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.
Generally, in encased flows, instantaneous velocity vectors are larger in magnitude in the middle of the profile due to the effect of friction from the material of the pipe, duct, or channel walls on nearby layers of fluid.
In tropospheric atmospheric flows, velocity increases with elevation from ground level due to friction from obstructions like trees and hills slowing down airflow near the surface.
In a turbulent flow, particles are traveling in random and chaotic directions which gives rise to curved, spiraling, and often intersecting streamlines.
The Reynolds number, a ratio indicating the relationship between viscous and inertial forces in a fluid, can be used to predict the transition from laminar to turbulent flow.
The range of Reynold's number that defines each type of flow depends on whether the air is moving through a pipe, wide duct, open channel, or around airfoils.
Air flow can be simulated using Computational Fluid Dynamics (CFD) modeling, or observed experimentally through the operation of a wind tunnel.
Because CFD models "also track the flow of solids through a system,"[6] they can be used for analysis of pollution concentrations in indoor and outdoor environments.
In outdoor environments, particulate matter comes from direct sources such as internal combustion engine vehicles’ (ICEVs) tailpipe emissions from burning fuel (petroleum products), windblow and soil, and indirectly from atmospheric oxidation of volatile organic compounds (VOCs), sulfur dioxide (SO2), and nitrogen oxide (NOx) emissions.
Total or static pressure rise, and therefore by extension airflow rate, is determined primarily by the fan speed measured in revolutions per minute (RPM).
[7] In control of HVAC systems to modulate the airflow rate, one typically changes the fan speed, which often come in 3-category settings such as low, medium, and high.
This may be achieved through mechanical means (i.e. the use of a louver or damper for air intake and a fan to induce flow through ductwork) or through passive strategies (also known as natural ventilation).
Natural ventilation also has the flaw that its feasibility is dependent on outdoor conditions; if outdoor air is significantly polluted with ground-level ozone concentrations from transportation related emissions or particulate matter from wildfires for example, residential and commercial building occupants may have to keep doors and windows closed to preserve indoor environmental quality (IEQ).
Wind-driven passive ventilation relies on building configuration, orientation, and aperture distribution to take advantage of outdoor air movement.
Varying rates of air movement may positively or negatively impact individuals’ perception of warmth or coolness, and hence their comfort.