UPCommons - E-prints UPC Universitat Politècnica de Catalunya
(74,555 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 1 - 20 de 136

1.
Singularity-free gravitational collapse and asymptotic safety - Torres Herrera, Ramon
A general class of quantum improved stellar models with interiors composed of non-interacting (dust) particles is obtained and analyzed in a framework compatible with asymptotic safety. First, the effective exterior, based on the Quantum Einstein Gravity approach to asymptotic safety is presented and, second, its effective compatible dust interiors are deduced. The resulting stellar models appear to be devoid of shell-focusing singularities. (C) 2014 The Author. Published by Elsevier B.V.

2.
Pressure effects on the performance of external gear pumps under cavitation - Campo Sud, David del; Castilla López, Roberto; Raush Alviach, Gustavo Adolfo; Gámez Montero, Pedro Javier; Codina Macià, Esteban
The numerical analysis of an external gear pump with cavitation effects has been validated with experimental data obtained by applying Time-Resolved Particle Image Velocimetry. The effect of inlet and outlet pressure on volumetric efficiency has been studied numerically. First, the Particle Image Velocimetry method was used to analyze the two-dimensional velocity field in the middle plane of the suction chamber of the gear pump. The main improvement, with respect to previous similar analysis is the use of alginate micro particles as tracers. It is seen that the two-dimensional model is able to characterize the flow field of the real pump...

3.
Pattern selection near the onset of convection in binary mixtures in cylindrical cells - Alonso Maleta, Arantxa; Mercader Calvo, María Isabel; Batiste Boleda, Oriol
We report numerical investigations of three-dimensional pattern formation of binary mixtures in a vertical cylindrical container heated from below. Negative separation ratio mixtures, for which the onset of convection occurs via a subcritical Hopf bifurcation, are considered. We focus on the dynamics in the neighbourhood of the initial oscillatory instability and analyze the spatio-temporal properties of the patterns for different values of the aspect ratio of the cell, 0.25 less than or similar to Gamma less than or similar to 11 (Gamma equivalent to R/d, where R is the radius of the cell and d its height). Despite the oscillatory...

4.
Observational and Numerical Simulation Study of a Sequence of Eight Atmospheric Density Currents in Northern Spain - Soler Duffour, Maria Rosa; Udina, Mireia; Ferreres Soler, Enriqueta
A sequence of eight atmospheric density current fronts occurred in consecutive days are identified and analyzed using micrometeorological time series and numerical simulations. Observations were collected in the context of the INTERCLE project, which took place from September 2002 to November 2003 at the CIBA (Research Centre for the Lower Atmosphere) site located over the northern Spanish plateau. Numerical simulations used the Weather Research and Forecast (WRF) model with fine horizontal resolution (1 km). Both observations and simulations agree that the arrival of the density currents are characterized by a sharp change in temperature, wind velocity, wind direction and specific...

5.
Observational and Numerical Simulation Study of a Sequence of Eight Atmospheric Density Currents in Northern Spain - Soler Duffour, Maria Rosa; Udina, Mireia; Ferreres Soler, Enriqueta
A sequence of eight atmospheric density current fronts occurred in consecutive days are identified and analyzed using micrometeorological time series and numerical simulations. Observations were collected in the context of the INTERCLE project, which took place from September 2002 to November 2003 at the CIBA (Research Centre for the Lower Atmosphere) site located over the northern Spanish plateau. Numerical simulations used the Weather Research and Forecast (WRF) model with fine horizontal resolution (1 km). Both observations and simulations agree that the arrival of the density currents are characterized by a sharp change in temperature, wind velocity, wind direction and specific...

6.
Three-dimensional instabilities in a discretely heated annular flow: onset of spatio-temporal complexity via defect dynamics - Marqués Truyol, Francisco; Lopez, Juan M
The transition to three-dimensional and unsteady flow in an annulus with a discrete heat source on the inner cylinder is studied numerically. For large applied heat flux through the heater (large Grashof number Gr), there is a strong wall plume originating at the heater that reaches the top and forms a large scale axisymmetric wavy structure along the top. For Gr approximate to 6 x 109, this wavy structure becomes unstable to three-dimensional instabilities with high azimuthal wavenumbers m similar to 30, influenced by mode competition within an Eckhaus band of wavenumbers. Coexisting with some of these steady three-dimensional states,...

