The definition of duct flow is the movement of air or other gases through a duct or pipe. It is a type of fluid flow that occurs in a closed conduit, such as a pipe or duct. Duct flow is typically characterized by a relatively high Reynolds number, which is a measure of the ratio of inertial forces to viscous forces in a fluid. This type of flow is important in many engineering applications, such as air conditioning and ventilation systems, as well as in industrial processes. Duct flow is also used in medical and scientific applications, such as measuring the flow of blood through a vessel. In duct flow, the flow rate is determined by the pressure difference between the two ends of the duct, as well as the cross-sectional area of the duct. The flow rate is also affected by the friction between the air and the walls of the duct, as well as the density of the air. Additionally, the shape of the duct and the presence of any obstructions can also affect the flow rate.