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INTERVIEW QUESTIONS BOOK > Fluid machenics > Flashcards

Fluid machenics Flashcards

1
Q

What are the laws affecting fluid motion?

(ESE 2009

A

(a) The principle of conservation of mass.
(b) Newton’s laws of motion and viscosity.
(c) The first and second laws of thermodynamics.

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2
Q

Why mercury is preferred for use in barometer

and in other pressure measuring devices?

A

The low vapour pressure along with its high

density.

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3
Q

1.25 Define kinematic viscosity and ~ .. ‘

viscosity.

A

Kinematic V1scos1ty is the mea . su~ ~ ~e
resist1ve flow of a fluid under the .
1
nfl . uence of
gravity. It may also be defined as the ab
1
t . f fl . sou e viscosity o a Uld divided by its denslt .
Absolute viscosity is the resistance t~ flow
encountered when one layer or plane of fluid
attempts to move over another identical layer
of plane of fluid at a given speed, Absolute
viscosity is also called dynamic viscosity

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4
Q

Differentiate between solid and liquid?

(ESE 2012

A

Solid, which is a state of matter that has a fixed
volume and shape with atoms that are bound
tightly to each other that makes It resistant to
change. They can only change if they are cut or
broken by force,
Liquid,which is another state of matter that .has
no shape and takes the shape of the conta1ner
that holds it.’ Atoms in a liquid, although also
bound together but are temporary thus allowing
them to move freely and flow.

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5
Q

What is hydraulic engineering?

A

Hydraulic engineering is a sub-discipline of civil
engineering and is concerned with the flow and
conveyance of fluids, principally water and
sewage. One feature of these systems is the
extensive use of gravity as the motive force to
cause the movement of the fluids. This area of
civil engineering is intimately related to the design
of bridges, dams, channels, canals, and levees
and to both sanitary and environmental
engineering

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6
Q

What is viscosity?

ESE 2016

A

It is the property of fluid by virtue of which it
offers resistance to the movement of one layer
to adjacent layer. It occurs mainly due to
cohesion between the fluid particles and the
exchange of momentum between the fluid
layers.
It is expressed as shear stress per unit rate of
change of shear strain i.e.
‘t
J.l = dv!dy
Its Sl unit is N.s/m2 or Pa.s.

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7
Q

How viscosity varies with velocity?

ESE 2016

A

Viscosity is the property of a fluid which is
independent of fluid velocity. However it
depends on the fluid temperature.

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8
Q

Explain various properties of fluid?

ESE 2016

A

The various properties of fluid are as under:
Mass density: It is the mass of fluid per unit
its volume i.e.
Mass density {p) = Mass of the fluid (M) I
Volume of the fluid (V).
Its Sl unit is kglm3.
Specific weight: It is the weight of fluid per
unit volume of the fluid i.e.
Specific weight (w) = Weight of the fluid (W) I
Volume of the fluid (V)
Its Sl unit is Nlm3.
Specific volume: It is the volume of the fluid
per unit weight of the fluid i.e.
Specific volume = Volume of fluid (V)I Weight
of the fluid (W)
Its Sl unit is m3IN.
Viscosity: It is the property of fluid by virtue of
which it offers resistance to the movement of
one layer to adjacent layer. It occurs mainly due
to cohesion between the fluid particles and the
exchange of momentum between the fluid
layers.
It is expressed as shear stress per unit rate of
change of shear strain i.e.
‘t
11 = dv! dy
Its Sl unit is N.slm2 or Pa.s.
Surface tension: It is the property of fluid film
by virtue of which it exerts tension and Is
expressed as,
Surface tension (a)= Force (F)Ilength(L)
Its Sl unit is N/m.

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9
Q

What Is Pascal’s law?

ESE 2012

A

According to Pascal’s law the intensity of
pressure at any point in a static fluid is same In
all directions.

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10
Q

What Is stable equilibrium?

A

If centre of buoyancy lies directly above the

centre of gravity.

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11
Q

What do you understand by, stagnation

pressure and dynamic pressure?

A

Stagnation pressure: The point where the fluid
is at rest is called as stagnation point (like in
Pitot’s tube) and the pressure at that point is
called as stagnation pressure.
Dynamic pressure: The pressure anywhere in
the flowing fluid is called as dynamic pressure.

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12
Q

What is velocity potential function?

A

It is a scalar function of space and time, its
negative derivative with respect to any direction
gives the fluid velocity in that direction.

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13
Q

What Is convective acceleration?

A

It is the rate of change of velocity due to the
change of position of fluid particles in a fluid
flow.

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14
Q

How can you say that the fluid flow is

irrotational?

A

If velocity potential exists, the flow should be
irrotational or if stream function satisfies the
Laplace equation, then also the fluid flow is
irrotational

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15
Q

What is a stream function?

