Settling dynamics of Kolmogorov-scale sediment particles in homogeneous isotropic turbulence using two-way coupled point--particle model*

Settling dynamics of slightly heavier-than-fluid particles in turbulence is of critical importance for suspended sediment transport in coastal environments. These interactions typically involve particle sizes on the order of or larger than the Kolmogorov scale, moderate-to-dense volume loadings, and small particle-to-fluid density ratios.  Of particular interest is the mean particle settling speed in turbulence, which can be influenced by fast tracking, vortex trapping, and loitering mechanisms. A two-way coupled point-particle model based on the complete Maxey-Riley equation is used to investigate settling dynamics of particles in a two-way coupled, forced, homogeneous isotropic turbulence by varying the turbulence intensity relative to the settling speed in quiescent flow for multiple Stokes numbers. To correctly capture the motion of Kolmogorov-scale particles, a self-disturbance corrected fluid velocity, obtained using the advection-diffusion-reaction (ADR) model (Keane et al., IJMF 2023) is used. For small-to-moderate Stokes numbers, increased settling speed is observed with increasing turbulence intensity. Preferential sweeping is found to be the main mechanism of increase in settling speed. However, for larger Stokes numbers, the settling speed is decreased even with increasing turbulence intensity. Larger particles with higher Stokes numbers tend to sample upward moving fluid more frequently resembling loitering mechanism resulting in decreased settling speed. A multiscale wavelet analysis of particle curvature trajectories as well as the vertical fluid velocity sampled by the particles at different scales is conducted to further elucidate the different mechanisms affecting settling dynamics.