Chapter 2 Fluid Properties
Mass and Weight, Thermal Energy, Viscosity, Bulk Modulus of Elasticity, Surface Tension, Vapor Pr.
CE 204
FLUID MECHANICS
Onur AKAY
Assistant Professor
Onur Akay, Ph.D. CE 204 Fluid Mechanics 1
Assistant Professor
Okan University
Department of Civil Engineering
Akfırat Campus
34959 Tuzla-Istanbul/TURKEY
Phone: +90-216-677-1630 ext.1974
Fax: +90-216-677-1486
E-mail: [email protected]
Chapter 2 Fluid Properties
Mass and Weight, Thermal Energy, Viscosity, Bulk Modulus of Elasticity, Surface Tension, Vapor Pr.
A fluid has certain characteristics by which its physical condition may be described. These
are called properties of the fluid.
Mass Density, ρ: Defined as the ratio of mass to volume at a point
Mass density has units of kilograms per cubic meter (kg/m3).
Onur Akay, Ph.D. CE 204 Fluid Mechanics 2
Mass density has units of kilograms per cubic meter (kg/m ).
Mass density of water at 277 K (4oC) is 1000 kg/m3, and it decreases slightly with increasing
temperature.
The mass density of air at 293 K (20oC) and standard atmospheric pressure is 1.2 kg/m3, and
it changes significantly with temperature and pressure.
The densities of common fluids are given in Tables A.2 and A.5.
Chapter 2 Fluid Properties
Mass and Weight, Thermal Energy, Viscosity, Bulk Modulus of Elasticity, Surface Tension, Vapor Pr.
Specific Weight, γ: Defined as the gravitational force per unit volume of fluid, or simply the
weight per unit volume.
γ = ρ.g
Water at 293 K has a specific weight of 9790 N/m3.
Specific weights of common liquids are given in Table A.4.
Variation in Liquid Density: For most applications, liquids can be considered incompressible
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and can be assumed to have constant density.
A mixture of salt in water changes the density of the water without changing its volume. A
fluid wherein density varies spatially is described as nonhomogeneous.
Specific Gravity, S: Defined as the ratio of the specific weight of a given fluid to the specific
weight of water at the standard reference temperature 4oC.
Chapter 2 Fluid Properties
Mass and Weight, Thermal Energy, Viscosity, Bulk Modulus of Elasticity, Surface Tension, Vapor Pr.
Thermal Energy
Specific Heat, c: The property that describes the capacity of a substance to store thermal
energy. It is the amount of thermal energy that must be transferred to a unit mass of
substance to raise its temperature by one degree (J/(kg K)).
cv = Specific volume v of the gas (v = 1/ρ) remains constant
cp = Pressure of the gas remains constant Table A.2
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Internal Energy: The energy that a substance possesses because of the state of the
molecular activity. In the SI system, the specific internal energy, u, is given in J/kg.
Enthalpy: The combination u + p/ρ (specific enthalpy)
Chapter 2 Fluid Properties
Mass and Weight, Thermal Energy, Viscosity, Bulk Modulus of Elasticity, Surface Tension, Vapor Pr.
Viscosity, μ: (Also called dynamic viscosity , or absolute viscosity) is a measure of a fluid’s
resistance to deformation under shear stress.
For example, crude oil has a higher resistance to shear than does water.
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Onur Akay, Ph.D. CE 204 Fluid Mechanics 5
{shear stress} = {viscosity} x {rate of strain}
Chapter 2 Fluid Properties
Mass and Weight, Thermal Energy, Viscosity, Bulk Modulus of Elasticity, Surface Tension, Vapor Pr.
{shear stress} = {viscosity} x
{rate of strain}
Onur Akay, Ph.D. CE 204 Fluid Mechanics 6
no-slip
condition
Chapter 2 Fluid Properties
Mass and Weight, Thermal Energy, Viscosity, Bulk Modulus of Elasticity, Surface Tension, Vapor Pr.
The viscosity of water at 293 K is 10-3 N.s/m2. The unit of viscosity in the SI system is Pa.s.
Kinematic Viscosity, ν: Many equations of fluid mechanics include the ratio μ/ρ.
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The kinematic viscosity of water at 293 K is 10-6 m2/s.
Temperature Dependency
As the temperature increases the viscosity of
Liquids: Decreases
Gases: Increases
Chapter 2 Fluid Properties
Mass and Weight, Thermal Energy, Viscosity, Bulk Modulus of Elasticity, Surface Tension, Vapor Pr.
