UPCommons - E-prints UPC Universitat Politècnica de Catalunya
(72,177 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 135

41.
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...

42.
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...

43.
Travelling convectons in binary fluid convection - Mercader Calvo, María Isabel; Batiste Boleda, Oriol; Alonso Maleta, María Aránzazu; Knobloch, Edgar
Binary fluid mixtures with a negative separation ratio heated from below exhibit steady spatially localized states called convectons for supercritical Rayleigh numbers. With no-slip, fixed-temperature, no-mass-flux boundary conditions at the top and bottom stationary odd- and even-parity convectons fall on a pair of intertwined branches connected by branches of travelling asymmetric states. In appropriate parameter regimes the stationary convectons may be stable. When the boundary condition on the top is changed to Newton’s law of cooling the odd-parity convectons start to drift and the branch of odd-parity convectons breaks up and reconnects with the branches of asymmetric states. We explore...

44.
Computation of azimuthal waves and their stability in thermal convection in rotating spherical shells with application to the study of a double-Hopf bifurcation - Sánchez Umbría, Juan; García González, Fernando; Net Marcé, Marta
A methodology to compute azimuthal waves, appearing in thermal convection of a pure fluid contained in a rotating spherical shell, and to study their stability is presented. It is based on continuation, Newton-Krylov, and Arnoldi methods. An application to the study of a double-Hopf bifurcation of the basic state is shown for Ekman and Prandtl numbers E=10−4 and σ=0.1, respectively, radius ratios η∈[0.32,0.35], Rayleigh numbers R∈[1.8×105,6×105], and nonslip and perfectly conducting boundary conditions. The knowledge of the bifurcation diagrams, including the unstable solutions, allows one to understand the coexistence of stable thermal Rossby waves of different azimuthal wave numbers at...

45.
A parallel algorithm for the computation of invariant tori in large-scale dissipative systems - Sánchez Umbría, Juan; Net Marcé, Marta
A parallelizable algorithm to compute invariant tori of high-dimensional dissipative systems, obtained upon discretization of PDEs is presented. The size of the set of equations to be solved is only a small multiple of the dimension of the original system. The sequential and parallel implementations are compared with a previous method (Sánchez et al. (2010)) [11], showing that important savings in wall-clock time can be achieved. In order to test it, a thermal convection problem of a binary mixture of fluids has been used. The new method can also be applied to problems with very low rotation numbers, for which...

47.
Nocturnal offshore precipitation near the Mediterranean coast of the Iberian Peninsula - Mazón Bueso, Jordi; Pino González, David
While nocturnal offshore precipitation, which
produces rain cells and bands, has been studied in tropical
areas, few studies have analyzed the occurrence of this
phenomenon at higher latitudes. Using radar reflectivity,
nocturnal rainfall in the western Mediterranean area has
been detected near the coast of the Iberian Peninsula and
North Africa. More than 50 events have been recorded
since 2009. MM5 mesoscale simulations of some of the
recorded events allow us to establish that the most likely
causes for these precipitation events are: (1) the interaction
between cold air masses conducted by drainage and kata-
batic winds, and (2) a wetter and warmer synoptic wind.
Two different episodes are presented: one in...

48.
The role of sea-land air thermal difference, shape of the coastline and sea surface temperature in the nocturnal offshore convection - Mazón Bueso, Jordi; Pino González, David
Nocturnal precipitation cells and lines occur near the coastline in the whole Mediterranean basin in all seasons.
The precipitation events are mainly located in areas where coastal mountain ranges and rivers enhance
convergence though the interaction of nocturnal mesoscale and local flows (land breeze, katabatic and
drainages winds) with prevailing synoptic wind or with other mesoscale and local flows. The methodology used
here to study this phenomenon consists of three stages. First, the Tropical Rainfall Measuring Mission
(TRMM) radar satellite database is used to detect nocturnal precipitation near the coastline, from 18 to 09
UTC. An event is included in the study if the 3 hours...

