Turbulence modification in a homogeneous turbulent shear flow laden with small heavy particles

Numerical simulations have been conducted for a homogeneous turbulent shear flow laden with small heavy particles in order to investigate the modification of turbulence structures in shear flows due to particles. The effects on the turbulence modification of gravitational settling of particles in the sheared direction are examined for the particles whose response times are comparable to the Kolmogorov time-scale of turbulence. It is found that the growth rate of turbulence energy of the carrier fluid, which is reduced by the particles in zero gravity, is increased by the effect of weak gravity through the enhancement of Reynolds shear stress, but decreased by the effect of strong gravity through the increase in drag dissipation. It is shown in finite gravity that particle clusters are generated due to the accumulation of particles in two types of regions; downward flows sandwiched between counter rotating quasi-streamwise vortex tubes and regions just beneath the vortex layers with negative spanwise vorticity. The particle clusters between the pair vortex tubes intensify the downward flows between them to enhance the Reynolds shear stress, while the particle clusters beneath the vortex layers enhance the drag dissipation of fluid turbulence energy in the streamwise direction. The downward flows induced by particle clusters activate the tilting of the spanwise vorticity toward the vertical direction.