Originally Posted by **needhel** Hello,
When i studied a little fluid mechanics i learned that in a certain section in a tube (with no energy changes) the fluid flow will be consistent (Q=VA and it is consistable). I can see it if i take a garden hose and fill up a bucket, if i'll squeeze the end of it, the rate of filling it won't change (the section area reduced but the velocity increased). The problem is that when you open your sink tap at your kitchen it is not maintaining the same rate of fluid flow and the bucket will not be filling at the same time if it fully open, or half open for example.
The question is what in a valve or a tap causes the reduction in energy in the same section? Is it only friction, or is it something really simple in the theory i really missing here?
I asked a few mechanical engineers this question but couldn't get a satisfied answer, i also looked in some fluid mechanic books but nobody asked this same question.
Thanks for the helpers |

Welcome to PHF

Gosh consistable?

In all my years I have never come across that expression, where in the world are you that they use that?

The idea of 'consistent' is however correct and the conventional term is 'continuity'.

The equation QA = a constant is called the equation of continuity in physics and fluid mechanics texts, worldwide.

This equation is coupled with a second equation, usually called Bernoulli's equation in fluids problems.

Bernoulli's equation is really the law of conservation of energy applied to the fluid and can be applied to standing or static fluids or flowing fluids, along with the continuity equation.

I find the easiest method for calculation with Bernoulli is to convert all energies to pressure heads, because you simply add them up.

There is a difference, however between the application of continuity and Bernoulli however.

QA = a constant applies directly throught the flow regime.

But the pressure head varies along that same flow regime and here is the important bit.

Total pressure head = a constant

**provided there are no energy inputs or ouputs along the way.**
This is the simple situation that is normally described first.

But if there are devices like pumps (which add pressure head) or valves (which subtract pressure head) then there is a difference of pressure head either side of that device.

Ultimately such devices operate by doing work on the fluid (pumps) or extracting work from the fluid (valves).

And yes for the ordinary tap this extraction is by friction.

Does this help?