Effect of a restriction on fluid flow
2019-06-11
When an object is put in the way of a flowing fluid (gas or liquid) in a pipe its presence obstructs the passage of the fluid. The fluid is forced to go around it. The flow diversion requires energy to power the motion and it is supplied from the fluid by its pressure falling as it squeezes past.
The smaller the hole, the less that can get through it, and the greater the back-pressure in the pipe. That is how a faucet controls the flow of water into a drink cup when it is opened. Just cracking the tap open produces a trickle.
The high back-pressure in the pipe forces a small amount of water out of the tiny opening. As the tap is opened further, the hole gets bigger and the flow increases. The back-pressure in the pipe falls as the restriction is removed.