DJJ2093 FLUID MECHANICS

COURSE LEARNING OUTCOMES

• Analyze problems related to the fluid mechanics and data from the experiments in relation to the theoretical aspects. (C4, PLO2)

• Organize appropriately experiments in groups according to the Standard Operating Procedures. (P4, PLO4)

• 3. Demonstrate team work skill in assigned task. (A3, PLO7)

CHAPTER 1:FLUID AND PROPERTIES

LESSON LEARNING OUTCOMES

INTRODUCTIONWhat is Fluid Mechanics???• A section of applied mechanics, concerned

with the static and dynamics of liquid and gases

• Deals with the study of the motion of general behavior of a fluid governed by the law of conversation of mass, laws of mechanics and of thermodynamics

FLUID……………What is fluid ?????

a substance which deforms continuously under the action of shearing forces, however small they may be.

if a fluid is at rest, there can be no shearing forces

all forces in the fluid must be perpendicular to the planes upon which they act.

Free body diagram

Particles can move past one another Particles can move past one

another

Rigid - Particles locked into place

(a) Solid (b) Liquid (c) Gas

k

k k

k

Free surface

PRESSURE

Units : Newton’s per square metre, Nm-2,kgm-1s-2.(The same unit is also known as Pascal, Pa i.e 1 Pa = 1Nm-2)

Also frequently used is the alternative SI unit the bar, where 1 bar = 105Nm-2

Atmospheric Pressure, patm– The pressure due to this atmosphere at the

surface of the earth depends upon the head of the air above the surface.

– The air is compressible, therefore the density is different at different height.

– atmospheric pressure is measured by the height of column of liquid that it can support.

– Atmospheric pressure at sea level is about 101.325 kN/m2, which is equivalent to a head of 10.35 m of water or 760 mm of mercury approximately, and it decreases with altitude.

Gauge Pressure, pG– measured with the help of a pressure measuring

instrument, in which the atmospheric pressure is taken as datum; in other words the atmospheric pressure at the gauge scale is marked zero.

– can be either positive or negative depending on whether the pressure is above atmospheric pressure (a positive value) if the fluid is in the state of compression or below atmospheric pressure (a negative value).

Absolute Pressure, pA

• It is the pressure equal to the algebraic sum of the atmospheric and gauge pressures.

Vacuum, pv

• In a perfect vacuum which is a completely empty space, the pressure is zero.

EXAMPLE• What is the pressure gauge of air in the

cylinder if the atmospheric gauge is 101.3 kN/m2 and absolute pressure is 460 kN/m2 ?

• Calculate the absolute pressure of air in the compressor cylinder if the pressure gauges is 2500 N/m2

(Assume atmospheric gauge is 101.3 kN/m2 )

LESSON LEARNING OUTCOMES

PHYSICAL PROPERTIES OF FLUID

Fluid properties are intimately related to fluid behaviour

Physical Properties

viscosity

Mass densityFluid

compressibility

Relative density

Specific weight

Specific volume

Viscosity• Viscosity, µ, is a measure of resistance to fluid flow .In other

words, viscosity can be seen as internal friction to fluid motion which can then lead to energy loss.

• Different fluids deform at different rates under the same shear stress. The ease with which a fluid pours is an indication of its viscosity. Fluid with a high viscosity such as syrup deforms more slowly than fluid with a low viscosity such as water. The viscosity is also known as dynamic viscosity.

Units: N.s/m2 or kg/m/s

Typical values: Water = 1.14x10-3 kg/m/s; Air = 1.78x10-5 kg/m/s

Kinematic viscosity, ν Definition: is the ratio of the viscosity to the density;

• will be found to be important in cases in which significant viscous and gravitational forces exist.

Units: m2/s

Typical values: Water = 1.14x10-6 m2/s; Air = 1.46x10-5 m2/s;

In general, viscosity of liquids with temperature, whereas

viscosity of gases with in temperature.

ρµν /=

Fluid Compressibility• How easily can the volume (density) of a given mass of the

fluid be changed when there is a change in pressure.• Is the resistance to change in volume.• Bulk Modulus @ Modulus of Elasticity defined to be the ratio

of the stress (change in pressure) to strain (change in volume divided by the original volume).

Bulk Modulus @ Modulus of Elasticity, K:

K= -ΔP ΔP = change in Pressure

(ΔV/V) ΔV = change in Volume

V = original volume

Mass density, ρ

SI units: kg/m3

Vvolume

mmass

,

,=ρ

Specific gravity or relative density, s

water

cesubssω

ω tan=

water

cesubssρ

ρ tan=

Specific volume, v

• SI units: m3/kg

ρν 1=

mmass

Vvolume

,

,=ν

Specific weight, ω

• SI units:N/m3

• In SI units the specific weight of water is

9.81 x 1000 = 9810 N/m3

Vvolume

Wweight

,

,=ω

V

mg=ω

gρω =

EXAMPLE

E.g. 1 : What is the mass density, ρ of fluid (in kg/m3) if

mass is 450 g and the volume is 9 cm3.

E.g. 2 :• What is the specific weight, ω of fluid (in kN/m3) if

the weight of fluid is 10N and the volume is 500 cm2.

E.g. 3

What is the specific gravity of fluid in E.g.2

E.g. 4

What is the specific volume, v of fluid in E.g.1

EXERCISES!!!!!!!1. Pressure of a barometer is 480kN/m2.

Calculate absolute pressure of barometer if the atmospheric pressure is 1.03 bar.

2. Volume of lubricant oil in a tank is 5.5 m3 and its weight is 50kN.

Calculate the following :-

a)Mass density of oil

b)Specific weight of oil

c)Specific volume of oil

d)Specific gravity of oil

THE END………