Turbulent flow terminal settling velocity

The aerodynamic diameter is one of the most common equivalent diameters. It can be defined as the diameter of a unit den.sity sphere with the same terminal settling velocity as the particle being measured. The aerodynamic diameter is commonly used to describe the mt)lioii of particles in collection devices such as cyclone separators and impactors. However, in shear flows, the motion of irregular particles may not be characterized accurately by the equivalent diameter alone because of the complex rotational and translational motion of inegular particles compared with spheres. That is, the path of the irregular particle may not follow that of a particle of the same aerodynamic diameter. It is of course possible that there may be a. sphere of a certain diameter and unit density that deposits at the same point this could be an average point of deposition because of the effects of turbulence or the. stochastic behavior of irregular particles. 

The flow velocity has to be sufficiently low so that the tuibulence in the flowing stream will not suspend the settled particles. According to Diyden the settled particles will not be resuspended provided the velocity of the turbulent eddies (refer to Section 3.2) does not exceed the particle terminal velocity. On this basis, the flow velocity can be about 20 times the teiminal velocity however, it is reasonable to use only about 10 times the terminal velocity. On this basis. 

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