When a tube narrows, the same volume occupies a greater length, as schematically shown in below figure. A1 is the cross section of the broader pipe and that of narrower pipe is A2 . By the equation of continuity, V2 = (A1/A2)V1
Since A1 /A2 > v2 > v1 . For the same volume to pass points 1 and 2 in a given time, the speed must be greater at point 2.
The process is exactly reversible. If the fluid flows in the opposite direction, its speed decreases when the tube widens.
