Three Dimensional Flexural–Gravity Waves

Waves propagating on the surface of a three–dimensional ideal fluid of arbitrary depth bounded above by an elastic sheet that resists flexing are considered in the small amplitude modulational asymptotic limit. A Benney–Roskes–Davey–Stewartson model is derived, and we find that fully localized wavepacket solitary waves (or lumps) may bifurcate from the trivial state at the minimum of the phase speed of the problem for a range of depths. Results using a linear and two nonlinear elastic models are compared. The stability of these solitary wave solutions and the application of the BRDS equation to unsteady wave packets is also considered. The results presented may have applications to the dynamics of continuous ice sheets and their breakup.     

Three-dimensional gravity waves in a channel of variable depth

We consider existence of three-dimensional gravity waves traveling along a channel of variable depth. It is well known that the long-wave small-amplitude expansion for such waves results in the stationary Korteweg–de Vries equation, coefficients of which depend on the transverse topography of the channel. This equation has a single-humped solitary wave localized in the direction of the wave propagation. We show, however, that there exists an infinite set of resonant Fourier modes that travel at the same speed as the solitary wave does. This fact suggests that the solitary wave confined in a channel of variable depth is always surrounded by small-amplitude oscillatory disturbances in the far-field profile.     

Two-Way Interactions Between Equatorially-Trapped Waves and the Barotropic Flow

Lateral energy exchange between the tropics and the midlatitudes is a topic of great importance for understanding Earth＇s climate system. In this paper, the authors address this issue in an idealized set up through simple shallow water models for the interactions between equatorially trapped waves and the barotropic mode, which supports Rossby waves that propagate poleward and can excite midlatitude teleconnection patterns. It is found here that the interactions between a Kelvin wave and a fixed meridionai shear （mimicking the jet stream） generates a non-trivial meridional velocity and meridional convergence in phase with the upward motion that can attain a maximum of about 50%, which oscillates on frequencies ranging from one day to 10 days. When, on the other hand, the barotropic flow is forced by slowly propagating Kelvin waves a complex flow pattern emerges; it consists of a phase-locked barotropic response that is equatoriaily trapped and that propagates eastward with the forcing Kelvin wave and a certain number of planetary Rossby waves that propagate westward and toward the poles as seen in nature. It is suggested here that the poleward propagating waves are to some sort of multi-way resonant interaction with the phase locked response. Moreover, it is shown here that a numerical scheme with dispersion properties that depend on the direction perpendicular to the direction of propagation, namely the 2D central scheme of Nessyahu and Tadmor, can artificially alter significantly the topology of the wave fields and thus should be avoided in climate models.     

Long-time Solutions of the Ostrovsky Equation

The Ostrovsky equation is a modification of the Korteweg-de Vries equation which takes account of the effects of background rotation. It is well known that the usual Korteweg-de Vries solitary wave decays and is replaced by radiating inertia gravity waves. Here we show through numerical simulations that after a long-time a localized wave packet emerges as a persistent and dominant feature. The wavenumber of the carrier wave is associated with that critical wavenumber where the underlying group velocity is a minimum (in absolute value). Based on this feature, we construct a weakly nonlinear theory leading to a higher-order nonlinear Schrödinger equations in an attempt to describe the numerically found wave packets.     

Dynamics of Vortex Rossby Waves in Tropical Cyclones,Part 1: Linear Time‐Dependent Evolution on an f‐Plane

Analyses of observational data on hurricanes in the tropical atmosphere indicate the existence of spiral rainbands which propagate outward from the eye and affect the structure and intensity of the hurricane. These disturbances may be described as vortex Rossby waves. This paper describes the evolution of barotropic vortex Rossby waves in a cyclonic vortex in a two‐dimensional configuration where the variation of the Coriolis force with latitude is ignored. The waves are forced by a constant‐amplitude boundary condition at a fixed radius from the center of the vortex and propagate outward. The mean flow angular velocity profile is taken to be a quadratic function of the radial distance from the center of the vortex and there is a critical radius at which it is equal to the phase speed of the waves. For the case of waves with steady amplitude, an exact solution is derived for the steady linearized equations in terms of hypergeometric functions; this solution is valid in the outer region away from the critical radius. For the case of waves with time‐dependent amplitude, asymptotic solutions of the linearized equations, valid for late time, are obtained in the outer and inner regions. It is found that there are strong qualitative similarities between the conclusions on the evolution of the vortex waves in this configuration and those obtained in the case of Rossby waves in a rectangular configuration where the latitudinal gradient of the Coriolis parameter is taken into account. In particular, the amplitude of the steady‐state outer solution is greatly attenuated and there is a phase change of across the critical radius, and in the linear time‐dependent configuration, the outer solution approaches a steady state in the limit of infinite time, while the amplitude of the inner solution grows on a logarithmic time scale and the width of the critical layer approaches zero.     

