TOPIC 1 Without Tutorial

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INTRODUCTION & BASICCONCEPT


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INTRODUCTION:Mechanics:

Stationary & moving bodies subjected to forces

Statics:

At rest(stationary)

Dynamics:

In motion(moving)

Fluid Mechanics:

Behaviour of fluids & interaction ofsolids & other fluids at the boundaries


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INTRODUCTION:

Fluid Mechanics

Gas dynamics:

Fluids that undergo density changes

Meteorology; oceanography & hydrology:

Naturally occurring flows

Aerodynamics:

Gas flows over bodies at high/low speeds

Hydrodynamics:

Motion of incompressible fluids

Hydraulics:

Liquid flows in pipes &open channels


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WHAT IS A FLUID?: Fluids:

A substance in the liquid or gas phase.

The differences between solid & liquid:

Solid Liquid

Can resist applied shear stress by

deforming

Deforms continuously due to even the

smallest shear stress

Stress proportional to strain Stress proportional to strain rate

Stops deforming when a constant

shear force is applied (at fixed strain

angle)

Never stop deforming and approaches

a certain rate of strain


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WHAT IS A FLUID?:

In a fluid at rest,

the normal stress is called as Pressure.

Zero shear stress.

AAreaFForceofcomponentTangentialstressShear

AArea

FForceofcomponentNormalstressNormal

t

n

,,,

,

,,


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WHAT IS A FLUID?: Solids & fluids may not be so clear in some borderline cases

e.g. asphalt (resist shear stress for a while; deforms slowly if

forces exerted over extended period of time), some plastics,

lead & slurry mixtures. [beyond the scope!]

Fluids that are dealt in this course are easily recognizable.

Intermolecular bonds are strongest in solids, but weakest in

gas [Reason: in solids, molecules are closely packed together].

Because the small distance between molecules in solids

attractive forces are large & keep molecules at fixed position.


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WHAT IS A FLUID?: Molecule spacing in liquids are somewhat similar with solids,

except molecules are not in fixed position & can rotate freely.

The distance between molecules generally increase slightly as

solids turns liquid [e.g. water].

Gas molecules moves at random, collides with each other &

the walls of the container which they are confined.

Molecules at gas phase are at higher energy level; must

release large amount of energy before it can condense or

freeze.


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WHAT IS A FLUID?: Gas & vapour are often used synonymously:

Gas: A vapour phase of a substance when above the critical

temperature.

Vapour: The current phase is not far from a state of

condensation.


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APPLICATION AREAS:


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THE NO-SLIP CONDITION: Fluid flow are often confined by solid surfaces important to

understand how these solid surfaces affects fluid flow.

Consider the flow of a fluid in a stationary pipe/over a solid

impermeable surface.

All experimental observations indicate that a fluid in motion

comes to a complete stop at the surface (assumes zero

velocity to the surface).

That is, a fluid is in direct contact with the a solid sticks to

the surface [no slip].


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THE NO-SLIP CONDITION:


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THE NO-SLIP CONDITION: Viscosity is responsible for the no-slip condition & the

development of boundary layer.

The no-slip condition is also responsible for the development

of the velocity profile.

Boundary layer: the flow region where the viscous effects (and

the velocity gradient) are significant.

No-slip conditions also responsible for the surface dragor skin

friction drag.

If a fluid is forced to flow over a curved surface, the boundary

layer no longer attached to the surface & separate from the

surfaceflow separation.


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THE NO-SLIP CONDITION:The Velocity Profile: Flow Separation:


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CLASSIFICATION OF FLUID FLOWS:

Classificationof Fluid

Flows

Viscous vs. InviscidRegions of Flow

Compressible vs.Incompressible flow

Laminar vs. Turbulent

Steady vs. UnsteadyFlow

1-, 2- & 3-dimensional Flows

Internal vs. ExternalFlow

Natural (Unforced) vs.Forced Flow


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VISCOUS VS. INVISCID: The development of viscous & inviscid regions of flow can

be seen by inserting a flat plate parallel into a fluid stream

of uniform velocity.

The fluid sticks to the plate on both sides because of the

no-slip condition.

Viscous flow region: The thin boundary layer in which the

viscous effects are significant (near the plate) Inviscid flow region: The region of flow on both sides

away from the plate and affected by the plate.


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VISCOUS VS. INVISCID:

INVISCID INVISCIDVISCOUS


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COMPRESSIBLE VS.

INCOMPRESSIBLE FLOW: Incompressibility: an approximation in which a flow is said to

be incompressible if the density remains nearly constant

throughout.

Volume remains unchanged when the flow is incompressible.

