Flow Regimes and Pipe Flow

Question 1

What contribution did Reynolds postulate?

Answer 1

That there’s two types of flows; Laminar and Turbulent Flow.

Question 2

Describe what a Laminar Flow is:

Answer 2

A laminar flow occurs at low velocities.

The particles of the fluid are moving entirely on the same line, albeit the velocity along each line is different.

Question 3

Describe the term: Turbulent Flow

Answer 3

Paths of fluid particles are no longer orderly but random in nature.

Turbulence can exist in a fluid that appears to be flowing very smoothly.

Question 4

What is the Reynolds number?

Answer 4

It’s the ratio that’s a fundamental characteristic of flow in which inertial and viscous forces are present.

It’s a means of comparing one flow with another.

Question 5

What does it mean when you have a 1.) High RN and a 2.) Low RN

In addition to this, what forces dominate in each flow?

Answer 5

High RN:

• Turbulent flow occurs
• Viscous Forces Predominate

Low RN:

• Laminar flow occurs
• Inertia Forces Predominate

Question 6

What’s an eddy?

Answer 6

The swirling of a fluid.

Question 7

What does the increase in velocity say about the Reynolds experiment?

Answer 7

• When velocity was increased, the eddies began suddenly rather than gradually.
• Flow is unstable at this point.
• Small disturbances are enough to instigate turbulent flow.
• ‘Critical velocity’ very sensitive to initial disturbances.

Question 8

What do you call the point at which the flow changes from laminar to turbulent?

Answer 8

The upper critical velocity.

Question 9

What do you call the flow change from turbulent to laminar at a slightly lower velocity?

Answer 9

Lower critical velocity.

Question 10

In normal practice with rough pipes, what is the critical Reynolds number (lower critical velocity).

Answer 10

2000

Question 11

When looking at the flow through pipes and ducts, what do we need to pay attention to?

And why do we need to pay attention to it?

Answer 11

The friction caused by the flow.

• This is related to the pressure drop and head loss.

Note: Head loss - Head loss is potential energy that is converted to kinetic energy. Head losses are due to the frictional resistance of the piping system (pipe, valves, fittings, entrance, and exit losses).

Question 12

What is used to determine pumping power of a pipe?

Answer 12

The pressure drop.

Question 13

Why are liquids generally transported in circular pipes?

Answer 13

A circular cross-section can withstand large pressure differences between the inside and outside without undergoing significant distortion.

Question 14

Fill in the blanks:

In both laminar and turbulent flows, the effects of viscosity of the fluid are greatest _ __ __ _ __ ___.

Answer 14

in regions closest to solid boundaries.

Question 15

What is meant by a ‘solid boundary’.

Answer 15

Solid body immersed in fluid (boat in water, aircraft in air)

Alternatively, a solid body enclose the fluid (pipe or duct)

Question 16

What is the entry length?

Answer 16

The distance along the pipe to the point at which the maximum velocity is only 1% different from the final value.

Question 17

What entry length does a pipe with laminar flow and turbulent flow need.

Answer 17

Laminar Flow: 114 diameters

Turbulent Flow: 50 diameters

Question 18

What is the Darcy - Weisbach equation?

Answer 18

An equation that shows the [dissipation of mechanical energy] by [fluid friction] that results in a [fall of piezometric pressure] in the [direction of the flow].

dissipation of mechanical energy by fluid friction = fall of piezometric pressure

Piezometric head - Piezometric head indirectly measures the potential energy of water by measuring the height of water at a given point.

Question 19

What is considered the total head loss?

Answer 19

The sum of the ‘normal’ friction losses for the length of the pipe considered plus the additional losses

sum(‘Normal’ friction losses for length of pipe + additional losses)