On the mechanisms of modifying the structure of turbulent homogeneous shear flows by dispersed particles

The present study is concerned with answering the question: What are the physical mechanisms responsible for the modification of the turbulence structure by solid particles dispersed in a homogeneous shear flow? We employ DNS (direct numerical simulations) to examine the effects of the two-way interaction between the two phases on the turbulence structure. Our results indicate that particles affect the rate of production of turbulence energy via modifying the vorticity dynamics. It is known that regions of large production rate of turbulence energy are sandwiched between counter rotating vortices whose vorticity, ωs, is aligned with the axes of the longitudinal vortex tubes. These longitudinal vortex tubes are strongly inclined toward the streamwise direction due to the imposed mean shear. The stronger ωs is, the larger the production rate. The dispersed solid particles modify the alignment of the local vorticity vector, ω, with the axis of the longitudinal vortex tube. Increasing this alignment increases ωs, which in turn augments the turbulence production rate, and vice versa. In addition, due to the enhanced strain rate of the carrier fluid by the particles, the dissipation rate of turbulence energy is always increased. The particles also reduce the alignment of the vorticity vector with the intermediate eigenvector (β) of the strain rate tensor. This reduction in alignment is due to an increase in the rotational term and particle-source term in the governing equation of the cosine of the angle between the vorticity vector and the intermediate strain eigenvector.