  We have conducted a direct numerical simulation (DNS) study of dilute
turbulent particulate flow in a vertical plane channel, considering thousands
of finite-size rigid particles with resolved phase interfaces. The particle
diameter corresponds to approximately 11 wall units and their terminal Reynolds
number is set to 136. The fluid flow with bulk Reynolds number 2700 is directed
upward, which maintains the particles suspended upon average. Two density
ratios were simulated, differing by a factor of 4.5. The corresponding Stokes
numbers of the two flow cases were O(10) in the near-wall region and O(1) in
the outer flow. We have observed the formation of large-scale elongated
streak-like structures with streamwise dimensions of the order of 8 channel
half-widths and cross-stream dimensions of the order of one half-width. At the
same time, we have found no evidence of significant formation of particle
clusters, which suggests that the large structures are due to an intrinsic
instability of the flow, triggered by the presence of the particles. It was
found that the mean fluid velocity profile tends towards a concave shape, and
the turbulence intensity as well as the normal stress anisotropy are strongly
increased. The effect of varying the Stokes number while maintaining the
buoyancy, particle size and volume fraction constant was relatively weak.
