Experimental study of finite-size spheres settling in homogeneous turbulence

Many particles transported in natural turbulent flows are large compared to the smallest scale of fluid motion and weakly negatively buoyant. For such finite-size particles, whose size is greater than Kolmogorov length scale, it has been reported that turbulence reduces the settling velocity relative to the terminal velocity in quiescent fluid. Although several qualitative explanations on the reduced settling exist, clear quantitative evidence remains lacking. This is partially due to limitations of the measurement systems where particle trajectories and background flow are not measured simultaneously. To solve this, we conduct laboratory experiments on weakly negatively buoyant, finite-sized particles settling in homogeneous and nearly isotropic turbulence generated by four random-jetarrays with simultaneous measurements of particle trajectories and ambient turbulent flow fields using three synchronized cameras. We use spherical particles with identical settling velocities but varying sizes. By examining the kinematics of settling particles with ambient flow structures, we identify the conditions that lead to particle acceleration and deceleration in the settling direction. We also compare the mean slip velocity with the terminal settling velocity to assess the reduced settling in turbulence. Finally, we examine how these effects vary with particle size when the settling velocity remains constant.