This document discusses fluid mechanics and its applications in various engineering systems. It begins by defining fluids and their properties such as being shapeless and transmitting pressure equally in all directions. It then discusses the two main types of fluids - hydraulic and pneumatic - and provides examples. Applications of fluid mechanics principles like Pascal's principle and Archimedes' principle are explained. Case studies on braking systems, lifting devices and aeronautical engineering are provided to illustrate how these principles are applied. Hydraulic and pneumatic systems for brakes in vehicles and trains are described. Innovation in ABS braking systems is also covered.
2. The term fluid applies to both liquids and gases. Fluid mechanics is
the study of gases and liquids, their physical behaviour, and their role
in engineering systems.
3. Fluids are:
Shapeless and do not resist being sheared
When a force is exerted on fluid the pressure increases, whereas
the force is directional the pressure is omnidirectional ( exerted in all
directions)
Viscous (Oil has a high viscosity whilst water has a low viscosity)
Oil has a higher viscosity when cold. As the temperature increases
the viscosity becomes lower so the oil becomes thinner
Subject to turbulence when force is applied
There are two types of fluids
Hydraulic fluids are:
Incompressible ( when a pressure is exerted no volumetric change
occurs). Oil is often used as a hydraulic fluid.
Pneumatic fluids are:
Gases can be compressed. An example is Liquid Petroleum Gas
(LPG). This is pressurised into a gas tank to be sored as a liquid.
When released it turns back to a gas.
4. Advantages of hydraulic systems include:
Appropriate method of power transmission over long distances
(Example: trucks use hydraulic power instead of fuel)
Good flexibility
Variable speed control
Safe and reliable
Disadvantages:
Need to be in a confined space
Fire hazard
Leaks can pose a safety hazard or environmental hazard
Oil filtration must be maintained
5. Archimedes Principle
In 212 B.C., the Greek scientist Archimedes discovered the
following principle:
When an object is completely or partly immersed in a fluid it
experiences a force thrusting it up.
The force (upthrust on object) is equal to the weight of the fluid
displaced by the object.
6. Archimedes Principle cont.…d
If the density of the object is greater than that of the fluid, the object
will sink.
If the density of the object is equal to that of the fluid, the object will
neither sink or float.
If the density of the object is less than that of the fluid, the object will
float.
7. Pascal's Principle cont.…d
Pascals principle states that pressure exerted anywhere in a confined
fluid is transmitted equally in all directions throughout the fluid.
A good example of this is when two pistons are fitted into two glass
cylinders filled with oil and connected to one another with an oil filled pipe.
If you applied a downward force on one of the pistons then the force is
transmitted to the second piston through the oil in the pipe. Since oil is
incompressible, efficiency is very good so most of the applied force
appears at the second piston.
8. Pascals Principle cont.…d
Therefore the application of a force (F1) in a cylinder of cross sectional
area (A1), an equal pressure will be transmitted to the other piston and
cylinder, of area (A2), causing a thrust or force in this piston, of
magnitude F2.
If A2 is very large compared to A1 a comparatively smaller force
applied to the smaller piston can overcome a large resistance acting
on the larger piston. Additionally, this can apply to a number of
different cylinders and pistons attached to the sealed system.
9. Pascal's Principle
So we see that Pascals principle states that pressure exerted
anywhere in a confined fluid is transmitted equally in all directions
throughout the fluid.
What is meant by pressure?
Pressure is force per unit area
Thus the total force or thrust on a surface is the area of the surface, times
a pressure exerted on that surface
F=pxA
Basic unit of pressure is the Pascal (Pa)
Pascal’s Principle
F1 = F2
A1 A2
F2 = F1 x F2
A1
10. Case Study: Braking Systems in Private Vehicles
Brakes are the most important feature of any modern vehicle.
A typical modern vehicle weighs around 1.4 tonnes, has a 3.5 litre
engine, and accelerates from 0 to 100 kph in approximately 10
seconds.
To do this it has a sophisticated engine, transmission and drive line
system. This system has thousands of parts and takes up nearly half
the vehicles weight. In contrast the braking system of a car has only
approximately 200 parts weighing less than 40 kilos and has to be
able to stop the vehicle from 100kph to 0 in 3 to 5 seconds.
11. We all know that a car slows down and stops when we apply brakes.
How does this happen?