A

It is that function whose partial derivative with
respect to any direction gives the velocity
component at right angles to that direction.

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16
Q

What is flow net?

A

A grid obtained by drawing a series of
equipotential lines and stream lines is called
flow net

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17
Q

What is streamlined and bluff body?

A

A body whose surface coincides with the
streamlines, when the body is placed in a ftow
is called streamlined body. If the surface of the
body does not coincide with the stream lines,
the body is called bluff body.

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18
Q

Is the flownet analysis applicable to

rotational flow? If not, why?

A

Flow net analysis is applicable to rotational ftow
as well however if the flow is irrotational then
the problem can be solved graphically. Flow
net is very useful in analyzing two dimensional
irrotational flow problems.

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19
Q

A cylindrical tank half filled with a liquid is
rotated about its vertical axis with a certain
constant speed. If the liquid at the sides
reaches at the top of the tank, what would
be the level of water at the axis of rotation?

A

In this case, the depth of water at the middle
on the vertical axis of rotation is equal to twice
the rise of liquid on the sides

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20
Q

What is Euler’s equation of motion?

ESE 2012

A

Euler’s equation of motion considers only two
forces, namely the pressure force and the fluid
weight or the body force.

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21
Q

How can you determine the flow rate?

ESE 2008

A

The numerical difference in stream function
between two streamlines is equal to the flow
rate per unit width passing between them.

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22
Q

What is forced vortex?

ESE 2016

A

When a fluid is rotated about a vertical axis at
constant speed, every particle of it has the same
angular velocity and the fluid moves as if it were
a solid. This type of motion is known as forced
vortex.

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23
Q

What is continuity equation?

ESE 2012

A

Continuity equation is a mathematical
expression of conservation of mass . This
equation is based on the principle that the mass
of any matter can neither be created nor be
destroyed.

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24
Q

What are assumptions of continuity
equation?
(ESE 2016)

A

In continuity equation, It is assumed that flow
rate is constant. It Is based on principle of
conservation of mass I.e. mass can neither be
created nor be destroyed.

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25
Q

What Is free vortex?

ESE 2016

A

A free vortex is characterized by velocity which
YClries invensely with the distance from the axis.
e.g. Flow in a wash basin.

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26
Q

What is uniform and non-uniform flow?

ESE 2017

A

Flow is said to be uniform, when the velocity of
flow does not change either in magnitude or in
direction at any point in a flowing fluid, for a
given time.
Flow is said to be non-uniform, when there is a
change in velocity of the flow at different points
in a flowing fluid, for a given time.

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27
Q

What Is steady and unsteady flow?

ESE 2017

A

A flow that is not a function of time is called
steady flow. Steady-state flow refers to the
condition where the fluid properties at a point in
the system do not change over time. Time
dependent flow is known as unsteady and also
called transient.

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28
Q

What are the various methods to draw flow

nets?

A
  • Hydraulic models
  • Analytical method
  • Electrical Analogy method
  • Graphical method
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29
Q

What type of flow exists in rivers?

ESE 2017

A

Normally unsteady flow exists in rivers.

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30
Q

What Is the D’ Alembert’s paradox

A

The steady flow of an Ideal fluid around
immersed bodies leads to zero resistance, except In those cases where vortex trail exists
in the works of the body.

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31
Q

How the ogee spillway profile determined?

A

For designed head it follows the profile of the

lower nappe.

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32
Q

What are the limitations of the Bernoulli’s

equation?

A

Limitations of Bernoulli’s equation:
(a) It is applicable for steady flows and thus
cannot be applied during transients, start
ups and shut down periods.
(b) Velocity of liquid particle at the pipe centre
is maximum and it decreases as distance
from the pipe centre line increases. Thus
only a mean velocity is taken into account
and not the actual velocity of individual
fluid particles.
(c) External forces acting on the liquid are
always neglected but this is not the case
at all the times. External forces do act on
the liquid.
(d) In turbulent flow, some kinetic energy is
converted into heat energy and in viscous
flow, some energy is lost due to shear
stresses. All such losses are neglected in
Bernoulli’s equations.
(e) If the liquid is flowing through a curved
path then energy due to centrifugal forces
should also be taken into account.
(f) This equation was derived from a force
balance on a particle moving along a
streamline. Thus it is not applicable where
shaft work is involved like in turbines,
pumps, fans, impellers etc.
(g) Another approximation made in the
Bernoulli’s equation was that density of
fluid remains constant. This condition can
be satisfied by liquids but not gases.

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33
Q

How Is the Bernoulli’s equation related to the

first law of thermodynamics?

A

Both Bernoulli’s equation and the first law of
thermodynamics are the alternate forms of
principle of conservation of energy.

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34
Q

Explain how the modern rockets are
launched? How is the tremendous thrust
required to send it into a pre-determined
trajectory achieved?

