Archive ouverte UNIGE | last documents for author 'Maura Brunetti'https://archive-ouverte.unige.ch/Latest objects deposited in the Archive ouverte UNIGE for author 'Maura Brunetti'engStabilization of Unsteady Nonlinear Waves by Phase-Space Manipulationhttps://archive-ouverte.unige.ch/unige:151325https://archive-ouverte.unige.ch/unige:151325We introduce a dynamic stabilization scheme universally applicable to unidirectional nonlinear coherent waves. By abruptly changing the waveguiding properties, the breathing of wave packets subject to modulation instability can be stabilized as a result of the abrupt expansion a homoclinic orbit and its fall into an elliptic fixed point (center). We apply this concept to the nonlinear Schrödinger equation framework and show that an Akhmediev breather envelope, which is at the core of Fermi-Pasta-Ulam-Tsingou recurrence and extreme wave events, can be frozen into a steady periodic (dnoidal) wave by a suitable variation of a single external physical parameter. We experimentally demonstrate this general approach in the particular case of surface gravity water waves propagating in a wave flume with an abrupt bathymetry change. Our results highlight the influence of topography and waveguide properties on the lifetime of nonlinear waves and confirm the possibility to control them.Wed, 28 Apr 2021 13:45:37 +0200Separatrix crossing and symmetry breaking in NLSE-like systems due to forcing and dampinghttps://archive-ouverte.unige.ch/unige:145692https://archive-ouverte.unige.ch/unige:145692We theoretically and experimentally examine the effect of forcing and damping on systems that can be described by the nonlinear Schrödinger equation (NLSE), by making use of the phase-space predictions of the three-wave truncation. In the latter, the spectrum is truncated to only the fundamental frequency and the upper and lower sidebands. Our experiments are performed on deep water waves, which are better described by the higher-order NLSE, the Dysthe equation. We therefore extend our analysis to this system. However, our conclusions are general for NLSE systems. By means of experimentally obtained phase-space trajectories, we demonstrate that forcing and damping cause a separatrix crossing during the evolution. When the system is damped, it is pulled outside the separatrix, which in the real space corresponds to a phase-shift of the envelope and therefore doubles the period of the Fermi–Pasta–Ulam–Tsingou recurrence cycle. When the system is forced by the wind, it is pulled inside the separatrix, lifting the phase-shift. Furthermore, we observe a growth and decay cycle for modulated plane waves that are conventionally considered stable. Finally, we give a theoretical demonstration that forcing the NLSE system can induce symmetry breaking during the evolution.Wed, 02 Dec 2020 13:56:44 +0100Stabilization of uni-directional water-wave trains over an uneven bottomhttps://archive-ouverte.unige.ch/unige:139237https://archive-ouverte.unige.ch/unige:139237We study the evolution of nonlinear surface gravity water-wave packets developing from modulational instability over an uneven bottom. A nonlinear Schr"odinger equation (NLSE) with coefficients varying in space along propagation is used as a reference model. Based on a low-dimensional approximation obtained by considering only three complex harmonic modes, we discuss how to stabilize a one-dimensional pattern in the form of train of large peaks sitting on a background and propagating over a significant distance. Our approach is based on a gradual depth variation, while its conceptual framework is the theory of autoresonance in nonlinear systems and leads to a quasi-frozen state. Three main stages are identified: amplification from small sideband amplitudes, separatrix crossing, and adiabatic conversion to orbits oscillating around an elliptic fixed point. Analytical estimates on the three stages are obtained from the low-dimensional approximation and validated by NLSE simulations. Our result will contribute to understand dynamical stabilization of nonlinear wave packets and the persistence of large undulatory events in hydrodynamics and other nonlinear dispersive media.Tue, 11 Aug 2020 10:44:00 +0200Reconciling different formulations of viscous water waves and their mass conservationhttps://archive-ouverte.unige.ch/unige:137162https://archive-ouverte.unige.ch/unige:137162The viscosity of water induces a vorticity near the free surface boundary. The resulting rotational component of the fluid velocity vector greatly complicates the water wave system. Several approaches