The flow of a fluid becomes turbulent when
- A. the speed falls below a critical value
- B. the speed exceeds a critical value and the Reynolds number becomes large
- C. the pipe is perfectly smooth
- D. the fluid is very viscous
Explanation
Turbulence sets in when inertial effects dominate viscous ones, which is what a large Reynolds number expresses, and it appears as eddies and rapid mixing. A high viscosity favours laminar flow rather than turbulence, since it damps the disturbances out. Turbulent flow carries far more resistance, which matters in pipelines, aircraft design and blood vessels alike.