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Aerosol Transport – Inertia


:: Section 3

Newton's Resistance Law and Stokes's Law

  Section Contents
I. Newton's Law

If you drop a ball, it falls due to the gravitational force. Meanwhile, the resistance of gas molecules (drag) tend to slow down the movement.

To describe the drag force resulting from gas resistance, FD, the following equation can be used:

drag force eq

where CD is the drag coefficient. This is the general form of Newton's resistance equation.

This Newton's resistance equation is valid in the turbulent flow regime (i.e. Rep > 1000), where inertial forces are much larger than viscous forces. The drag coefficient CD has a nearly constant value of 0.44.


Which of the following condition(s) can the Newton's Law be applied?
(a) A particle with a diameter of 10 μm having a velocity of 1 m/s.
(b) A particle with a diameter of 100 μm having a velocity of 10 m/s.
(c) A particle with a diameter of 1 mm (1000 μm) having a velocity of 20 m/s.

To calculate the Rep values, you can use the web-calculator below:

ρg (kg/m3)

  V (m/s)   dp (μm)   η (Pa·s)   Rep