Bruce Sutherland | Departments of Physics and of Earth & Atmospheric Sciences, University of Alberta
Particles in Fluid Flows: Small Things Making a Big Difference
Through laboratory experiments, we examine the transport, settling and resuspension of sediments as well as the influence of floating particles upon damping wave motion. Salt water is shown to enhance flocculation of clay and hence increase their settling rate. In studies modelling sediment-bearing (hypopycnal) river plumes, experiments show that the particles that eventually settle through uniform-density fluid toward a sloping bottom form a turbidity current. Meanwhile, even though the removal of particles should increase the buoyancy and hence speed of the surface current, in reality the surface current stops. This reveals that the removal of fresh water carried by the viscous boundary layers surrounding the settling particles drains the current even when their concentration by volume is less than 5%. The microscopic effect of boundary layer transport by particles upon the large scale evolution is dramatically evident in the circumstance of a mesopycnal particle-bearing current that advances along the interface of a two-layer fluid. As the fresh water rises and particles fall, the current itself stops and reverses direction. As a final example, the periodic separation and consolidation of particles floating on the surface as perturbed by surface waves is shown to damp faster than exponentially to attain a finite-time arrest as a result of efficiently damped flows through interstitial spaces between particles - a phenomenon that may be important for understanding the damping of surface waves by sea ice in the Arctic Ocean.