49.
A conceptual framework to quantify the influence of convective boundary layer development on carbon dioxide mixing ratios - Pino González, David; Vilà-Guerau de Arellano, Jordi; Peters, Wouter; Schröter, Joel; van Heerwaarden, Chiel C.; Krol, Maarten
Interpretation of observed diurnal carbon dioxide (CO2) mixing ratios near the surface requires knowledge of the local dynamics of the planetary boundary layer. In this paper, we study the relationship between the boundary layer dynamics and the CO2 budget in convective conditions through a newly derived set of analytical equations. From these equations, we are able to quantify how uncertainties in boundary layer dynamical variables or in the morning CO2 distribution in the mixed-layer or in the free atmosphere (FA) influence the bulk CO2 mixing ratio.
We find that the largest uncertainty incurred on the mid-day CO2 mixing ratio comes from...

50.
The role of nocturnal Low-Level-Jet in nocturnal convection and rainfalls in the west Mediterranean coast: the episode of 14 December 2010 in northeast of Iberian Peninsula - Mazón Bueso, Jordi; Pino González, David
The night of 14 December 2010 radar images of the Spanish Weather Agency recorded a large rain
band that moved o
ff
shore at the Northeast coast of the Iberian Peninsula. MM5 mesoscale model is used to
study the atmospheric dynamics during that day. A Nocturnal Low Level Jet (NLLJ) generated by an inertial
oscillation that brings cold air to the coast from inland has been simulated in the area. This cold air interacts
with a warmer air mass some kilometers o
ff
shore. According to the MM5 mesoscale model simulation, the cold
air enhances upward movements of the warm air producing condensation. Additionally, there is a return flow...

52.
The upper-mass limit for the formation of super-agb stars and the dredge-out phenomenon - Gil Pons, Pilar; Doherty, Carolyn L.
We have computed the evolution of Super-AGB stars from the main sequence
and up to a few hundred thermal pulses, with special attention to the low metallicity cases
(Z = 10 10; 10 5; 10 4 and 10 3). Our computations have been performed using time–
dependent mixing and new opacity tables that admit variations in the abundances of carbon
and oxygen. By following the evolution along the main central burning stages and the
early TP-SAGB, we resolve the upper mass limits for the formation of TP-SAGB stars and
determine the mass range at which the dredge-out phenomenon occurs. This phenomenon
involves the merger of a convective...

53.
On the fate of extremely low metallicity stars - Gil Pons, Pilar; Gutierrez Cabello, Jorge Luis; García-Berro Montilla, Enrique; Althaus, Leandro Gabriel
We have computed and analyzed the evolution of intermediate-mass stars of metallicity
Z = 10−5 from the main sequence until the early stages of the TP-(S)AGB phase. In order to check
the influence of mixing we have performed our calculations using two different evolutionary codes,
EVOLVE — which does not include diffusion but allows for overshooting— and LPCODE, which
includes diffusion and overshooting “à-la-Herwig”. Important differences appear during the TP–
AGB phase, as the calculations done with EVOLVE lead to an almost negligible third dredge-up,
whereas the sequences computed with LPCODE show important third dredge-up and, therefore,
significant enrichment in metals of the envelope. This fact has...

54.
Spiral vortices between concentric cylinders - Sánchez Umbría, Juan; Crespo Artiaga, Daniel; Marqués Truyol, Francisco
Spiral vortices appearing in Couette- Taylor flows are studied by means of numerical simulation. Transition curves from Couctte to spiral vortices for different radius ratios and wavenumbers have been calculated in order to test our technique. Critical Reynolds numbers, angular velocities and slopes of the spirals at the onset of the instability agree with previous results [1]. Non-linear solutions obtained by a pseudospectral collocation method are studied, and they show a weak net axial ftow. In arder to counteract this effect, which is absent in the usual experimental set-up, an axial pressure gradient has been included. This procedure has proved...

