Drag reduction in flows with clay mineral particles was experimentally
studied, Experiments with clay suspensions flowing over smooth gravel
, and stone beds proved that resistance of the flow may be considerabl
y reduced if the flow boundary is rough. The friction factor of hows o
f high clay concentrations over the gravel bed was less than half of t
he clear-water flow at the same discharge and energy slope. At low con
centrations, a flee of clay particles having a long-chain structure ma
y damp turbulence and result in drag reduction. At high concentrations
, all flocs in clay suspension connect together and form a three-dimen
sional net structure, which affects the resistance in two ways: (1) By
damping turbulence; (2) by increasing the viscous resistance. For flo
ws over rough boundaries, turbulent shear dominates the resistance and
high clay concentrations cause drag reduction. For flows over a smoot
h bed, the effect of damping turbulence was counterbalanced by the eff
ect of increasing viscous resistance; therefore, no drag reduction occ
urred.