A

A rocket is a machine that develops thrust by
rapid expulsion of matter. A rocket engine differs
from other engines in the sense that it carries
its fuel and oxidizer internally which will be burnt
in vacuum of space as well as in earth’s
atmosphere. The thrust generated by the first
stage must be greater than the complete weight
of the launch vehicle while standing on a launch
pad. Once moving upwards, thrust continues
to be generated to accelerate the launch vehicle
against the force of gravity. Additional thrust
must be generated so that the satellite can be
placed in orbit.
Propellant of the rocket engine is either liquid
or solid or hybrid. Propellant generally consists
of a fuel and an oxidizer. Liquid propellant rocket
engines burn two separately stored liquids viz.
a fuel and an oxidizer, to produce the thrust.
Typical fuel-oxidizer combinations are: liquid
hydrogen-liquid oxygen, kerosene-liqui.d
oxygen, aniline-nitric acid, hydrazine which 15
a monopropellant

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35
Q

What are the various methods for measuring
flow in pipelines? Discuss their advantages
and disadvantages

A

Various methods of measuring flow in pipes are:
(a) Pi tot tube: It can be used in both channels
and conduits. These tend to get clogged
unless the flowing fluid is very clean.
(b) Differential producers: This class of flow
measurement device is used for full pipe
flow. They cause a pressure differential
which can be measured and correlated to
velocity to give flow rate.
Example: Venturimeter, Nozzles, Orifices

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36
Q

Under what conditions will a flow through reentrant mouthpiece occur? What is Borda’s
mouthpiece?

A

An internal mouthpiece extending into the fluid
inside the vessel is called as Borda’s
mouthpiece or re-entrant mouthpiece. Two types
of flow occur in this viz. mouthpiece running
full and mouthpiece running free. In free
running mouthpiece, the length of the
mouthpiece is small and thus the jet after
converging at vena contracta cannot expand
to fill the mouthpiece to the full before it
emerges from the mouthpiece. In mouthpiece
running full, the length of mouthpiece is large
so that the jet expands after contracting at vena
contracta and expands to fill the mouthpiece
completely

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37
Q

Will the discharge get affected by changing

the orientation of the venturlmeter?

A

Discharge does not depend upon the orientation
of the venturimeter. It may be kept horizontal,
Inclined at any angle or even vertical without
affecting the discharge.

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38
Q

What are the factors influencing the
co-efficient of discharge?
{ESE 2009)

A

Inside roughness, Reynold’s number, diameter
ratio, placement of pipe fittings such as bends,
valves etc.

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39
Q

Which device gives more accurate value for
measurement of discharge: Orificemeter or
Venturimeter? Give reason?
{ESE 2006)

A

Venturimeter gives more precise discharge
measurement as compared to orifice meter. In
venturimeter, the tube smoothly converges and
then diverges with minimum loss of energy. In
orifice meter, a sharp edged plate is fitted by
clamps and is put in the flow field, pressure
drop occurs with substantial loss of energy.

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40
Q

How can you derive Bernoulli’s equation?

ESE 2012

A

Bernoulli’s equation can be derived by
integration of Euler’s equation of motion and by
the principle of conservation of energy.

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41
Q

What are the applications of Bernoulli’s
equation?
{ESE 2012)

A

Bernoulli’s equation combined with continuity
equation can be used in very wide range of fluid
problems like flow measuring devices (venturi
meter, orifice meter, pitot tube ), free liquid jet
etc.

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42
Q

what is bernoulli’s principle

A

Bernoulli’s principle Is In reot anothm torm ot
law of conservation of energy. It states rr1ot riQ,
(or fall) In pressure of flowing fluid must bo
accompanied by a fall (or rlso) in tho fllJid
velocity. Thus an Increase (or decreaoo) in tho
fluid velocity causes a decrease (or lnr:roaeo)
in the fluid pressure and vice versa. It lo
expressed as:

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43
Q

State Naiver Stoke’s equation and type of
forces involved in lt.
(ESE 2016)

A

In Navier-Stoke’s equation, the forces that are
considered in fluid flow are the gravitational
force (F9
) , viscous force (F) and the pressure
force (F). Thus as per Navier- Stoke’s equation, p
Ma= F + F + F g v p
Navier-Stoke’s equations are useful for analysis
of viscous flow

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44
Q

Enumerate distinguishing characteristics of

laminar flow.

A

Characteristics of laminar flow
(a) In laminar flow, the fluid particles move
along straight paths in layers or laminas
such that the path of individual particles
do not cross each-other.
(b) This type of flow occurs at low velocity so
that viscous forces predominate over
Inertial forces.
(c) Due to dominating viscous forces, shear
stress occurs which is maximum at the
boundary and decreases as distance frorn
the boundary Increases.

