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EAH 221 July 12th

FLUID MECHANICSfor

CIVIL ENGINEERING

Dr H Md

AzamathullaLecturer,REDAC, USM

Solid State Fluid State

Liquids Gases

Physical Properties

Behaviour of fluids

FM is the study of behaviour of fluids,either at rest (Fluid Statics) or in motion(Fluid dynamics)

Fluid Mechanics

•

Fluid Mechanics: the study of forces thatdevelop when an object moves through a

fluid medium.

•

Two fluids of interest–

Water

– Air

Why should study Fluid Mechanics ?

• There are many ways fluids affect our life

for e.g., Fluid Storage Tanks, Water distribution system in Homes, Systems that delivers fluids to industrial processes., fluid power systems, etc.

•

This FM course will help you to under stand how fluids can be controlled.

Development of FM•

The earlier development in the science of Fluid Flow has been influenced by two basic schools of thought.

•

One dealing with theoretical aspect and the other with the practical aspect of fluid flow.

•

In 17th

Century, mathematicians and mathematical physicists studied the behaviour of the ideal fluid.

•

A fluid is a substance that flows under the action of shearing forces. If a fluid is at rest, we know that the forces on it are in balance.

•

A gas is a fluid that is easily compressed. It fills any vessel in which it is contained.

•

A liquid is a fluid which is hard to compress. A given mass of liquid will occupy a fixed volume, irrespective of the size of the container.

•

A free surface is formed as a boundary between a liquid and a gas above it.

Definition of a Fluid

Density

The density of a fluid is defined as its mass per unit volume. It is denoted by the Greek symbol, ρ.

ρ =V m3kgm-3

If the density is constant (most liquids), the flow is incompressible.If the density varies significantly (eg some gas flows), the flow is compressible.(Although gases are easy to compress, the flow may be treated as incompressible if there are no large pressure fluctuations)

ρ water = 998 kgm-3

ρair =1.2kgm-3

kgm

Pressure

Pressure is the force per unit area, where the force is perpendicular to the area.

p=A m2

Nm-2

(Pa)

NF

This is the Absolute pressure, the pressure compared to a vacuum.

pa = N/m2

The pressure measured in your Car tyres is the gauge pressure, p-pa .

Pressure in a Fluid

SHEAR STRESS

•

Shear stress, greek letter ζ

(tau) , can be defined as the force required to slide one unit area layer of a substance over another.

•

ζ

= force/area N/ m2

(Pa)

Shear in Different Fluids•

Shear-stress relations for different types

of fluids•

Newtonian fluids: linear relationship

•

Slope of line (coefficient of proportionality) is “viscosity”

dydV

dydV

μτ

τ

=

∝

Viscosity

•

viscosity (μ) can be defined resistance to flow of a liquid

•

Viscosity generally decreases as temperature increases; (for liquids)

•

In case of gases viscosity increases when temperature increases

Basic UnitsIn any Technical work the units in which physical properties are measured must be stated.A system of units specifies the units of the BASIC quantities of LENGTH, TIME, FORCE, and MASS.

Length = meter (m)Time = second (s)

Mass =Kilogram (Kg)Force = Newton (N) or Kg.m/s2

F = ma

Different type of units

•

The international system of Units (SI)

•

FPS system (foot –

pound-second)

Rules & Conversions in Using SI units

•

The equations are often complex, and it is extremely important that the results be dimensionally correct.

•

They must have their proper units•

Otherwise the answers will have the wrong numerical value (if units in the equation are not consistent).

•

Unit Cancellation procedure

SI Units for common quantities used in FM

•

Quantity Standard SI Units •

Length

meter (m)

•

Time

seconds (s)•

Mass

Kilogram (Kg)

•

Force or weight Newton (N)•

Energy N-m or Joule (J)

•

Power

N-m/s or Joule (J)•

Volume flow rate (discharge) m3/s

•

Specific weight N/m3

•

Density Kg/m3

Prefix SI symbol Factor

•

Giga

G 109

= 1000 000 000

•

Mega

M 106

= 1000 000

•

Kilo

K 103 = 1000

•

Milli m 10

-3 = 0.001

•

Micro

μ

10-6

= 0.000 001

•

Nano n

10 -9 =0.000 000 001

Specific weightSpecific weight y (gamma-Greek letter)=w/vUnits N/m3

W=m.g

Specific gravity

Specific gravity is the ratio of the density of a substance to the density of water at 40 C

Specific gravity is the ratio of the specific weight of a substance to the specific weight of water at 40

C

Relation between density and specific water

•

y=ρg•

g (acceleration due to gravity)

•

Also we know y=W/V ,•

Now we will multiply both the numerator & denominator of this equation by g we obtain y=wg/Vg , but m=w/g

so becomes y=mg/VWe also know ρ

=m/V. y= ρg

Test

•

July 14th

2008 1200 to 1230 PM

•

Recommended Text Book

•

Mott, Applied Fluid Mechanics’, Sixth edition, Prentice Hall, 2006