Welcome to the course of Introduction to Fluid
Mechanics and Fluid Engineering. This
particular course will be spread into various
lectures, and today we will have the Lecture
1. Before going into the course I think what
is important for all of us, is to recognize
that
there is a motivation behind learning fluid
mechanics. So, we will first go through a
few
examples that will illustrate us the motivation,
and then we will get in to the fundamental
topics that we intend to cover.
Now, fluid mechanics is almost everywhere
in human life. And let me give you some
examples to illustrate what are the important
applications of fluid mechanics. Let us
think of automobiles: when we think of automobiles,
I mean automobiles are the basic
elements which many times motivate young minds
to study fluid mechanics. And really
there is a whole lot of challenge in designing
automobiles based on the requirements of
fluid mechanics, based on the constraints
given by some considerations of fluid
mechanics.
For example: if you want to design a car,
design the shape of a car. The shape of a
car
should be such that it should minimize the
drag force or the resistance force. We will
come in to what is drag force and how it can
be minimized later on in our course, but it
gives one of the important motivations.
Now when we think of vehicles, not just automobiles
which run on land are important
but we have also watercrafts, boats, and ships.
And as you can see here in these
visualizations that there can be nice flow
patterns, very interesting flow patterns that
can
be generated as the watercrafts are propelling
in water. And there can be again a whole
lot of analysis that can go on in the background
to make sure that one can minimize the
resistances against the motion of the watercraft.
Now, we come to the third example which deals
with spacecrafts. Spacecraft are the
most fascinating of all the examples that
we intend to highlight. And you can clearly
see
that; let us say that when a aircraft is taking
off or landing or when a space shuttle is
moving. So, you can see nice visualization
of flow and this nice visualization of flow
is a
very natural way of visualizing the flow.
So, what is happening is that the smokes or
products of combustion are coming out, and
these are basically highlighting the flow
patterns that are surrounding the aircraft
or the spacecraft.
So, this is a very nice way of visualizing
the flow not just qualitatively but one can
get a
quantitative insight on the details of the
fluid flow that is taking place. And again,
the
very basic principles of motion of these aircrafts
or spacecrafts rely on fluid mechanics.
Many times again the issue of not just a drag
force but a lift force that is important,
because the lift force pulls the aircraft
up in the sky. And many times one may need
to
use fundamental considerations like say law
of conservation of linear momentum and
some other basic principles or Newton’s
Laws of Motion, and some modified versions
of
these which can be used for fluid flow analysis.
Now, similar to the concept of flow around
automobiles and watercrafts and aircrafts,
we
can have interesting interaction between fluid
flow and sports balls. And all of us
experience that sports balls under certain
conditions can spin or can swing, and there
is a
interesting fluid dynamics that goes behind
the swing of spin of sports balls. It is a
very
involved topic, and in one of the lectures
later on in this course we will discuss in
details
about the sports wall dynamics.
Coming to the material world, I mean in engineering
we deal with lots of industries. And
industries many times are basically comprising
various plants power, plants and process
plants for example. So, you can see that there
can be interesting flow patterns that can
be
observed, because of emissions of products
of combustion from the chimney. That you
can see in one case and maybe also similar
flows can be visualized in process plants
as
well, like effluent treatment plants.
Now fluid dynamics; it is not just in the
material world of automobiles, and power plants,
and process plants like that. Fluid dynamics
is there in nature, and it is such a beautiful
pattern or gallery of patterns of fluid flow
that can be visualized if we really observe
nature in a vivid way. And what you can see
here is that in tornadoes, in rivers, in
raindrops, what interesting flow patterns
can be observed.
Now, nature is not just made of inanimate
objects. There are animate objects, like
animals and you can see interesting flow around
butterfly, snake, fish, and all these are
really giving rise to very intriguing fluid
flow pattern which can be observed in nature.
Now when we discuss about nature, I mean we
basically come in to the domain of
biological sciences and a life sciences. And
one of the important follow ups is signs of
human bodies or signs of living systems or
medical science. The human body for
example, is a paradise for fluid mechanist
to make their own analysis for studying the
respiratory system, pulmonary system, cardiovascular
system, swimming of sperms and
all these are very intricate. And if one gets
a complete understanding of this it gives
rise
to not only a fundamental physical insight,
but also maybe strategies to combat various
life threatening diseases.
