2. Introduction of fluid
• 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 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 free surface is formed as a boundary between a
liquid and a gas above it.
3. • Solid :When external forces stress and deform solids,
the solid will regain their original shape when these
external forces are removed [Elasticity].
• Fluid : it will continue to change shape in time even
after the removal of the external forces causing the
deformation.
• Ideal fluid: is a substance that is unable to resist
internal shear and tensile forces.
• Fluids can be classified into 2 forms of matter
• 1) Liquids
• 2) Gases
4. PROPERTIES OF FLUID
Adhesion is the property of different molecules or surfaces to
cling to each other. For example, solids have high cohesive
properties so they do not stick to the surfaces they come in
contact.
Cohesion is the property of like molecules (of the same
substance) to stick to each other due to mutual attraction.
a. Adhesive Forces = polar liquid has intermolecular
forces with polar surface.
b. Cohesive Forces = intermolecular forces of the liquid
for itself
Applications
Adhesion is used for the proper functioning of glue, paint, tar,
cement, ink etc. Adhesive and cohesive forces together induce
capillary action which is the principle used in lamp wicks.
Synthetic fibers use wicking to remove sweat from the skin.
5. Ideal Fluid :
-is unable to resist F
-the relative velocity would remain constant, even F is
removed
Real Fluid :
-is able to resist F due to
1) internal molecular activity
2) friction between the layers
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6. VISCOSITY
Viscosity: It is the property of the fluid by virtue of
which it offers resistance to shear or angular
deformation. Viscosity is a measure of a fluid's
resistance to flow. It describes the internal friction of a
moving fluid. A fluid with large viscosity resists motion
because its molecular makeup gives it a lot of internal
friction. A fluid with low viscosity flows easily because
its molecular makeup results in very little friction when
it is in motion.
Mollases,tar, glycerine are highly viscous fluids.
Water,air,petrol have very small viscosity and are called
thin fluids
7. Newton’s Law of Viscosity
a liquid’s resistance to flow
= (du/dy)
Where, du/dy =velocity gradient
= coefficient of viscosity, absolute viscosity or dynamic
viscosity.
Viscous fluids tend to cling to a solid surface.
• Syrup and honey are more viscous than water.
• Grease is more viscous than engine oils.
• Liquids are more viscous than gases.
• Lava is an example of a very viscous material
8. Viscosity is the measure of a fluid’s resistance to internal shear
stresses.
Dynamic Viscosity ()
= Dynamic viscosity, N.s/m2 or Pa.s
= Internal shear stress, N ( =F/A)
v = Velocity, m/s
= h
v
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9. Let
-A = the horizontal area of each
layer
-d = the vertical distance
between their centerlines
-F = internal shear force
A thin layer of fluid
The top layer is acted upon by F
The top layer will move with a
velocity, v relative to the bottom
layer
9
Fig.:-1
10. The kinematic viscosity of a fluid is the ratio of its dynamic viscosity
to its density.
Kinematic Viscosity ()
= kinematic viscosity, m2/s
= Dynamic viscosity, N.s/m2 or Pa.s
= Density of fluid, kg/m3
=
10
11. The specific weight is the weight of the fluid per unit volume
= Specific weight, N/m3
W = Weight of fluid, N (W=mg)
V = Unit volume, m3
= W
V
The specific weight of water at 40C = 9.81 kN/m3
Specific Weight ()
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12. SPECIFIC VOLUME
Specific Volume is the inverse of the mass density.
v= V/m
or v=1/
The concept of specific volume is found to be more useful in the
study of flow of compressible fluids i.e gases
Specific Volume: Represents the volume per unit mass of fluid.
13. The specific gravity is the ratio of the density or specific weight of the
fluid to the density or specific weight of water, at a temperature of 4oC
Specific Gravity/Relative Gravity (S)
S = w
=
w
Dimensionless
Specific Gravity
The ratio of the density of any substance to the density of some other sub
stance taken as standard, waterbeing the standard for liquids and solids, a
nd hydrogen or air being the standard for gases.
13
14. Capillary action
Capillary action is the ability of a fluid to flow in narrow
spaces without the assistance of, and in opposition to,
external forces like gravity.
14Fig.:-2
15. Capillary action
• Capillary Action = spontaneous rising of a liquid up a narrow tube
15
For a contact angle less than 90o, the liquid will rise in
the tube, but the liquid can also fall if the contact angle
is greater than 90o. If the liquid is water, solids with a
contact angle less than 90o are called hydrophilic, the
opposite is hydrophobic.
16. APPLICATION OF CAPILLARY ACTION
• Capillary action is found in thermometer where fluid used in
it automatically rises when comes in contact with higher
temperature or falls down with lower ones.
• Capillary action can be performed to transfer fluid from one
vessel to another on its own.
• Capillary action can be experienced in the half dipped cloth
as well as on lantern
16
17. Compressibility
• : to compress something into a small space.
Liquids
• Liquids are considered to be incompressible.
• Liquids only change in volume even when
subjected to very high pressure.
Gases
• Gases are very compressible.
• Their volume can increase/decrease when
subjected to slight variation in pressure.
17
19. CONTENT REFERENCES
A TEXT BOOK OF FLUID MECHANICS AND HYDRAULIC
MACHINES , Dr. R.K.BANSAL , LAXMI PUBLICATION
FLUID MECHANICS AND FLUID POWER ENGINEERING ,
Dr. D.S.KUMAR , S.K.KATARIA & SONS
A TEXT BOOK OF HYDRAULICS,FLUID MECHANICS AND
HYDRAULIC MACHINES , R.S.KHURMI , S.CHAND
PUBLISHING
