Dispersion of inertial particles in cellular flows in the small-Stokes, large-P\'eclet regime

Abstract

We investigate the transport of inertial particles by cellular flows when advection dominates over inertia and diffusion, that is, for Stokes and P\'eclet numbers satisfying $\mathrm{St} \ll 1$ and $\mathrm{Pe} \gg 1$. Starting from the Maxey--Riley model, we consider the distinguished scaling $\mathrm{St} \, \mathrm{Pe} = O(1)$ and derive an effective Brownian dynamics approximating the full Langevin dynamics. We then apply homogenisation and matched-asymptotics techniques to obtain an explicit expression for the effective diffusivity $\overline{D}$ characterising long-time dispersion. This expression quantifies how $\overline{D}$, proportional to $\mathrm{Pe}^{-1/2}$ when inertia is neglected, increases for particles heavier than the fluid and decreases for lighter particles. In particular, when $\mathrm{St} \gg \mathrm{Pe}^{-1}$, we find that $\overline{D}$ is proportional to $\mathrm{St}^{1/2}/(\log ( \mathrm{St} \, \mathrm{Pe}))^{1/2}$ for heavy particles and exponentially small in $\mathrm{St} \, \mathrm{Pe}$ for light particles. We verify our asymptotic predictions against numerical simulations of the particle dynamics.