Fluid Dynamics

Question 1

What is a fluid ?

Answer 1

Something that flows; a substance that constantly deforms under shear stress, no matter how small

Question 2

Examples of fluids

Answer 2

Liquids / gasses / plasmas

Question 3

Density equation

Answer 3

Volume (kgm^-3) = mass / unit volume

p = m / V

Question 4

What is the density of pure water ?

Answer 4

1000 kgm^-3 at 4 °C

Question 5

What is relative density ?

Answer 5

The ratio of the density of a substance to the density of a given reference material.

Question 6

Relative density equation for solids

Answer 6

RD = density of substance at 20°C / density of water at 4°C (1000kgm^-3)

Question 7

Relative density equation for gases

Answer 7

RD = density of substance at 20°C / density of dry air (1.00)

Question 8

Pressure definition

Answer 8

When a force acts perpendicular to a surface, the pressure exerted is the ratio between the magnitude of the force and the area of the surface

Question 9

Pressure equation

Answer 9

Pressure = Force / Area

P = F / A

1 Pa = 1 N/m^2

Question 10

Pressure unit conversions

Answer 10

1 bar = 10^5 Pa

1 atm = 1013.25 millibars , 14.69 psi = 260 mmHg

Question 11

Absolute pressure equation

Answer 11

Absolute Pressure = gauge pressure + atmospheric pressure

Question 12

What is a barometer ?

Answer 12

A pressure gauge that measures the pressure of the atmosphere itself

Question 13

What do pressure gauges measure?

Answer 13

The difference between an unknown pressure and atmospheric pressure.

Question 14

How does Pressure act in a fluid container? (2)

Answer 14

The forces that a fluid exerts into the walls of a container, and those that the wall exerts on the fluid, always act perpendicular to the walls.

And

The force exerted by the fluid is the same at all points at a given depth.

Question 15

What is Archimedes principle ?

Answer 15

An object immersed in fluid is acted upon by an upward force that arises because pressures in a fluid increase with depth.

(Upward force on the bottom is larger than the downward force on the top).

Question 16

What is buoyant force?

Answer 16

The difference between the upward force from the bottom and the downward force on the top of an object immersed in a fluid.

Question 17

What is the buoyant force equal to?

Answer 17

The buoyant force is equal to the volume of the fluid displaced.

Question 18

What does upthrust (upward force) equal?

Answer 18

The weight of the water displaced.

Question 19

What is Streamline flow?

Answer 19

The direction of motion of individual particles is the same as that of the fluid as a whole.

Each particle of the fluid that passes a point follows the same path as those particles which passed the point before.

Question 20

What is laminar flow?

Answer 20

Streamlined flow in which the fluid moves in layers without fluctuations or turbulence.

Question 21

What is turbulent flow?

Answer 21

Characterised by the presence of irregular whirls and Eddies; it occurs at high velocities and when the fluids path changes direction sharply (an obstruction).

Question 22

Viscosity definition

Answer 22

An internal friction that prevents adjacent layers of the fluid from sliding freely past each other when subjected to shear stress.

Question 23

As the temp of a liquid increases, Viscosity …..

Answer 23

Decreases.

Question 24

As the temp of a gas increases, viscosity….

Answer 24

Increases.

Question 25

What is required to keep a viscous liquid moving though a pipe?

Answer 25

Differential pressure across the pipe inlet and the pipe outlet. (Greater pressure at the inlet than the outlet)

Question 26

As a viscous fluid flows though a pipe, where is the velocity at its maximum?

Answer 26

At the centre of the fluid/pipe. Velocity is ZERO at the walls.

Question 27

What is skin friction?

Answer 27

The air is slowed and brought to a stand-still when very close to a surface.

The viscosity of the air causes the layers near the skin to retard the surrounding layers along with it.

Question 28

What is the boundary layer of air?

Answer 28

The layer(s) of air in which the shearing / slowing action takes place between the surface and the full velocity of airflow.

It can be laminar or turbulent.

Question 29

What are the two types of aerodynamic flow?

Answer 29

Compressible and incompressible.

Question 30

What is compressible flow?

Answer 30

Flow where the density of the fluid elements can change from point to point (p2 ≠ p1).

The variability of density in aerodynamic flow is important at high speeds.

Question 31

What is incompressible flow?

Answer 31

Flow in which the density of the fluid elements is always constant.

This does not happen in reality. It is only used to simplify analysis of real physical systems.

Question 32

Flow rate equation

Answer 32

Flow rate = velocity x cross-sectional area.

FR = v x A

(Litres per second or cubic meters per second)

Question 33

When is the rate of flow of a fluid constant, even if the pipe or channel varies in size?

Answer 33

When the fluid is incompressible. (Approximately true for most liquids)

Question 34

Continuity equation

Answer 34

v1 x A1 = v2 x A2

Question 35

How does a Venturi work?

Answer 35

The fluid velocity in the throat is increased and the pressure is therefore reduced.

The pressure drop can be measured and the flow rate within the throat can be calculated.

Question 36

As velocity increases, pressure…

Answer 36

Decreases

Question 37

What two components make up the total pressure exerted by moving liquids ?

Answer 37

Static component (at rest) and dynamic component (pressure equivalent of its velocity).

Question 38

What is the make up of a pitot-static tube?

Answer 38

Measure total pressure by consisting of two tubes, one with an opening facing the moving fluid and the other with an opening 90° to the direction of flow.

Question 39

What is the stagnation region of a pitot-static tube?

Answer 39

Where the fluid at the open end of the tube is at rest. The total pressure is also called stagnation pressure.