Non-monotonic effect of mass loading on turbulence modulations in particle-laden channel flow

The effect of mass loading on turbulence modulation in channel flows is investigated in two-way coupled direct numerical simulations with the Lagrangian point-particle method. We carried out the simulations of turbulent channel flows at Re-tau = 180, and the particle mass loading ranges from 0 to 0.96 with a fixed Stokes number St = 30. The statistics show that the intensity of the streamwise velocity fluctuation augmented non-monotonically with the increase in mass loading. The maximum enhancement of the streamwise turbulence intensity observed at a mass-loading of phi(m) = 0.75 is 60% greater than that of the unladen flow. However, the velocity fluctuations in the spanwise and wall-normal direction as well as the Reynolds shear stress are attenuated monotonically with mass loading. To better interpret the non-monotonic effect of mass loading on the streamwise turbulence intensity, we further examined the near-wall vortices and streaks and proposed two competitive mechanisms. First, we found that the near-wall vortices become fewer and weaker and the spacing between streaks become wider as phi(m) increases, which results in the damping effect on the fluctuation of the streamwise velocity. Second, the vortex structures and the streaks become more organized and straightly aligned in the streamwise direction with an increase in the streak strength, which enhances the streamwise velocity fluctuations. In addition, we observed a non-monotonic change in turbulent kinetic energy transfer between particle and fluid phases. The statistics of energy budgets show that particles extract energy from the fluid phase at phi(m) < 0.6 but transfers energy back to the fluid phase at phi(m) > 0.75 in the stream-wise direction.