A Perturbation Analysis of an Interaction between Long and Short Surface Waves

The interaction of finite-amplitude long gravity waves with a small-amplitude packet of short capillary waves is studied by a multiple-scale method based on the invariance of the perturbation expansion under certain translations. The result of the analysis is a set of equations coupling the complex amplitude of the packet of short waves with the long-wave velocity potential and surface elevation. The short wave is described by a Ginzburg-Landau equation with coefficients that depend on properties of the long wave. The long-wave potential and surface elevation satisfy the usual free-surface conditions augmented by forcing terms representing effects of the short waves. The derivation removes some of the restrictions imposed in earlier studies.     

A Model Equation for Wavepacket Solitary Waves Arising from Capillary-Gravity Flows

A model equation governing the primitive dynamics of wave packets near an extremum of the linear dispersion relation at finite wavenumber is derived. In two spatial dimensions, we include the effects of weak variation of the wave in the direction transverse to the direction of propagation. The resulting equation is contrasted with the Kadomtsev–Petviashvilli and Nonlinear Schrödinger (NLS) equations. The model is derived as an approximation to the equations for deep water gravity-capillary waves, but has wider applications. Both line solitary waves and solitary waves which decay in both the transverse and propagating directions—lump solitary waves—are computed. The stability of these waves is investigated and their dynamics are studied via numerical time evolution of the equation.     

地形作用下的非线性Rossby波

本文利用一个受地形强迫作用的半地转正压模式讨论了非线性Rossby波的稳定度和解.结果发现,东西向地形和南北向地形对非线性Rossby波的稳定度和相速的影响很不相同.同时也发现,地形强迫下的非线性Rossby波可用著名的KdV方程描述.     

Plane wave approximation in linear elasticity

We consider the approximation of solutions of the time-harmonic linear elastic wave equation by linear combinations of plane waves. We prove algebraic orders of convergence both with respect to the dimension of the approximating space and to the diameter of the domain. The error is measured in Sobolev norms and the constants in the estimates explicitly depend on the problem wavenumber. The obtained estimates can be used in the h- and p-convergence analysis of wave-based finite element schemes.     

Lyapunov function method for control law synthesis under one integral and several phase constraints

We consider the synthesis of linear control laws under one integral and several phase constraints on the basis of the Lyapunov function method and the technique of linear matrix inequalities. In particular, our method permits one to obtain suboptimal state or output feedbacks providing the minimum upper bound for a quadratic functional under phase and control constrains or the minimum bound for the maximum deviation of the controlled variable under one integral and several phase constraints. The synthesis is generalized to the case of nonstationary parametric perturbations in the plant.     

Contributions to the Theory of Waves in Non-linear Dispersive Systems

This paper makes contributions to the general theory of wavepropagation in conservative systems under conditions when theproportional change in amplitude or wavenumber over a distanceof one wavelength is very small. For linear systems, such propagationis governed by the well-known theory of group velocity; thereis "frequency dispersion", in the sense that energy in componentsof different frequency is propagated at different group velocities.For non-linear systems without frequency dispersion, e.g. acousticsystems, a different, but also well-known, modification of thewaveform occurs. It may be called "amplitude dispersion", inthat different values of an amplitude variable like the pressureare propagated at different speeds. A much more general theory of non-linear systems, where frequencydispersion and amplitude dispersion would be expected to bein competition, has been given by Whitham (1965b). Energy doesnot play a key role in the theory, because it is easily transferredbetween components of different frequencies. The fundamentalequation follows from Hamilton's principle in an averaged form. In examples given by Whitham, changes in, for example, wavenumber(or amplitude) are propagated at two different velocities, becausethe fundamental equation is hyperbolic. However, in the limitingcase of infinitesimal amplitude, the equation is parabolic andonly one velocity of propagation (the group velocity) occurs.Thus, Whitham showed that non-linearity can "split" the groupvelocity. This paper is concerned with the inference of detailed conclusionsfrom Whitham's theory, to enable comparisons with experimentthat will show the range of applicability of the theory. Itattempts to obtain these in the simplest case, namely, thatof one-dimensional propagation when Whitham's "pseudo-frequencies"are absent. If the relationship between frequency and wavenumber k forinfinitesimal amplitude is = f(k), then for finite amplitudethe equation is shown to be hyperbolic or elliptic respectively,according as [–f(k)]f^*(k) takes positive or negative values.For gravity waves on deep water this product is negative andthese, it is inferred, may be good for comparison of theorywith experiment in the elliptic case. A new non-linear non-perturbationaltheory of waves under the combined action of gravity and surfacetension is used to indicate that waves at 9.6 c/s on mercurymay be suitable for comparison with experiment in the hyperboliccase. When non-linear effects are only moderate, approximate transformationsof Whitham's equation to the axisymmetric Laplace and wave equationsrespectively, in the elliptic and hyperbolic cases, are usedto obtain particular solutions for comparison with experiment.A feature of these solutions is the appearance of discontinuitiesin wavelength. For example, when a wavemaker creates gravity waves of fixedfrequency whose amplitude first increases and then decreases,the theory predicts that the length of waves in the group decreasesahead of the point of maximum amplitude and increases behindit. This produces in turn a concentration of energy towardsthe centre of the group, which continues during the whole periodbefore a discontinuity in wavelength actually forms. This solutionin the elliptic case is obtained with the aid of the theoryof imaginary characteristics.     

