A Venturi meter with a throat diameter $d=2 \mathrm{~cm}$ measures the flow rate in a pipe of diameter $D=6 \mathrm{~cm}$, as shown in the figure. A U-tube manometer is connected to measure the pressure drop. Assume the discharge coefficient is independent of the Reynolds number and geometric ratios. If the volumetric flow rate through the pipe is doubled $Q_{2}=2 Q_{1}$, the corresponding ratio of the manometer readings $\Delta h_{2} / \Delta h_{1}$, rounded off to the nearest integer, is $\_\_\_\_\_\_\_\_$
