Effect of Shape and Size on the Transport of Particles Over The Turbulent Free Surface of a Natural Stream

Abstract

Here we investigate experimentally the motion of floating particles of different shape and size on the turbulent free surface of a field-scale meandering stream, using particle tracking velocimetry. Millimetre-sized spheres are used as tracers to obtain mean and fluctuating velocity fields of the flow surface. We focus on an approximately homogeneous region, where the single-point and two-point velocity statistics follow the classic phenomenology of three-dimensional turbulence. We then consider centimetre-sized discs and rods, much larger than the dissipative scales but much smaller than the integral scales of the turbulence. As finite-sized inertial particles, these exhibit similar velocities as the small tracers but weaker and less intermittent accelerations. Consequently, the motion of the larger particles along their trajectories is more time-correlated, and their diffusion coefficient is larger. This is confirmed by the mean square displacement of single particles and mean square separation between particle pairs, both of which grow faster in time compared to the tracers. The discs are also found to disperse faster than rods of the same size, pointing the important role of the objects' shape in the transport dynamics.