This paper deals with turbidity currents in a circular settling tank. A mathematical model with a k-epsilon turbulence model has been developed. Using this mathematical model, the following unique properties of turbidity currents in a circular settling tank are demonstrated: turbulence induced by the turbidity currents remains after most sediment particles have settled down. This residual turbulent diffusivity has a serious effect on the settling of finer particles. This phenomenon is a very important result in this study. Especially, in the case of a smaller densimetric Froude number, which is a stronger density effect, this residual turbulence effect increases, and also decreases the removal ratio in the downstream with low concentration. Generally, the bottom density current enhances the sediment transport near the tank bottom, while the bottom shear gives reversal influence. When the settling velocity is high, the settling ends under the developing stage both of the turbidity current and of the bottom boundary layer. On the contrary, if the settling velocity is low, the sediment travels a long distance, where the boundary layer is built up, resulting in the reduction of sediment transport near the tank bottom. The overall properties of the density-affected settling tank are also investigated in terms of the removal ratio.

Density current in a rectangular settling tank is investigated mathematically. The basic equation consists of equation of motion for horizontal and vertical direction together with the mass conservation equation of both fluid and sediment.As a turbulence model, the standard k-€ model is employed. Flow velocity profiles and sediment concentration distribution are illustrated. When the temperature of inlet suspension is warmer than that of the tank water, the suspension flows downward with the positive buoyancy. This density current increases the turbulence in its accelerating stage, which prevents the settling of sediments. On the other hand, in the case of high sediment concentration, the turbidity current accelerates rather low region in a tank, which may bring the resuspension of bottom sediment. It is also shown that the flow field is very complicated one, because the density current brings up under the developing stage of bottom boundary layer. If the sediment particle has higher fall velocity, they settle down within a short distance. In such case, the density current gives its effect on the sediment motion only in the developing stage and this effect leads to the decrease of removal ratio.

Effect of inlet structure on flow in rectangular settling tank.

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