How does pressure alter when flow is restricted through an aperture?

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Offline thedoc

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Tony Hue asked the Naked Scientists:
As a liquid flows through a narrow section, it accelerates since the same amount of liquid has to go through the narrow section.  And, the liquid pressure in the narrow section decreases.
Does the distance between molecules increase as the liquid accelerates? But, liquid is nearly incompressible. Is this explanation OK?

Some say that molecules have less time colliding with the side wall since the liquid is moving faster in the flow direction. Which one is the correct answer? Or, are both answers correct?
I am looking for a physical explanation of the decrease in fluid pressure as flow accelerates without using such equation as the Bernoulli equation.

Some say it's the decrease in pressure that causes acceleration of fluid. But, that doesn't explain why pressure is lowered.
I would very much appreciate your reply.
Tony Hue

What do you think?
« Last Edit: 25/07/2013 10:30:01 by _system »