to close this system have been proposed. Our analysis compares three common sets of model equations. The first set has a rotational kinematic boundary condition at the surface. In the second set, a gauge choice for the velocity vector is made that cancels the rotational contribution in the kinematic boundary condition, at the cost of rotational velocity in the bulk and a rotational pressure. The third set circumvents the problem by introducing two domains: the irrotational bulk and the vortical boundary layer. This comparison puts forward the link between rotational pressure on the surface and vorticity in the boundary layer, addresses the existence of nonlinear vorticity terms, and shows where approximations have been used in the models. Furthermore, we examine the conservation of mass for the three systems, and how this can be compared to the irrotational case.Tue, 16 Jun 2020 15:31:06 +0200Nonlinear propagation of light pulses and water waves under the influence of damping and forcinghttps://archive-ouverte.unige.ch/unige:127104https://archive-ouverte.unige.ch/unige:127104This work reports results on the relationship between the nonlinear propagation of optical pulses and of water waves. What unites these two phenomena is that the envelope of the electric ﬁeld of the light pulse, and of the water wave, can both be described by the same propagation equation: the nonlinear Schrödinger equation (NLSE). In the practical context of rogue waves on the ocean the optical system can serve as a table-top experiment. On a more fundamental level, the comparison allows cross-fertilization of insights. For water waves, the influence of viscous damping is examined with respect to the dispersion relation and the mass conservation. In addition, the effect of wind forcing on water waves is examined, specifically with respect to the spectral shift. Finally, triggering of optical filamentation by a turbulent atmosphere is demonstrated.Wed, 27 Nov 2019 11:32:51 +0100Co-existing climate attractors in a coupled aquaplanethttps://archive-ouverte.unige.ch/unige:122768https://archive-ouverte.unige.ch/unige:122768The first step in exploring the properties of dynamical systems like the Earth climate is to identify the different phase space regions where the trajectories asymptotically evolve, called ‘attractors’. In a given system, multiple attractors can co-exist under the effect of the same forcing. At the boundaries of their basins of attraction, small changes produce large effects. Therefore, they are key regions for understanding the system response to perturbations. Here we prove the existence of up to five attractors in a simplified climate system where the planet is entirely covered by the ocean (aquaplanet). These attractors range from a snowball to a hot state without sea ice, and their exact number depends on the details of the coupled atmosphere–ocean–sea ice configuration. We characterise each attractor by describing the associated climate feedbacks, by using the principal component analysis, and by measuring quantities borrowed from the study of dynamical systems, namely instantaneous dimension and persistence.Mon, 09 Sep 2019 10:57:22 +0200Single-spectrum prediction of kurtosis of water waves in a non-conservative modelhttps://archive-ouverte.unige.ch/unige:121099https://archive-ouverte.unige.ch/unige:121099We study statistical properties after a sudden episode of wind for water waves propagating in one direction. A wave with random initial conditions is propagated using a forced-damped higher order Nonlinear Schr"odinger equation (NLS). During the wind episode, the wave action increases, the spectrum broadens, the spectral mean shifts up and the Benjamin-Feir index (BFI) and the kurtosis increase. Conversely, after the wind episode, the opposite occurs for each quantity. The kurtosis of the wave height distribution is considered the main parameter that can indicate whether rogue waves are likely to occur in a sea state, and the BFI is often mentioned as a means to predict the kurtosis. However, we find that while there is indeed a quadratic relation between these two, this relationship is dependent on the details of the forcing and damping. Instead, a simple and robust quadratic relation does exist between the kurtosis and the bandwidth. This could allow for a single-spectrum assessment of the likelihood of rogue waves in a given sea state. In addition, as the kurtosis depends strongly on the damping and forcing coefficients, by combining the bandwidth measurement with the damping coefficient, the evolution of the kurtosis after the