Enhancement of wall-bounded turbulence by solid particles under gravity

When we add solid particles to turbulent flow, the turbulence can be either attenuated or enhanced. Both phenomena are caused by the wakes of particles. More specifically, turbulence attenuation is due to the additional energy dissipation in the wakes, while turbulence enhancement is due to the additional turbulent energy by the wakes. In the present study, we propose a scenario for turbulence modulation in terms of particle wakes and verify it using direct numerical simulation data of turbulent channel flow in the presence of gravity. We show that the total turbulent kinetic energy is enhanced when the relative velocity becomes larger so that a significant turbulent energy is introduced locally by particle wakes. This does not happen in the absence of gravity. • We describe a scenario that describes the physical mechanism of turbulence enhancement and attenuation by solid particles. • We verify the proposed scenario using the DNS in the presence of gravity. • We show that the slip velocity between particles and fluid plays a key role in both the enhancement and attenuation of turbulence.