Z7

where: V = flow rate

AP = driving pressure difference (PB - Palv) r = airway radius L = length of airway rj = breathing gas viscosity

Turbulent flow

Turbulent flow occurs when the flow rate is high, and when the quotient of gas density and gas viscosity is large. As a rule of thumb, gas flows are laminar when Reynold's number is below 1000, and gas flows are turbulent when Reynold's number is over 1500 (see equation below). But turbulent flows may occurr at smaller values of Reynold's number at sharp bends and bifurcations, or at irregular surfaces and in varying calibres3.

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