Investigation of the hydrodynamic behaviour of marine aggregates using particle image velocimetry (PIV)

Abstract

The hydrodynamic behaviour of particles has a significant influence on the interactions and the kinetics of flocculation between suspended particles. However, characterisation of the hydrodynamics of particles and particle aggregates in water is rather difficult because of the small scale of the hydraulic field, a wide range of particle sizes and the dynamic situation of moving particles. In this laboratory study, an advanced visualisation technique in particle image velocimetry (PIV) was employed to investigate the hydrodynamic properties of settling particles in water. The experiments were conducted in a settling column filled with a suspension of fluorescent polymeric beads as seed tracers. A laser light sheet was generated by the PIV setup to illuminate a thin vertical planar region in the settling column, while the motions of particles were recorded by a high speed CCD camera. The PIV technique was able to capture the trajectories of the tracers when a large particle settled through the tracer suspension, which gave the direct flow information, e.g., streamlines, surrounding the settling particles. Three types of particles, including large solid spheres, flocs of standard microspheres, and marine diatom aggregates, have been characterised for their hydrodynamic behaviours. The PIV laboratory results indicate directly the curvilinear feature of the streamlines around falling particles. The rectilinear collision model largely overestimates the collision areas of the settling particles. However, the available curvilinear model underestimates the collision potentials by one order of magnitude or two for the solid spheres. The collision areas of the microsphere flocs are more than an order of magnitude greater than the similarly-sized solid spheres. Marine aggregates appear to be highly porous and fractal, which allows streamlines to penetrate into the aggregate interior. The permeable feature of marine aggregates can significantly enhance the collisions and flocculation between the aggregates and other small particles including algal cells in water.