Periodic solutions of coupled Boussinesq equations and Ostrovsky-type models free from zero-mass contradiction

TL;DR

This paper develops a new asymptotic approach to model coupled Boussinesq equations in high-contrast bi-layer systems, overcoming the zero-mass constraint of traditional Ostrovsky models and enabling analysis of non-zero mean initial conditions.

Contribution

The authors construct a novel asymptotic solution for coupled Boussinesq equations that avoids the zero-mass contradiction by focusing on deviations from the evolving mean, applicable to non-zero mean initial conditions.

Findings

Asymptotic solutions accurately describe counter-propagating and co-propagating waves.

Numerical validation confirms the validity of the derived models.

Conservation laws are established to ensure numerical accuracy.

Abstract

Coupled Boussinesq equations describe long weakly-nonlinear longitudinal strain waves in a bi-layer with a soft bonding between the layers (e.g. a soft adhesive). From the mathematical viewpoint, a particularly difficult case appears when the linear long-wave speeds in the layers are significantly different (high-contrast case). The traditional derivation of the uni-directional models leads to four uncoupled Ostrovsky equations, for the right- and left-propagating waves in each layer. However, the models impose a ``zero-mass constraint'' i.e. the initial conditions should necessarily have zero mean, restricting the applicability of that description. Here, we bypass the contradiction in this high-contrast case by constructing the solution for the deviation from the evolving mean value, using asymptotic multiple-scale expansions involving two pairs of fast characteristic variables and two slow-time variables. By construction, the Ostrovsky equations emerging within the scope of this derivation are solved for initial conditions with zero mean while initial conditions for the original system may have non-zero mean values. Asymptotic validity of the solution is carefully examined numerically. We apply the models to the description of counter-propagating waves generated by solitary wave initial conditions, or co-propagating waves generated by cnoidal wave initial conditions, as well as the resulting wave interactions, and contrast with the behaviour of the waves in bi-layers when the linear long-wave speeds in the layers are close (low-contrast case). One local (classical) and two non-local (generalised) conservation laws of the coupled Boussinesq equations for strains are derived, and these are used to control the accuracy of the numerical simulations.