  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.
