On the adiabatic invariance of the action of a trapped wave

TL;DR

This paper demonstrates that the adiabatic invariant of a trapped wave in a linear solid system can be expressed as the ratio of its energy to frequency, simplifying analysis of localized oscillations.

Contribution

It introduces a simplified method to calculate the adiabatic invariant for trapped waves, generalizing the concept from Hamiltonian systems and linking it to energy and frequency.

Findings

Adiabatic invariant equals energy divided by frequency.

Simplified approach to amplitude evolution law.

Connection to Hamiltonian system concepts.

Abstract

Recently, it has been shown (Gavrilov et al., Nonlinear Dyn, 112, 2024) that in a linear solid discrete-continuous system with several slowly time-varying parameters, the amplitude of a strongly localized mode (a trapped wave) can be calculated as a function of current parameter values and does not depend on the history of the parameter change. This result allows us to introduce the adiabatic invariant for such a system according to the general definition as a quantity that remains approximately constant if the parameters vary slowly. In this paper, we show that, defined in this manner, the adiabatic invariant can be calculated as the ratio of the total energy of the trapped wave to its frequency. This yields a significantly simplified approach to solving a class of problems concerning localized oscillation of continuous systems with discrete inclusions, although the definition of the wave energy can be ambiguous. Thus, we can consider the newly introduced adiabatic invariant as a straightforward generalization of the concept known to Hamiltonian systems. Finally, we introduce an effective Hamiltonian system, which is characterized by the same adiabatic invariant as the trapped wave. This yields another highly straightforward approach to deriving the amplitude evolution law, although further investigation is required.