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Old Jan 7th 2016, 01:54 PM   #1
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Driving force for fluid flow

Hello,
I have a question. Total pressure is a measure of the amount of energy put into the fluid system. Then what drives fluid flow? Is it the difference in total pressure or the difference in static pressure.
I feel it should be static pressure difference but if we take the energy route then shouldn't the total pressure difference be responsible for fluid flow?
Thanks a lot!
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Old Jan 7th 2016, 03:25 PM   #2
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Originally Posted by ssrivas3 View Post
Hello,
I have a question. Total pressure is a measure of the amount of energy put into the fluid system. Then what drives fluid flow? Is it the difference in total pressure or the difference in static pressure.
I feel it should be static pressure difference but if we take the energy route then shouldn't the total pressure difference be responsible for fluid flow?
Thanks a lot!
I suppose that depends how you define 'total pressure' and what you have added up to obtain your 'total'.

There are other sources of energy input to a fluid than pressure.

Perhaps you would like to describe further the circumstances of the fluid in your question?
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Old Jan 8th 2016, 06:12 AM   #3
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There are certainly mechanisms other than static pressure that can cause a fluid to flow. Consider that Bernoulli's equation has three terms, which involve static pressure, velocity, and change in potential energy. You could have a process where static pressure remains constant as a fluid flows downhill and gains velocity - think for example of a thin stream of water flowing down a sluice, where static pressure is constant (equal to air pressure). Another example: consider the circulation of hot water in a pot on a stove that is being heated from below - that fluid flow is driven by changes in density of the water as it heats, flows upwards, cools, and sinks again. Static pressures at various depths in the pot are essentially constant throughout this process.
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Old Jan 8th 2016, 03:16 PM   #4
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Thank you studiot and ChipB for the help. By total pressure I meant the sum of Static pressure, Dynamic pressure and potential energy.
I guess in my mind I had a sort of horizontal pipe. If I start to flow some fluid in it and at the other end I create a diverging section so that the Static pressure in the diverging section becomes more than the Static pressure pressure at the inlet. Would the flow still go on? The total pressure stays the same.
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Old Jan 9th 2016, 12:14 PM   #5
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OK a pipe or system of pipes.

First and most important.

Analysis of fluid flow in pipes means that the pipe is full at all times.

If the pipe is only partly full the analysis is quite different and the analysis is mostly the same as open channel flow.

Secondly the analysis refers to flowing fluid.
Again it can be different for static fluid.
This implies there is an entry section and an exit section where the conditions are set externally, not just by the conditions of the fluid in the pipe.
The length of pipework (including all the branches and changes of section) between the entry section and the exit section is known as the control volume.

ChipB is right then you are referring to a basic version of Bernoulli's Equation for an incompressible fluid with no friction.

You also need to use the continuity equation in these circumstances.

But returning to your question, as stated.

There is no physical reason whatsoever for the fluid to be flowing at all, unless external agents impose forces at the entry and exit sections. These therefore have to be known to solve the situation.
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Old Jan 10th 2016, 11:14 AM   #6
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Aspects of Pressure...

Hi Ssrivas3

I've wrestled with these terms as I used text after text. For ideal (assumed frictionless) fluids, pressure is a property "at a point" provided the point is sufficiently large for the fluid to be a continuum.

Pressure is sometimes postulated by envisaging an aquarium full of water.
A constant momentum-exchange is argued to occur as the water molecules impact the aquarium right glass wall. Equilibrium "static condition" occurs by a constant "force per area" of the glass. The same can be said for the water condition on the symmetric, left side of the aquarium. So if there is "pressure on the left," and "pressure on the right," there is pressure at any point within the static fluid.

"Static" does not mean motionless. Static means "undisturbed." Hence fluids "not moving or moving in a straight line" are said to be static.

If I wrote adequately well, this is all there is. The nature of pressure as a property is clear. There is no other "pressure." What is "total" pressure. Are there two pressures at a point - We must add them?

Dynamic pressure? Pressure is not dynamic, it is static.

#########################
I offer a reference about flow

http://www.thermospokenhere.com/wp/0...rricelli.htmls

Good luck with your studies. JP
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