if by Poiseuille’s Law, the greater the radius/diameter of a tube the faster the flow, why does water flow faster out of a hose when you put your thumb over the nozzle?

Question

if by Poiseuille’s Law, the greater the radius/diameter of a tube the faster the flow, why does water flow faster out of a hose when you put your thumb over the nozzle?

In: 7

Answer 1

I had this same problem in a fluids class in the early 90s and it stumped me too! When you put your thumb over the end of a hose you restrict the flow of water. Cover it completely and the flow stops entirely. There is then no flow, and no ‘drag’ on the walls of the hose. The pressure is then the same all the way from the source/manifold/pump/whatever to the tip of the hose. Open it a bit and viola! High pressure creating a powerful jet at the hose tip. Open it more and the flow rate, and hence viscous flow-induced drag, increases. Your hose doesn’t squirt quite as far.