introduction fluid

8/12/2019 Introduction Fluid

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Introduction

Fluid Mechanics and Fluid Properties

What is fluid mechanics? As its name suggests it is the branch of applied mechanics concerned

with the statics and dynamics of fluids - both liquids and gases. The analysis of the behaviour of

fluids is based on the fundamental laws of mechanics which relate continuity of mass and energy

with force and momentum together with the familiar solid mechanics properties.

There are two aspects of fluid mechanics which make it different to solid mechanics

1. The nature of a fluid is much different to that of a solid2. In fluids we usually deal with continuous streams of fluid without a beginning or end. In

solids we only consider individual elements.

We normally recognize three states of matter: solid; liquid and gas. However, liquid and gas are

both fluids: in contrast to solids they lack the ability to resist deformation. Because a fluid cannot

resist the deformation force, it moves, itflows under the action of the force. Its shape will change

continuously as long as the force is applied. A solid can resist a deformation force while at rest,

this force may cause some displacement but the solid does not continue to move indefinitely.

Fluid mechanics is the branch of physics that studies fluids (liquids, gases, and plasmas) and the forces

on them. Fluid mechanics can be divided into fluid statics, the study of fluids at rest; fluid kinematics,

the study of fluids in motion; and fluid dynamics, the study of the effect of forces on fluid motion. It is a

branch of continuum mechanics, a subject which models matter without using the information that it is

made out of atoms, that is, it models matter from a macroscopic viewpoint rather than from a

microscopic viewpoint. Fluid mechanics, especially fluid dynamics, is an active field of research with

many unsolved or partly solved problems. Fluid mechanics can be mathematically complex, and can best

be solved by numerical methods, typically using computers. A modern discipline, called computational

fluid dynamics (CFD), is devoted to this approach to solving fluid mechanics problems. Particle image

velocimetry, an experimental method for visualizing and analyzing fluid flow, also takes advantage of the

highly visual nature of fluid flow.


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Brake System

A brake is a mechanical device which inhibits motion. The rest of this article is dedicated to

various types of vehicular brakes. Most commonly brakes use friction to convert kinetic energy into

heat, though other methods of energy conversion may be employed. For example regenerative braking

converts much of the energy to electrical energy, which may be stored for later use. Other methodsconvert kinetic energy into potential energy in such stored forms as pressurized air or pressurized oil.

Eddy current brakes use magnetic fields to convert kinetic energy into electric current in the brake disc,

fin, or rail, which is converted into heat. Still other braking methods even transform kinetic energy into

different forms, for example by transferring the energy to a rotating flywheel.

Disc Brakes The disc brake is the most common type of brake used in modern personal and

public transport vehicles such as cars and buses. Disc brakes consist of a rotating disc (rotor)that is

connected to the axle. Connected to the suspension is a backing plate with a caliper attached. The

caliper wraps over the disc and houses two pads that are forced laterally against the disc by a

hydraulically operated piston. The frictional resistance created retards the rotor. The disc brake offers

better heat distribution than the drum brake and also offers better wet-weather performance as water

is thrown off the disc by centrifugal force. Initially, the disc (rotor) was solid but now they have vents

through them (ventilated discs) or they are drilled to further improve heat distribution.

Hydraulic brake system

When the brake pedal is pressed, a pushrod exerts force on the piston(s) in the master cylinder,

causing fluid from the brake fluid reservoir to flow into a pressure chamber through a compensating

port. This forces fluid through the hydraulic lines toward 4 caliper pistons then apply force to the brake

pads, which pushes them against the spinning rotor, and the friction between the pads and the rotor

causes a braking torque to be generated, slowing the vehicle. Heat generated by this friction is either

dissipated through vents and channels in the rotor or conducted through the pads, which are made of

specialized heat-tolerant materials such as kevlar or sintered glass. Subsequent release of the brake

pedal/lever allows spring(s) to return the master piston(s) back into position. This relieves the hydraulic

pressure on the caliper, allowing the brake piston in the caliper assembly to slide back into its housing

and the brake pads to release the rotor. In figure , pedal ratio=5


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Brake Hydraulic Theory

Most people will be be starting with a brake caliper from a particular car. The goal is to find out what

size master cylinder should be used to give acceptable pedal force, and also to figure out how to

proportion the braking forces between the front and rear wheels. Before we jump into that lets look at

brake system basics.

