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Aerosol Transport – Inertia


:: Section 4

Settling Velocity

Once we can determine the force acting on an aerosol, we can then investigate how an aerosol moves. When a particle is released, it settles due to gravity, and the velocity increases. As the velocity increases, the drag also increases which counterbalances the gravitational force. Eventually, when these two forces are equal to each other, i.e. the net force is zero, there is no more acceleration and the velocity reaches a constant value. This is called terminal settling velocity, i.e.

By solving the equation above, the terminal settling velocity can be obtained as:

This equation is valid for Rep < 1.0. For most aerosols we'll encounter, this is the case. If Rep is rather large, Newton's resistance equation should be used for FD in the equation above.


1. Does a larger or a smaller aerosol settle faster? Why?
2. Why does a particle settle faster in air than in water? Why?