Fluids and their Properties

Question 1

What is the definition of a fluid?

Answer 1

A fluid is a substance which deforms continuously under the action of shearing forces, however small they may be

Question 2

What does it mean if a fluid is at rest?

Answer 2

If a fluid is at rest, there can’t be any shearing forces acting. This means that all forces in the fluid must be perpendicular to the planes upon which they act

Question 3

Will there be any shear stresses formed in a fluid if all of the fluid particles are moving at the same velocity?

Answer 3

No, because their relative velocities to each other will equal zero`

Question 4

What happens to the velocity of a fluid moving parallel to a boundary?

Answer 4

The fluid in contact with the surface boundary adheres to it, so will have the same velocity of it.
The layers will stick to each other, so the velocity will increase as you move further away from the surface boundary

Question 5

What is the equation for shear stress?

Answer 5

Force / Area

Question 6

What is the deformation measured as an angle proportional to?

Answer 6

The shear stress

Question 7

How does the angle of deformation vary in solids and liquids?

Answer 7

• Solids: The angle will be a fixed quantity for a given value of the shear stress since a solid can resist shear stress permanently
• Liquids: The angle will continue to increase with time and the fluid will flow. It’s found experimentally that for a true fluid, the rate of sheer strain is directly proportional to the shear stress

Question 8

What is the equation for the shear stress considering dynamic viscosity?

Answer 8

Shear Stress = Dynamic Viscosity * du/dy

du/dy is the differential of the change of velocity with y, where y is the distance from the boundary in contact

Question 9

What are the differences between the behaviours of solids and fluids under an applied force?

Answer 9

1. For a solid, the strain is a function of the applied stress, provided that the elastic limit is not exceeded. For a fluid, the rate of strain is proportional to the applied stress
2. The strain in a solid is independent of the time over which the force is applied, and if the elastic limit is not exceeded, the deformation disappears when the force is removed. A fluid continues to flow for as long as the force s applied and will not recover its original form when the force is removed

Question 10

Why does glass appear to be solid?

Answer 10

Because their rate of deformation under their own weight is very small

Question 11

Does a plastic substance count as a true fluid?

Answer 11

No, because the shear stress must exceed a certain minimum before flow occurs

Question 12

What fluids qualify as Newtonian?

Answer 12

Fluids with a constant value of dynamic viscosity

Question 13

What types of Non-Newtonian fluids are there?

Answer 13

1. Plastic, where the shear stress must reach a minimum for flow to occur
2. Psuedo-Plastic, where the dynamic viscosity decreses as the rate of shear increases (Milk and Cement)
3. Dilatant substances where the dynamic viscosity increases as the shear increases (Quicksand)
4. Thioxotropic substances, where the dynamic viscosity decreases with the time for which shearing forces are applied
5. Rheopetic materials, where the dynamic viscosity increases with the time for which shearing forces are applied
6. Viscoelastic materials, which behave in a manner similar to Newtonian fluids under time-invariant conditions but, if the shear stress changes suddenly, behave as if plastic

Question 14

What are some of the key differences between liquids and gases?

Answer 14

• A liquid is difficult to compress and can be considered incompressible for certain purposes, whereas a gas is easy to compress
• A given mass of liquid occupies a fixed volume, whereas a given mass of gas will expand to completely fill any container

Question 15

What happens if two pieces of the same material are far apart?

Answer 15

There is no detectible force exerted between them

Question 16

How can we weld two pieces of the same material together?

Answer 16

They need to be forced into very close contact. Under these conditions, the forces are attractive when the separation is very small

Question 17

What do we need to compress solid or liquids?

Answer 17

We need very large forces to overcome the repulsive forces between molecules

Question 18

How does the relative values of the kinetic energy of the particles in a material and the dissociation or binding energy determine if a material is a solid, liquid or gas?

Answer 18

• If the average value of the kinetic energy is greater than the dissociation energy, then the material will have no stable pairs, meaning that it will behave as a gas
• If the average value of the kinetic energy is less than the dissociation energy, then no dissociation pairs exist, so we will see a solid
• If the average value of the kinetic energy is equal to the dissociation energy, then we will have an intermediate between the conditions of the gas and solid, leaving us with a liquid

Question 19

What is a continuum?

Answer 19

A hypothetical continuous substance

Question 20

Why do we treat materials as a continnum?

Answer 20

Because treating them all as individual molecules introduces too many variables. Using the continnum allows us to assume constant properties such as velocity and pressure.
Furthermore, we can also consider variations acting smoothly from point to point.

Question 21

What is the definition of relative density?

