Nonhydrodynamic modes and a priori construction of shallow water lattice Boltzmann equations.

Lattice Boltzmann equations for the isothermal Navier-Stokes equations have been constructed systematically using a truncated moment expansion of the equilibrium distribution function from continuum kinetic theory. Applied to the shallow water equations, with its different equation of state, the same approach yields discrete equilibria that are subject to a grid scale computational instability. Different and stable equilibria were previously constructed by Salmon [J. Marine Res. 57, 503 (1999)]. The two sets of equilibria differ through a nonhydrodynamic or "ghost" mode that has no direct effect on the hydrodynamic behavior derived in the slowly varying limit. However, Salmon's equilibria eliminate a coupling between hydrodynamic and ghost modes, one that leads to instability with a growth rate increasing with wave number. Previous work has usually assumed that truncated moment expansions lead to stable schemes. Such instabilities have implications for lattice Boltzmann equations that simulate other nonideal equations of state, or that simulate fully compressible, nonisothermal fluids using additional particles.