Bernoulli’s Principle seems to say that increased flow velocity causes less pressure. This does make sense, since pressure takes energy and so does motion, so with a fixed amount of energy of course there’s a tradeoff. But in every example I can think of, the opposite seems to be true. If you put your thumb over a hose nozzle, the water goes faster and there’s also increased pressure on your thumb. The higher you turn the water flow, the faster the water flows and the stiffer the hose becomes. What am I missing?
In: Physics
Conservation of energy is the key principle.
If one variable is changed, the other elements must change to compensate. In our case, speed has increased so pressure must drop to keep the energy budget consistent.
It’s mind bending, and whilst I accept this as fact, I don’t intrinsically “get it” either.
If I’m not mistaken the water escapes the higher pressure caused from your thumb in the nozzle into the much lower atmospheric pressure which as per Bernoullis principle would come with an increase in velocity
The pressure increases downstream of your thumb, and the velocity decreases. Less water flows out of the hose.
But as the water passes the restriction, your thumb, the pressure drops and the velocity increases.
You can only feel the pressure increase, because you don’t feel the water after it leaves.
It’s a bit easier to imagine with the [nozzle of a rocket engine](https://en.wikipedia.org/wiki/Rocket_engine_nozzle#/media/File:De_laval_nozzle.svg): The throat of the nozzle throttles the flow of gas out of the combustion chamber, that is why there is a large pressure inside. Inside the nozzle, the gas can expand and speed up, turning the energy from its heat and pressure into kinetic energy.
In the example of the garden hose, your finger is the throat of the nozzle. It restricts flow of water, and therefore increases pressure inside the hose. The gap between the garden hose and your finger, where the water can flow through, is where the water can turn the pressure into kinetic energy.
Bernoullis Principal is basically conservation of mechanical energy for fluids. It should not be applied in a situation where the fluid is flowing through a long, narrow pipe, since viscosity (analogous to friction) is a dominating factor. It doesn’t help that, when learning about Bernoullis principal, fluid flowing through pipes is the go to example.
When fluid flows through a pipe, it experiences “head loss”, which is a decrease in the mechanical energy due to the viscosity/friction between the fluid and the walls of the pipe. All else being equal, the faster the fluid flows, the more head is lost. By putting your thumb over the end of the hose, you are forcing the fluid in the hose to flow slower overall, meaning it has more head when it reaches you, and thus greater pressure where you thumb is, and greater velocity where your thumb isn’t.
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