Small heavy particles on top of contours of the tangential gas velocity at a cross section of the pipe (particle volume fraction is 0.005%). The particles attenuate the turbulence. A hard-sphere collision model was used.

Small heavy particles in the boundary layer (particle volume fraction is 1.5%). The color refers to the location in the pipe (radius). Only colliding particles are shown, during short time intervals around the collisions.

Trajectories of a few randomly chosen particles for particle volume fractions of 0.01% and 1.5%, respectively. The trajectories are projected on a cross section of the pipe (circle). A red flash means a collision with another particle, while a yellow flash means a collision with the boundary.

Turbulence characteristics of particle-laden pipe flow. A.W. Vreman, J. Fluid Mech. 584, 235-279 (2007). PDF