Turbulent flow
Definition
Chaotic, swirling fluid flow full of eddies and mixing — like whitewater rapids or the gusty wake behind a truck on the highway.
Technical definition
Flow regime characterized by chaotic, three-dimensional velocity fluctuations and eddies across a wide range of sizes. Occurs at high Reynolds numbers (typically Re > 4000 in pipes), where inertial forces overwhelm viscous damping.
Example
The roar inside an airplane cabin is largely turbulent airflow over the wings and fuselage. Golf ball dimples deliberately trip the airflow into turbulence, which paradoxically reduces drag and makes the ball fly farther.
Turbulence is not always bad. While it increases drag and mixing losses, engineers deliberately create it — in combustion chambers to mix fuel and air, or on golf balls to keep airflow attached longer. Whether turbulence helps or hurts depends on the goal.