by sir Asif zaman at Abasyn uni Islamabad

1. Fluid Mechanics I
Course Instructor: Muhammad Asif Zaman
Email: masifzaman@gmail.com

2. Recommended Books
Text Book:
• A textbook of Hydraulics, Fluid Mechanics and Hydraulic
Machines by R.S. Khurmi
Reference Book:
• Fluid Mechanics with Engg. Application by Daugherty and
Franzini.

3. Introduction to Fluid
Mechanics

4. Definition of Fluid Mechanics
• Mechanics is the oldest physical science that deals with both
stationary and moving bodies under the influence of forces.
• The branch of mechanics that deals with bodies at rest is called
statics, while the branch that deals with bodies in motion is
called dynamics.
• The subcategory fluid mechanics is defined as the science that
deals with the behavior of fluids at rest(fluid statics) or in
motion (fluid dynamics), and the interaction of fluids with solids
or other fluids at the boundaries.
• The study of fluids at rest is called fluid statics.

5. Application areas of Fluid Mechanics
• Biomechanics
• Blood flow through arteries and veins
• Airflow in the lungs
• Households
• Piping systems for cold water, natural gas, and sewage
• Piping and ducting network of heating and air- conditioning
systems
• refrigerator, vacuum cleaner, dish washer, washing machine,
water meter, natural gas meter, air conditioner, radiator, etc.
• Mechanical Engineering
• Design of pumps, turbines, air-conditioning equipment, pollution-
control equipment, etc.
• Design and analysis of aircraft, boats, submarines, rockets, jet
engines, wind turbines, biomedical devices, the cooling of electronic
components, and the transportation of water, crude oil, and natural
gas.

6. Application areas of Fluid Mechanics
• Civil Engineering
• Transport of river sediments
• Pollution of air and water
• Design of piping systems
• Flood control systems

7. Application areas of Fluid Mechanics

8. What is a Fluid?
• A substance exists in three primary phases: solid, liquid, and gas. A
substance in the liquid or gas phase is referred to as a fluid.
• Distinction between a solid and a fluid is made on the basis of the
substance’s ability to resist an applied shear (or tangential) stress that
tends to change its shape.
• A solid can resist an applied shear stress by deforming, whereas a fluid
deforms continuously under the influence of shear stress, no matter how
small.
• In solids stress is proportional to strain, but in fluids stress is
proportional to strain rate.

9. Distinction between a Solid and a Fluid
Solid
• Definite Shape and definite
volume.
• Does not flow easily.
• Molecules are closer.
• Attractive forces between the
molecules are large enough to
retain its shape.
• An ideal Elastic Solid deform
under load and comes back to
original position upon removal of
load.
• Plastic Solid does not comes back
to original position upon removal
of load, means permanent
deformation takes place.
Fluid
• Indefinite Shape and Indefinite
volume & it assumes the shape
of the container which it
occupies.
• Flow Easily.
• Molecules are far apart.
• Attractive forces between the
molecules are smaller.
• Intermolecular cohesive forces in
a fluid are not great enough to
hold the various elements of
fluid together. Hence Fluid will
flow under the action of applied
stress. The flow will be
continuous as long as stress is
applied.

10. Distinction between a Gas and Liquid
 The molecules of a gas are
much farther apart than
those of a liquid.
 Hence a gas is very
compressible, and when
all external pressure is
removed, it tends to
expand indefinitely.
 A liquid is relatively
incompressible.
 If all pressure is removed,
the cohesion between
molecules holds them
together, so that the liquid
does not expand
indefinitely.

11. SI Units

12. FPS Units

13. Properties of Fluids
 Density (r):
Mass per unit volume of a substance.
kg/m3 in SI units
Slug/ft3 in FPS system of units
Density of water at 4c is 1000 kg/m3
Specific weight (g):
Weight per unit volume of substance.
N/m3 in SI units
lbs/ft3 in FPS units
• specific weight of water at 4c is 9810 N/m3
• Density and Specific Weight of a fluid are related as:
• Where g is the gravitational constant having value 9.8m/s2 or
32.2 ft/s2.
grg 
V
w
g
V
m
r

14. Properties of Fluids
 Specific Volume (v):
Volume occupied by unit mass of fluid.
 It is commonly applied to gases, and is usually expressed in
cubic feet per slug (m3/kg in SI units).
 Specific volume is the reciprocal of density.
r/1 vlumeSpecificVo

15. Properties of Fluids
• Specific gravity:
It can be defined in either of two ways:
a. Specific gravity is the ratio of the density of a substance
to the density of water at 4°C.
b. Specific gravity is the ratio of the specific weight of a
substance to the specific weight of water at 4°C.
c. Specific gravity is unitless.
w
l
w
l
r
r
g
g
liquids

