$7\mathrm{.}3\mathrm{.}3$ At a sudden contraction in a pipe the diameter changes from $D_1$ to $D_2.$ The pressure drop, $p,$ which develops across the contraction is a function of $D_1$ and $D_2,$ as well as the

velocity, $V,$ in the larger pipe, and the fluid density, $,$ and viscosity, $.$ Use $D_1,V,$ and $$ as repeating variables to determine a suitable set of dimensionless parameters. Why would it be

incorrect to include the velocity in the smaller pipe as an additional variable?