Question

Two pipes, each of diameter d, converge to form a pipe of diameter D. What should be the relation between d and D such that the flow velocity in the third pipe becomes double of that in each of the two pipes?

(a) D = d

(b) D = 2d

(c) D = 3d

(d) D = 4d

I got this question during an interview for a job.

I'd like to ask this question from Continuity Equation in section Kinematics of Flow and Ideal Flow of Fluid Mechanics

Answer 1

The correct choice is (a) D = d

Explanation: According to the Continuity Equation,

where a represents flow area, v represents flow velocity, i is for inlet conditions and o is for outlet conditions. Thus,

A1v1 + A2v2 = Av

d^2v + d^2v = D^2v

D = d.