I know it's a common question but I can't find an explanation that can clearly show how it happens. If we take Bernoulli's equation, being aware of its hypothesis, it states that energy is constant between 2 given points. So if pressure drops, velocity should rise.
I know flow mass should be conserved but one thing is the mathematical explanation and also the mechanism itself. How exactly does this happen? If velocity increases, should that be due to a force. Not gravitational not surface force so, which one?
Furthermore, if liquids can't be compressed and the temperature is constant. Where is this “pressure” energy stored?
EDIT: My question stems from working with hydraulic pumps in which diffusers are used to transform velocity into pressure.
It must have something to do with the geometry of the pipe but I can't understand how a liquid flowing with velocity drops some of it to increase its pressure in a wider segment of the pipe. More space should lead to less pressure and more velocity as it has more space available.
I am looking for a more “atomistic” answer such as this one (it doesn't satisfy me completely):
Best Answer
Q: What is the mechanism that transforms pressure into velocity?
A: Pushing.
Seriously, that's the answer. Bernoulli's equation is usually stated as “pressure drops when velocity increases”, but I find it much clearer when stated the other way around: velocity increases in a narrow part of a pipe because before the narrow part, there's a big pressure pushing the liquid into the narrow part. Inside the narrow part, the fluid is already accelerated, so that pressure isn't needed anymore to keep up the speed.
(In fact more accurately it's the pressure gradient that accelerates the fluid, i.e. the particles at the entry are strongly being pushed from behind but not so strongly resisted from ahead.)