Which type of flow is characterized by a velocity gradient from slowest at the walls to fastest at the center?

The type of flow characterized by a velocity gradient from slowest at the walls to fastest at the center is laminar flow. In laminar flow, the fluid moves in parallel layers, with minimal disruption between them. This layered movement means that fluid particles closest to the channel boundaries (like the walls of a pipe or vessel) experience more friction and therefore move more slowly than those in the center, which have less resistance to motion.

This velocity gradient is a key feature of laminar flow, and it distinguishes it from turbulent flow, where fluid moves chaotically and without a defined flow pattern. In turbulent flow, the velocities of the fluid particles vary significantly more than in laminar flow, and there is no clear gradient.

Other types of flow, such as vortex flow, involve rotation and swirling motion, which also disrupt a simple velocity gradient. Stagnant flow refers to a condition where there is little or no movement, resulting in a uniform velocity of zero rather than a defined gradient. Thus, the defining characteristic of a changing velocity gradient is fundamentally unique to laminar flow, making it the correct answer in this context.