# Fluid mechanics and pipe flow questions

• AwfulPhysicist

#### AwfulPhysicist

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Pics are attached, my concerns are with question 4 and 5.

For q4

-which way is flow? How do we know?
-the working then goes on to say that energy loss is compensated by increasing in pressure energy, however the question says that pressure is constant...clear contradiction
-last bit he equated the shear force to fluid weight, but then for fluid weight he says that weight =density x g x area isn't weight density x g x volume??

For q5

-he stays that hf= (pressure1-pressure2)/densityxg but that would assume that velocity is constant ??question says nothing about constant velocity !

Thank you for any help!

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< Mentor Note -- thread moved to HH from the technical engineering forums, so no HH Template is shown >

Pics are attached, my concerns are with question 4 and 5.

For q4

-which way is flow? How do we know?
-the working then goes on to say that energy loss is compensated by increasing in pressure energy, however the question says that pressure is constant...clear contradiction
-last bit he equated the shear force to fluid weight, but then for fluid weight he says that weight =density x g x area isn't weight density x g x volume??

The weight of the fluid and the shear force are both calculated assuming a unit length here. The internal area of the pipe is πD2/4 and the internal circumference is πD.
For q5

-he stays that hf= (pressure1-pressure2)/densityxg but that would assume that velocity is constant ??question says nothing about constant velocity !

There doesn't appear to be anything like a pump or such shown located between P1 and P2, so why would you assume that the fluid must be changing velocity between these two points?

-which way is flow? How do we know?
There is only one direction that allows a constant pressure.
-the working then goes on to say that energy loss is compensated by increasing in pressure energy, however the question says that pressure is constant...clear contradiction
Energy loss is compensated by gravity is probably a better description.
-last bit he equated the shear force to fluid weight, but then for fluid weight he says that weight =density x g x area isn't weight density x g x volume??
It is weight per pipe length here.

-he stays that hf= (pressure1-pressure2)/densityxg but that would assume that velocity is constant ??question says nothing about constant velocity !
The fluid does not disappear in between and it is assumed to be incompressible I think. Everything that flows in has to flow out, therefore velocity has to be constant.

Thank you for all the answers,

For q4
-mfb states that there can only be one direction for constant pressure, could you please elaborate?

For q5
- velocity must be constant because Qin=Qout BUT if you look at the pressure in p1 and p2 you shall see that pressure is different.. Now using Bernoulli equation we can see that an increase in pressure can only mean a changing velocity as the pipe is horizontal! How can pressure change but velocity doesn't?

What can be contributed to the changing pressure?

For q5
- velocity must be constant because Qin=Qout BUT if you look at the pressure in p1 and p2 you shall see that pressure is different.. Now using Bernoulli equation we can see that an increase in pressure can only mean a changing velocity as the pipe is horizontal!

The pressure changes in order to overcome the viscous forces in the oil. Bernoulli's equation applies only when the viscous forces are negligible.

How can pressure change but velocity doesn't?

What can be contributed to the changing pressure?

The viscosity of the oil.

The pressure changes in order to overcome the viscous forces in the oil. Bernoulli's equation applies only when the viscous forces are negligible.

The viscosity of the oil.
yeah but the answer applied the Bernoulli equation when it equated hf=p1-p2/rho g

So it can be applied in this situation ..

yeah but the answer applied the Bernoulli equation when it equated hf=p1-p2/rho g

So it can be applied in this situation ..
Yes, but not by itself. It must be modified to account for the losses due to viscosity. One way is by application of Poiseuille's law to a circular pipe:

http://en.wikipedia.org/wiki/Hagen–Poiseuille_equation

This is how the head loss hf is calculated.

Yes, but not by itself. It must be modified to account for the losses due to viscosity. One way is by application of Poiseuille's law to a circular pipe:

http://en.wikipedia.org/wiki/Hagen–Poiseuille_equation

This is how the head loss hf is calculated.
Right ok, thank you for your help! Is there a way you could explain q4 about flow direction?

For q4
-mfb states that there can only be one direction for constant pressure, could you please elaborate?
Pressure decreases along the flow direction (due to viscosity), increases if the fluid moves downwards (due to graviy) and decreases if it moves upwards (again due to gravity).
If the fluid would move upwards, both effects would lead to a lower pressure on the upper side, so pressure cannot be constant.
If the fluid moves downwards, the effects can cancel if viscosity, flow rate and so on fit together.

Pressure decreases along the flow direction (due to viscosity), increases if the fluid moves downwards (due to graviy) and decreases if it moves upwards (again due to gravity).
If the fluid would move upwards, both effects would lead to a lower pressure on the upper side, so pressure cannot be constant.
If the fluid moves downwards, the effects can cancel if viscosity, flow rate and so on fit together.
That's brilliant thank you for explaining so simply