Question
Why does velocity increase when water flowing in broader pipe enters a narrow pipe?
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Answer
When a tube narrows, the same volume occupies a greater length, as schematically shown in below figure.
$A_1$ is the cross section of the broader pipe and that of narrower pipe is $A_2$.
By the equation of continuity, $V_2 = (A_1/A_2)V_1$
Since $A_1/A_2 > v_2 > v_1$. 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.
$A_1$ is the cross section of the broader pipe and that of narrower pipe is $A_2$.
By the equation of continuity, $V_2 = (A_1/A_2)V_1$
Since $A_1/A_2 > v_2 > v_1$. 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.
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