Question

Why does velocity increase when water flowing in broader pipe enters a narrow pipe?

Answer 1

When a tube narrows, the same volume occupies a greater length, as schematically shown in Fig.  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

Fig. Speed of fluid increases as it enters a narrower pipe

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