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45
Q

What Is velocity defect?

A

It is the difference between the maximum

velocity and local velocity at any point.

46
Q

How shear stress varies In the laminar sublayer?

A

Shear stress In laminar sublayer Is constant and

Is equal to boundary shear stress

47
Q

What is a siphon and under what conditions

would it stop functioning?

A

Siphon: Siphon is a long bent pipe used to
carry water from one reservoir at higher elevation
to another reservoir at lower elevation when the
two reservoirs are separated by a hili or a high
level ground.
When HGL is above the pipe centre line, then
pressure is above atmospheric pressure and if
HGL Is below pipe centre line then pressure is
below atmospheric pressure i.e. negative gauge
pressure. Thus, for portion of the siphon above
C and D, the pressure is below atmospheric
pressure, lowest pressure being at summit. Now
zero absolute pressure implies - 10.3 m of water (since atmosphere pressure is 10.3 m of water).
If pressure is reduced to 2.5 m of water absoiut
or 7.8 m of water vacuum, then air will come ou~ of water and will collect at summit to form an a· • • 1r
lock which hinders the continUity of flow and
flow ceases

48
Q

What is energy gradient?

A

It is the slope of total energy line and it is
expressed as sine of the angle which the line
makes with the horizontal.

49
Q

What causes head loss in pipes?

{ESE 2006)

A

Causes of head loss in pipes can be classified
as:
(a) Major losses: Frictional loss due to
roughness of flow boundary.
(b) Minor losses: It includes head loss due to
pipe bends, sudden change in pipe cross·
section values etc.

50
Q

How the value of a varies for laminar and

turbulent flow?

A

For Laminar flow being the purely viscous flow,
the value of ex is maximum and equals to 2. For
turbulent flow. its value may be taken to vary
from 1.01 to 1.15 depending upon the surface
roughness and the Reynolds number. The lower
value is applicable to relatively rough surface
and high Reynolds number.

51
Q

What is hydrant?

ESE 2013

A

Hydrant is an outlet from a fluid main often
consisting of an upright pipe with a valve
attached, from which fluid (like water, fuel etc.)
can be tapped.

52
Q

What Is the Re of laminar flow in pipe flow
and open channel flow?
(ESE 2014)

A

If R9 < 2000- laminar in pipe flow.

R9 < 500 -laminar in open channel flow.

53
Q

Describe the factor which affect the
transition of flow from laminar to turbulent.
(ESE 2016)

A

Although in the transition zone from laminar to
turbulent flow, there is no definite relationship
between the friction factor ‘f’ and Reynold’s
number but the transition is believed to depend
on flow velocity, pipe diameter, relative
roughness and viscosity of the fluid.

54
Q

What factors influence the establishment of
fully developed flow? What do you
understand by entrance length?
(ESE 201’6)

A

The following factors influence the establishment
of fully developed flow in a pipe:
(a) Type of flow i.e. laminar or turbulent.
(b) Type of entrance i.e. sharp or smooth.
(c) Surface roughness characteristics of the
pipe material.
(d) Other factors affecting the entrance of flow
in the pipe.
Entrance Length: The length of the pipe from
its entrance along which the velocity profile
changes continuously, up to the point where it
attains fully developed velocity profile and
beyond which it remains unaltered for the
remaining length is called as entrance length.

55
Q

What is the •stalling angle” and what
happens when the angle of attack Is
Increased further?

A

Stalling Angle: When angle of attack of an airfoil
increases to about 1 oo then lift coefficient
attains a maximum value which decreases with
further increase in the angle of attack. This point
corresponding to maximum lift coefficient is
called as stall angle and the angle of attack
corresponding to this point is called as stalling
angle

56
Q

Give reasons for the following observed
phenomena:
{a) Telephone wires produce a humming
noise {singing) in wind.
{b) Air planes generally take off and land
against wind.
(c) Drag on a submerged body in an ideal
fluid under steady flow conditions is zero
whereas in real fluid flow the drag force
is significant.
(d) A pitted golf ball moves faster than a
smooth ball of similar size and weight.
(e) For the same given area, a square plate
experiences a less drag than a
rectangular one when placed normal to
the flow.
(f) High winds tend to lift roofs of buildings
than to press them down

