The interaction of localised solitary waves with large-scale, time-varying dispersive mean flows subject to nonconvex flux is studied in the framework of the modified Korteweg-de Vries (mKdV) equation, a canonical model for nonlinear internal gravity
wave propagation in stratified fluids. The principal feature of the studied interaction is that both the solitary wave and the large-scale mean flow -- a rarefaction wave or a dispersive shock wave (undular bore) -- are described by the same dispersive hydrodynamic equation. A recent theoretical and experimental study of this new type of dynamic soliton-mean flow interaction has revealed two main scenarios when the solitary wave either tunnels through the varying mean flow that connects two constant asymptotic states, or remains trapped inside it. While the previous work considered convex systems, in this paper it is demonstrated that the presence of a nonconvex hydrodynamic flux introduces significant modifications to the scenarios for transmission and trapping. A reduced set of Whitham modulation equations, termed the solitonic modulation system, is used to formulate a general, approximate mathematical framework for solitary wave-mean flow interaction with nonconvex flux. Solitary wave trapping is conveniently stated in terms of crossing characteristics for the solitonic system. Numerical simulations of the mKdV equation agree with the predictions of modulation theory. The developed theory draws upon general properties of dispersive hydrodynamic partial differential equations, not on the complete integrability of the mKdV equation. As such, the mathematical framework developed here enables application to other fluid dynamic contexts subject to nonconvex flux.
The main goal of this paper is to put together: a) the Whitham theory applicable to slowly modulated $N$-phase nonlinear wave solutions to the focusing nonlinear Schrodinger (fNLS) equation, and b) the Riemann-Hilbert Problem approach to particular s
olutions of the fNLS in the semiclassical (small dispersion) limit that develop slowly modulated $N$-phase nonlinear wave in the process of evolution. Both approaches have their own merits and limitations. Understanding of the interrelations between them could prove beneficial for a broad range of problems involving the semiclassical fNLS.
We quantify the notion of a dense soliton gas by establishing an upper bound for the integrated density of states of the quantum-mechanical Schrodinger operator associated with the KdV soliton gas dynamics. As a by-product of our derivation we find t
he speed of sound in the soliton gas with Gaussian spectral distribution function.
We consider the step Riemann problem for the system of equations describing the propagation of a coherent light beam in nematic liquid crystals, which is a general system describing nonlinear wave propagation in a number of different physical applica
tions. While the equation governing the light beam is of defocusing nonlinear Schrodinger equation type, the dispersive shock wave (DSW) generated from this initial condition has major differences from the standard DSW solution of the defocusing nonlinear Schrodinger equation. In particular, it is found that the DSW has positive polarity and generates resonant radiation which propagates ahead of it. Remarkably, the velocity of the lead soliton of the DSW is determined by the classical shock velocity. The solution for the radiative wavetrain is obtained using the WKB approximation. It is shown that for sufficiently small initial jumps the nematic DSW is asymptotically governed by a Korteweg-de Vries equation with fifth order dispersion, which explicitly shows the resonance generating the radiation ahead of the DSW. The constructed asymptotic theory is shown to be in good agreement with the results of direct numerical simulations.
We propose a novel, analytically tractable, scenario of the rogue wave formation in the framework of the small-dispersion focusing nonlinear Schrodinger (NLS) equation with the initial condition in the form of a rectangular barrier (a box). We use th
e Whitham modulation theory combined with the nonlinear steepest descent for the semi-classical inverse scattering transform, to describe the evolution and interaction of two counter-propagating nonlinear wave trains --- the dispersive dam break flows --- generated in the NLS box problem. We show that the interaction dynamics results in the emergence of modulated large-amplitude quasi-periodic breather lattices whose amplitude profiles are closely approximated by the Akhmediev and Peregrine breathers within certain space-time domain. Our semi-classical analytical results are shown to be in excellent agreement with the results of direct numerical simulations of the small-dispersion focusing NLS equation.
We derive generalised multi-flow hydrodynamic reductions of the nonlocal kinetic equation for a soliton gas and investigate their structure. These reductions not only provide further insight into the properties of the new kinetic equation but also co
uld prove to be representatives of a novel class of integrable systems of hydrodynamic type, beyond the conventional semi-Hamiltonian framework.
