Particle dispersion in flows over rough surfaces

The dispersion of particles in turbulent flows is a very common, yet not fully understood problem. In this work we deal with a fully developed turbulent channel flow, at Reτ = 180. The influence of wall roughness on the velocity field and on the particle dispersion is analysed. Three types of surface are studied: a smooth wall, a series of staggered cubes and triangular riblets parallel to the flow direction. The cubes strongly increase the pressure gradient necessary to maintain a constant mass flow rate, whereas the riblets yield drag reduction with respect to the smooth channel case. The particle statistics are analysed in terms of their mean square separation, for which a theoretical reference is provided by the Richardson law, which applies to isotropic turbulence. Significant deviations are found in the channel flow, dependent on the type of roughness, the Stokes number and the initial position of dispersion.