A

(a) When wind pushes past the telephone wire,
it c;:reates a low pressure zone on the downward side of the wire. Air rushes into
this low pressure zone, resulting in
collisions between pressure fronts which
creates vibrations In the air which is
perceived as sound.
{b) Airplanes fly due to lifting force generated
by the wings. The wings do this by moving
forward through the air. Faster the wings
move through the air, greater the lift it will
get. Wings flying against the wind, move
through it more quickly without increasing
the speed with respect to the ground. Thus
aircraft flying against the wind get into the
air more quickly as compared to ground.
For instance, if an aircraft needs 120km/hr
of speed in order to generate enough lift
and if wind is blowing at 20 km/hr, then
ground speed needed for the aircraft will
be 1 OOkm/hr and less runway length is
required.
While landing against the wind, the speed
necessary to generate enough lift to stay
afloat is small compared to landing in the
direction of wind.
(c) In ideal fluid, it is assumed that the fluid
does not have any viscosity but drag on a
body occurs due to formation of boundary
layer. Boundary layer formation takes place
with real fluids and not the ideal flujds.
(d) When a body moves through the air, drag
comes into play. Drag is of two types viz.
pressure drag and the skin friction drag.
Skin friction drag depends largely on the
pattern of airflow around the body. If flow
is smooth or laminar, it will have lower skin
friction but less able to stay attached to
the rear surface of ball. A turbulent
boundary layer gives more skin friction and
is better able to stay attached to the back
of ball. By pitting the golf ball. boundary
layer is forced to change from laminar to
turbulent. Greater the mixing of air in the
turbulent boundary layer, greater it will allow
the passing air to remain clanged to the
flying ball to a little bit longer before it
separates, which in turn narrows the ball’s
wake. A smaller wake means less air

57
Q

How does circulation originate around an the turbulent flow.

airfoil?

A

Circulation around an airfoil: When a uniform
stream of a real fluid flows past an airfoil, then
initially the flow pattern is same as that of an
ideal fluid flowing past an airfoil i.e. an
irrotational flow takes place. But this irrotational
flow cannot exist too long. The fluid which flows
along the lower surface of the airfoil is required
to have a turn around the sharp trailing edge of
the airfoil to reach the rear stagnation point which
is located on the upper surface of airfoil slightly
upstream of its trailing edge.
But immediately after the commencement of
flow due to boundary layer development, the
retarded fluid of the lower surface boundary
layer has no enough energy to take such a
sharp turn at the trailing edge. Thus boundary
layer separates from the lower surface of the
airfoil at the trailing edge and on the upper
surface; flow is induced from the stagnation
point towards the trailing edge. This flow is in
the opposite direction to that of an ideal fluid
which results in the formation of eddy called as
starting vortex on the upper surface in the region
of trailing edge. The starting vortex has some
strength of circulation associated with it. It is
quite unstable. Thus starting cortex is rapidly
washed away from the trailing edge of the airfoil
but it generates an equal and opposite reaction
in the form of circulation around the airfoil. This
circulation produced around the airfoil as a
reaction to the starting vortex brings the rear
stagnation point near to the trailing edge.

58
Q

Why is the drag of a hollow hemisphere
greater when held with the concave side
upstream than when held with the convex
side upstream?

A

Hemisphere when kept with concave side
upstream offers more drag as there is large
pressure difference created on the upstream
and downstream faces as compared to when
the hemisphere is placed with convex side
upstream.

59
Q

What do you understand by ‘no slip’

condition?

A

No SliP Condition: When a real fluid flows over
a boundary, the layer of fluid in contact with the
boundary comes to a complete relative halt due
to viscosity of the real fluid. This layer of fluid
cannot slip away from the boundary and attains
~he same velocity as that of the boundary. ThiS
IS called as “no-slip” condition .

60
Q

What will be the flow characteristics outside
the boundary layer and within the boundary
layer?

A

Row outside the boundary layer has irrotatlonal
chara?teristi~s while .that within the boundary
layer .1s rotat1?nal. W1thin the boundary layer,
veloc1ty grad1ent exists.

61
Q

What is momentum thickness?

A

It is defined as the distance measured
perpendicular to the boundary surface by which
boundary should be displaced to compensate
for the reduction in momentum on account of
boundary layer formation.

62
Q

What Is displacement thickness?

A

The distance measured perpendicular to the
boundary of the solid body, by which the
boundary should be displaced to compensate
for the reduction in flow rate on account of
boundary layer formation.

63
Q

‘How can you say that the boundary layer

has separated?

A

lf the pressure gradient is positive, the
boundary layer separates from the surface and
backflow and eddies formation take place due
to which a great loss of energy occur.

64
Q

What are the methods of preventing the

separation of boundary layer?

A

(a) Providing guide blades in a bend.
(b) Rotating boundary in the direction of flow.
(c) Supplying additional energy from a blower

65
Q

What Is boundary layer theory?
Ans.
(ESE 2016)

A

In physics and fluid mechanics, a boundary layer is the layer of fluid in the immediate vicinity of a bounding surface where the effects of viscosity are significant. The liquid or gas in the boundary layer tends to cling to the surface.

66
Q

Why do pipes behave as hydrodynamically
smooth or rough when turbulent flow takes
place through them?