Proof of Modulational Instability of Stokes Waves in Deep Water

It is proven that small-amplitude steady periodic water waves with infinite depth are unstable with respect to long-wave perturbations. This modulational instability was first observed more than half a century ago by Benjamin and Feir. It has been proven rigorously only in the case of finite depth. We provide a completely different and self-contained approach to prove the spectral modulational instability for water waves in both the finite and infinite depth cases. © 2022 Courant Institute of Mathematics and Wiley Periodicals LLC.     

Transient Metastability and Selective Decay for the Coherent Zonal Structures in Plasma Drift Wave Turbulence

The emergence of persistent zonal structures is studied in freely decaying plasma flows. The plasma turbulence with drift waves can be described qualitatively by the modified Hasegawa–Mima (MHM) model, which is shown to create enhanced zonal jets and more physically relevant features compared with the original Charney–Hasegawa–Mima model. We analyze the generation and stability of the zonal state in the MHM model following the strategy of the selective decay principle. The selective decay and metastable states are defined as critical points of the enstrophy at constant energy. The critical points are first shown to be invariant solutions to the MHM equation with a special emphasis on the zonal modes, but the metastable states consist of a zonal state plus drift waves with a specific smaller wavenumber. Further, it is found with full mathematical rigor that any initial state will converge to some critical point solution at the long-time limit under proper dissipation forms, while the zonal states are the only stable ones. The selective decay process of the solutions can be characterized by the transient visits to several metastable states, then the final convergence to a purely zonal state. The selective decay and metastability properties are confirmed by numerical simulations with distinct initial structures. One highlight in both theory and numerics is the tendency of Landau damping to destabilize the selective decay process.

     

The Signaling Problem in Nonlinear Hyperbolic Wave Theory

This work is devoted to investigating weakly nonlinear hyperbolic waves arising from the action of small-amplitude, high-frequency boundary disturbances. By directly introducing a nonlinear phase variable corresponding to the leading wavefront and specifying a single-wave-mode boundary disturbance, we are able to construct an asymptotic solution. Furthermore, our result shows that, by properly arranging the relation of small amplitude to high frequency, a systematic procedure can be provided for constructing weakly nonlinear wave solutions with interior shocks and determining the shock initiation position (and time) when there is a local linear degeneracy at the leading wavefront.     

Long waves in inviscid compressible atmosphere II

Solitary waves have been found in an adiabatic compressible atmosphere which, in ambient state, has winds and temperature gradient, generalizing our earlier results for the isothermal atmosphere. Explicit results are obtained for the special case of linear temperature and linear wind distributions in the undisturbed conditions. An important result of the study is that the number of possible critical speeds of the flow depends crucially on whether the maximum Richardson number (which is variable in the present example) is greater or less than 1/4.     

Long Internal Waves in Fluids of Great Depth

An equation is derived that governs the evolution in two spatial dimensions of long internal waves in fluids of great depth. The equation is a natural generalization of Benjamin's (1967) one-dimensional equation, and relates to it in the same way that the equation of Kadomtsev and Petviashvili relates to the Kortewegde-Vries equation. The stability of one-dimensional solitons with respect to long transverse disturbances is studied in the context of this equation. Solitons are found to be unstable with respect to such perturbations in any system in which the phase speed is a minimum (rather than a maximum) for the longest waves. Internal waves do not have this property, and are not unstable with respect to such perturbations.     

The critical speed of a moving load on a prestressed plate resting on a prestressed half-plane

Within the framework of a piecewise homogeneous body model, with the use of the three-dimensional linearized theory of elastic waves in initially stressed bodies, the dynamical response of a system consisting of a prestressed covering layer and a prestressed half-plane to a moving load applied to the free face of the covering layer is investigated. Two types (complete and incomplete) of contact conditions on the interface are considered. The subsonic state is considered, and numerical results for the critical speed of the moving load are presented. The influence of problem parameters on the critical speed is analyzed. In particular, it is established that the prestressing of the covering layer and half-plane increases the critical speed. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 43, No. 2, pp. 257–270, March–April, 2007.