wind episode can be predicted.Tue, 23 Jul 2019 10:28:30 +0200Quantitative analysis of self-organized patterns in ombrotrophic peatlandshttps://archive-ouverte.unige.ch/unige:114088https://archive-ouverte.unige.ch/unige:114088We numerically investigate a diffusion-reaction model of an ombrotrophic peatland implementing a Turing instability relying on nutrient accumulation. We propose a systematic and quantitative sorting of the vegetation patterns, based on the statistical analysis of the numbers and filling factor of clusters of both Sphagnum mosses and vascular plants. In particular, we define the transition from Sphagnum-percolating to vascular plant-percolating patterns as the nutrient availability is increased. Our pattern sorting allows us to characterize the peatland pattern stability under climate stress, including strong drought.Fri, 08 Feb 2019 15:15:10 +0100Viscous damping of gravity-capillary waves: Dispersion relations and nonlinear correctionshttps://archive-ouverte.unige.ch/unige:112386https://archive-ouverte.unige.ch/unige:112386We discuss the impact of viscosity on nonlinear propagation of surface waves at the interface of air and a fluid of large depth. After a survey of the available approximations of the dispersion relation, we propose to modify the hydrodynamic boundary conditions to model both short and long waves. From them, we derive a nonlinear Schrödinger equation where both linear and nonlinear parts are modified by dissipation and show that the former plays the main role in both gravity and capillary-gravity waves while, in most situations, the latter represents only small corrections. This provides a justification of the conventional approaches to damped propagation found in the literature.Tue, 18 Dec 2018 09:46:02 +0100Nonlinear stage of Benjamin-Feir instability in forced/damped deep-water waveshttps://archive-ouverte.unige.ch/unige:101274https://archive-ouverte.unige.ch/unige:101274We study a three-wave truncation of a recently proposed damped/forced high-order nonlinear Schrödinger equation for deep-water gravity waves under the effect of wind and viscosity. The evolution of the norm (wave-action) and spectral mean of the full model are well captured by the reduced dynamics. Three regimes are found for the wind-viscosity balance: we classify them according to the attractor in the phase-plane of the truncated system and to the shift of the spectral mean. A downshift can coexist with both net forcing and damping, i.e., attraction to period-1 or period-2 solutions. Upshift is associated with stronger winds, i.e., to a net forcing where the attractor is always a period-1 solution. The applicability of our classification to experiments in long wave-tanks is verified.Tue, 09 Jan 2018 11:22:43 +0100Spectral up- and downshifting of Akhmediev breathers under wind forcinghttps://archive-ouverte.unige.ch/unige:98057https://archive-ouverte.unige.ch/unige:98057We experimentally and numerically investigate the effect of wind forcing on the spectral dynamics of Akhmediev breathers, a wave-type known to model the modulation instability. We develop the wind model to the same order in wave steepness as the higher order modification of the nonlinear Schrödinger equation, also referred to as the Dysthe equation. This results in an asymmetric wind term in the higher order, in addition to the leading order wind forcing term. The derived model is in good agreement with laboratory experiments within the range of the facility's length. We show that the leading order forcing term amplifies all frequencies equally and therefore induces only a broadening of the spectrum, while the asymmetric higher order term in the model enhances the higher frequencies more than the lower ones. Thus, the latter term induces a permanent upshift in the spectral mean. On the other hand, in contrast to the direct effect of wind forcing, wind can indirectly lead to frequency downshifts, due to dissipative effects such as wave breaking, or through the amplification of the intrinsic spectral asymmetry of the Dysthe equation. Furthermore, the definitions of the up- and downshift in terms of peak frequency and mean frequency, which are critical to relate our work to previous results, are highlighted and discussed.Wed, 25 Oct 2017 14:21:42 +0200How to reduce long-term drift in present-day and deep-time simulations?https://archive-ouverte.unige.ch/unige:96855https://archive-ouverte.unige.ch/unige:96855Climate models are often affected by long-term drift that is revealed by the evolution of global variables such as the ocean temperature or the