Volume of the liquid flowing per second through a pipe of diameter d is V. At a point where the radius is d, the velocity of flow is  

Using $V=\pi r^{2}v\Rightarrow v=\frac{V}{\pi d^2}$

$$\begin{gathered} \text{ At radius }r\text{Volume flow rate of flowing fluid }{V}_1=\pi r^2\times \text{ velocity } \\ {V}_1=\pi r^2\times v \\ Where, radius becomes double or equal to the diameter \\ and velocity becomes 2v then volume flow rate \\ {V}_2=\pi 4r^2\times 2v \\ {V}_2=8V_1 \end{gathered}$$