Important Implications:
1. The nonappearance of pressure shows that both τ and μ are independent of pressure.
- Fluid friction is different from that between moving solids.
2. Any shear stress τ, will cause flow because applied tangential forces must produce a
Onur Akay, Ph.D. CE 204 Fluid Mechanics 8
2. Any shear stress τ, will cause flow because applied tangential forces must produce a
velocity gradient.
3. The shearing stress in viscous fluids at rest will be zero.
4. The velocity profile cannot be tangent to a solid boundary.
5. The equation is limited to laminar fluid motion, in which viscous action is strong.
6. The velocity at a solid boundary is zero (no-slip condition).
Chapter 2 Fluid Properties
Mass and Weight, Thermal Energy, Viscosity, Bulk Modulus of Elasticity, Surface Tension, Vapor Pr.
Onur Akay, Ph.D. CE 204 Fluid Mechanics 9
Chapter 2 Fluid Properties
Mass and Weight, Thermal Energy, Viscosity, Bulk Modulus of Elasticity, Surface Tension, Vapor Pr.
Newtonian versus Non-Newtonian Fluids:
Fluids for which the shear stress is directly proportional to the rate of strain are called
Newtonian fluids.
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Shear-thinning fluids/ pseudoplastic : Toothpaste, paints, printer’s ink
Pseudoplasticity can be demonstrated by the manner in which shaking a bottle of ketchup
causes the contents to undergo an unpredictable change in viscosity. The force causes it to
go from being thick like honey to flowing like water.
Shear-thickening fluids: mixtures of glass particles in water, traction control, body armor
Chapter 2 Fluid Properties
Mass and Weight, Thermal Energy, Viscosity, Bulk Modulus of Elasticity, Surface Tension, Vapor Pr.
Bulk Modulus of Elasticity, Ev:
Relates changes in pressure to changes in volume (e.g., expansion or contraction)
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The elasticity is often called the compressibility of the fluid.
The bulk modulus of elasticity of water is approximately 2.2 GN/m2, which corresponds to
a 0.05% change in volume for a change of 1 MN/m2 in pressure.
For most purposes a liquid may be considered as incompressible.
Chapter 2 Fluid Properties
Mass and Weight, Thermal Energy, Viscosity, Bulk Modulus of Elasticity, Surface Tension, Vapor Pr.
Surface Tension, σ:
A material property whereby a liquid at a material interface, exerts a force per unit length
along the surface.
Membrane effect: Molecules near the surface have a greater attraction for each other than
they do for molecules below the surface because of the presence of a different substance
above the surface.
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Because of the membrane effect, each portion of the liquid surface exerts “tension” on
objects that are in contact with the liquid surface.
Surface tension for a water-air surface is 0.073 N/m
(at room temperature).
Surface tension decreases with increasing temperature.
Chapter 2 Fluid Properties
Mass and Weight, Thermal Energy, Viscosity, Bulk Modulus of Elasticity, Surface Tension, Vapor Pr.
Capillary Action: Rise above a static water level at atmospheric pressure.
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Chapter 2 Fluid Properties
Mass and Weight, Thermal Energy, Viscosity, Bulk Modulus of Elasticity, Surface Tension, Vapor Pr.
Cases where surface tension force is balanced by the internal pressure:
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Chapter 2 Fluid Properties
Mass and Weight, Thermal Energy, Viscosity, Bulk Modulus of Elasticity, Surface Tension, Vapor Pr.
Vapor Pressure: The pressure at which a liquid will vaporize, or boil, at a given temperature,
is called its vapor pressure.
-Boiling occurs whenever the local pressure equals the vapor pressure.
-Vapor pressure increases with temperature.
There are two ways to boil a liquid:
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1. Raise the temperature, assuming that the temperature is fixed (For water at
101.3 kPa, the boiling temperature is 373 K).
2. Reduce the pressure in liquid to its vapor pressure (If the pressure in water at
323 K (10oC)is reduced to 1.23 kPa, the water boils).
Boiling can occur in localized low-pressure zones of flowing liquids. They can then collapse
in regions of high pressure. This phenomenon, which is called cavitation, can cause
extensive damage to fluid systems.
Chapter 2 Fluid Properties
Mass and Weight, Thermal Energy, Viscosity, Bulk Modulus of Elasticity, Surface Tension, Vapor Pr.
Cavitation
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