A

The head loss in pipes depend on friction factor
‘f’. For a fully developed turbulent flow, the
friction factor T depends on Reynold’s number
{R6 ) or ratio of pipe roughness height to pipe
diameter (kiD) or both depending upon whether
the pipe boundary is hydro dynamically smooth
or rough or it is in transition.

67
Q

In what way does the flow through a rough

pipe differ from that in a smooth pipe?

A

In smooth pipes, the friction factor for
computing the pipe head loss depends on
Reynold’s number while for rough pipes, it
depends on relative roughness.

68
Q

What do you understand by Intensity of

turbulence and Isotropic turbulence?

A

Intensity of turbulence: It is defined as the
ratio of root mean square of velocity fluctuations
to the mean flow velocity.
Isotropic turbulence: A turbulence is said to
be isotropic if its statistical characteristics have
no correlation to any direction. In an isotropic
turbulent flow, no average shear stress can
occur and thus there are no gradients in the
mean fluid velocity. The viscosity effects result
in conversion of kinetic energy of flow Into heat.

69
Q

How the loss of pressure head Is related to
mean velocity of flow In laminar and turbulent
flow?

A

Loss of pressure head oc mean velocity
4 laminar flow
Loss of pressure head oc (mean velocity)^2
4 Turbulent flow

70
Q

What Is advantage of Buckingham method

over Rayleigh’s method?

A

Buckingham method helps in letting us know,
in advance of the analysis, as to how many
dimensionless groups are to be expected.

71
Q

What are applications of dimensional

analysis?

A

It finds Its applications In nearly all fields of
engineering and Is most extensively used in fluid
mechanics and heat transfer.

72
Q

How do you calculate the number of

dimensionless groups?

A

The number of dimensionless products in a
complete set is equal to the total number of
variable minus the maximum number of these
variables that will not form a dimensionless
product.

73
Q

Where Is Reynold’s model law applicable?

A

It Is applicable to:
(a) Pipe flow.
(b) Resistance experienced by submarines.
airplanes, fully Immersed bodies etc,

74
Q

9 Where Is Froude’s model law applicable

A 
It is applicable when gravity force is
predominant like:
(a} Free surface flow such as flow over
spillways, weirs etc.
(b} Flow of jet from an orifice.
(c) Where fluids of different densities flow over
one another.
(d) Where waves are likely to be formed on
surface.
75
Q

Where Is Reynold’s model law applicable?

A

It Is applicable to:
(a) Pipe flow.
(b) Resistance experienced by submarines.
airplanes, fully Immersed bodies etc

76
Q

When ls Mach model law applicable?

A

Applicable where elastic force is predominant:

(a) Flow of aeroplane at supersonic speed.
(b) Aerodynamic testing
(c) Under water testing of torpedoes
(d) Water hammer problems

77
Q

What is Weber model law’s applicability?

A

Applicable when surface tension is predominant
like:
(a) Capillary rise
(b) Flow over weirs for small heads

78
Q

What Is dimensional homogeneity?

{ESE 2007

A

An equation is said to be dimensionally
homogeneous If the form of the equation does
not depend upon the units of measurement

79
Q

What is the effect of sudden expansion of

pipe?

A

Due to sudden expansion in pipe, eddies are
generated In the corner space developed
between the junction of small and large pipe.
This lead to loss of head in the pipe flow.

80
Q

What are the criteria for the flow in the model
and prototype to be dynamically similar?
(ESE 2009)

A

When the two are geometrically similar,
kinematically similar and the forces at similar
points in the two systems have the same ratio
throughout the flow field.

81
Q

What Is model analysis?

ESE 2007

A

It is an experimental method of finding solutions
of complex flow problems. A model is a small
scale replica of the actual machine or
structure. The actual machine or structure is called
prototype.

82
Q

What are undistorted and distorted models?

ESE 2017

A

Undistorted model: Those models which are
geometrically similar to their prototypes.
Distorted model: Geometrically not similar to
their prototype.

83
Q

What is Reynold’s number, Froude number,
Euler number, Mach number and Weber
number?
(ESE 2016)

A

(a) Reynold’s number: It is ratio of inertia force
and viscous force.
(b) Froude number: Ratio of square root of
inertia force and gravity force.
(c) Euler number: Ratio of square root of inertia
force and pressure force.
(d) Mach number: Ratio of square root of
inertia force and elastic force.
(e) Weber number: Ratio of square root of
inertia force of a flowing fluid to the surface
tension force.

84
Q

What are model laws or laws of similarity?

ESE 2017

A

The laws on which models are designed for

dynamic similarity are called model laws.

85
Q

What are undistorted and distorted models?

ESE 2017

A

Undistorted model: Those models which are
geometrically similar to their prototypes.
Distorted model: Geometrically not similar to
their prototype.

86
Q

Can we apply the momentum equation at

sudden contraction point?

