Air flow through a duct exhibits the highest velocity where?

The highest air velocity in a duct system is found in the middle of the duct due to the principles of fluid dynamics. In a rectangular duct, air encounters friction with the duct walls, which slows down the air particles nearest to the surface. This results in a gradient of velocity, with the air moving faster toward the center where there is less friction.

This phenomenon is consistent with the concept of a parabolic velocity profile commonly observed in laminar and turbulent flow in ducts. While the edges of the duct experience lower velocity because of increased viscous drag and boundary layer effects, the center, having less interaction with the walls, allows the air to travel more freely, achieving higher speeds.

In different scenarios, such as the entrance and exit of the duct, the airflow may also be affected by pressure changes and flow expansion but does not typically result in higher velocities than those found in the middle of the duct.