The fundamental concept behind hydraulics is the incompressible fluid. A fluid is a material that can flow

into any volume. Gases and liquids are all fluids, the principle difference being the amount of

compressibility they exhibit. The other unique characteristic that all fluids share is that the pressure is

the same everywhere within the fluid region (neglect the effects of gravity, it doesnt play on the size

scales we are talking about). For example, lets fill a 55 gallon drum completely with water, so that there

is no air in the tank. If I push on the bottom of the tank the top of the tank will start to bulge (so will the

sides, but not as much since they are thicker material). The force on the bottom of the tank got

transmitted to every part of the tank. Now lets get another 55 gallon drum and completely fill it with

water as well. Connect it to the first tank with a hose, and get rid of all the air from the system. Again if I

push on the bottom (or the top) of the first tank, the force will be felt every within the fluid, even the

second tank that we just connected. Now we are starting to see our hydraulic system, the drums are the

master and slave cylinders and the hose is, well, the hose.

This is our first hydraulic system. On the left is our master cylinder, on the right our slave

cylinder. The key to remember is that the hydraulic fluid is incompressible. It will always have

the same volume no matter what we do. If we move the master cylinder piston, then the volume

inside the master cylinder changes, the volume of the rest of the system has to change in order

to keep the total volume constant. Assuming that the hydraulic lines are perfect and never

change the volume change in the master cylinder is going to have to be matched by a volume

change in the slave cylinder.

The volume change in the master cylinder is

Where is the distance the master cylinder piston moved, and is the cross

sectional area of the master cylinder. Since the volume change in the slave cylinder is the same

as the volume change in the master cylinder the distance the slave cylinder moves must be:

OK, but what about pressure? Thats easy, the pressure is the same everywhere. And we find it

by dividing the force on the master cylinder piston by the master cylinder area:


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Now what is the force exerted by the slave cylinder piston? We know the pressure in the slave

cylinder, it is the same as in the master cylinder. The force on the slave cylinder piston is the

pressure times the area of the slave cylinder:

So now we have the entire picture. If the slave cylinder is bigger (in diameter, and therefore

area) than the master cylinder we get a bigger force out of the slave than we put into the

master, but a smaller movement.

The Master Cylinder

Our ideal hydraulic system is a pretty good model, but we need to add some things to it to

make it applicable to automotive systems. The first addition is to the master cylinder. We do

not act directly on the master cylinder piston, but rather, through a brake pedal that adds

mechanical advantage. If we apply a force on the brake pedal we get a higher force on the

master cylinder:

is the pedal ratio.

If we press on the pedal with 70lbf and the pedal ratio is 7, then the

force on the piston is 490lbf.


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Conclusion

In a hydraulic brake system, the master cylinder moves brake fluidthrough the system. The lines used to

carry the liquid may be pipes hoses, or a network of internal bores or passages in a single housing, suchas those found in a master cylinder. Valves are used to regulate hydraulic pressure and direct the flow of

the liquid. The output devices are brake drum cylinders and disc brake calipers. Hydraulics is the study of

liquids in motion. Liquids are considered non-compressible fluids.Pascals Law saysthat pressure at any

one point in a confined liquid is the same in every direction and applies equal force on equal areas. If a

liquid is confined and a force applied, pressure isproduced. If the pressure on the fluid is applied to a

movable output piston, it creates output force. In a brake system, a small master cylinder piston is used

to apply pressure to larger pistons at the wheel brake units to increase braking force. Most brake

systems with front discs and rear drums have large-diameter master cylinders (large piston area) and a

power booster to increase the input force.

introduction fluid

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