This document discusses fluid mechanics concepts taught in a mechanical engineering course. It defines key terms like fluid, density, viscosity, vapor pressure, and cavitation. It explains that fluids continuously deform under shear stress, unlike solids, and discusses properties of ideal and real fluids. Surface tension and capillarity are also summarized. The document appears to be class notes for an introductory fluid mechanics course in mechanical engineering.

1. Department of Mechanical Engineering
Jahangirabad Institute of Technology, Barabanki-225203
Course- B. Tech
Branch- Mechanical Engineering
Subject- Fluid Mechanics
Subject Code-NCE-301
Author
Mr Md Ateeque Khan
Asst. Professor
MED 6/10/2017MAK(ME)

2. Basics of Fluid
 Fluid: A substance in the liquid or gas phase known as fluid.
 A solid can resist an applied shear stress by deforming.
 A fluid deforms continuously under the influence of a shear stress, no matter how
small shear stress applied.
 In solids, stress is proportional to strain, but in fluids, stress is proportional to
strain rate.
 When a constant shear force is applied, a solid eventually stops deforming at some
fixed strain angle, whereas a fluid never stops deforming and approaches a
constant rate of strain.
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3. Properties of Fluid
 Density or Mass Density: Density or mass density of fluid is defined as the ratio of
the mass of a fluid to its volume. Thus mass per unit volume of a fluid is called
density.
The value of density of water is
Or
fluidofVolume
fluidofMass

cmgm
3
/1
mkg
3
/1000
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4. Properties of Fluid
 Specific Weight or Weight Density: Weight per unit volume of a fluid is called
weight density.
fluidofVolume
fluidofMass
w
gw  
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5. Properties of Fluid
 Specific Gravity: Specific density is defined as the ratio of the weight density of a
fluid to the weight density of standard fluid. For liquid, water ia taken as standard
fluid and for gasses, air is taken as standard fluid.
 If the specific gravity of the fluid is known, the density of that fluid will be equal to
the specific gravity of fluid multiplied by the density of fluid.
 It has no unit.
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6. Properties of Fluid
 Viscosity: Viscosity is defined as the property of the fluid which offers resistance to
the movement of one layer of fluid over another adjacent layer of the fluid.
Mathematically,
µ is known as coefficient of dynamic viscosity.
Poise is another unit of dynamic viscosity in
CGS system.
dy
du
 
m
Ns
2
viscosityofunitSI 
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7. Properties of Fluid
 Kinematic Viscosity: It is defined as the ration between dynamic viscosity and
density of fluid.
 Unit of kinematic Viscosity is meter squire per second.
 In CGS system unit is STOKE.


 
Density
Viscosity
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8. Types of Fluid
 Ideal Fluid
 Real Fluid
 Newtonian Fluid
 Non-Newtonian Fluid
 Ideal Plastic Fluid
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9. Surface Tension
 The cohesive forces between liquid molecules are responsible for the phenomenon known as surface tension. The molecules at
the surface do not have other like molecules on all sides of them and consequently they cohere more strongly to those directly
associated with them on the surface. This forms a surface "film" which makes it more difficult to move an object through the
surface than to move it when it is completely submersed.
 Unit of surface tension in SI is N/m.
 Surface tension on liquid droplet:
 Surface Tension on Hollow Bubble:
 Surface Tension on liquid jet:
d
p
4

d
p
8

d
p
2
 6/10/2017MAK(ME)

10. Capillarity
 Capillary action, or capillarity, is the ability of a liquid to flow in narrow spaces without the assistance of, and in opposition
to, external forces like gravity.
 The effect can be seen in the drawing-up of liquids between the hairs of a paint-brush, in a thin tube, in porous materials such
as paper, in some non-porous materials such as liquified carbon fiber, and in a cell.
 It occurs because of intermolecular attractive forces between the liquid and solid surrounding surfaces.
 If the diameter of the tube is sufficiently small, then the combination of surface tension (which is caused by cohesion within
the liquid) and adhesive forces between the liquid and the container act to lift the liquid.
 With some pairs of materials, such as mercury and glass (see ), the intermolecular forces within the liquid exceed those
between the solid and the liquid, so a convex meniscus forms, and capillary action works in reverse.
Capillarity rise in clean glass full of water:
Capillarity fall for mercury: gd
h

4

gd
h

 cos4
 6/10/2017MAK(ME)

11. Vapor Pressure
 The process of evaporation in a closed container will proceed until there are as many molecules returning to the
liquid as there are escaping. At this point the vapor is said to be saturated, and the pressure of that vapor (usually
expressed in mmHg) is called the saturated vapor pressure.
 Since the molecular kinetic energy is greater at higher temperature, more molecules can escape the surface and
the saturated vapor pressure is correspondingly higher. If the liquid is open to the air, then the vapor pressure is
seen as a partial pressure along with the other constituents of the air. The temperature at which the vapor
pressure is equal to the atmospheric pressure is called the boiling point.
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12. Cavitation
 Cavitation is the formation and abrupt collapse of vapour-filled bubbles. This
process takes place at points inside the pump where the pressure falls below the
vapour pressure of the pumped medium. The vapour pressure of a liquid is the
pressure at which the liquid begins to boil or evaporate. Cavitation, which may
damage the pump, occurs when the net positive suction head (NPSHR) needed by
the pump is not available.
 In order to avoid cavitation, a minimum pressure referred to as the net positive suction
head available (NPSHA) must be present at the suction port so that the liquid does not boil
or evaporate. It must be ensured that the pressure applied at the suction port is always
greater than the vapour pressure of the liquid at a given temperature of the medium
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