And, I will just give you one example to illustrate
the complicacies involved, like let us
think of flow of blood through arteries and
veins. Like this is a very common thing in
human body mechanics. Now, think of an analog
in a industrial system, like flow of
water through pipes. Now can you tell, what
are the basic differences between flow of
water through pipes and flow of blood through
arteries and veins? You will immediately
come up with some obvious differences like
blood is a much more complex fluid than
water, but the complexity of blood as the
fluid is a not a mystery now I mean it is
somewhat appreciated and understood not to
the fullest extent, but to some extent it
has
been well understood.
But the problem is that it is not just the
issue of blood as a complex fluid, but think
of
blood vessels. These blood vessels are flexible;
their diameters vary with local blood
pressure. And till now it is a mystery in
fundamental science that how the diameter
of a
blood vessel should vary with locally with
blood pressure. This is not yet fundamentally
completely understood; I mean one can go through
empirical formula to express this, but
it is not yet fundamentally well understood.
So, you can well appreciate that an apparently
an elusively simple problem, like flow of
blood through arteries and veins can he rise
to very complicated considerations in fluid
dynamics. Now coming from human body mechanics
to something let us consider as an
example of not so humanistic, like nuclear
explosion. So, you can see an example like
off the flow that is taking place, because
of nuclear explosion in the view graph that
is
being presented.
Then like when there is a fluid there is also
an interacting structure that is interacting
with the fluid. So, there is often a very
interesting interaction between fluid and
structure.
And critical situations may occur when there
is a fluid let us say wind blowing past a
beach with imposed frequencies that impose
frequency matches with the natural
frequency of the structure. Then there is
something called as resonance, and because
of
this resonance the structure may oscillate
or vibrate vigorously.
And there can be failure of the structure.
In this example the bridge has totally collapsed
and eventually it is going to collapse in
this view graph. And that collapsing of the
bridge, collapsing of the structure is because
of the intricate interaction between the fluid
and the structure. So, fluid structure interaction
is also a very important and interesting
modern day topic.
Let us come to more common day to day examples
like food or drinks. Of course, we
need to understand that; I mean this issue
we will discuss later on. That sometimes it
is
very difficult to demarcate between a food
and a drink right, whether it is a fluid or
its a
solid or it is something in between fluid
and solid, these kind of questions come and
hard
demarcations are many times difficult.
For example gel like matters, now what we
call them? Should we call them fluids, should
we call them solids? I mean of course, there
are very standard descriptions and theories
to describe these matters, but these are important
and interesting topics in fluid
mechanics which deal with the constitutive
behavior or the behavior of the substance
as
it is. And typically it belongs to the study
of Rheology of fluid flows.
Even something not so fluid: now looking in
to this particular view graph that is being
shown, can you tell what does it represent?
Yes, you are correct; it is traffic. So, if
you
visualize the traffic from altitude you will
see that the traffic in a typical city will
be
moving like this. So, traffic flow although
it is not the physical flow of fluid has some
resemblance with the physical flow of a fluid.
And there is a lot of research that is
currently going on and has in the past being
going on in the area of traffic flow.
Now, the issue is that should we study fluid
mechanics just because there are so many
applications in the industry, in the nature
and so on. But sometimes we may study fluid
mechanics just because we are fascinated with
beautiful flow patterns. So, you can see
these examples, these flow patterns which
are being demonstrated here. These flow
patterns are so interesting and so fascinating
that if one is interested to study the
structures of these flow patterns, demonstrate
these flow patterns to experiments it is like
fluid mechanics gives us a structured way
of understanding this pattern.
To summarize this discussion that we had had
so far, we can conclude that fluid
mechanics is everywhere. Fluid mechanics is
not just in inanimate objects or in animals,
but fluid mechanics is everywhere in the world.
So, there is a lot of motivation in
studying and understanding fluid mechanics.
Now, when we say fluid mechanics is everywhere,
and we have given certain examples.
The examples that we have given so far are
somewhat traditional. Now we can give
some more examples where fluid mechanics in
a different way is relevant for modern
day applications. Like, the first example
that I want to give you is cooling of microchips
through fluid flow and phase change.
The motivation of this is as follows: as we
have come to the modern era what we find is
that the sizes of the electronic devices are
getting smaller and smaller. Despite the fact
that sizes of the electronic devices are getting
smaller and smaller the power dissipation
is not getting progressively reduced. So what
it means is that the power then dissipation
per unit volume is getting significantly increased,
because of reduction in volume and
that makes the devices overheated. So, you
may not be surprised to know that many of
the electronic devices actually fail not because
of the failure in electronic design, but
failure in thermal design. That is those materials
cannot withstand that high temperature.
