CWR Lecture 1

8/3/2019 CWR Lecture 1

1/19

HYDRAULICS: CWR 3201

SPRING 2012

LECTURE 1

INTRODUCTION & PROPERTIESOF FLUID


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FLUID PROPERTIES

-Definition: Fluid has certain characteristics by which itsphysical properties are described.

-These properties expressed in terms of length, mass, force,

time temperature.

-The traditional system of unit in US is ft-lb-s system.

-SI system: kg-m-s system.

-This course will use both SI & traditional units


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BASIC UNITS

SI Unit:

-Mass: kg, Length: m, Time: s

-Force: Newton= 1kg x 1m/s2

-Temperature: Kelvin K= 273+ oC


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BASIC UNITS

SI Unit:

-Work or Energy: Joule (J)= N.m (Force x Distance)

-Power: Watt= Joule/s

-Prefixes: -G (giga) = 109 c (centi) = 10-2

-M (mega) = 106 m (milli) = 10-3

-K (kilo) = 103 (micro) = 10-6


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BASIC UNITS

FPS System / US Customary Units:

-FPS (Foot-Pound-Second)

-Mass: lbm

, Length: ft, Time: s

Also,

-Mass: slug (14.59 kg),

-Force: lbf= 1slug x 1ft/s2

-Conversion: 1 slug = 14.59 kg x (1 lbm / 0.453 kg) = 32.2 lbm


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Properties Involving Mass or WeightMass Density ():

Mass per unit volume.

Mass density of water = 1000 kg/m3 or 62.4 lbm/ft3

Specific Weight ():

Gravitational force per unit volume

Specific weight of water is 9.8 kN/m3

Specific Gravity (S):

Ratio of specific weight of a fluid to the specific weight of water.

V

m

g

Water

Fluid

Water

FluidS


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EQUATION OF STATE & DENSITY OF GASESIdeal Gas Law:

p= Absolute Press.; V= Volume, n= Number of moles,Ru=Universal gas constant, T= Absolute temperature

Eqn may be rewritten as

M= molecular wt. of the gas. The product of number of molesand molecular wt. is the mass of the gas.

Ru/M= R, is the gas constant

TnRpV u

TM

R

V

nMp u


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EQUATION OF STATE & DENSITY OF GASESSo the Equation of State becomes

Or mass density of gas

* Problem (Handout)

RTp

RT

p


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VISCOSITY

-Viscosity (also called dynamic or absolute viscosity):-Measure offluids resistance to deformation under shear stress.

-Shear stress proportional to time rate of strain

-Unit: Poise= 0.1 N.s/m2

-Viscosity of water @ 20oC is 1 CP (10-2 poise)

* Problem (Handout)

dydV

2

2

/./)/(

/

/msN

msm

mN

dydV


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VISCOSITY-Kinematic Viscosity

smmkg

msN/

/

/. 23

2


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NEWTONIAN VS. NON-NEWTONIAN FLUIDS

Newtonian FluidShear stress is directly proportional to rate of strain

Non-Newtonian Fluid

Shear stress is NOT directly proportional to rate of strain

-Shear-thinning fluids: decreases with

-Shear thickening Fluids: gypsum in water

-Bingham plastic: Acts like solid

dy

dV/ dy

dV


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SURFACE TENSION

-Property by which a liquid at a material interface exerts a force per unitlength along the surface

- Due to Molecular attraction

-Produces a surface that acts like a stretched membrane.

-The tension that acts on the plane of surface is calculated by:

F = L L=Length over-which the surface tension acts

-Capillary action shows the effect of surface tension


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PRESSURE

Pressure:

- Ratio of normal force to area at the point.

- Scalar quantity: Only magnitude

Units:

- SI: N/m2 (Pascal)- Traditional unit: psi, psf

- Engineers frequently states the pressure as height of column of liquid (ft,inch, m)

- Example: 50 psi= 127 ft of water column or just 127

dA

dF

A

FLimp normalnormalA

||

0


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ABSOLUTE PRESSURE, GAGE PRESSURE,

VACUUM PRESSURE

Absolute Pressure:

- Pressure measured relative to zero pressure

- Pressure in perfect vacuum is called zero pressure

- Absolute pressure is referenced to regions such as outer space (region is

devoid of gas)

Gage Pressure:

-Pressure measured relative to prevailing to atmospheric pressure

-e.g.: Gage pressure 50 kPa recorded at a place, where atmospheric press.=

100 kPa then that pressure can be expressed as

p= 50 kPa gage or p= 150 kPa abs


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VACUUM PRESSUREVacuum Pressure:

-Difference between atmospheric press. & actual press.

-When press. is less than atmospheric, press. can be described as vacuum

press.

Standard Atmospheric Pressure at sea level

1 atm = 101.3 kPa = 14.7 psi = 33.9water = 760 mm Hg = 2116 psf


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VACUUM PRESSURE

Relationship

Example

gageatmabs PPP

vacuumatmabs

PPP

gagevacuum PP


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PRESSURE MEASUREMENT

Five scientific instruments:

- Barometer

- Bourdon-Tube Gage

- Piezometer

- Manometer

- Pressure Transducer

Today


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Barometer- Measure atmospheric pressure.

- Mercury barometer

-What is atm. press (kPa) when h=760 mm (SHg= 13.6)

- Aneroid barometer: Works mechanically

hphp HgvHgatm


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Bourdon-Tube Gage

Measures pressure by sensing deflection of a coiled tube.

Performed lab experiment

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CWR Lecture 1