How does the force exerted on the foot pedal stop or slow down a
car?
How does it multiply the force enough to stop something as big as a
car?
The basic idea behind any hydraulic system is very simple. The force
applied at one point is transmitted to another point (as stated by
Pascal's principle) using an incompressible fluid, generally oil. Most
brake systems multiply the force in the process.
The advantages of hydraulic systems are the pipe connecting the two
cylinders can be of any length and shape allowing to choose any path
separating the two pistons and the force applied is multiplied.
12. Here you can see the hydraulic
brake system of a car.
It consists of a pipeline containing fluid.
One end of which is connected
to the master cylinder fitted with
a piston attached to the foot pedal.
The other end of the pipeline is
connected to the wheel cylinder
which has two steel caliper pistons
on either side of it. Attached to the
pistons is the brake drum and within
the brake drum is the brake shoes.
The area of cross-section of
the wheel cylinder is greater
than the area of the cross-section
of the master cylinder
13. Let us see what
happens when brakes are applied.
When the brakes are applied
the foot pedal is pushed exerting
pressure on the fluid in
the master cylinder.
14. This pressure is transmitted equally and
undiminished throughout the fluid and to
the pistons of the wheel cylinder.
This pushes the pistons outwards forcing
the brake shoes to press against the
rim of the wheel due to which the motion
retards. On releasing the pressure
on the pedal the return spring
forces the pistons of the
wheel cylinder back and the
fluid flows back into the master cylinder.
15. Case Study: Air Brake System used in Trains (Pneumatic System)
The air brake is the standard, fail-safe, train brake used by
railways all over the world
It is based on the simple physical properties of compressed
air
A moving train has kinetic energy which needs to be removed
in order for it to stop.
The majority of trains still use the compressed air braking
system.
These systems are known as air brakes or pneumatic brakes
16. Air Brake System used in Trains (Pneumatic System) cont.…d
The force of the air pushes blocks
or pads onto the train wheels.
The compressed air is fed through
the train by a brake pipe.
Varying the level of air pressure
in the pipe causes change in the
state of the brake on each vehicle.
The driver can apply the brake,
release it or hold it on after
a partial application.
17. Air Brake System used in Trains (Pneumatic System) cont.…d
When the driver places the brake valve in the application position
this causes air pressure in the brake pipe to escape.
This loss of pressure is detected by the slide vale in the triple valve
Due to the loss of pressure on one side, the brake side, one side of
the valve has fallen causing the auxiliary reservoir pressure to push
the valve towards the right so that the feed groove over the valve
closes.
This in turn causes the connection between the brake cylinder and
the exhaust to be closed
The connection between the auxiliary reservoir and the brake
cylinder has become open.
Auxiliary air feeds through into the brake cylinder
This air forces the piston to move against the spring putting
pressure on the brake blocks which then are applied to the wheels.
Air will still pass through the reservoir to the brake cylinder until the
pressure in both equalises.
18. Case Study: Innovation in Braking Systems
Anti-Lock Braking system (ABS)
Anti- lock braking(ABS) systems first came about around the 1920’s
when it was applied to the concept of an automatic override system for
aircraft brakes.
ABS was primarily used up until the 1950’s for aircraft braking
technology
Car manufactures started to experiment with ABS in the 1960’s
however it became an expensive project which was soon abandoned
In the 70’s saw the addition of computer-controlled sensors which led
to the revival of ABS for safety purposes.
Advantages
Effective way to prevent crashing due to the sensors detecting lockup
thus reducing hydraulic pressure at the wheel
Disadvantages
Debate on whether the driver should have full control of the car and not
rely on a braking system that could fail
Drivers tend to drive aggressively knowing they have the ABS to rely on
19. Innovation in Braking Systems
Anti-Lock Braking system (ABS)
The existing hydraulic braking
system which consists of the
master cylinder, calipers,
wheel cylinders, pads, shoes
and associated connecting valves,
line and hoses has the ABS system
incorporated into the car as well.
The computer receives a signal from
the individual sensors which are
located at each wheel
It compares the speed of each
wheel with the other wheels
If the comparison indicates wheel
lock up is present signals are sent
to valves and actuators which raise or
lower the hydraulic pressure to each
wheel which corrects the skid.
