In general, the maximal mesh size hmax is dictated by the smallest wavelength (the slowest wave) such that
where fmax is the maximal frequency to resolve in the model, given by the frequency content of the source, and
cmin is the slowest waves speed in the model. This is typically the shear wave speed
cs, but for problems with a free interface or a material discontinuities, interfacial waves also exist. For example, the classical estimates of the Rayleigh wave speed
vR is
where ν is the Poisson’s ratio and
cs is shear wave speed.
When several materials are used in a model, the use of assemblies and pair features is recommended (Continuity and
Pair Acoustic-Structure Boundary, Time Explicit). The advantage of using a pair feature is that the mesh does not need to be conforming on the two sides of the interface (the two parts pf the assembly). This is especially advantageous for the time explicit discontinuous Galerkin method as the time step depends on mesh size and local speed of sound.