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

Abstract

Coupled Boussinesq equations are used to describe long weakly nonlinear longitudinal strain waves in a bi-layer with soft bonding between the layers (e.g., a soft adhesive). From a 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 behavior 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 (generalized) conservation laws of the coupled Boussinesq equations for strains are derived and used to control the accuracy of the numerical simulations.

Studying the effect of delamination on Ostrovsky wave packet propagation in waveguides

“…The BKG equation can arise in the context of oceanic waves in a rotating ocean [22], but its leading order uni-directional approximation, the Ostrovsky equation, requires that any regular localized solution has zero mass, a restriction that is not enforced by the BKG equation. This contradiction has been resolved on the periodic domain [23] and more recently a similar resolution is found for the coupled Boussinesq equations [24]. This has yet to be explored in the context of wave scattering in delaminated bars, and so the initial conditions are chosen to have zero mass to circumvent this restriction.…”