Dispersion of inertial finite-size particles in turbulent open-channel flow

Plastic pollution in our aquatic systems is a pressing issue, and the spread of these particles is determined by several factors. In this study, the advection and dispersion of negatively buoyant finite-size particles of four different shapes (spheres, circular cylinders, square cylinders and flat cuboids) and two sizes (6 and 9 mm) are investigated in turbulent open-channel flow. The volume, mass and characteristic length are fixed for each size. Four different turbulent conditions are considered, varying the free stream velocity $U_{\infty }=$ 0.25 and 0.38 m s –1 and turbulence intensity ( $(u'/U)_\infty =4$ % and 9 %). The particles are released individually from below the water surface. A catch-grid is placed along the bottom floor to mark the particle landing location. The average particle advection distance remains unchanged between the turbulence levels, suggesting that the mean settling velocity is independent of turbulence in this regime. Based on the root mean square of the landing locations, the particle dispersion varies with particle shape, size, settling velocity and turbulent flow conditions. For the square cylinders investigated in this work, the effect of particle shape on dispersion is difficult to predict at low flow velocities and turbulence intensities. As the turbulent fluctuations increase, the dispersion becomes more predictable for all shapes. An empirical expression is proposed to relate turbulent velocity fluctuations, integral length scales, particle settling velocity and particle size to streamwise dispersion. It is found that finite-size inertial particles do not disperse per simple turbulent diffusion, meaning that particle geometry has to be incorporated into dispersion models.