Settling Velocity

The terminal settling velocity of an airborne particle is an important quantity for characterizing the settling behavior of the particle. The aerodynamic diameter of a particle, a key property for characterizing particle deposition, is dependent upon the settling velocity.

A particle will reach its settling velocity when the drag force and buoyancy force balance with the gravitational force on the particle.

Free Body Diagram
Sum the forces!

Solving for V, the settling velocity is therefore: