The effect of gravity on flocculation of cohesive fine sediment is an important issue. Since it is difficult to observe, this complicated process was numerically simulated in 3-D space based on fractal cluster growing theory. The impacts of gravity on fractal dimension and size distribution of flocs were studied with different boundaries and sediment concentrations. On the basis of the data obtained from simulation experiments, it can be concluded that: when gravity is considered, the variation of fractal dimension is similar in both boundaries, that is, at the beginning, it is much the same as in Brown motion, and then, the fractal dimension appears the trend from increasing to decreasing. Finally, it reaches a stable value, which is 2.12 in closed boundary and 2.05 of periodic boundary; Range of floc size becomes large and the heterogeneity of it is bigger. Lastly, the ratio of flocs with large diameter increases with the increase of sediment concentration.
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