MDME: MANUFACTURING, DESIGN, MECHANICAL ENGINEERING

# FLUID PROPERTIES

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

Lecture Notes Fluid-Properties.pdf    Fluid-Properties.one

### Definitions

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

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

Wikipedia

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.

http://www.physicalgeography.net/fundamentals/7d.html

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.

## Questions:

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.