UPCommons - E-prints UPC Universitat Politècnica de Catalunya
(63,417 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 119

41.
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...

42.
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...

43.
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...

44.
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.

45.
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...

46.
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...

47.
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...

48.
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...

49.
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...

50.
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...

51.
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...

52.
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...

53.
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...

54.
Instabilities and inertial waves generated in a librating cylinder - López, J.M.; Marqués Truyol, Francisco
A librating cylinder consists of a rotating cylinder whose rate of rotation is modulated.
When the mean rotation rate is large compared with the viscous damping rate, the flow may support inertial waves, depending on the frequency of the modulation. The modulation also produces time-dependent boundary layers on the cylinder endwalls and sidewall, and the sidewall boundary layer flow in particular is susceptible to instabilities which can introduce additional forcing on the interior flow with time scales different from the modulation period. These instabilities may also drive and/or modify the inertial waves. In this paper, we explore such flows numerically using...

55.
The influence of wave direction on the morphological response of a double sandbar system - Thiebot, Jerome; Idier, Deborah; Garnier, Roland; Falqués Serra, Albert; Ruessink, B.G.
Nearshore sandbars are often characterized by three-dimensional bed patterns. To analyze the influence of wave direction on the morphological response of a double sandbar system, this paper uses the 2DH nonlinear surf zone model MORFO55. Depending on wave direction, different morphologies emerge as free instabilities. These morphological responses differ in terms of geometry (shape of the alongshore rhythmic patterns) and temporal evolution. Nearly shore-normal waves favor the emergence of crescentic patterns along both the inner and outer bars. These instabilities arise from “bedsurf” coupling, which is the positive feedback between the bed perturbations and the wave-breaking patterns resulting in rip-cell...

56.
Dissipative solitons in binary fluid convection - Mercader Calvo, María Isabel; Batiste Boleda, Oriol; Alonso Maleta, María Aránzazu; Knobloch, Edgar
A horizontal layer containing a miscible mixture of two fluids can produce dissipative solitons when heated from below. The physics of the system is described, and dissipative solitons are computed using numerical continuation for three distinct sets of experimentally realizable parameter values. The stability of the solutions is investigated using direct numerical integration in time and related to the stability properties of the competing periodic state.

57.
Observations and large-eddy simulations of entrainment in the sheared sahelian boundary layer - Canut, Guylaine; Couvreux, Fleur; Lothon, Marie; Pino González, David; Saïd, Frédérique
At the top of the planetary boundary layer, the entrainment of air, which incorporates
dry and warm air from the free troposphere into the boundary layer, is a key process
for exchanges with the free troposphere since it controls the growth of the boundary layer.
Here, we focus on the semi-arid boundary layer where the entrainment process is analyzed
using aircraft observations collected during the African Monsoon Multidisciplinary Analysis
experiment and large-eddy simulations. The role of the entrainment is specifically enhanced
in this region where very large gradients at the planetary boundary-layer top can be found
due to the presence of the moist, cold monsoon flow on...

59.
Transitions to three-dimensional flows in a cylinder driven by oscillations of the sidewall - Panadès Guinart, Carles; Marqués Truyol, Francisco; Lopez, Juan M.
The transition from two-dimensional to three-dimensional flows in a finite circular
cylinder driven by an axially oscillating sidewall is explored in detail. The complete
symmetry group of this flow, including a spatio-temporal symmetry related to the
oscillating sidewall, is Z2xO(2). Previous studies in flows with the same symmetries,
such as symmetric bluff-body wakes and periodically forced rectangular cavities,
were unable to obtain the theoretically predicted bifurcation to modulated travelling
waves. In the simpler cylindrical geometry, where the azimuthal direction is physically
periodic, we have found these predicted modulated travelling waves as stable fully
saturated nonlinear solutions for the first time. A careful analysis of the base states and...