Why do velocities measured at different points in the same cross-section of a pipe or duct vary?

Velocities measured at different points in the same cross-section of a pipe or duct can vary primarily because molecules may slide or slip against each other as they flow through the medium. This phenomenon is a result of the viscous nature of the fluid, where the layers of fluid move at different speeds. In a flow with a velocity gradient, the fluid near the pipe wall experiences friction, which slows it down compared to the fluid in the center that moves more freely. This differential movement creates a velocity profile across the cross-section of the duct or pipe, leading to variations in velocities observed at different points.

While pressure levels, fan speed, and air density can influence overall flow characteristics, the fundamental reason for variations in velocity at a cross-section is tied to the interactions and behavior of the fluid molecules themselves. The viscosity and resultant behavioral characteristics of the fluid play a crucial role in determining these velocity discrepancies.