Assume that the flow at the level of the trachea is 500 milliliter per second, and that trachea diameter is 2 cm.
What is the gas speed in the trachea?
OK, here is the anwer.
Speed (mm/s) is volume flow (cm³/s) divided by surface area (cm²). Transsectional surface area is π multiplied by radius squared (π×r², or 0.25×π×d², where r and d are radius and diameter respectively, and π = 3.14). Volume flow is 500 mL/s or 500 cm³ per second. The velocity at the trachea is therefore 500 cm³ per second divided by 0.25×3.14×2×2 cm² = 500/3.14 or 159 cm/s, or 5.7 km/h. That is pretty fast, don't you think? During quiet breathing these speeds are easily reached.