surface air temperature. This spurious trend reduces the fidelity to initial conditions and has a great influence on the equilibrium climate after long simulation times. Useful insight on the nature of the climate drift can be obtained using two global metrics, i.e. the energy imbalance at the top of the atmosphere and at the ocean surface. The former is an indicator of the limitations within a given climate model, at the level of both numerical implementation and physical parameterisations, while the latter is an indicator of the goodness of the tuning procedure. Using the MIT general circulation model, we construct different configurations with various degree of complexity (i.e. different parameterisations for the bulk cloud albedo, inclusion or not of friction heating, different bathymetry configurations) to which we apply the same tuning procedure in order to obtain control runs for fixed external forcing where the climate drift is minimised. We find that the interplay between tuning procedure and different configurations of the same climate model provides crucial information on the stability of the control runs and on the goodness of a given parameterisation. This approach is particularly relevant for constructing good-quality control runs of the geological past where huge uncertainties are found in both initial and boundary conditions. We will focus on robust results that can be generally applied to other climate models.Tue, 19 Sep 2017 14:51:24 +0200Recurrence in the high-order nonlinear Schrödinger equation: A low dimensional analysishttps://archive-ouverte.unige.ch/unige:95765https://archive-ouverte.unige.ch/unige:95765We study a three-wave truncation of the high-order nonlinear Schrödinger equation for deep-water waves (also named Dysthe equation). We validate the model by comparing it to numerical simulation; we distinguish the impact of the different fourth-order terms and classify the solutions according to their topology. This allows us to properly define the temporary spectral upshift occurring in the nonlinear stage of Benjamin-Feir instability and provides a tool for studying further generalizations of this model.Mon, 31 Jul 2017 09:20:37 +0200Triggering filamentation using turbulencehttps://archive-ouverte.unige.ch/unige:87516https://archive-ouverte.unige.ch/unige:87516We study the triggering of single filaments due to turbulence in the beam path for a laser of power below the filamenting threshold. Turbulence can act as a switch between the beam not filamenting and producing single filaments. This 'positive' effect of turbulence on the filament probability, combined with our observation of off-axis filaments suggests the underlying mechanism is modulation instability caused by transverse perturbations. We hereby experimentally explore the interaction of modulation instability and turbulence, commonly associated with multiple-filaments, in the single-filament regime.Tue, 20 Sep 2016 09:42:07 +0200Diffusion in barred-spiral galaxieshttps://archive-ouverte.unige.ch/unige:86870https://archive-ouverte.unige.ch/unige:86870We characterize the radial migration of stars in the disk plane by calculating the diffusion coefficient and the diffusion time-scale for a bulge-disk N-body self-consistent system with a marginally-stable Toomre-Q parameter. We find that diffusion is not constant in time, but follows the evolution of the bar, and becomes maximum near the corotation region and in the external disk region, where asymmetric patterns develop.Wed, 07 Sep 2016 14:08:17 +0200Beyond scale separation in gyrokinetic turbulencehttps://archive-ouverte.unige.ch/unige:86869https://archive-ouverte.unige.ch/unige:86869This paper presents the results obtained with a set of gyrokinetic codes based on a semi-Lagrangian scheme. Several physics issues are addressed, namely, the comparison between fluid and kinetic descriptions, the intermittent behaviour of flux driven turbulence and the role of large scale flows in toroidal ITG turbulence. The question of the initialization of full-F simulations is also discussed.Wed, 07 Sep 2016 14:03:32 +0200A drift-kinetic Semi-Lagrangian 4D code for ion turbulence simulationhttps://archive-ouverte.unige.ch/unige:86868https://archive-ouverte.unige.ch/unige:86868A new code is presented here, named Gyrokinetic SEmi-LAgragian (GYSELA) code, which solves 4D drift-kinetic equations for ion temperature gradient driven turbulence in a cylinder (r, θ, z). The code validation is performed with the slab ITG mode that only depends on the parallel velocity. This code uses a semi-Lagrangian numerical