A Fluid Rushing Through A Pipe: Does It Create A Pulling Force on The Inside Walls of The Pipe ?

Question

A water main.

Water is cruising through it in a real big hurry.

Is that not creating a vacuum / 'negative pressure' that pulls the inside of the pipe toward the water that is rushing over its surface ?

Potentially odd but related thought:

If such a pipe were 'rifled' would the fluid build and maintain angular momentum ?

If so, would that not be a centrifugal force pushing against the inner walls of the pipe ?

Addition: I am real hip to Bernoulli's Principle. The 'rifling' is really just an odd curiosity. I am really interested in the impact upon the pipe. Halve the thickness of the metal skin on air airplane wing and the air rushing over it will yank it apart.

Moving sideways here:

How about the same dynamic, but now it is blood in our blood vessels. Increased rate of flow, any deterioration of the 'pipe', and variance in elasticity - produce Cardio-Vascular problems ?

Answer 1

if you are "hip" to bournouli's equation then you know that the pressure in the pipe is calcuable

mostly, water rushing through a pipe reduces its pressure compared to water not rushing through the pipe but in most cases the net pressure is still higher than the pressure outside the pipe

so your water main still has positive pressure inside

do the test, make a hole in the pipe, does water rush out or does air rush in

you can make a venturi (a device shpaed kind of like an hour glass) that reduces the diameter of the pipe down to something sufficiently small, and then increases it back to the nominal pipe size

you can size that venturi so that a hole drilled in the small diameter part of the venturi will suck air in if the water is flowing fast enough

if the water slows down sufficiently, then water will leak out

there are many industrial vacuum sources that use this principle

they are usually called "eductors" or "vacuum jets"
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you can create a momentum change with something like rifling

this could potentially develop some force against the inside surface of the pipe

in industry, a cyclone separator uses that technique to make a centrifuge type device (the water spins instead of the vessel)

these cyclone separators are generally made by introducing the fluid tangentially into the separator vessel and thus developing a "whirlpool" like effect

Answer 2

>>>>>>>Is that not creating a vacuum / 'negative pressure' that
>>>>>>> pulls the inside of the pipe toward the water that is
>>>>>>> rushing over its surface ?

Yes it is. This is called the Bernoulli effect.

>>>>If such a pipe were 'rifled' would the fluid build and maintain >>>>angular momentum ?

Yes but less than you might expect because fluid is by its nature amorphous.

>>>If so, would that not be a centrifugal force pushing against the >>>inner walls of the pipe ?

Yes, but the increase in pressure will come primarily from the momentum lost to impart that spin. A series of baffles that redirected an equal amount of forward momentum without imparting spin would achieve the same effect.

Remember that centifugal force isn't a true force, it is just a manifestation of momentum. In your rifled pipe you have converted some of your 'forward' momentum into momentum at an angle to the stream flow, but the total momentum remains approximately constant. What you will have done is slow down the flow rate. That means that the Bernoulli effect is reduced, thus the pressure would rise even without any spin.

>>>>How about the same dynamic, but now it is blood in our
>>>>blood vessels. Increased rate of flow, any deterioration of
>>>>the 'pipe', and variance in elasticity - produce
>>>>Cardio-Vascular problems ?

Nope.

The heart is a positive pressure pump and any pressure sensitive controls in the vascular system measure net pressure. As a result the pressure in the vessels is constantly positive. If increased flow rate does reduce the pressure measurably that will be detected and balanced by either a decrease in the diameter of the vessel or increased volume output from the heart.

Answer 3

The water pressure is pushing the pipe outwards from the inside. Of course there must be a reaction force pushing towards the water but these are in equilibrium. Water being a fluid, the force has little affect on the water.

Answer 4

I tend to agree with enginerd.

For Bernoulli to be in play there must be a change in velocity.

Assuming steady state flow, the velocity at the pipe wall will be constant and therefore there is no chance for a reduction in pressure or a vacuum.

Only if you have a situation like a venturi will there be an increase in velocity at the pipe wall and the chance for a pressure reduction.

Answer 5

Well, if you rifled the inside of the pipe by cutting grouves in it like a rifle barrel, the only fluid that would be affected by the grouves would be that fluid on the outside edges of the main flow. So, lots of friction and turbulence in the fluid flow would be generated.

The suction effect you mentioned is present and can bve useful in adding bleach to a high pressure hose line for washing the exterior walls of buildings, or concrete slabs.

Answer 6

Good questions. My answers; yes, Bernouilli's theorem says that moving fluid lowers its pressure and yes, rifling the pipe to make the fluid swirl round should produce an outward centrifugal force on the pipe. Many years ago I read a Scientific American article about negative hydrostatic pressure. You may be able to google this topic.

Answer 7

I work in pipeline, and deal with gas and oil more than water in a pipe, however I do believe that even in a water main, there is positive pressure pushing against the pipe rather than the other way around.

Answer 8

You are atrue thinker.
Yes I have to agrre it would in both cases.
There are frictional forces that tend to retard the water flow. The reaction to this must be a force that pulls the pipe. (never occured to me though) Yes angular momentum. you should look into cyclonic dust extraction for air borne dust. This uses centrifugal force to extract dust.

Answer 9

I think it maters if it's a siphon or forced pressure, if its forced the pressure is outward and siphon is inward , the rifleing might be benificial in the siphon mode but not in pressureized. but this is all conjecture, never thought about it till now!

Answer 10

You know....I never thought of these things. Now that I have, I honestly don't care. Thanks.