges span several kilometers, are a few meters high, and they evolve on a timescale of centuries. A process-based model is used to gain a fundamental insight into the response of these ridges to extraction of sand. Different scenarios of sand extraction (depth, location, and geometry of the extraction area; multiple sand extractions) are imposed. For each scenario, the response timescale as well as the characteristics of the new equilibrium state are determined. Results show that ridges partially restore after extraction, i.e., the disturbed bathymetry recovers...

43.
Singularity free gravitational collapse in an effective dynamical quantum spacetime - Torres Herrera, Ramon; Fayos Vallés, Francisco
We model the gravitational collapse of heavy massive shells including its main quantum corrections. Among these corrections, quantum improvements coming from Quantum Einstein Gravity are taken into account, which provides us with an effective quantum spacetime. Likewise, we consider dynamical Hawking radiation by modeling its back-reaction once the horizons have been generated. Our results point towards a picture of gravitational collapse in which the collapsing shell reaches a minimum non-zero radius (whose value depends on the shell initial conditions) with its mass only slightly reduced. Then, there is always a rebound after which most (or all) of the mass evaporates...

44.
Numerical simulations of thermal convection in rotating spherical shells under laboratory conditions - García González, Fernando; Sánchez Umbría, Juan; Net Marcé, Marta
An exhaustive study, based on numerical three-dimensional simulations, of the Boussinesq thermal convection of a fluid confined in a rotating spherical shell is presented. A moderately low Prandtl number fluid (ro = 0.1) bounded by differentially-heated solid spherical shells is mainly considered. Asymptotic power laws for the mean physical properties of the flows are obtained in the limit of low Rossby number and compared with laboratory experiments and with previous numerical results computed by taking either stress-free boundary conditions or quasi-geostrophic restrictions, and with geodynamo models. Finally, using parameters as close as possible to those of the Earth's outer core,...

45.
How kilometric sandy shoreline undulations correlate with wave and morphology characteristics: preliminary analysis on the Atlantic coast of Africa - Idier, Deborah; Falqués Serra, Albert
Sandy coasts are characterized by a number of rhythmic patterns like, amongst others, shoreline undulations or sandwaves at a kilometric scale. One hypothesis for their formation is that high angle waves (large incidence angle with respect to shore normal) could induce an instability of the shoreline (Ashton et al., 2001). More recently, a scaling for their wavelength has also been proposed (van den Berg et al., 2014). The existing studies rely mainly on modelling but quantitative field tests are lacking. We aim at investigating how both the formation hypothesis of these shoreline undulations and the theoretical scaling do fit with...

46.
On the entrainment coefficient in a forced plume: quantitative effects of source parameter - Matulka, Anna Magdalena; Lopez Gonzalez-Nieto, Pilar; Redondo Apraiz, José Manuel; Tarquis Alfonso, Ana Maria
The behavior of a forced plume is mainly controlled by the source buoyancy and momentum fluxes and the efficiency of turbulent mixing between the plume and the ambient fluid (stratified or not). The interaction between the plume and the ambient fluid controls the plume dynamics and is usually represented by the entrainment coefficient aE. Commonly used one-dimensional models incorporating a constant entrainment coefficient are fundamental and very useful for predictions in geophysical flows and industrial situations. Nevertheless, if the basic geometry of the flow changes, or the type of source or the environmental fluid conditions (e.g., level of turbulence, shear,...

47.
Transition of the stellar initial mass function explored using binary population synthesis - Suda, Takuma; Komiya, Yutaka; Yamada, Shimako; Katsuta, Yutaka; Aoki, Wako; Gil Pons, Pilar; Doherty, Carolyn L.; Campbell, Simon W.; Wood, Peter R.; Fujimoto, Masayuki Y.
The stellar initial mass function (IMF) plays a crucial role in determining the number of surviving stars in galaxies, the chemical composition of the interstellar medium, and the distribution of light in galaxies. A key unsolved question is whether the IMF is universal in time and space. Here we use state-of-the-art results of stellar evolution to show that the IMF of our Galaxy made a transition from an IMF dominated by massive stars to the present-day IMF at an early phase of the Galaxy formation. Updated results from stellar evolution in a wide range of metallicities have been implemented in...

48.
Secondary flows in a laterally heated horizontal cylinder - Mercader Calvo, María Isabel; Sánchez Casals, Odalys de la Caridad; Batiste Boleda, Oriol
In this paper we study the problem of thermal convection in a laterally heated, finite, horizontal cylinder. We consider cylinders of moderate aspect ratio (height/diameter approximate to 2) containing a small Prandtl number fluid (sigma < 0.026) typical of molten metals and molten semiconductors. We use the Navier-Stokes and energy equations in the Boussinesq approximation to calculate numerically the basic steady states, analyze their linear stability, and compute some nonlinear secondary flows originated from the instabilities. All the calculated flows and the stability analysis are characterized by their symmetry properties. Due to the confined cylindrical geometry, -presence of lateral walls...

49.
Rapidly rotating cylinder flow with an oscillating sidewall - López Moscat, Juan Manuel; Marqués Truyol, Francisco
We present numerical simulations of a flow in a rapidly rotating cylinder subjected to a time-periodic forcing via axial oscillations of the sidewall. When the axial oscillation frequency is less than twice the rotation frequency, inertial waves in the form of shear layers are present. For very fast rotations, these waves approach the form of the characteristics predicted from the linearized inviscid problem first studied by Lord Kelvin. The driving mechanism for the inertial waves is the oscillating Stokes layer on the sidewall and the corner discontinuities where the sidewall meets the top and bottom end walls. A detailed numerical...

50.
Confined rotating convection with large Prandtl number: centrifugal effects on wall modes - Curbelo Hernández, Jezabel; López Moscat, Juan Manuel; Mancho Sánchez, Ana María; Marqués Truyol, Francisco
Thermal convection in a rotating cylinder with a radius-to-height aspect ratio of G=4 for fluids with large Prandtl number is studied numerically. Centrifugal buoyancy effects are investigated in a regime where the Coriolis force is relatively large and the onset of thermal convection is in the so-called wall modes regime, where pairs of hot and cold thermal plumes ascend and descend in the cylinder sidewall boundary layer, forming an essentially one-dimensional pattern characterized by the number of hot and cold plume pairs. In our numerical study, we use the physical parameters corresponding to aqueous mixtures of glycerine with mass concentration...

51.
Rapid and sudden advection of warm and dry air in the Mediterranean Basin - Mazón Bueso, Jordi; Pino González, David; Barriendos, Mariano
Rapid advection of extremely warm and dry air is studied during two events in the Mediterranean Basin. On 27 August 2010 a rapid advection of extremely warm and dry air affected the northeast Iberian Peninsula during a few hours. At the Barcelona city center, the temperature reached 39.3 ° C, which is the maximum temperature value recorded during 230 yr of daily data series. On 23 March 2008 a rapid increase of temperature and drop of relative humidity were recorded for a few hours in Heraklion (Crete). During the morning on that day, the recorded temperature reached 34 °C for...

52.
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....

53.
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)...

54.
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.

55.
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...

56.
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...

57.
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...

58.
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...

59.
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...

60.
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...