55.
Numerical study of the onset of thermosolutal convection in rotating spherical shells - Net Marcé, Marta; García González, Fernando; Sánchez Umbría, Juan
The influence of an externally enforced compositional gradient on the onset of convection of a mixture of two components in a rotating fluid spherical shell is studied for Ekman numbers E = 10−3 and E = 10−6, Prandtl numbers σ = 0.1, 0.001, Lewis numbers τ = 0.01, 0.1, 0.8, and radius ratio η = 0.35. The Boussinesq approximation of the governing equations is derived by taking the denser component of the mixture for the equation of the concentration. Differential and internal heating, an external compositional gradient, and the Soret and Dufour effects are included in the model. By neglecting...

56.
Edge state in pipe flow experiments - De Lozar, Alberto; Mellibovsky Elstein, Fernando; Avila, M.; Hof, Björn
Recent numerical studies suggest that in pipe and related shear flows, the region of phase space separating laminar from turbulent motion is organized by a chaotic attractor, called an edge state, which mediates the transition process. We here confirm the existence of the edge state in laboratory experiments. We observe that it governs the dynamics during the decay of turbulence underlining its potential relevance for turbulence control. In addition we unveil two unstable traveling wave solutions underlying the experimental flow fields. This observation corroborates earlier suggestions that unstable solutions organize turbulence and its stability border.

57.
On the invariant causal characterization of singularities in spherically symmetric spacetimes - Fayos Vallés, Francisco; Torres Herrera, Ramon
The causal character of singularities is often studied in relation to the existence of naked singularities and the subsequent possible violation of the cosmic censorship conjecture. Generally, one constructs a model in the framework of general relativity described in some specific coordinates and finds an ad hoc procedure to analyze the character of the singularity. In this paper, we show that the causal character of the zero-areal-radius (R = 0) singularity in spherically symmetric models is related to some specific invariants. In this way, if some assumptions are satisfied, one can ascertain the causal character of the singularity algorithmically through...

58.
On the use of linear stability model to characterize the morphological behaviour of a double bar system. Application to Truc Vert beach (France) - Brivois, Olivier; Idier, Déborah; Thiébot, Jérôme; Castelle, Bruno; Le Cozannet, Gonéri; Calvete Manrique, Daniel
Sandy barred beaches are often characterized by the presence of rhythmic patterns such as crescentic bars. In this article, a linear stability analysis (LSA) model is used to characterize the morphological behaviour of the double bar system of Truc Vert beach. Using a limited number of combinations of representative bathymetries, wave classes and water levels, the morphodynamic response of the system is analysed, focussing on the geometrical characteristics of 3D patterns generated with the model. These characteristics are described and then compared with available observations. The shapes and the wavelengths of the instabilities predicted by the model compare well with...

59.
A model for grain-size sorting over tidal sand ridges - Walgreen, M; de Swart, Huib E.; Calvete Manrique, Daniel
A model was developed and analyzed to quantify the effect of graded sediment on the formation of tidal sand ridges. Field data reveal coarse (fine) sediment at the crests (in the troughs), but often phase shifts between the mean grain-size distribution and the bottom topography occur. Following earlier work, this study is based on a linear stability analysis of a basic state with respect to small bottom perturbations. The basic state describes an alongshore tidal current on a coastal shelf. Sediment is transported as bed load and dynamic hiding effects are accounted for. A one-layer model for the bed evolution...

60.
Modelling the formation and the nonlinear evolution of crescentic bars of the Aquitanian coast - Garnier, Roland; Bonneton, P.; Falqués Serra, Albert; Calvete Manrique, Daniel
Crescentic bars emerge as free instabilities of the coupling between topography and water motion. Their long term behaviour will be studied in the real case of the French Aquitaine beaches by using the 2DH numerical model MORFO55. For the first time, the equilibrium state of a crescentic bar system is obtained, in the case of steady incident wave conditions. Due to non linear interactions, the final wave length of the system does not inevitably increase with the incident wave height. Finally, the study of variable incident wave conditions suggests that the equilibrium state may depend on the initial conditions.