Settling Velocity Measurement

The settling velocity of fractal aggregates is one of their most significant features in many environmental systems. Settling velocities of fractal aggregates have been observed to be significantly higher than would be expected from impermeable spheres with the same mass and spatial extent as the aggregates [23, 64, 65]. The reason for this behaviour is that the fractal structures are often highly permeable, so that the fluid drag on the aggregates is reduced. This is clearly a function of the geometry of the aggregates and the relative velocity of the aggregates and the fluid.

The fact that the drag is geometry dependent is highly significant to many applications involving settling of aggregates because it implies that transport properties of fractally aggregated matter should depend on the fractal dimension of the system. In other words, it is important in practice to have models of settling velocity that properly account for the effects of fractal dimension. The geometry dependence of drag is also significant, in that it potentially gives us a measurement that can be used to infer structural information about aggregates.

Using settling velocity to infer fractal dimension and modelling settling velocity accurately are really two different problems with a common underlying physical basis. The first of these problems will be dealt with here; the second is beyond the scope of this chapter.

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