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 flow. 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. Particles with St+ = 0.5 act as an energy source in the streamwise direction in the high-speed regions and low-speed streaks near the wall, which may increase the instability of the low-speed streaks responsible for the birth of new quasistreamwise vortices. However, particles that have been just swept into the low-speed streaks with a long memory due to turbophoresis act as an energy sink in the streamwise direction. This streamwise interaction is maximized for St+ = 35. On the other hand, maximum preferential concentration outside vortical regions occurs when St+ = 5, but their streamwise interaction with the fluid is weak, rather similarly to the case of St+ = 0.5. Thus, the turbulence is less suppressed than St+ = 35.

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