20. Innovation in Braking Systems cont...d
Anti-Lock Braking system (ABS)
This process is produced thousands of times per second enabling
maximum stopping ability under any condition
All of these actions go unnoticed by the driver unless warnings lights
are shown signalling failure of the braking system.
When the driver applies the brakes and ABS kicks the driver will feel a
shudder or vibration. This is normal, however the driver tends to ease
of the brakes. The driver should carry on applying the brakes which
will eventually stop the car skidding.
21. Case Study: Fluid Mechanics in Lifting Devices
Prior to the introduction of the hydraulic jack in 1851 by Richard
Dudgeon, screw jacks were being used. Screw jacks took more time
and effort to raise the desired object.
Scissor screw jacks are usually used to lift a car to change a flat tyre
The bottom of the jack rests on the ground while the top fits under the
body of a car. A screw is inserted in the center of the scissor system
and is turned to the right to raise the jack and lift the car. After the tire
is replaced, the screw is turned to the left to lower the car back to the
ground.
22. Case Study: Fluid Mechanics in Lifting Devices
Hydraulic Bottle Jacks are extremely adaptable since they can be
placed in restricted spaces and provide good leverage.
They have a longer handle as compared to rest of the hydraulic jacks
and push up against a lever that gives a lift to the main lift arm.
With their use, it is possible to give a greater lift per stroke.
They are extensively used in the construction of buildings and
repairing the foundation of houses.
It has also been found to be very useful in search and rescue
operations.
23. Case Study: Fluid Mechanics in Lifting Devices
Hydraulic jacks have revolutionised the way we lift heavy objects and
are widely used all across the globe.
They make our life much more comfortable than it was before.
These jacks have outweighed conventional screw jacks that were in use
at some point of time.
They have two cylinders which are joined together and are filled with a
fluid usually oil.
The hydraulic jack works on the principle of Pascal's law
24. Case Study: Fluid Mechanics in Lifting Devices
The jack basically consists of two cylinders, one small, one large.
The two cylinders are each filled with oil, and there is a passage
between them. Inside each cylinder is a piston.
The oil in the jack is a liquid, so it’s incompressible.
When you push down on the jack’s lever, you create a force, F1, on the
small piston.
This then creates equal pressure in the oil under both the small and
large pistons.
25. Case Study: Fluid Mechanics in Lifting Devices
We know that pressure is force divided by area p = F
A
In the diagram the large piston is going to lift the weight of the car.
Because the large piston has a greater surface area than the small
piston, the fluid in the large cylinder will create a much larger force to
push against the weight of the car hence lifting it off the ground.
26. Case Study: Hydraulic Systems in Aeronautical Engineering
Hydraulics are used for different aircraft applications.
Brakes
Landing gear
Flight control
Flaps
Speed brakes
Nose wheel tillers
Hydraulic Fluid
Superior hydraulic fluid should be:
Incompressible
Flows with minimal friction
Has strong lubricating properties
Resistant to foaming
Maintain properties at high temperatures
Should never be mixed
Flammable at 5606 C
27. Case Study: Hydraulic Systems in Aeronautical Engineering
System Components
Hydraulic pumps are usually engine or electrically driven gear type
pumps that provide system pressure
Large aircraft will have more than one interconnected hydraulic
systems with backup pumps in case of failure
Hydraulic motors utilise hydraulic pressure to provide mechanical
power to flaps or landing gear
Hydraulic cylinders use pistons to translate hydraulic pressure into
linear mechanical movement for brakes
Hydraulic lines deliver hydraulic power from pump to motor or
actuator
Pressure gauge supplies the pilot with system pressure information.
28. Case Study: Hydraulic Systems in Aeronautical Engineering
Valves direct the flow of hydraulic fluid and control and regulate
pressure
Actuators convert hydraulic pressure to move components to a
desired position, also helps maintain a constant pressure within the
system. Absorbs the shocks due to rapid pressure variations
Reservoir store adequate hydraulic fluid fro system
Standpipe is designed into the reservoir to guard against system
leakage.
The diagram represents a
hydraulic landing gear system
in a aeroplane
29. Case Study: Hydraulic Systems in Aeronautical Engineering
Landing gear
The aircraft landing gear is a combination of mechanical
structure, pneumatics (air springs) and hydraulic damping.
A good landing gear design reduces the loads produced into the
airframe during landing and take-off.
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Prepared by Yanake Tennant SID No: 3159851