Force on inertial particles crossing a two layer stratified turbulent/non-turbulent interface

We study the peculiar motion of inertial solid particles across stratified turbulent/non-turbulent interfaces (STNTI). Previous studies in quiescent stratified layers demonstrated that inertial particles slow down substantially due to an additional force term related to the stratification. Here we report for the first time a similar effect on inertial particles moving across a two-layer STNTI of finite thickness. This problem is addressed both experimentally and numerically: we utilize the three-dimensional particle tracking velocimetry (3D-PTV) in a index-matched STNTI experiment under an oscillating grid, and two direct numerical simulation (DNS) cases of STNTI. The DNSs test the effects of different turbulent forcings on inertial spheres in the turbulent layer and across STNTI and extends the parameter ranges of Reynolds and Froude numbers unfeasible in the experiments. Turbulence is produced in the DNSs using a convective forcing (heat source at the domain boundary) in one case, and a forcing that mimics a vertically oscillating grid in the other. The numerical spheres are tracked, through one-way coupling approach, using a modified Basset–Boussinesq–Oseen equation which includes a stratification-induced term. The stratification force is modelled as an additional buoyancy of a caudal wake with varying density. This algorithm creates Lagrangian trajectories that resemble the motion of inertial particles across stratified interfaces in quiescent and turbulent experiments. Furthermore, numerical results for the STNTI cases help to distinguish the essential features observed in the experiments that are caused by stratification from those that relate to turbulence–particle interactions.