scheme, which exhibits good properties of energy conservation in non-linear regime as well as an accurate description of fine spatial scales. The code has been validated in the linear and non-linear regimes. The GYSELA code is found to be stable over long simulation times (more than 20 times the linear growth rate of the most unstable mode), including for cases with a high resolution mesh (δr ∼ 0.1 Larmor radius, δz ∼ 10 Larmor radius).Wed, 07 Sep 2016 14:01:38 +0200Fine‐Scale Structures and Negative‐Density Regions: Comparison of Numerical Methods for Solving the Advection Equationhttps://archive-ouverte.unige.ch/unige:86867https://archive-ouverte.unige.ch/unige:86867A common feature of the Vlasov equation is that it develops fine‐scale filamentation as time evolves, as observed, for example, in global nonlinear simulations of the ion‐temperature‐gradient instability. From a numerical point of view, it is not trivial to simulate nonlinear regimes characterized by increasingly smaller scales, which eventually become smaller than the (finite) grid size. When very small structures occur, higher order interpolation schemes have a tendency to produce overshoots and negative‐density regions unless some additional dissipative procedure is applied. Different interpolation schemes for the distribution function are compared and discussed.Wed, 07 Sep 2016 13:58:15 +0200Full radius linear and nonlinear gyrokinetic simulations for tokamaks and stellarators: zonal flows, applied E x B flows, trapped electrons and finite betahttps://archive-ouverte.unige.ch/unige:86588https://archive-ouverte.unige.ch/unige:86588The aim of this paper is to report on recent advances made in global gyrokinetic simulations of ion temperature gradient (ITG) modes and other microinstabilities. The nonlinear development and saturation of ITG modes and the role of E × B zonal flows are studied with a global nonlinear δf formulation that retains parallel nonlinearity and thus allows for a check of the energy conservation property as a means of verifying the quality of the numerical simulation. Due to an optimized loading technique, the conservation property is satisfied with an unprecedented quality well into the nonlinear stage. The zonal component of the perturbation evolves to a quasi-steady state with regions of ITG suppression, strongly reduced radial energy flux and steepened effective temperature profiles alternating with regions of higher ITG mode amplitudes, larger radial energy flux and flattened effective temperature profiles. A semi-Lagrangian approach free of statistical noise is proposed as an alternative to the nonlinear δf formulation. An ASDEX-Upgrade experiment with an internal transport barrier is analysed with a global gyrokinetic code that includes trapped electron dynamics. The weakly destabilizing effect of trapped electron dynamics on ITG modes in an axisymmetric bumpy configuration modelling W7-X is shown in global linear simulations that retain the full electron dynamics. Finite β effects on microinstabilities are investigated with a linear global spectral electromagnetic gyrokinetic formulation. The radial global structure of electromagnetic modes shows a resonant behaviour with rational q values.Mon, 05 Sep 2016 11:13:24 +0200A semi-Lagrangian code for nonlinear global simulations of electrostatic drift-kinetic ITG modeshttps://archive-ouverte.unige.ch/unige:86583https://archive-ouverte.unige.ch/unige:86583A semi-Lagrangian code for the solution of the electrostatic drift-kinetic equations in straight cylinder configuration is presented. The code, CYGNE, is part of a project with the long term aim of studying microturbulence in fusion devices. The code has been constructed in such a way as to preserve a good control of the constants of motion, possessed by the drift-kinetic equations, until the nonlinear saturation of the ion-temperature-gradient modes occurs. Studies of convergence with phase space resolution and time-step are presented and discussed. The code is benchmarked against electrostatic Particle-in-Cell codes.Mon, 05 Sep 2016 11:08:44 +0200Motion of extended vortices in an inhomogeneous pure electron plasmahttps://archive-ouverte.unige.ch/unige:86580https://archive-ouverte.unige.ch/unige:86580The motion of extended vortices in a pure electron plasma with an inhomogeneous, centrally peaked, density in a Penning–Malmberg trap is studied by means of a two-dimensional electrostatic Eulerian code that solves the evolution equation for the electron distribution function in the guiding center approximation, coupled to the Poisson equation