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Old Jan 14th 2018, 01:14 AM   #1
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An easy question

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
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Old Jan 14th 2018, 02:43 AM   #2
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Originally Posted by needhel View Post
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?
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Old Jan 14th 2018, 03:15 AM   #3
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Thank you for your detailed answer

Sorry for my english, it ain't my tongue language. I don't think anyone uses it, just searched myself for the correct word.

So if friction is the cuase for the decreased/increase of the flow, how it is specifically being done in a kind of uterputz (i don't know the english term) or other home tap/valve? After all, the main control on those valves are changing the area of the section, isn't? I still doesn't underatand how the change in friction is hapenning there.
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Old Jan 14th 2018, 03:54 AM   #4
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Originally Posted by needhel View Post
Thank you for your detailed answer

Sorry for my english, it ain't my tongue language. I don't think anyone uses it, just searched myself for the correct word.

So if friction is the cuase for the decreased/increase of the flow, how it is specifically being done in a kind of uterputz (i don't know the english term) or other home tap/valve? After all, the main control on those valves are changing the area of the section, isn't? I still doesn't underatand how the change in friction is hapenning there.
Did you mean unterputz?

This is German for 'hidden' or 'concealed' or probably best in this case 'underlying'?

Every section of pipe or apparatus that the water flows through has what is called a 'friction factor'.
This friction factor depends upon the geometry and roughness of the section.

The loss of energy head due to friction depends upon both the friction factor and the velocity.
The larger the velocity or the larger the friction factor the greater the friction loss.

A valve causes a restriction (area or bore reduction) which greatly increases the velocity through the tap section.

It also has a very large friction factor compared to a simple straight smooth bored pipe.

So for both reasons there will be a large drop in energy head due to friction through the valve.

Now it take a pressure difference or pressure head to force the water through the pipe or valve.

This difference is the loss in pressure head between the incoming water and the outflowing water.

Because this is by far the largest pressure drop in the system, it will control the available flow rate.

Note I do not agree that if you restrict (squeeze or put your thumb over the end) a hosepipe you will not slow the flow rate.
Have you tried an experiment with a bucket and a stopwatch?
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