Turbulent channel flow of a dense binary mixture of rigid particles
  We study turbulent channel flow of a binary mixture of finite-size neutrally-buoyant rigid particles by means of interface-resolved direct numerical simulations. We fix the bulk Reynolds number and total solid volume fraction, $Re_b = 5600$ and $\Phi=20\%$, and vary the relative fraction of small and large particles. The binary mixture consists of particles of two different sizes, $2h/d_l=20$ and $2h/d_s=30$ where $h$ is the half channel height and $d_l$ and $d_s$ the diameter of the large and small particles. While the particulate flow statistics exhibit a significant alteration of the mean velocity profile and turbulent fluctuations with respect to the unladen flow, the differences between the mono-disperse and bi-disperse cases are small. However, we observe a clear segregation of small particles at the wall in binary mixtures, which affects the dynamics of the near wall region and thus the overall drag. This results in a higher drag in suspensions with a larger amount of large particles. As regards bi-disperse effects on the particle dynamics, a non-monotonic variation of the particle dispersion in the spanwise (homogeneous) direction is observed when increasing the percentage of small/large particles. Finally, we note that particles of the same size tend to cluster more at contact whereas the dynamics of the large particles gives highest collision kernels due to a higher approaching speed. 
Iman Lashgari, Francesco Picano, Pedro Costa, Wim-Paul Breugem and   Luca Brandt
physics.flu-dyn