Settling and dispersion of Lagrangian particles in the presence of stratified Kelvin-Helmholtz instability and turbulence*

This study investigates the influence of the Kelvin-Helmholtz (KH) instability on particle settling and dispersion in stratified fluid environments, addressing gaps in our understanding of how these instabilities alter particle settling and dispersion compared to classic Stokes settling in a quiescent environment. We conducted direct numerical simulations to analyze particle motion in a Lagrangian framework, focusing on the effects of the KH instability on particles ranging in size from 20-100 micrometers in diameter. Our results show that the settling of larger particles is slower than pure Stokes settling because particles become trapped within KH billows; this effect diminishes for particles larger than about 80 micrometers. Conversely, small particles (20 micrometers) exhibit enhanced settling in the presence of KH billows, with velocities up to seven times greater than Stokes settling. These smaller particles are preferentially trapped in the downward-moving portion of a billow, which dominates their vertical motion. Additionally, we observed a significant increase in spanwise dispersion following the breakdown of a KH billow to turbulence, while vertical dispersion increases immediately upon KH billow formation and decreases with increasing particle size. This research offers new insights into the interaction between the KH instability and particle dynamics, highlighting the significant role of the instability in altering sediment transport in stratified fluids.