Air flow modeling generally refers to a scaled physical reproduction of a gas flow system, possibly involving a dispersed second phase (liquid or solid). Air flow modeling is routinely performed for flue gas ducts, air pollution control equipment, chimneys and petrochemical process equipment such as FCC (Fluidized Catalytic Cracker) units, cyclone separators, and reactors. Large cost savings can be reaped by including a modeling effort in the design process for new plants or retrofits. When combined with 3D computational modeling, air flow modeling is a valuable tool for detailed design.