turbulent flow

Meaning: Turbulent flow is fluid motion in which velocity fluctuates irregularly and the fluid forms eddies and swirls, rather than sliding in smooth parallel layers as in laminar flow. It occurs when inertial effects outweigh the viscous forces that damp disturbances. Turbulence dissipates extra kinetic energy as heat, so a larger pressure difference is needed to move the same volume per second.

Equation: Re = ρvD/η, where Re is the Reynolds number (dimensionless), ρ is fluid density (kg/m³), v is average flow speed (m/s), D is pipe diameter (m), and η is viscosity (Pa·s). In a long straight pipe, flow is typically laminar below Re of about 2000 and turbulent above about 3000, with an unstable range between.

On the MCAT: A passage changes the speed, vessel radius, or fluid and asks whether flow becomes turbulent. Poiseuille's law, Q = πr⁴ΔP/8ηL, assumes laminar flow, so it does not hold once flow is turbulent. For a fixed flow rate, narrowing a tube raises v by continuity and raises Re.

Common confusion: Students assume high speed alone causes turbulence. Re also depends on density, diameter, and viscosity. A more viscous fluid has a lower Re and stays laminar more easily. The cutoff values are approximate, not exact.