An experimental study on the motion of buoyant particles in the free-surface vortex flow.

Experiments are conducted in a Ø600 mm tank to analyse the 3D motion of buoyant particles in the free-surface vortex flow. The experiments revealed two stages in the particle motion: stage 1 is the helical motion along the vortex air core and stage 2 is the axial motion inside the vortex core. The stage 1 motion is sensitive to the particle's initial conditions by showing a chaotic behaviour and quantified by determining the largest Lyapunov exponent. Consequently, the predictability (forecast horizon) of the particle's motion is limited. A motion parameter is proposed that indicates if a continuous downward motion along the air core occurs. The dynamics in the stage 2 motion is determined by the imbalance between the particle's buoyancy force and fluid drag force. The drag appears to be determined by the Taylor column drag force as present in rotating fluids. Based on this force, a motion condition is proposed that indicates if axial motion inside the vortex core occurs. [ABSTRACT FROM AUTHOR]