By Poiseuille law (Q ∝ r⁴/L), if a tube radius is doubled and its length is also doubled, the flow rate changes by a factor of:

Since the same volume of fluid must pass every cross-section per second (continuity), the fluid speeds up where the pipe narrows; Bernoulli's equation then says this faster-moving fluid has lower pressure - the principle behind a venturi meter and an aircraft wing's lift.