Resistance and Drag Reduction of Flows of Clay Suspensions

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 considerably reduced if the flow boundary is rough. The friction factor of flows of high clay concentrations over the gravel bed was less than half the clear-water flow at the same discharge and and energy slope. At low concentrations, a floc of clay particles having a long-chain structure may damp turbulence and result in drag reduction. At high concentrations, all flocs in clay suspension connect together and form a three-dimensional net structure, which affects the resistance in two ways: (1) By damping turbulence; (2) by increasing the viscous resistance. For flows over rough boundaries, turbulent shear dominates the resistance, and high clay concentrations cause drag reduction. For flows over a smooth bed, the effect of damping turbulence was counterbalanced by the effect of increasing viscous resistance; therefore, no drag reduction occurred.