A

No, as streamlines in up stream are curved &
accelerated, so pressure cannot be determined
there.

87
Q

How many types of similarities are there
between model and prototype?
(ESE 2017)

A 
There are three types of similarities:
(a) Geometric (b) Kinematic (c) Dynamic.
Similarity of motion between model and
prototype. Similarity of forces between model
and prototype.
88
Q

What is a weir?

ESE 2012

A

A weir is a concrete or masonry structure built
across a river (or stream) in order to raise the
level of water on the upstream side to allow the
excess water to flow over its entire length to the
downstream side.

89
Q

What is the advantage of increasing the specific speed of Pelton wheel turbine?

A

For a given condition, Pelton turbines have a
‘~de. range o~ speed. There is a definite speed
yJeldmg . ma:<1mum efficiency. If the speed of
the turb1ne IS made higher then the specific
speed will increase

90
Q

What do you mean by forebay?

A

It is an enlarged section of a canal spread out
to accommodate the required width of intake:
• It is provide with intake structure, to direct
water into the penstocks.
• Intake should be provide with trash racks
so as to prevent the entry of debris into the
penstock and thus avoid the possible
damage to turbine runnes.
• Its function is store temporarily the water
rejected by the plant when the electrical
load Is reduced and also to meet the
Instantaneous increased demand of water
due to sudden increase load

91
Q

What is the purpose of guide vane?

A

Guide vane changes the angular movement of

water hitting the runner and controls discharge.

92
Q

What are the disadvantages of high specific

speed of Pelton wheel?

A

(a) The disadvantages of higher N is that it . s
necessitates muti-jets for which the
governing becomes complicated and more
expensive.
(b) The speed directly coupled generator will
increase. This means that small number of
pair of poles are required and hence the
generator will also be less costly.
(c) Material employed for high speed
machines will the costly, as high speed
cause great stresses in revolving parts.

93
Q

What do you mean by efficiency of draft

tube?

A

The efficiency of a draft tube is defined as the
ratio of actual conversion of kinetic head into
pressure head in the draft tube to the kinetic
head at the inlet of the draft tube.

94
Q

What are the hydraulic functions of spiral

casing and guide vanes?

A

Spiral Casing: The casing is designed to
reduce the velocity as the fluid leaves tt¥.3
impeller of a centrifugal pump. This decrease
in kinetic energy results in an increase in
pressure energy.
Guide Vanes: In reaction turbines, guide var:es
act as a nozzle to accelerate the flow and tum
the fluid in appropriate direction as fluid enters
the rotor. A part of pressure drop occurs across
the guide vanes.

95
Q

What is air vessel?

A

Air vessel is a closed chamber fitted on the
suction as well as on the delivery side, near the
pump cylinder, to reduce the accelerating head.
Function of vessel:

  1. Reduce the possibility of separation.
  2. Pump can be run at higher speed.
  3. Constant rate of discharge can be ensured.
  4. Save power required to drive the pump. For
    single acting cylinder pump percentage
    work saved is 84.8% and for double acting
    single cylinder the percentage work save
    is 39.2%.
96
Q

State the principle on which turbomachines

are based.

A

Turbomachine is any device that extracts
energy from or imparts energy in to a
continuously moving stream of fluid. e.g,
Windmills, water mills, pumps, compressors etc.
A turbomachine works on the principle of energy
transfer carried out by the action of one or more
rotating blade rows. The dynamic action of the

97
Q

Why should it be necessary for the relative
velocity vector at inlet to be tangential to the
blade tip?

A 
This is done so because the head (i.e. energy)
produced by the blade is proportional to the
tip speed (i.e. u) and the tangential velocity.
98
Q

Which type of turbine Is best suited to
conditions of changing loads on the turbine,
and why?

A

Pelton wheel is most suited for changing load

conditions

99
Q

Differentiate turbines on basis of head?

ESE 2007, 2016

A

Turbine classification on the basis of head:
On the basis of head, turbines are classified as:
(a) High head turbines which work under a
head ranging from several hundred meters
to few thousand meters. e.g. Pelton wheel.
(b) Medium head turbines which work under
a head of 60 to 250 m.e.g. Francis turbine.
(c) Low head turbines which work under a
head of lower than 60 m. e.g. Kaplan
turbine

100
Q

Under what conditions of flow and head
would you use axial flow, radial flow and
impulse turbines?
{ESE 2017)

A

Axial Flow Turbines: In this, water flows parallel
to the axis of rotation of the shaft. At the turbine
inlet, the total energy is in the form of pressure
energy and kinetic energy. During the flow
through turbine, a part of pressure energy gets
converted to kinetic energy. The turbine shaft
is vertical. Propeller and Kaplan turbines are
axial flow turbines. This is suitable where large
volume of water at low head is available.
Radial Flow Turbines: Here, the water flows in
a radial direction either from outwards to inwards
or inwards to outwards and accordingly the
turbine is called as inward flow radial turbine or
outward flow radial turbine respectively. The
~ate~ possesses both pressure energy and
k1net1c ene b . rgy afore entering into the turbine.
Impulse Turbines: For high heads (about more
than 250 ) . m and at relatively low flow, the
reaction turbine would require too high speed

101
Q

What is a Hydraulic Machine?

