New Form of Mixing in Turbulent Sedimentation

We study the sedimentation of finite-size inertial particles in a Rayleigh-Taylor-like setup using state-of-the-art direct numerical simulations. The falling particles are observed to produce two distinct regions: a leading mixing layer with a linear concentration profile followed by a bulk region of uniform density. Unlike classical RT turbulence, the mixing layer extension accelerates with an anomalous, noninteger exponent, while the bulk region moves at a constant velocity. A one-dimensional model based on a local hindered settling law accurately captures the observed dynamics and its dependence on the particle-to-fluid density ratio. The present Letter identifies a new regime of convective mixing which develops at the front of particle suspensions in sedimentation processes.