So, how can we address this? Of course, you
may say that we can employ a fan. But yes,
we have to understand that if we have a small
miniaturized device, your entire parts
purpose of miniaturization will be lost if
you have a very small device and to cool that
small device you require a large fan. So,
you require a compatible cooling arrangement.
So what you can do, you can for example employ
various strategies. One particular
strategy is you can employ change of phase.
So, you can have a liquid which takes heat
from hot spots of the electronic device and
can change its phase. The liquid can flow
through our micro channel which is a very
small channel, a channel of the order of micro
meter dimensions, and then when this liquid
gets evaporated the evaporated fluid moves
to a different place in the channel and can
get condensed.
So, there can be an evaporation condensation
simultaneously going on to complete the
loop and this mechanism is used even industrial
applications this is known as heat pipe.
And in a miniaturized environment this is
known as micro heat pipe. So, there is a lot
of
interesting fluid mechanics that goes behind.
We do not have scope for discussing that at
this moment, but it is just to let you know
that these kinds of interesting applications
do
exist.
There is also another technology which is
called as Droplet Based Microfluidics. So,
what you can do? Basically you take small
droplets, you arrange for small droplets and
these droplets will go and sit on hot spots
on the electronic device and absorb heat from
that hot spot. So, it is very interesting
to design the movement of droplets so that
they
can move in an optimal path. And in the shortest
time they go and sit on the right hot
spot and take away heat from that hot spot.
We have to understand that fluid dynamics
is so interdisciplinary as a subject that
it is
not a subject just within the jurisdiction
of mechanical engineering, chemical
engineering, aerospace engineering, civil
engineering like that. If you are interested
to
design an optimal path and make chips for
transmission of the droplet according to that
optimal path design then you require to interface
with electrical engineers and computer
scientists. So, it is really emerging as an
interdisciplinary subject.
Now, I will give you a couple of; sorry, I
will give you a couple of more interesting
applications and these applications essentially
deal with a health care engineering. Now
what is health care engineering? Health care
engineering is an interface of health care
with engineering. And let us see that how
fluid mechanics plays a role towards that.
So, in health care engineering many times
what we require is rapid diagnosis of a
disease. And this is a classical problem;
it is a problem relevant in many countries
especially in the underdeveloped countries.
That let us say that a person is suspected
to
have a certain disease. Now the person cannot
go to a pathological laboratory, because
he or she does not have access to high class
pathological laboratories. So, it takes time
to
take the blood sample, let us say as an example
and that blood sample is tested in a
sophisticated laboratory. By that time the
result of that test comes and it is quite
expensive to get the result of the test, this
time consuming. And by the time the result
of
the test comes the patient may be under very
serious condition.
As an alternative one can go for various technologies.
So, one can have small devices
which are called as lab-on-a-chip or a device
which is like a rotating disk that is called
as
lab-on-a-CD. It is like the compact disc for
external data storage in a computer. So, what
we can do is, you can have micro channels
or small channels are cut in the disk and
you
put a droplet of blood, just a small droplet
of blood not a huge volume of blood drawn
from the patient. And then you spin the disc
in a small motor and that is the portable
system so that can be taken to the patient
itself this is called a point of care way
of
handling medical diagnostics.
And then this blood sample is dropped, and
then within the channels there are various
reactants, and because of reactions there
can be a for example change of color of the
blood sample. And a particular change of color
can give rise to the indication of a
particular disease. Here fluid mechanics comes
in a big way, because you need to have a
proper design of what is the rotational speed
of the compact disc for most efficient
transport of the blood sample. So this is
one example, there can be several other
examples given.
And then once the test result comes, immediately
this test result can be conveyed to a
medical doctor maybe through SMS in the mobile
phone system. And then the medical
doctors can immediately advice for a treatment.
And this entire process can take place
within a few minutes. So, it is very rapid,
it is inexpensive, it is portable, and if
this kind
of system comes in to the market it can really
solve some of the challenging problems in
medical diagnostics in many places in the
world.
Another example like which is related to the
medical sciences is DNA hybridization.
And DNA hybridization basically refers to
like, identification of a particular sequence
of
bases in a DNA which can indicate the existence
of a certain disease. Like, all of us
know that DNA is a linear polymer made up
of a sequence of repeating units known as
nucleotides. And each monomer is composed
of a phosphate group which is
schematically shown in the view graph, which
is responsible for a negative charge on the
DNA, deoxyribose sugar and a nitrogen containing
base. So, there are four different
bases found in DNA: A. T. C and G.
