Experimental investigation of single particle settling in turbulence generated by oscillating grid

In this study, turbulence influence on the settling behavior of solid particles was investigated experimentally in confined turbulent aquatic environment generated by oscillating grid. An enhanced PIV system was employed to conduct simultaneous velocity measurements of individual settling particles and ambient fluid. Grains varying in shape (spherical and cylindrical) and diameter (2.78–7.94 mm) were tested with different turbulent conditions.The results showed clearly that the settling behavior of particles subjected to turbulence is significantly modified. First, the settling velocity modification is closely correlated to the mean vertical velocity of the fluid zone (very close to the settling particle), which in size is in the order of a few particle diameters. Second, the relative settling velocity is smaller than the still water terminal velocity for the most cases. Lastly, the fluctuation in the settling velocity is significantly increased, as compared to the still water conditions, and clearly dependent on the turbulence intensity.The experimental data were also analyzed with dimensional considerations. By comparing to literature, turbulence effects on the relative settling velocity were discussed with regard to Stokes number, Richardson number and dimensionless turbulence length scale. Finally, a simple analytical model was proposed for estimating the turbulence-modified settling velocity.