Experimental evidence of settling retardation in a turbulence column
Résumé
Settling experiments were conducted in a turbulence column to investigate the effect of turbulence on the effective fall velocity of solid particles slightly denser than the fluid (ρp≳ρf). Five types of particles of different materials and shapes were tested, their size ranging between o(1)η and o(10)η, where η is the Kolmogorov viscous length scale. Thus, the particles were of finite size with an unknown analytical form for the fluid-particle forces. The density ratio ranged as (ρp−ρf)/ρf={0.13:1.6}, and the still-fluid particle Reynolds number as Re0p={75:981}. The turbulence levels characterized with the integral-scale Reynolds number ranged as ReL={34:510}. Two-dimensional (2D) particle image velocimetry was used to obtain flow statistics, the residual mean circulation, and the turbulence statistics, while 2D particle tracking was performed to measure particle settling velocities. For all types of particles tested, settling retardation is observed as the turbulence intensity is increased. It is found that if both the effective fall velocity Ws and the turbulent fluid velocity Wf,rms are nondimensionalized by the still-fluid particle terminal velocity W0, the settling retardation can be described by a unique relation independent of the particle type, Ws/W0=f(Wf,rms/W0), for the given range of flow regimes. Using analytical descriptions of the loitering and nonlinear drag effects, this scaling is shown to have a solid physical basis.