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.