The densities of liquids are constant, thus liquid is typically

incompressible. [incompressible substance].

Gases are highly compressible. Modelling gas flows as

incompressible depends on the Mach number, usually the case

when Ma < 0.3.


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COMPRESSIBLE VS.

INCOMPRESSIBLE FLOW: Mach number,

Small density changes of liquid corresponding to large

pressure changes still have important consequences, e.g.

water hammer.

soundofSpeed

flowofSpeed

c

VMa

Ma = 1 Sonic

Ma < 1 Subsonic

Ma > 1 Supersonic

Ma >> 1 Hypersonic


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LAMINAR VS. TURBULENT: Laminar: The highly ordered fluid motions characterized

by smooth layers.

The flow of high-viscosity fluids (e.g. oil) at low velocities

are typically laminar.

Turbulent: The highly disordered fluid motions that

typically occurs at high velocities.

Transitional: A flow that alternates between beinglaminar and turbulent.

Reynolds number, Re: the key parameter for the

determination of flow regimes in pipes.


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LAMINAR VS. TURBULENT:


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NATURAL VS. FORCED FLOW: Forced Flow: A fluid is forced to flow over a surface or in

a pipe by external means e.g. pump or fan.

Natural/Unforced Flow: Fluid motion is due to natural

means e.g. the bouyancy effect (the rise of warmer &

lighter fluid and the fall of cooler & denser fluid).


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STEADY VS. UNSTEADY FLOW: Steady: No change at a point of time.

Unsteady: Change at a point of time (opposite ofsteady).

Uniform: No change with location over a specified region. Non-uniform: Change with location over a specified

region.

Transient: developing flows.


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1-, 2- & 3-DIMENSIONAL FLOWS A flow field is best characterized by its velocity

distribution.

A flow is said to be one-, two-, or three-dimensional if the

flow velocity varies in one, two, or three primary

dimension, respectively.


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SYSTEM & CONTROL VOLUME: System: A quantity of matter / a region in space chosen

for study.

Surroundings: The mass/region outside the system.

Boundary: The real/imaginary surface that separates the

system from its surroundings. Can be fixed or moveable.

SYSTEM

SURROUNDINGS

BOUNDARY


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SYSTEM & CONTROL VOLUME: Closed system (or control mass):

consists of a fixed amount of mass, and no mass can cross

the boundary.

Energy can cross the boundary & the volume of a closed

system does not have to be fixed.

Isolated system: If even energy is not allowed to cross the

boundary.


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SYSTEM & CONTROL VOLUME: Open system (or control volume):

encloses a device that involves mass flow e.g. compressor,

turbine or nozzle.

Flow through these device is studied by selecting the region

within a device as the control volume.

Both mass & energy can cross the boundary (the control

surface)

CV

(a nozzle)

Imaginary

boundary

Real

boundary


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IMPORTANCE OF DIMENSIONS

AND UNITS: Any physical quantity can be characterized by dimensions.

The magnitude assigned to the dimensions are called units.

Some basic dimensions e.g. mass m, length L, time tare called

primary/fundamental dimensions.

Velocity V, and energy Eare expressed in terms of primary

dimensions are called secondary/derived dimensions.

Two sets of units are still in common use today: the English

system(United States Customary System, USCS), and the

metric SI (Le Systme International dUnits), also known as

International System.


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AND UNITS: The SI:

is a simple & logical based on a decimal relationshipbetween various units.

is used for scientific & engineering work in most of theindustrialized nations, including England.

The English system:

has no systematic numerical base; various units in this

system are related to each other rather arbitrarily. confusing & difficult to learn.

The United States is the only industrialized country usingthis system.


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AND UNITS:The seven fundamental (or primary) dimensions and their

units in SIDimension Unit

Length Meter (m)

Mass Kilogram (kg)

Time Second (s)

Temperature Kelvin (K)

Electric current Ampere (A)Amount of light Candela (cd)

Amount of matter Mole (m)


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DIMENSIONAL HOMOGENEITY: In engineering, all equations must be dimensionally

homogeneous: every term in an equation must have the

same dimensions.

If two quantities that have different dimensions (or units)

are added, it is an indication that error is made.

Thus,

ALWAYSCHECK THE UNITS IN YOUR CALCULATIONS!!


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UNITY CONVERSION RATIOS: Take for example:

They can also be expressed more conveniently as unity

conversion ratios:

If a box of breakfast cereal written as net weight = 454 g, theactual weigh of the cereal on earth is:

2s

mkgN

1/

2

smkg

N

Ng

kg

smkg

NsmgmgW 49.4

1000

1

/.1

1)/81.9)(6.453(

2

2

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TOPIC 1 Without Tutorial