CE 204 FLUID MECHANICS Onur AKAY 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: onur.akay@okan.edu.tr Onur Akay, Ph.D. CE 204 Fluid Mechanics 1
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/m 3 ). Mass density of water at 277 K (4 o C) is 1000 kg/m 3, and it decreases slightly with increasing temperature. The mass density of air at 293 K (20 o C) and standard atmospheric pressure is 1.2 kg/m 3, and it changes significantly with temperature and pressure. The densities of common fluids are given in Tables A.2 and A.5. Onur Akay, Ph.D. CE 204 Fluid Mechanics 2
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/m 3. Specific weights of common liquids are given in Table A.4. Variation in Liquid Density: For most applications, liquids can be considered incompressible 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 4 o C. Onur Akay, Ph.D. CE 204 Fluid Mechanics 3
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)). c v = Specific volume vof the gas (v = 1/ρ) remains constant c p = Pressure of the gas remains constant Table A.2 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) Onur Akay, Ph.D. CE 204 Fluid Mechanics 4
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. Movie {shear stress} = {viscosity} x {rate of strain} Onur Akay, Ph.D. CE 204 Fluid Mechanics 5
{shear stress} = {viscosity} x {rate of strain} no-slip condition Onur Akay, Ph.D. CE 204 Fluid Mechanics 6
The viscosity of water at 293 K is 10-3 N.s/m 2. The unit of viscosity in the SI system is Pa.s. Kinematic Viscosity, ν: Many equations of fluid mechanics include the ratio μ/ρ. The kinematic viscosity of water at 293 K is 10-6 m 2 /s. Temperature Dependency As the temperature increases the viscosity of Liquids: Decreases Gases: Increases Onur Akay, Ph.D. CE 204 Fluid Mechanics 7
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 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). Onur Akay, Ph.D. CE 204 Fluid Mechanics 8
Onur Akay, Ph.D. CE 204 Fluid Mechanics 9
Newtonian versus Non-Newtonian Fluids: Fluids for which the shear stress is directly proportional to the rate of strain are called Newtonian fluids. 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 Onur Akay, Ph.D. CE 204 Fluid Mechanics 10
Bulk Modulus of Elasticity, E v : Relates changes in pressure to changes in volume (e.g., expansion or contraction) The elasticity is often called the compressibility of the fluid. The bulk modulus of elasticity of water is approximately 2.2 GN/m 2, which corresponds to a 0.05% change in volume for a change of 1 MN/m 2 in pressure. For most purposes a liquid may be considered as incompressible. Onur Akay, Ph.D. CE 204 Fluid Mechanics 11
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. 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. Onur Akay, Ph.D. CE 204 Fluid Mechanics 12
Chapter 2 Fluid Properties Capillary Action: Rise above a static water level at atmospheric pressure. Onur Akay, Ph.D. CE 204 Fluid Mechanics 13
Cases where surface tension force is balanced by the internal pressure: Onur Akay, Ph.D. CE 204 Fluid Mechanics 14
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: 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 (10 o C)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. Onur Akay, Ph.D. CE 204 Fluid Mechanics 15
Cavitation Movie Onur Akay, Ph.D. CE 204 Fluid Mechanics 16