UPCommons - E-prints UPC Universitat Politècnica de Catalunya
(67,447 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 126

41.
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...

43.
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...

44.
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...

45.
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...

46.
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...

47.
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.

48.
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...

49.
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...

50.
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...

51.
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.

52.
Understanding coastal morphodynamics using stability methods - Dodd, N.; Blondeaux, P.; Calvete Manrique, Daniel; Swart, Huib E. de; Falqués Serra, Albert; Hulscher, S.J.M.H.; Rózynski, G.; Vittori, G.
Stability methods, as they are applied in describing the initiation, growth and long term evolution of morphological features, are discussed. In particular, their use in describing large-scale, long-term rhythmic morphological features is highlighted. The analysis of such models indicates that many rhythmic bottom features arise from an inherent instability of a morphodynamical system, rather than being forced by external conditions. A synopsis of their theoretical
basis is given, and the assumptions commonly pertaining to their use are described. These models, which can be applied more efficiently than many other process-oriented models, are categorized, and the kind of information that they can...

53.
Nonlinear modelling of shoreface-connected ridges; impact of grain sorting and interventions - de Swart, Huib E.; Walgreen, M; Calvete Manrique, Daniel; Vis-Star, N C
The evolution of shoreface-connected sand ridges on micro-tidal inner shelves and the variations in the mean grain size over these ridges are investigated with process-based models. A review of previous studies is presented, as well as new results that concern the influence of grain sorting on the finite-amplitude behaviour of the ridges, the application of the model to La Barrosa beach and the role of wave-topography feedbacks. The ridges initially form due to morphodynamic self-organisation, in which the presence of waves and a storm-driven current are crucial. Predicted growth time scales, migration speeds, topography and spatial pattern of the mean...

54.
Non-linear response of shoreface-connected sand ridges - de Swart, Huib E.; Calvete Manrique, Daniel
A non-linear morphodynamic model of a microtidal coastal shelf is used to study the response of shoreface-connected sand ridges and the net sand balance of the shelf to large-scale interventions. The model describes the interaction between storm-driven currents and the erodible bottom. The transport of sediment comprises both bedload and suspended load contributions and is due to the joint action of waves (stirring of sediment from the bed) and net currents (causing transport). Three basic types of interventions are studied: extracting sand from ridges, nourishing sand at the shelf and constructing navigation channels. The model results indicate that for all...

55.
Growth of large-scale bed forms due to storm-driven and tidal currents: a model approach - Walgreen, M.; Calvete Manrique, Daniel; Swart, Huib E. de
An idealized morphodynamic model is used to gain further understanding about the formation and characteristics of shoreface-connected sand ridges and tidal sand banks on the continental shelf. The model consists of the 2D shallow water equations, supplemented with a sediment transport formulation and describes the initial feedback between currents and small amplitude bed forms. The behaviour of bed forms during both storm and fair weather conditions is analyzed. This is relevant in case of coastal seas characterized by tidal motion, where the latter causes continuous transport of sediment as bed load.
The new aspects of this work are the incorporation of...

56.
A field test of a linear stability model for crescentic bars - Tiessen, M C H; Leeuwen, S M Van; Calvete Manrique, Daniel; Dodd, N
A morphodynamical linear stability analysis is used to predict the natural development of crescentic bed patterns and rip channels. The purpose is to investigate whether this technique, which is useful for understanding the physics of emerging bed-forms, can be used to make quantitative predictions in the field, which may then be of use for coastal engineers.
To this end a morphodynamical linear stability model (Morfo60) is used to describe the development of crescentic bed patterns at the coast at the USACE Field Research Facility in Duck, North Carolina, USA. Wave, tide and alongshore-averaged bathymetry data recorded at Duck over a 2...

57.
Effect of depth-dependent wave stirring on the final amplitude of shoreface-connected sand ridges - Calvete Manrique, Daniel; Swart, Huib E. de; Falqués Serra, Albert
A nonlinear morphodynamic model is analysed to gain fundamental knowledge about the initial growth and long-term behaviour
of observed shoreface-connected sand ridges. Themodel describes quasi-steady, depth-averaged flow on a storm-dominated inner shelf with an erodible bottom and a transverse slope. Both bed load and suspended load sediment transport are incorporated. The formulations are linear with respect to the current and account for depth dependent stirring of sediment by waves as well as for the effect of local bed slopes.
A linear stability analysis has already revealed the initial growth of bed forms that resemble observed shoreface-connected
ridges. Here, a nonlinear analysis is carried...

58.
Linear evolution of a shoreface nourishment - Leeuwen, Van S.; Dodd, N.; Calvete Manrique, Daniel; Falqués Serra, Albert
The morphological evolution of a shoreface nourishment is investigated by interpreting the nourishment as a linear perturbation of the natural system. The nourishment is projected onto the subset of linear eigenmodes with negative growth rates of the morphodynamical system. The evolution of these linear modes then determines the temporal behaviour of the shoreface nourishment. The method is presented, and results are shown for shoreface nourishments of different length scales on a straight coast and subject to normal incidence. Shoreface nourishments are represented by their expansions according to the projection method on a 1:50 plane beach profile. All nourishments are shown...

59.
Rhythmic surf zone bars and morphodynamic self-organization - Falqués Serra, Albert; Dodd, N.; Garnier, Roland; Ribas Prats, Francesca; Machardy, L.C.; Larroude, P.; Calvete Manrique, Daniel; Sancho, F.
Work undertaken in the EU HUMOR project on morphodynamical modelling, particularly with regard to simulating and understanding
rhythmic surf zone bars and related morphodynamic self-organization, is presented. These features are reviewed and their engineering context stated. Hydrodynamical and morphodynamical models developed and/or applied within the HUMOR project in order to address these issues are
briefly presented. The linear stability modelling concept and stability studies using fully nonlinear models are contrasted. The stability of a shoreparallel bar under normal or oblique wave incidence is chosen as a test case for the different models. The results are compared and discussed. Lastly, modelling efforts and...

60.
Shoreline instability under low-angle wave incidence - Idier, Deborah; Falqués Serra, Albert; Ruessink, B.G.; Garnier, Roland
The growth of megacusps as shoreline instabilities is investigated by examining the coupling between wave transformation in the shoaling zone, long-shore transport in the surf zone, cross-shore transport, and morphological evolution. This coupling is known to drive a potential positive feedback in case of very oblique wave incidence, leading to an unstable shoreline and the
consequent formation of shoreline sandwaves. Here, using a linear stability model based on the one-line concept, we demonstrate that such instabilities can also develop in case of low-angle or shore-normal incidence, under certain conditions (small enough wave height and/or large enough beach slope). The wavelength and...