turbulent
Summary
Turbulent flow is irregular, swirling fluid motion with eddies, in contrast to smooth laminar layers. It becomes likely at a high Reynolds number (fast, wide, dense, or low-viscosity fluid), and Poiseuille's law no longer applies.
Detail
Meaning: Turbulent flow is irregular, chaotic fluid motion with eddies and swirls, instead of smooth parallel layers (laminar flow). It occurs when inertia outweighs viscous damping: high speed, wide tube, high density, or low viscosity. Turbulence dissipates extra energy as heat, so a larger pressure difference is needed for the same flow rate.
Equation: Re = ρvD/η, where Re is the Reynolds number (dimensionless), ρ is fluid density (kg/m³), v is average speed (m/s), D is tube diameter (m), and η is viscosity (Pa·s). Roughly, Re below about 2000 is laminar and above about 3000 is turbulent, with an unstable range between.
On the MCAT: A passage describes flow through a pipe or blood vessel, and you predict how the flow type changes when speed, radius, or viscosity changes by using Re. Poiseuille's law and the usual Bernoulli treatment assume smooth laminar flow, so they fail or only approximate in turbulence.
Common confusion: Students expect a narrower tube to keep flow laminar because its resistance is higher. For a fixed flow rate Q, a smaller cross-sectional area raises speed (v = Q/A), so Re rises and turbulence becomes more likely at a constriction.
Sources
- Physics (Cutnell and Johnson)
- Fundamentals of Physics (Halliday, Resnick, Walker)
- University Physics (Young and Freedman)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.