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

Reread the law – it has a lot to do with pressure. Specifically, very little pressure. I’m not extremely familiar with fluid dynamics (more than average tho) but I don’t think this law applies to the situation you are mentioning.

When you put your finger over the end of the hose you are decreasing the cross section for the same volume of fluid – thus skyrocketing the pressure.