So, if you want to identify a particular disease
it may be related to the sequence of base,
a particular sequence of bases like A A T
T G G C C like that. And it is known that
A
and T want to get combined with the help of
hydrogen bond and G wants to get
combined with C with the help of hydrogen
bond. So, A is complementary to T, and G is
complementary to C. So, if you want to identify
whether a particular sequence of DNA
bases is present in a DNA sample then what
you can do, you can put a complement of
that interrogating sequence on the wall of
a small channel and pump a DNA sample with
single strands.
So what you can do is that; first you break
the cell, which is called a cell Lysis and
bring
the DNA out of it and then you heat the DNA
sample so that double stranded DNA gets
broken in to single strands. Then you pass
the sample through a fluid flow. When you
pass the sample through a fluid flow there
is an interesting interaction between fluid
dynamics and the transport of DNA. And that
can control effectively that how fast you
can achieve this hybridization reaction. And
if you can achieve this hybridization
reaction fast, then it is possible to get
an answer whether a particular DNA sample
base
sequence is there in a DNA sample or not and
a rapid diagnosis of certain diseases can
be
made.
Next example is to track the dynamics of a
biological cell. Now biological cell is a
very
interesting object in general. And there are
several motivations of studying biological
cells in a small confinement. In human bodies
there are hierarchical structures of blood
vessels: you have large arteries large veins,
small arteries small veins, arterioles, venules
and micro capillaries. The micro capillaries
are of the order of micrometer dimensions,
and cells are also of the order of micrometer
dimension. Like, typical length scale of cell
in a human body may be around 10 micron.
So, when these cells are moving through human
bodies; let us think of a challenging
problem of like how to understand cancer progression.
So, one of the little stages of
cancer progression comes when a cancer cell
from its origin moves to a distant location
within the human body cutting across are removed,
not cutting across moving across the
blood vessels. So, when it moves across the
blood vessels it has to also move through
micro capillaries. And there is a tremendous
resistance that comes from fluid dynamics
considerations for moving against moving of
cells through micro capillaries. Despite that
cancer cells are able to survive under that
stressful condition, where normal cells are
not
able to survive.
So, can fluid dynamics give an answer to this
question; that why cancer cells can survive
effectively in a micro fluidic confinement,
where the normal cells are not able to do.
So,
there are several possible answers. And some
of the answers, I am not going in to the
answer; this is not a research presentation.
So, I am not going in to the answer to this
question or possible answer to this question.
I am just giving you some clues where fluid
dynamics find this relevance in this application.
So, the cell membrane if you look in to the
cell, the cell membrane in its composition
is
somewhat fluidic in nature. So, the fluidity
of the cell membrane has something to do
with the malleability of the cell. And the
manner in which a cell membrane can control
its fluidity based on that 
it depends on whether a cell can adopt or
adjust its shape
effectively to withstand a stressful condition.
And a cancer cell possibly does it in a
much more better way than a normal cell.
So, that is how a cancer cell survives in
a stressful condition. And it is a very important
and interesting area of research, because
if one understands the proper fluid dynamic
mechanism that goes in a run around the cancer
cell which controls the adaptation of
cancer cell then possibly newer and newer
drugs can be discovered that can inhibit the
survival capacity of cancer cells in a stressful
environment.
Coming from a biological example; I will give
you another example which just illustrates
that fluid dynamics can be multi physics.
So, multi physics means that you just do not
require only fluid physics, only flow physics.
The flow physics maybe we need to be
combined with electricity, electro hydrodynamics,
magneto hydrodynamics that is
electrical sciences or electromagnetic theory
or sometimes optics. So, this is called as
multi physics, where physics from multiple
disciplines need to be converts together to
solve a fluid dynamics problem.
Let us look in to this slide where we intend
to show that you can bend water or move
water by using light. So, what is the strategy?
Briefly the strategies as follows: you coat
the surface of a channel with a metal oxide
semiconductor say- titanium dioxide or zinc
oxide and you shine ultraviolet light on that.
When you shine ultraviolet light on that,
because of the typical energy gap you have
its compatible energies that is provided by
the ultraviolet radiation, and immediately
electron hole reactions will start. So, based
on
that the surface will either acquire a positive
charge or a negative charge depending on
whether it will have excess holes or excess
electrons.
I am not going to the exact details what happens
in this specific case, but the net effect
is
that the surface energy gets altered. Because
the surface energy gets altered a surface
which was earlier disliking water may start
liking water. That is from so called
hydrophobic it becomes hydrophilic and water
we move in to that direction. So, you do
not require a physical pump to drive water.
You just require a source of light to drive
water. And you can even bend water by light.