Answer 21

This is the ratio of the mass density of a substance to some standard mass density.
For solids and liquids, we use the maximum density of water (Occurs at 4 Degrees Celcius at Atmospheric Pressure)
For gases, we don’t have a standard mass density as this term isn’t used often

Question 22

How can we define the coefficient of dynamic viscosity?

Answer 22

The shear force per unit area required to drag one layer of fluid with unit velocity past another layer a unit distance away from it in the fluid

Question 23

What are the two useful rearrangements of the equation to find the coefficient of dynamic viscosity?

Answer 23

• (Force * Time) / Area
• Mass / (Length * Time)

Question 24

What are the units for the coefficient of dynamic viscosity?

Answer 24

• Newton Seconds per Square Metre
• Kilograms per Metre per Second

Question 25

What is the dynamic viscosity?

Answer 25

It’s a ratio of dynamic viscosity to mass density
Kinematic Viscosity = Dynamic Viscosity / Mass Density

Question 26

What is Newton’s Law of Viscosity?

Answer 26

If the temperature of a gas increases, the molecular interchange will increase. The viscosity of a gas will, therefore, increase as the temperature increases.
According to the kinetic theory of gases, viscosity should be proportional to the square root of the absolute temperature: in practice, it increases more rapidly. Over the normal range of pressures, the viscosity of a gas is found to be independent of pressure, but is affected by very high pressures

Question 27

Why does surface tension of a liquid occur?

Answer 27

Because the intermolecular forces become unbalanced at the surface of the liquid, meaning that the molecules are pulled towards the bulk of the liquid, forming something similar to an elastic membrane under tension

Question 28

How does the temperature impact the surface tension?

Answer 28

A greater temperature causes a lower surface tension

Question 29

What is the effect of the surface tension?

Answer 29

To reduce the surface of a free body of liquid to a minimum. This is because otherwise, the molecules will have to fight against the strong attractive force to the centre of the liquid.
This causes an icrease in internal pressure and means drops of liquid tend to take on a spherical shape

Question 30

What is the force due to internal pressure?

Answer 30

Internal Pressure * pi * radius^2

Question 31

What is the force due to surface tension around the perimeter?

Answer 31

2 * pi * radius * Surface Tension

Question 32

What equation can we use when a droplet of water is in equilibrium?

Answer 32

Internal Pressure = (2 * Surface Tension) / Radius

Question 33

If a fine tube open at both ends is lowered vertically into a liquid which wets the tube, what will happen to the level of liquid in the tube?

Answer 33

It will increase

Question 34

If a fine tube open at both ends is lowered vertically into a liquid, but the tube doesn’t become wet, what happens to the level of the liquid inside the tube?

Answer 34

It will be depressed below the level of the free surface outside

Question 35

What is the equation for the upward pull due to surface tension in a capillary tube?

Answer 35

Upward pull = Component of surface tension acting upwards * Perimeter of tube

Question 36

What is the weight of the column raised in a capillary tube?

Answer 36

Density * g * (pi / 4) * Diameter^2 * Height of column

Question 37

What is the vapour pressure within a confined space?

Answer 37

It’s the pressure caused by molecules of liquid that dissociate from the fluid and float above, causing a partial pressure

Question 38

How does the vapour pressure depend on the temperature?

Answer 38

If the temperature increases, the degree of molecular activity increases, meaning that the vapour pressure will also increase

Question 39

When will boiling occur?

Answer 39

When the vapour pressure is equal to the pressure above the liquid

Question 40

How can we boil at temperatures lower than the standard temperatures known at atmospheric pressure?

Answer 40

By reducing the pressure, we can boil at lower temperatures e.g. if the pressure is 0.2 bar, then water will boil at 60 degrees celcius

Question 41

What is Cavitation?

Answer 41

Under certain conditions, areas of low pressure can occur locally in a flowing fluid. If the pressure in such areas falls below the vapour pressure, there will be local boiling and a cloud of vapour bubbles will form

Question 42

Why is Cavitation an issue in a flowing fluid?

Answer 42

The flow of liquid can sweep the cloud of bubbles that form into an area of higher pressure, causing the bubbles to collapse suddenly. If this happens in contact with a solid surface, damage can occur

Question 43

What is the equation for the bulk modulus of a material?

Answer 43

Bulk Modulus = Change in Pressure / Volumetric Strain

Bulk Modulus is represented by K

Question 44

What is the equation of state of a perfect gas?

Answer 44

p = Rho * R * T

p = Absolute Pressure
T = Absolute Temperature
R = Gas Constant for the gas concerned
Rho = Mass Density

Question 45

What is the work done during the expansion of a gas?

Answer 45

The area under a p-V curve between the initial and final volumes

The integral of p dV with the limits of the maximum and minimum volumes