ESE 2017, 2012

A

Hydraulic machines are machinery and tools
that use liquid fluid power to do simple work.
Hydraulic fluid is transmitted throughout the
machine to various hydraulic motors and
hydraulic cylinders and becomes pressurized
according to the resistance present.

102
Q

Explain Cavitation.

ESE 2017, 2007

A

Cavitation is the formation of vapour bubbles
within a liquid at low-pressure regions that occur
in places where the liquid has been accelerated
to high velocities, as in the operation of
centrifugal pumps, water turbines, and marine
propellers

103
Q

What precautions are to be taken while

starting and closing the centrifugal pump?

A

Operating the centrifugal pump: First priming
is required keeping the discharge valve closed
and ensuring that no air is entrapped. As
pressure is built up, discharge valve is opened
gradually. Large size pumps are provided with
star-delta starter. Frequent start and stop can
damage the pump. The suction vessel must
be large enough to have sufficient inventory of
liquid being pumped. A minimum NPSH is
required to be maintained in the pump. NPSH
takes into account any potential head loss that
might occur between the pump’s suction nozzle
and impeller, thus ensuring that the pressure
does not drop below vapour pressure of the
liquid

104
Q

Why centrifugal pumps are less efficient as

compared to turbines?

A

Centrifugal pump is inherently less efficient than
turbine because the rotating impeller creates
forced vortex in the passage of pump and
efficiency depends on the extent to which the
velocity head is converted to pressure head.

105
Q

Under what conditions would you suggest
use of double-suction pump and a multistage
pump?

A

Double suction pump: In double suction
pump, the flow rate is distributed over several
impellers arranged on the pump shaft. Double
suction pump with impellers arranged in parallel
can be used to increase the discharge at
constant head.
Double suction pumps are used when the flow
rate required from the centrifugal pump
becomes too large for the inlet cross section of
one impeller or when the flow velocity in the
inlet cross-section in the first impeller has to
be reduced to avoid cavitation.
Multi-stage pump: Multi-stage pump is used
where liquid that is to be pumped is. required
to be pumped at very high pressure which is
achieved by the use of multiple impellers or
stages of the body of the pump

106
Q

Explain the working prlnclpfe of centrifugal

pump?

A

Working Principle of Centrifugal Pump: The
very first step is priming wherein the suction
pipe, pump casing and delivery pipe up to the
delivery valve are filled with water to expel any
air/gas and no air pocket is left inside the pump.
Necessity of priming of centrifugal pump Is
because of the fact that pressure generated In
the impeller of the centrifugal pump is
proportional to the density of fluid that is in
contact with the Impeller. When Impeller Is made
to rotate in presence of air then a negligible
pressure Is developed and no liquid will be lifted
by the pump.

107
Q

specific speed of pump

A

The specific speed of pump Is given by,
N _NJQ
s- H%
In case of double suction pump. the discharge
Q is half of the total flow rate of the pump.
For multi-stage pumps, the specific speed is
computed for the first stage only.
The temperature of the liquid affects the
viscosity of the liquid and thus as temperature
is increased, the viscosity decreases and thus
the height of pump installation can be increased
and the power saved in overcoming the viscous
effect allows to have increased height of pump
installation.

108
Q

What Is Hydraulic RAM?

ESE 2016

A

It Is a sort of pump in which the anergy of large
quantity of water falling through a small height
Is used to lift a small quantity of this water to a
greater height. No external power is require.d to
operate the pump.

109
Q

Rotodynamic pump?

A 
Rotodynamic pump: This type of pump
contams a rotating element called as
impeller through which, as the liquid
passes its angular momentum changes
due to which pressure energy of the liquid
increases.
Some of the rotodynamic pumps are:
• Submersible pump
• Self- priming pump
• End - suction pump
• Horizontal multistage pump
• Vertical multistage pump
• Axial flow pump
110
Q

Define specific speed of pumps.?

ESE 2017

A

It is defined as the speed in revolutions per
minute at which a geometrically similar impeller
would operate if it were of such a size as to
deliver one meter cube per minute against one
meter of hydraulic head

111
Q

What Is a Backward Pump?

(ESE 2017

A

A backward running pump will deliver less water,
at a lower pres$ure, and will typically consume
‘Jess ~1ectri9it

INTERVIEW QUESTIONS BOOK (8 decks)

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