Fluid mechanics is about fluids - i.e. Liquids and gases.

Lecture Notes Fluid-Properties.pdf


See Chapter 2 (P33-35)

Density = r = Mass/Volume (kg/m3)

Relative Density = RD = r/1000 = Density / 1000 (Density relative to water) (No units)

Specific Volume = n = 1/r (m3/kg)

E.g. Air density is about 1.2 kg/m3, Specific Volume (air) = 5/6


Pressure = Force / Area

Pressure (Pa) = Force (N) / Area (m2)

Most often use kPa in Fluid mechanics.

Atmospheric Pressure: 101.3 kPa (average at sea level)

Sometime treated as 100kPa (1 Bar)

Gauge Pressure: Compared to atmosphere

Absolute Pressure: Compared to vacuum.

E.g. Car tyre 29.4psi. 14.7psi=1 atm. Car tyre is 2 bar = 2*100kPa = 200kPa (Gauge)

Absolute Pressure = atm+gauge = 101.3 + 200 = 301.3 kPa.

Temperature: p38

Celsius is metric temperature scale. Celsius (metric) is based on water (at standard pressure).

0C = freezing/melting(fusion) (ice), 100C = boiling (evaporate) / condensing.

Kelvin is used in thermodynamics and fluids. Special scale that starts at absolute zero temperature.

-273.15 oC = OK (Kelvin).

You cannot get colder than OK. (0 oC). Temperature is the amount of vibration of the atoms/molecules. At absolute zero the vibration is stopped.


Basic Properties of Fluids

  • Chapter 8 p179
  • Fluid  = Liquid and gas. (flow)
  • Solid: Rigid (cannot flow). Molecules/atoms are interlocked to each other. Metals, polymers, ceramics. "Not compessible"
  • Liquid: Fills container, free surface on the top. "Not compressible". Similar density to solid but molecules can move around.
  • Gas: Fills container completely (no surface). Compressible.
  • Hydraulics and pneumatics act differently according to the properties of a fluid. Air is compressible, oil is not. Hydraulics can go to much higher pressure. (Air too dangerous at hydraulic pressure) Pneumatics pressures 400-1000 kPa. Hydraulics pressures 10000kPa - 30000kPa. Highest pressures are in water jet cutting 200Mpa = 200000kPa.
  • Mass: The amount of stuff (atoms). F=ma find m=F/a. For weight, m=N/g. Here, m is actually inertial mass (resistance to acceleration).
  • Vapour: Gas of a particular fluid. e.g. Water vapour.
  • Slurry: Combination of liquid + solid particle.
  • Atomised liquid: Combination of liquid + gas where liquid is fine droplets. (liquid added to gas)
  • Foam: Combination of liquid + gas where gas is bubbles in the liquid. (gas added to liquid)
  • Solubility: How well one liquid (solute) dissolves into another (solvent). Gas usually mix perfectly.
  • Concentration: How much substance in the solution. (mass/volume) e.g. g/L or ppm = parts per million.
  • Saturated: When the solute is at its maximum concentration.
  • Surface tension: The "skin" on the surface of a liquid. Wetting the surface means the fluid sticks to the surface which makes the fluid pull upward (meniscus A). Non-wetting acts like tthe surface is "waterproof", where liquid does not stick to surface, which looks like meniscus B. When a metal surface is properly cleaned of all oil/grease, water will wet the surface. This is used as a cleanliness test, such as in preparation for applying adhesive.
  • left = water (wetting), right = mercury (not wetting)
  • Capillary = soaking of liquid caused by wetting (surface tension) - can pull liquid up. (Wicking)
  • Corrosiveness: Fluid attacks solids like metal / plastics. Acid attacks metal, solvents attack plastic.
  • Toxicity: Poisonous (to humans)



Viscosity: p183

Dynamic Viscosity (m) is the thickness of the fluid. (Resistance to shear velocity). "Friction"

Units are Pas (pascal seconds)

Imperial (Centipoise = 0.001 Pas)


Kinematic Viscosity (g) = m / density. Units are m2/s (in imperial units Centistokes)

1 Centistoke = 10^-6 m2/s


SAE (motor oil) is the dynamic viscosity in centipoise. e.g. SAE 60, RD = 0.92.

Dynamic viscosity (m): 60 Centipoise = 60*0.001 = 0.06 Pas

Kinematic Viscosity (g): 0.06 Pas / 920 kg/m3 = 65.2e-6 m2/s (p295)



Saturation Vapour Temperature and Pressure

As a liquid is heated it evaporates more molecules, which increases the vapour pressure.


Camping on Mount Everest



Atmospheric pressure is created by the pile of air above us. Since air has a density of about 1.2 kg/m3, the weight of air creates about 101.3 kPa of pressure at sea level.

Mount Everest is 8480m high. At this altitude, the air pressure is about 253 Torr (33.73 kPa), which is 337mb. That is only about 1/3 of normal atmospheric pressure at sea level.
By comparison, Mt Kosciusko is about 80kPa.

According to the saturation vapour pressure graph, at a pressure of 34kPa, water will boil at approx 72oC on Mount Everest.

Water boiling at 72oC on Mount Everest.


Whiteboard:  Go to page




Homework Assignment: Kinksy new edition
Do all questions; Chapter 8: Basic Properties of Fluids.
8.1 to 8.16 (page 186-188)

Do all questions.
Relevant pages in MDME

Web Links
  • Google search: Fluid Mechanics is a little tricky to search on the web. You will probably find most sites are highly technical, usually requiring advanced Engineering Mathematics. We are studying Fluid Mechanics at a level somewhere in between the mechanical trades (Hydraulics/Pneumatics/Fluid Power) and Engineering (Fluid Mechanics). 
  • Information on Pumps
  • Information on Valves