Ahmed A Sattar, PhD

Assistant Professor, GUC

FLUID MECHANICS

LECTURE TWO: FLUID PROPERTIES

CIW 402

Winter 2012 Semester

CIW 402- Fluid Mechanics Ahmed A Sattar, PhD Lecture 2- Slide 2

Fluid Properties

Any characteristic of a system is called a property.

Familiar

temp

pressure

volume

Not Familiar

Surface tension

viscosity

Vapor pressure

Intensive properties: independent of the mass of the system, e.g. T, P

Extensive properties: dependent on the size of the system, e.g. m,V

CIW 402- Fluid Mechanics Ahmed A Sattar, PhD Lecture 2- Slide 3

Density & Specific Gravity & weight

Density (ρ) mass per unit volume

ρ = m/V kg/m3

Specific Volume volume per unit mass

S.V.= V/m = 1/ρ m3/kg

Specific gravity density of fluid / density of ref fluid at T0

S.G.= ρliquid / ρwater or S.G.= ρgas / ρair

Specific weight weight per unit volume

S.W.= γ = W/V = mg/V = ρg N/m3

CIW 402- Fluid Mechanics Ahmed A Sattar, PhD Lecture 2- Slide 4

Density & Specific Gravity

Density of liquids determine which one stays up and which

goes down!!

Material Density

Rubbing Alcohol .82

Lamp Oil .80

Baby Oil .83

Vegetable Oil .92

Ice Cube .92

Water 1.00

Milk 1.03

Dawn Dish Soap 1.06

Light Corn Syrup 1.33

Maple Syrup 1.37

Honey 1.42

CIW 402- Fluid Mechanics Ahmed A Sattar, PhD Lecture 2- Slide 5

Ideal & Real Fluids

● A fluid with no friction

● An inviscid (zero viscosity) fluid

● No friction forces between layers

● Many flows approximate away from solid boundaries.

● Tangential or shearing forces always develop where

there is motion relative to solid body, thus friction appears

● Shear forces oppose motion of one particle past another

● Friction forces gives rise to a fluid property called

viscosity

Idea

l F

luid

R

eal

Flu

id

CIW 402- Fluid Mechanics Ahmed A Sattar, PhD Lecture 2- Slide 6

● A measure of a fluid's resistance to angular

deformation, e.g.,

– Motor oil: high viscosity, feels sticky

– Gasoline: low viscosity, flows “faster”

● Friction forces result from cohesion and

momentum interchange between molecules.

Fluid Viscosity

CIW 402- Fluid Mechanics Ahmed A Sattar, PhD Lecture 2- Slide 7

Liquids: viscosity decreases as temperature increases

Gases: viscosity increases as temperature increases

Fluid Viscosity

CIW 402- Fluid Mechanics Ahmed A Sattar, PhD Lecture 2- Slide 8

Viscosity represents the internal resistance of a fluid to motion.

The force a flowing fluid exerts on a body in the flow direction

is called the drag force, and the magnitude of this force depends, in

part, on viscosity

Experiments show that F is proportional to A.v/L

Fluid Viscosity

CIW 402- Fluid Mechanics Ahmed A Sattar, PhD Lecture 2- Slide 9

Experiments show that F is proportional to A.v/L

F = Constant Av/L

Shear stress between layers = τ = F/A

τ = Constant v/L

Thus, Newton equation for viscosity can be written as;

τ = μ v/L = μ dv/dy

μ coefficient of dynamic viscosity

Fluid Viscosity

0 Ideal fluids

Constant Newtonian fluids

e.g. air and water

Vary Non Newtonian fluids

e.g. paint, ink, gel, blood

CIW 402- Fluid Mechanics Ahmed A Sattar, PhD Lecture 2- Slide 10

Fluid Viscosity

- Metric unit of viscosity

- Named after Jean Poiseuille (1799-1869)

- 1 P = 0.10 N.s/m2

- 1 cP=0.01P=1 mN.s/m2

- For water at 20 degrees, μ =1cP

Units of μ Poise (P)

CIW 402- Fluid Mechanics Ahmed A Sattar, PhD Lecture 2- Slide 11

Kinematic viscosity (ν)

ν = ratio of dynamic viscosity to density

= μ / ρ m2/s

Called kinematics because it has no dynamic (force)

dimensions

Units of ν Stoke (St)

The metric unit for viscosity

Named after Sir George Stokes (1819-1903)

1cSt = 0.01St =10-6m2/s

Fluid Viscosity

CIW 402- Fluid Mechanics Ahmed A Sattar, PhD Lecture 2- Slide 12

Using a Viscometer

Two concentric cylinders with a fluid in the small gap ℓ.

Inner cylinder is rotating, outer one is fixed.

Measuring Viscosity

Courtesy of Steingraeber Corp., USA

Shear force = F/A = τ = μ v/l

F = Torque / R

A = 2π RL

V = ωr

CIW 402- Fluid Mechanics Ahmed A Sattar, PhD Lecture 2- Slide 13

Vapor Pressure

Liquids tend to evaporate or vaporize by projecting molecules

across free surface

When same number of molecules enter as leave free surface, vapor pressure is

reached

As molecular activity increases, P decreases

Saturation pressure = vapor pressure, Pvap. = boiling pressure (@ any temp)

CIW 402- Fluid Mechanics Ahmed A Sattar, PhD Lecture 2- Slide 14

Vapor Pressure

If P drops below Pvap., liquid is locally vaporized, creating

cavities of vapor.

Vapor cavities collapse when local P rises above Pvap.

Collapse of cavities is a violent process which can damage

machinery and is noisy also.

CIW 402- Fluid Mechanics Ahmed A Sattar, PhD Lecture 2- Slide 15

Surface Tension

Molecular attraction forces in liquids:

-cohesion: enables liquid to resist tensile stress

-adhesion: enables liquid to adhere to another body

At liquid-gas interface, out of balance attraction forces form

imaginary surface film that exerts tension force in the surface

Surface tension is computed as force per unit length

CIW 402- Fluid Mechanics Ahmed A Sattar, PhD Lecture 2- Slide 16

Surface Tension

Cohesion < adhesion, liquid wets solid, rises at point of contact

Cohesion > adhesion, liquid surface depresses at point of

Contact angle is the

angles between the

fluid surface and the

solid surface

CIW 402- Fluid Mechanics Ahmed A Sattar, PhD Lecture 2- Slide 17

Surface Tension

σ = surface tension,

θ = wetting angle,

γ = specific weight of liquid,

r = radius of tube,

h = capillary rise

Equilibrium of surface tension force and gravitational pull

on the water cylinder of height h produces:

CIW 402- Fluid Mechanics Ahmed A Sattar, PhD Lecture 2- Slide 18

Surface Tension

-For r > ¼ in (6 mm), capillarity is negligible

-Its effects are negligible in most engineering situations.

-Important in problems involving capillary rise, e.g., soil

water zone, water supply to plants

-When small tubes are used for measuring properties, e.g.,

pressure, account must be made for capillarity

CIW 402- Fluid Mechanics Ahmed A Sattar, PhD Lecture 2- Slide 19

Triple Point of Fluids

Triple point is the intersection on a phase diagram where three phases

coexist in equilibrium. The most important application of triple point

is water, where the three-phase equilibrium point consists of ice,

liquid, and vapor.

See You Next Time