Abstract:
The changes occurring to a turbulent shear flow when sediment of different densities and concentrations is in suspension was investigated in this study. Theoretical and experimental results were used to find the effect of the solid particles on the flow structure. Observations of the sediment patterns in the flow, and the most recent knowledge of the structure of a turbulent shear flow were used to explain the suspension mechanism. Theoretical studies, using the two-phase energy balance, and the motion of the solid particles in response to an oscillating fluid motion demonstrate that the low-frequency fluid motion is retarded for sediment heavier than the suspending fluid because the solid and the particle oscillate together at the same frequency and amplitude. For higher frequency motion, slip occurs between the fluid and the solid, so the fluid motion is not significantly affected, although the solid particles oscillate with a different amplitude to the fluid motion. As a result, these local effects of the sediment cause the fluid turbulent intensity to be reduced, and the turbulent microscale is reduced. Experimental studies using a hot-film probe in the sediment-laden flow demonstrate that the mean velocity gradient changes. The velocity is reduced near the bed, but is increased in the upper regions of the flow, although the logarithmic velocity profile still applies over most of the flow depth. The longitudinal fluid turbulence intensity is increased greatly when the sediment is heavier than the suspending fluid, although it is not altered significantly for almost neutrally buoyant sediment particles. By using information gained when drag-reducing polymers are added to a pipe flow, it is inferred that the longitudinal turbulent intensity is increased, the vertical turbulent intensity is reduced, and the turbulent shear stress becomes "decoupled". This decoupling of the shear stress and the decrease in the vertical turbulent intensity cause the change in the mean velocity gradient. The temporal microscale is slightly reduced in the region immediately adjacent to the bed where the concentration of heavy sediment is largest. For the lightweight sediment (S.G. = 1.18), the sediment transfer coefficient follows the momentum transfer coefficient over the flow depth, while for the heavier sediment (S.G. = 2.71), the sediment transfer coefficient follows the momentum transfer coefficient in the lower region of the depth (y/y <0.4), but is almost constant in the upper regions of the flow depth. This is considered to be due to the mechanism by which the sediment becomes suspended in the flow. The bursting-vortex motion which has been demonstrated to be responsible for two-dimensional shear flows is shown to be responsible for maintaining sediment in suspension. The lightweight sediment follows the fluid motion, while the heavier sediment does not follow the fluid motion exactly. Hence, the heavier particles do not follow the fluid vortex motion. Photographs of the sediment clouds are presented, showing the sediment "pulses".