On the spectral asymptotics of waves in periodic media with Dirichlet or Neumann exclusions

TL;DR

This paper develops a homogenization framework for scalar waves in periodic media with voids, capturing complex wave behaviors near band edges and applying it to specific lattice structures with numerical validation.

Contribution

It introduces a spectral asymptotic homogenization method for waves in periodic media with exclusions, accounting for various eigenvalue regimes and source terms, advancing wave modeling in metamaterials.

Findings

Effective models for wave behavior near band edges are derived.

Homogenization captures wave- and Dirac-type phenomena in phononic crystals.

Numerical examples validate the asymptotic dispersion approximations.

Abstract

We consider homogenization of the scalar wave equation in periodic media at finite wavenumbers and frequencies, with the focus on continua characterized by: (a) arbitrary Bravais lattice in $\mathbb{R}^d$, $d\!\geqslant\!2$, and (b) exclusions i.e. "voids" that are subject to homogenous (Neumann or Dirichlet) boundary conditions. Making use of the Bloch wave expansion, we pursue this goal via asymptotic ansatz featuring the "spectral distance" from a given wavenumber-eigenfrequency pair (within the first Brillouin zone) as the perturbation parameter. We then introduce the effective wave motion via projection(s) of the scalar wavefield onto the Bloch eigenfunction(s) for the unit cell of periodicity, evaluated at the origin of a spectral neighborhood. For generality, we account for the presence of the source term in the wave equation and we consider -- at a given wavenumber -- generic cases of isolated, repeated, and nearby eigenvalues. In this way we obtain a palette of effective models, featuring both wave- and Dirac-type behaviors, whose applicability is controlled by the local band structure and eigenfunction basis. In all spectral regimes, we pursue the homogenized description up to at least first order of expansion, featuring asymptotic corrections of the homogenized Bloch-wave operator and the homogenized source term. Inherently, such framework provides a convenient platform for the synthesis of a wide range of wave phenomena in metamaterials and phononic crystals. The proposed homogenization framework is illustrated by approximating asymptotically the dispersion relationships for (i) Kagome lattice featuring hexagonal Neumann exclusions, and (ii) "pinned" square lattice with circular Dirichlet exclusions. We complete the numerical portrayal of analytical developments by studying the response of a Kagome lattice due to a dipole-like source term acting near the edge of a band gap.