7.
La rubinada de Santa Tecla a Tàrrega (23 de Setembre de 1874) - Barriendos Valve, Mariano; Tuset Mestre, Jordi; Mazón Bueso, Jordi; Pino González, David; Ruiz-Bellet, Josep Lluis; Balasch Solanes, Josep Carles
A partir de la recopilación histórica de los datos documentales disponibles y aplicando una metodología multidisci-
plinar con modelos de simulación hidráulica, hidrológica y meteorológica, se han reconstruido las características prin-
cipales de la devastadora crecida hidrológica (o
rubinada
de Santa Tecla) que padeció la ciudad de Tàrrega en la
madrugada del 23 de septiembre de 1874, y se describe el contexto meteorológico que condicionó la tormenta. El
caudal punta fue excepcional, causando una de las mayores mortandades de la historia de Tàrrega. Las circunstan-
cias actuales no han cambiado, lo que no permite pensar que una avenida similar no pueda volver a ocurrir.
The main characteristics of...

8.
Evolution and CNO yields of Z = 10-5 stars and possible effects on carbon-enhanced metal-poor production - Gil Pons, Pilar; Doherty, Carolyn L.; Lau, Herbert B.; Campbell, Simon W; Suda, Takuma; Guilani, Shervin Mansouri; Gutierrez Cabello, Jorge Luis; Lattanzio, John C.
Aims. Our main goals are to get a deeper insight into the evolution and final fates of intermediate-mass, extremely metal-poor (EMP) stars. We also aim to investigate the C, N, and O yields of these stars.
Methods. Using the Monash University Stellar Evolution code MONSTAR we computed and analysed the evolution of stars of metallicity Z = 10-5 and masses between 4 and 9 M¿, from their main sequence until the late thermally pulsing (super) asymptotic giant branch, TP-(S)AGB phase.
Results. Our model stars experience a strong C, N, and O envelope enrichment either due to the second dredge-up process, the dredge-out...

9.
Streamwise-localized solutions at the onset of turbulence in pipe flow - Àvila Cañellas, Marc; Mellibovsky Elstein, Fernando; Roland, N.; Hof, Björn
Although the equations governing fluid flow are well known, there are no analytical expressions that describe the complexity of turbulent motion. A recent proposition is that in analogy to low dimensional chaotic systems, turbulence is organized around unstable solutions of the governing equations which provide the building blocks of the disordered dynamics. We report the discovery of periodic solutions which just like intermittent turbulence are spatially localized and show that turbulent transients arise from one such solution branch. 2013 American Physical Society.

10.
Subcritical equilibria in Taylor-Couette flow - Deguchi, Kengo; Meseguer Serrano, Álvaro; Mellibovsky Elstein, Fernando
Nonlinear equilibrium states characterized by strongly localized vortex pairs are calculated in the linearly stable parameter region of counterrotating Taylor-Couette flow. These subcritical states are rotating waves whose region of existence is consistent with the critical threshold for relaminarization observed in experiments. For sufficiently rapid outer cylinder rotation the solutions extend beyond the static inner cylinder case to corotation, thus exceeding, for the first time, the boundary defined by the inviscid Rayleigh's stability criterion.

11.
Role of the residual layer and large-scale subsidence on the development and evolution of the convective boundary layer - Blay Carreras, Estel; Pino González, David; Vilà Guerau de Arellano, Jordi; van de Boer, Anneke; de Coster, Olivier; Darbieu, Clara; Hartogensis, Oskar; Lohou, Fabienne; Lothon, Marie; Pietersen, Henk
Observations, mixed-layer theory and the Dutch Large-Eddy Simulation model (DALES) are used to analyze the dynamics of the boundary layer during an intensive operational period (1 July 2011) of the Boundary Layer Late Afternoon and Sunset Turbulence campaign. Continuous measurements made by remote sensing and in situ instruments in combination with radio soundings, and measurements done by remotely piloted aircraft systems and two manned aircrafts probed the vertical structure and the temporal evolution of the boundary layer during the campaign. The initial vertical profiles of potential temperature, specific humidity and wind, and the temporal evolution of the surface heat and...

12.
Effects of sea level rise on the formation and drowning of shoreface-connected sand ridges, a model study - Nnafie, Abdel; Swart, Huib E. de; Calvete Manrique, Daniel; Garnier, Roland
Shoreface-connected sand ridges occur on many storm-dominated inner shelves. These rhythmic features have an along-shelf spacing of 2-10 km, a height of 1-12 m, they evolve on timescales of centuries and they migrate several meters per year. An idealized model is used to study the impact of sea level rise on the characteristics of the sand ridges during their initial and long-term evolution. Different scenarios (rates of sea level rise, geometry of inner shelf) are examined. Results show that with increasing sea level the height of sand ridges increases and their migration decreases until they eventually drown. This latter occurs...

13.
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...

14.
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...

15.
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...

16.
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,...

17.
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...

18.
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,...

19.
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...

20.
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...