Fluids- Properties of Fluids and Key Concepts

Question 1

What is a free surface?

Answer 1

The surface of a fluid that is subject to zero parallel shear stress, such as the boundary between two homogeneous fluids.

Question 2

What is the normal stress on a fluid at rest?

Answer 2

Pressure

Question 3

What happens to a fluid under the influence of a shear stress?

Answer 3

It deforms continuously no matter this small the stress.

Question 4

In fluids, what is the stress proportional to?

Answer 4

The strain rate

Question 5

What is specific weight?

Answer 5

The weight of a unit volume of a substance

Question 6

What is specific gravity?

Answer 6

The ratio of the density of a substance to the density of some standard substance at a specified temperature (usually water at 4°C)

Question 7

What is viscosity?

Answer 7

A property that represents the internal resistance of a fluid to motion. It measures its resistance to deformation.

Question 8

What is absolute viscosity about?

Answer 8

Resistance to motion

Question 9

What is kinematic viscosity about?

Answer 9

Resistance to motion under gravity

Question 10

Formula for dynamic viscosity

Answer 10

μ=τ/(du/dy)
μ is coefficient of viscosity (Pa•s)
τ is shear stress
u is local velocity 
y is y axis when looking at profile of pipe

Question 11

Formula for kinematic viscosity

Answer 11

ν=μ/ρ

ν is kinematic viscosity (m^2/s)

Question 12

How do the dynamic and kinematic viscosités of liquids and gases depend on pressure?

Answer 12

For liquids, they are both practically independent of pressure except for extremely high pressures.
For gases, dynamic viscosity is independent of pressure, from low to moderate pressures, but kinematic depends on pressure since density is proportional to pressure.

Question 13

Explain the relation between viscosity of liquids and temperature

Answer 13

Viscosity of liquids is caused by cohesive forces between molecules. At higher temperatures, the molecules possess more energy and can oppose the large cohesive intermolecular forces more strongly so can move more freely. Therefore viscosity decreases when temperatures increase.

Question 14

Explain the relation between viscosity of gases and temperature

Answer 14

Viscosity in gases is caused by molecular collisions. At high temperatures, molecules move faster, meaning more collisions and therefore a greater resistance to flow. So viscosity increases with temperature.

Question 15

Formula for viscometer

Answer 15

T=FR=4π^2R^3n•L/lμ
T is torque (a moment)
R is radius of viscometer
n• is number of revolutions per second
L is length of cylinder
l is separation between the two cylinders

Question 16

Describe surface tension of a flui

Answer 16

For a free surface of a fluid at rest under gravity, the horizontal surface is in tension. The pulling force causing this tension acts parallel to the surface and is due to the attractive forces between molecules of the the liquid.

Question 17

Symbol for surface tension and units

Answer 17

σ subscript s (will refer to it as σ in other cards)

Units N/m

Question 18

Formula relating force and surface tension

Answer 18

F=2bσ
Where F and σ act in opposite directions
b is length of wire

Question 19

What is surface energy?

Answer 19

The work done per unit area. It is the same property as surface tension.

Question 20

Formulae for equilibrium of a droplet

Answer 20

Force due to surface tension = area x pressure difference

2πRσ=πR^2ΔP

Question 21

Formulae for equilibrium of a bubble

Answer 21

Force due to surface tension = area x pressure difference

2(2πRσ)=πR^2ΔP

Question 22

What does the behaviour of a liquid at the surface where it meets a solid depend on?

Answer 22

The adhesive force between liquid and solid molecules compared to the internal cohesive force between the liquid molecules

Question 23

What is a wetting fluid and describe the relation between the forces?

Answer 23

One where the L-S forces are greater than the L-L forces so the liquid rises slightly at the surface in contact with the solid.

Question 24

What is a non-wetting fluid and describe the relation between the forces?

Answer 24

One where the L-S forces are less than the L-L forces so the liquid falls slightly at the surface in contact with the solid.

Question 25

What is a capillary?

Answer 25

A narrow tube or confined flow channel

Question 26

What is the capillary effect?

Answer 26

The rise or fall of a liquid in a small-diameter tube inserted into the liquid

Question 27

What is a meniscus?

Answer 27

The curved free surface of a liquid in a capillary tube

Question 28

What is the wetting angle?

Answer 28

The angle that the tangent to the liquid surface makes with the solid surface at the point of contact

Question 29

Formula for capillary rise

Answer 29

h=(2σ/ρgR)Cosφ
φ is wetting angle
h is height change

Question 30

Define pressure

Answer 30

A normal force exerted by a fluid per unit area

Question 31

What can be said about the pressure at different points of equal depth in the same fluid?

Answer 31

It is the same

Question 32

Pascal’s law

Answer 32

The pressure applied to a confined fluid increases the pressure throughout by the same amount

Question 33

How does a hydraulic lift work?

Answer 33

A pressure is applied to a fluid over a small area. The pressure under the larger area under a heavy object is equal to this pressure so the force acting upwards on this larger area must be greater than that applied to the smaller area.

Question 34

Formula for hydraulic lift

Answer 34

F2=F1(A2/A1)

F2 and A2 under heavy object