Laminar Versus Turbulent Flow

Expansion

Reynolds number = (density x velocity x diameter) / viscosity

• Below about 2000: laminar flow, silent, with a parabolic velocity profile
• Above about 2000: turbulent flow, audible, with energy lost as heat and sound

Turbulence is favoured by high velocity, large diameter, high density and low viscosity.

This explains a range of clinical findings:

• Murmurs from stenotic or regurgitant valves, where velocity rises through a narrow orifice
• Bruits over arterial stenoses
• Flow murmurs in anaemia, pregnancy and thyrotoxicosis, where viscosity is low and velocity high, with structurally normal valves
• Korotkoff sounds, produced when a cuff narrows the brachial artery enough to create turbulence

Turbulent flow also increases resistance beyond the Poiseuille prediction, since energy is dissipated rather than driving forward flow.