laminar

Meaning: Laminar flow is fluid moving in smooth, parallel layers that do not mix. Viscosity makes the layers drag on each other, so in a pipe the speed is greatest at the center and falls to nearly zero at the wall. Turbulent flow has eddies and mixing, and becomes likely as speed or tube diameter rises, or as viscosity falls.

Equation: Re = ρvD/η, where ρ is density (kg/m³), v is average speed (m/s), D is tube diameter (m) and η is viscosity (Pa·s); Re has no units. Flow in a pipe is typically laminar below an Re of roughly 2000. For laminar flow, Q = πr⁴ΔP/(8ηL), where Q is volume flow rate (m³/s), r is radius (m), ΔP is pressure difference (Pa) and L is length (m).

On the MCAT: Passages about blood vessels or pipes ask how flow changes when radius, viscosity or speed changes. Because of the r⁴ term, halving the radius at fixed ΔP cuts Q by a factor of 16. If Re rises past the threshold, flow turns turbulent, and more pressure is needed for the same flow.

Common confusion: Laminar does not mean viscosity is absent. An ideal fluid is defined as nonviscous, and Bernoulli's equation assumes ideal, steady, streamline flow. Real laminar flow still has viscous losses, which Poiseuille's law describes.