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

There’s a restriction in flow before the water enters the hose, so with nothing blocking the end, the pressure in the hose is very close to air pressure.

If you put your thumb over the nozzle, the pressure can increase up to whatever the pressure is inside the water pipes before the earlier restriction. The velocity of the water increases, but the total flow decreases, because the cross sectional area is smaller.