The motion of the air relative to a body in which it is moving.
Q = V / A
Q: Air flow in m3/s or cfm; V: Air velocity in m/s or ft/min; A: Duct cross sectional area in m2 or Ft2
Air behaves in a fluid manner, meaning particles naturally flow from areas of higher pressure to those where the pressure is lower. It can be described as a volumetric flow rate (volume of air per unit time) or a mass flow rate (mass of air per unit time). What relates both forms of description is the air density, which is a function of pressure and temperature through the ideal gas law. The flow of air can be induced through mechanical means (such as by operating an electric or manual fan) or can take place passively, as a function of pressure differentials present in the environment.
Laminar flow occurs when air can flow smoothly, and exhibits a parabolic velocity profile;
Turbulent flow occurs when there is an irregularity (such as a disruption in the surface across which the fluid is flowing), which alters the direction of movement and exhibits a flat velocity profile.
To predict whether the flow tends to be Laminar or Turbulent, Reynolds number can help to determine the pattern.
Reynolds number is a dimensionless quantity that helps predict fluid flow patterns in different situations by measuring the ratio between inertial and viscous forces. At low Reynolds numbers, flows tend to be dominated by laminar flow, while at high Reynolds numbers, flows tend to be turbulent.
(See also, Renolds Number)