Experimental studies of turbulence in liquid-solid flows

Abstract

A series of laboratory experiments were performed to ascertain the extent and manner of turbulence modification induced by low concentration suspended loads of near neutral buoyancy. Hot-wire measurements of the fluid velocity field of free surface flows were obtained in a specially designed flume recirculating a dielectric liquid and 0.5 mm" diameter spherical plastic particles. In the fixed Reynolds number flow (16,800) the data obtained at six different concentration levels, ranging from 0 to 3.5% by volume, indicate that the presence of particles produces substantial turbulence changes. Even at this low level the mean velocity profile shows an increasing gradient near the bed and sharp deviation from a logarithmic profile. The rms level of each of the velocity components u', v' and w' increases, indicating a general rise in turbulence intensity. The Reynolds stress ρ u'v' increases, and its maximum value shifts away from the bed. The overall scale of turbulence appears to remain unchanged. The data indicate that offhand neglect of suspended particle presence is an oversimplification. There is a similarity between these data and those obtained under adverse pressure gradients. Some effort is made to clarify the altered turbulence production meechanism, and some future experimental work is proposed.