laminar flow

Meaning: In laminar flow, fluid moves in smooth parallel layers (streamlines) with no eddies or mixing between layers. In a tube, viscosity holds the layer at the wall nearly still, so speed is greatest at the center and the velocity profile is parabolic. Laminar flow occurs when viscous forces dominate over inertia. Turbulent flow has swirling eddies and dissipates more energy.

Equation: Re = ρvD/η, where ρ is density (kg/m³), v is average speed (m/s), D is tube diameter (m), η is viscosity (Pa·s), and Re is dimensionless; Q = πΔP r⁴/(8ηL), where Q is volumetric flow rate (m³/s), ΔP is the pressure difference (Pa), r is radius (m), and L is length (m).

On the MCAT: Passages describe blood or another fluid in a vessel and ask what happens when radius, viscosity or speed changes. Predict that raising v or D raises Re, pushing flow toward turbulence. Poiseuille's law gives Q ∝ r⁴, so doubling r raises flow 16-fold at fixed ΔP, and resistance scales as 1/r⁴. Re below about 2000 in a pipe is usually laminar.

Common confusion: Laminar does not mean every layer moves at the same speed. Speed varies across the cross-section and is zero at the wall in the standard model. Poiseuille's law needs laminar flow of a viscous fluid. Bernoulli's equation assumes an ideal, nonviscous fluid.