for the electrostatic potential. Vortices corresponding to electron density clumps propagate inward, as discussed in a recently proposed model for the case of point vortices, and carry inward both high and low density plasma. New, long-lived, structures consisting of a higher and of a lower density vortex pair are formed in the presence of a small amount of vorticity reconnection.Mon, 05 Sep 2016 11:03:28 +0200Asymptotic evolution of nonlinear Landau dampinghttps://archive-ouverte.unige.ch/unige:86482https://archive-ouverte.unige.ch/unige:86482The long-time evolution of nonlinear Landau damping in collisionless plasmas is analyzed by solving the Vlasov-Poisson system numerically. The value of the parameter marking the transition between Landau's and O'Neil's regimes is determined and compared with analytical results. The long-time evolution of a finite-amplitude electric field with wavelength λ equal to the length of the simulation box L is given by a superposition of two counterpropagating “averaged” Bernstein-Greene-Kruskal (BGK) waves. When L>λ and longer wavelength modes can be excited, the BGK waves correspond to an intermediate regime that is eventually modified by the excitation of the sideband instability. Ions dynamics is found not to affect these behaviors significantly.Tue, 30 Aug 2016 12:40:07 +0200Vlasov-Poisson Numerical Simulations of Wave-Particle Interactions in the Relativistic Regimehttps://archive-ouverte.unige.ch/unige:86481https://archive-ouverte.unige.ch/unige:86481The Landau damping of an electrostatic wave is analyzed in relativistic plasmas by solving the Vlasov–Poisson system numerically. The Jüttner–Synge distribution function is perturbed in order to obtain the numerical dispersion relation for short wavelenghts and for vph lesssim c, vph being the phase velocity of the wave. This is compared with the analytical results available in the literature. The ordering of the time-scales characterizing the system and the corresponding formation of vortices in phase space, as a function of the initial temperature, are investigated.Tue, 30 Aug 2016 12:37:46 +0200Cross section of a resonant-mass detector for scalar gravitational waveshttps://archive-ouverte.unige.ch/unige:86442https://archive-ouverte.unige.ch/unige:86442Gravitationally coupled scalar fields, originally introduced by Jordan, Brans and Dicke to account for a non constant gravitational coupling, are a prediction of many non-Einsteinian theories of gravity not excluding perturbative formulations of String Theory. In this paper, we compute the cross sections for scattering and absorption of scalar and tensor gravitational waves by a resonant-mass detector in the framework of the Jordan-Brans-Dicke theory. The results are then specialized to the case of a detector of spherical shape and shown to reproduce those obtained in General Relativity in a certain limit. Eventually we discuss the potential detectability of scalar waves emitted in a spherically symmetric gravitational collapse.Mon, 29 Aug 2016 11:17:22 +0200Gravitational wave radiation from compact binary systems in the Jordan-Brans-Dicke theoryhttps://archive-ouverte.unige.ch/unige:86441https://archive-ouverte.unige.ch/unige:86441In this paper we analyze the signal emitted by a compact binary system in the Jordan-Brans-Dicke theory. We compute the scalar and tensor components of the power radiated by the source and study the scalar waveform. Eventually we consider the detectability of the scalar component of the radiation by interferometers and resonant-mass detectors.Mon, 29 Aug 2016 11:15:58 +0200Modeling the Middle Jurassic ocean circulationhttps://archive-ouverte.unige.ch/unige:78981https://archive-ouverte.unige.ch/unige:78981We present coupled ocean-sea-ice simulations of the Middle Jurassic (~165 Ma) when Laurasia and Gondwana began drifting apart and gave rise to the formation of the Atlantic Ocean. Since the opening of the Proto-Caribbean is not well constrained by geological records, configurations with and without an open connection between the Proto-Caribbean and Panthalassa are examined. We use a sea-floor bathymetry obtained by a recently developed three-dimensional (3D) elevation model which compiles geological, palaeogeographical and geophysical data. Our original approach consists in coupling this elevation model, which is based on detailed reconstructions of oceanic realms, with a dynamical ocean circulation model. We find that the Middle Jurassic bathymetry of the Central Atlantic and Proto-Caribbean seaway only allows for a weak current of the order of 2 Sv in the