Chapter 1 and chapter 2

• Fluid

  A substance that deforms when subjected to a force, cannot offer permanent resistance to any force causing change of shape, flow under their own weight and take the shape of any solid body with which they are in contact, may be divided into liquids and gases
• Liquids

  Occupy definite volumes
• Gases


  Expand to occupy any containing vessel
• S.I. Units in Fluids

  Mass (kilogrammes), Length (metres), Time (seconds)
• Density

  Mass per unit volume, SI unit is kg/m³, temperature dependent, pressure dependent to a lesser extent
• Relative density (or specific gravity)


  Ratio of a fluid density to the density of a standard reference fluid maintained at the same temperature and pressure
• Viscosity


  Measure of a fluid's resistance to flow, related to the ease with which the molecules can move with respect to one another, depends on the strength of attractive forces between molecules and the kinetic energy of the molecules, decreases with temperature for liquids, increases with temperature for gases
• Viscosity measurement

  1. Moving plate at constant velocity V

  2. Shear stress T = F/A

  3. T = constant x (dv/dy)

  4. T = μ x (V/y)

  5. Kinematic viscosity v = μ/ρ
• Dynamic viscosity (μ)


  Constant of proportionality in the viscosity equation, units kg/ms or Pa s
• Kinematic viscosity (v)


  Ratio of dynamic viscosity to density, units m²/s
• Kinematic Viscosity

  Unit: 10-6 m2/s
• Typical liquid

  • Water
  • Vegetable oil
  • SAE 10 oil
  • SAE 30 oil
  • Glycerine
  • SAE 50 oil
  • SAE 70 oil
• Dynamic Viscosity


  Unit: Centipoise (cp)
• Kinematic Viscosity


  Unit: Centistokes (cSt)
• Table 1.1 shows the viscosity of selected fluids at standard temperature and pressure
• 1 cp = 10 kg/ms and 1cSt = 10-6 m²/s
• As temperature increases

  Liquid viscosity tends to decrease
• Predicting the viscosity of water

  Use the expression: μ (kg/ms) = 2.414*10-5*10^(247.8 K/(Temp - 140 K))
• The difference between the calculated and chart values is small, and observation errors may be part of it
• Fluid Pressure

  The force exerted by the fluid per unit area, transmitted with equal intensity in all directions and acts normal to any plane
• Pressure units

  • 1 bar = 10³ N/m²
  • 1 atmosphere = 101325 N/m²
  • 1 psi = 6895 N/m²
  • 1 Torr = 133.3 N/m²
• Pressure Head


  The pressure intensity at the base of a column of homogenous fluid of a given height in metres
• Vacuum


  A completely empty space where the pressure is zero
• Atmospheric Pressure


  The pressure at the surface of the earth due to the head of air above the surface, about 101.325 kN/m² at sea level
• Gauge Pressure


  The pressure measured above or below atmospheric pressure
• Absolute Pressure


  The pressure measured above absolute zero or vacuum
• Vapour Pressure

  The partial pressure created by vapour molecules during evaporation of a liquid, increases with temperature
• Compressibility


  The relationship between pressure and change in volume for a fluid, liquids are virtually incompressible while gases are easily compressed
• Pascal's law states that the pressure intensity at a point in a fluid at rest is the same in all directions
• The fluid-static law states that the pressure in a fluid increases with increasing depth
• Pressure Transducer


  A device that converts a pressure signal into an electrical output, either using an elastic element or an electric element
• Bourdon Gauge


  A mechanical pressure measurement device that uses a circular arc of elliptical cross-section tube (Bourdon tube) that changes shape under fluid pressure
• Types of Manometers

  • U-tube manometer
  • Well-type manometer
  • Inclined tube manometer
• Hydraulic brakes and hydraulic lifts are applications of Pascal's law
• 1. tube manometer


  Device connected to a closed tank to measure pressure
• Closed tank

  • Contains oil with density of 860 kg/m³
  • Air pressure above oil is 3500 Pa
• Pressure at point A in the oil

  14000 Pa
• Manometer fluid

  Relative density of 3
• Determining depth of oil, z

  1. Use pressure equation at point A

  2. Solve for z₁
• Determining differential reading, z, on the manometer

  1. Use pressure equilibrium at datum

  2. Solve for z₂