Modification of particle-laden near-wall turbulence: effect of Stokes number

Turbulent channel flows laden with particles are investigated using direct numerical simulation with a point-force approximation for small, heavy particles with a diameter smaller than the Kolmogorov length scale of the fluid. The Stokes numbers based on the wall units considered in our study are $St^+=0.5, 5, 35$ and 125. The main purpose of this study is to examine the effect of Stokes number on turbulence modification in a channel. We found that particles with $St^+=0.5$ enhance turbulence by increasing the occurrence of quasistreamwise vortices, while larger-Stokes-number particles attenuate turbulence. When $St^+=0.5$, kinetic energy is transferred from the particles to streamwise fluid velocity fluctuations in the high-speed regions and low-speed streaks, which may increase the instability of the low-speed streaks responsible for the birth of new quasistreamwise vortices. On the other hand, the preferential concentration of larger-Stokes-number particles in low-speed streaks is responsible for turbulence attenuation, and the slow response of the particles to the fluid produces feedback against the fluid velocity associated with quasistreamwise vortices.