16. Examples

17. Example
Calculate the specific weight, density, specific volume and
specific gravity of 1litre of petrol weights 7 N.
Solution:

18. Example
If the specific gravity of petrol is 0.70.Calculate its Density, Specific
Volume and Specific Weight in SI Units.
Solution:

19. Viscosity
• Viscosity is a measure of the resistance of a fluid to flow.
• Viscosity describes a fluid's internal resistance to flow and may
be thought of as a measure of fluid friction. Thus, water is
"thin", having a lower viscosity, while honey is "thick" having a
higher viscosity.
• The friction forces in flowing fluid result from the cohesion in
case of liquids and molecular momentum in case of gases
(collision) between molecules.
• All real fluids except super-fluids / ideal fluid (which have zero
viscosity) have some resistance to flow, but a fluid which has
no resistance to flow is known as an ideal fluid or super fluid.
• It is also known as Absolute Viscosity or Dynamic
Viscosity.

20. Viscosity
• Viscosity increases with temperatures in case of gases due to increase in
collision between gas molecules.
• While Viscosity decreases with increase in temperature in case of liquids.

21. Dynamic Viscosity
• Dynamic viscosity of the fluid is represented by m. The term
absolute viscosity is sometimes used.
dy
du
m 

22. Kinematic Viscosity
•The kinematic viscosity ν is defined as:
“Ratio of absolute / dynamic viscosity to density.”
r
m
 

23. Example
Find the kinematic viscosity of liquid in stokes whose specific
gravity is 0.85 and dynamic viscosity is 0.015 poise.
Solution:
Given S = 0.85
m = 0.015 poise
= 0.015 x 0.1 Ns/m2 = 1.5 x 10-3 Ns/m2
We know that S = density of liquid/density of water
density of liquid = S x density of water
r  0.85 x 1000  850 kg/m3
Kinematic Viscosity ,
u  m/ r  1.5 x 10-3/850
 1.76 x 10-6 m2/s = 1.76 x 10-6 x 104cm2/s
= 1.76 x 10-2 stokes.

24. Example

25. Types of Fluids
• Ideal Fluid. A fluid, which is incompressible and is having no viscosity, is
known as an ideal fluid. Ideal fluid is only an imaginary fluid as all the
fluids, which exist, have some viscosity.
• Real fluid. A fluid, which possesses viscosity, is known as real fluid. All
the fluids: in actual practice, are real fluids.
• Newtonian Fluid. A real fluid, in which the shear stress is directly,
proportional to the rate of shear strain (or velocity gradient), is known as
a Newtonian fluid.
• Non-Newtonian fluid. A real fluid, in which shear stress is not
proportional to the rate of shear strain (or velocity gradient), known as a
Non-Newtonian fluid.

26. Types of Fluids
• Ideal Plastic Fluid. A fluid, in which shear stress is more than the yield
value and shear stress is proportional to the rate of shear strain (or
velocity gradient), is known as ideal plastic fluid.

27. Surface Tension
• Cohesion: “Attraction between molecules of same surface”
It enables a liquid to resist tensile stresses.
• Adhesion: “Attraction between molecules of different surface”
It enables to adhere to another body.
• “Surface Tension is the tensile force acting on the surface of
liquid in contact with air (gas) or between two immiscible (not
mixable) liquids”.
It is denoted by sigma.
Unit is N/m.

28. Metric to U.S. System Conversions,
Calculations, Equations, and Formulas
• Millimeters (mm) x 0.03937 = inches (")(in)
• Centimeters (cm) x 0.3937 = inches (")(in)
• Meters (m) x 39.37 = inches (")(in)
• Meters (m) x 3.281 = feet (')(ft)
• Meters (m) x 1.094 = yards (yds)
• Kilometers (km) x 0.62137 = miles (mi)
• Kilometers (km) x 3280.87 = feet (')(ft)
• Liters (l) x 0.2642 = gallons (U.S.)(gals)

29. Calculations, Equations & Formulas
 Gallons (gals) x 3.78 = liters (l)
 Cubic feet x 28.316 = liters (l)
 Pounds (P) x 0.4536 = kilograms (kg)
 Square inches x 6.452 = square centimeters
 Square feet x 0.0929 = square meters
 Square miles x 2.59 = square kilometers
 Acres x 4046.85 = square meters
 Cubic inches x 16.39 = cubic centimeters
 Cubic feet x 0.0283 = cubic meters

30. Assignment 1

31. Assignment 1(Continued)