upper 1000 m even if the system is open to the West. The effect of closing the western boundary of the Proto-Caribbean is to increase transport related to barotropic gyres in the southern hemisphere and to change water properties, such as salinity, in the Neo-Tethys. Weak upwelling rates are found in the nascent Atlantic Ocean in the presence of this superficial current and we discuss their compatibility with deep-sea sedimentological records in this region.Tue, 05 Jan 2016 11:02:45 +0100Formation of rogue waves under forcing fieldshttps://archive-ouverte.unige.ch/unige:78980https://archive-ouverte.unige.ch/unige:78980abstract not availableTue, 05 Jan 2016 11:00:18 +0100Modeling proper motions beyond the Galactic bulgehttps://archive-ouverte.unige.ch/unige:55054https://archive-ouverte.unige.ch/unige:55054We analyse the radial and tangential velocity fields in the Galaxy as seen from the Sun by using as a first approximation a simple axisymmetric model, which we then compare with the corresponding fields in a barred N-body model of the Milky Way. This provides a global description of these quantities missing in the literature, showing where they take large values susceptible to be used in future observations even for sources well beyond the Galactic center. Absolute largest proper motions occur at a distance slightly behind the Galactic Center, which are there 1.5 times larger than the highest local proper motions due to the Galactic differential rotation. Large proper motions well beyond the Galactic center are well within the current astrometric accuracy.Wed, 01 Apr 2015 14:24:21 +0200Modulational instability in wind-forced waveshttps://archive-ouverte.unige.ch/unige:54921https://archive-ouverte.unige.ch/unige:54921We consider the wind-forced nonlinear Schroedinger (NLS) equation obtained in the potential flow framework when the Miles growth rate is of the order of the wave steepness. In this case, the form of the wind-forcing terms gives rise to the enhancement of the modulational instability and to a band of positive gain with infinite width. This regime is characterised by the fact that the ratio between wave momentum and norm is not a constant of motion, in contrast to what happens in the standard case where the Miles growth rate is of the order of the steepness squared.Mon, 30 Mar 2015 11:56:58 +0200Stellar diffusion in barred spiral galaxieshttps://archive-ouverte.unige.ch/unige:54918https://archive-ouverte.unige.ch/unige:54918We characterize empirically the radial diffusion of stars in the plane of a typical barred disk galaxy by calculating the local spatial diffusion coefficient and diffusion time-scale for bulge-disk-halo N-body self-consistent systems which initially differ in the Safronov-Toomre-Q_T parameter. We find different diffusion scenarios that depend on the bar strength and on the degree of instability of the disk. Marginally stable disks, with Q_T sim 1, have two families of bar orbits with different values of angular momentum and energy, which determine a large diffusion in the corotation region. In hot disks, Q_T> 1, stellar diffusion is reduced with respect to the case of marginally stable disks. In cold models, we find that spatial diffusion is not constant in time and strongly depends on the activity of the bar, which can move stars all over the disk recurrently. We conclude that to realistically study the impact of radial migration on the chemical evolution modeling of the Milky Way the role of the bar has to be taken into account.Mon, 30 Mar 2015 11:48:26 +0200Nonlinear fast growth of water waves under wind forcinghttps://archive-ouverte.unige.ch/unige:37855https://archive-ouverte.unige.ch/unige:37855In the wind-driven wave regime, the Miles mechanism gives an estimate of the growth rate of the waves under the effect of wind. We consider the case where this growth rate, normalised with respect to the frequency of the carrier wave, is of the order of the wave steepness. Using the method of multiple scales, we calculate the terms which appear in the nonlinear Schrödinger (NLS) equation in this regime of fast-growing waves. We define a coordinate transformation which maps the forced NLS equation into the standard NLS with constant coefficients, that has a number of known analytical soliton solutions. Among these solutions, the Peregrine and the Akhmediev solitons show an enhancement of both their lifetime and maximum amplitude which is in qualitative agreement with the results of tank experiments and numerical simulations of dispersive focusing under the action of wind.Mon, 16 Jun 2014 16:34:06 +0200