1. CE319F: Elementary Mechanics of Fluids
FLUID PROPERTIES
Chapter 2
CE319F: Elementary Mechanics of Fluids

2. Elasticity (Compressibility)
If pressure acting on mass of fluid increases: fluid contracts
If pressure acting on mass of fluid decreases: fluid expands
Elasticity relates to amount of deformation for a given change in pressure
Ev = bulk modulus of elasticity
How does second part of equation come about?
Small dV/V = large modulus of elasticity

3. Example
Based on the definition of Ev and the equation of state, derive an equation for the modulus of elasticity of an ideal gas.

4. Vapor Pressure (Pvp)
Vapor pressure of a pure liquid = equilibrium partial pressure of the gas molecules of that species above a flat surface of the pure liquid
Concept on board
Very strong function of temperature (Pvp up as T up)
Very important property of liquids
When Pvp exceeds total air pressure applied at surface, liquid will boil.
Pressure at which a liquid will boil for a given temperature
At 10 oC, vapor pressure of water = atm = 1200 Pa
If reduce pressure to this value - get boiling of water
Formation of cavitation bubbles
Harm to pipes, pumps, turbines, propellers
If Pvp > 1 atm compound = gas
If Pvp < 1 atm compound = liquid or solid

5. Vapor Pressure (Pvp) Water
Vapor pressure of water (and other liquids) is a strong function of temperature.

6. Vapor Pressure (Pvp) - continued
Pvp,H2O = Pexp( a – a2 – a3 – a4)
P = 101,325 Pa a = 1 – (373.15/T) T = oK
valid to +/- 0.1% accuracy for T in range of -50 to 140 oC
Equation for relative humidity of air = percentage to which air is “saturated” with water vapor.
How does RH affect drying of building materials, and why? Implications?
How does RH affect dust mites? Implications?

7. 300 mm
Dust Mites

10. Example (HW!): Relative Humidity
The relative humidity of air in a room is 80% at 25 oC.
What is the concentration of water vapor in air on a volume % basis?
If the air contacts a cold surface, water may condense (see effects on next slide). What temperature is required to cause water condensation?

11. Example: Relative Humidity
Continued