Assume that the flow at the level of the trachea is 500 milliliter per second, and that tracheal diameter is 2 cm.
What is the gas speed in the trachea?
Great, you know the answer.
500/π = 159 cm/s
500/(0.25×π) = 637 cm/s
(0.25×π)/500 = 0.02 cm/s
π/500 = 6.28 cm/s