Engineering mechanics powerpoint fluid mechanics

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Engineering Assignment - Case studies


  • 1.Braking SystemsLifting DevicesAeronautical Engineering

2. The term fluid applies to both liquids and gases. Fluid mechanics isthe study of gases and liquids, their physical behaviour, and their rolein engineering systems. 3. Fluids are: Shapeless and do not resist being sheared When a force is exerted on fluid the pressure increases, whereasthe force is directional the pressure is omnidirectional ( exerted in alldirections) Viscous (Oil has a high viscosity whilst water has a low viscosity) Oil has a higher viscosity when cold. As the temperature increasesthe viscosity becomes lower so the oil becomes thinner Subject to turbulence when force is appliedThere are two types of fluidsHydraulic fluids are: Incompressible ( when a pressure is exerted no volumetric changeoccurs). 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 reliableDisadvantages: 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 PrincipleIn 212 B.C., the Greek scientist Archimedes discovered thefollowing principle:When an object is completely or partly immersed in a fluid itexperiences a force thrusting it up.The force (upthrust on object) is equal to the weight of the fluiddisplaced by the object. 6. Archimedes Principle cont.d If the density of the object is greater than that of the fluid, the objectwill sink. If the density of the object is equal to that of the fluid, the object willneither sink or float. If the density of the object is less than that of the fluid, the object willfloat. 7. Pascals Principle cont.d Pascals principle states that pressure exerted anywhere in a confinedfluid is transmitted equally in all directions throughout the fluid.A good example of this is when two pistons are fitted into two glasscylinders 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 istransmitted to the second piston through the oil in the pipe. Since oil isincompressible, efficiency is very good so most of the applied forceappears at the second piston. 8. Pascals Principle cont.dTherefore the application of a force (F1) in a cylinder of cross sectionalarea (A1), an equal pressure will be transmitted to the other piston andcylinder, of area (A2), causing a thrust or force in this piston, ofmagnitude F2.If A2 is very large compared to A1 a comparatively smaller forceapplied to the smaller piston can overcome a large resistance actingon the larger piston. Additionally, this can apply to a number ofdifferent cylinders and pistons attached to the sealed system. 9. Pascals PrincipleSo we see that Pascals principle states that pressure exertedanywhere in a confined fluid is transmitted equally in all directionsthroughout the fluid.What is meant by pressure?Pressure is force per unit areaThus the total force or thrust on a surface is the area of the surface, timesa pressure exerted on that surfaceF=pxABasic unit of pressure is the Pascal (Pa)Pascals PrincipleF1 = F2A1 A2F2 = F1 x F2A1 10. Case Study: Braking Systems in Private VehiclesBrakes are the most important feature of any modern vehicle.A typical modern vehicle weighs around 1.4 tonnes, has a 3.5 litreengine, and accelerates from 0 to 100 kph in approximately 10seconds.To do this it has a sophisticated engine, transmission and drive linesystem. This system has thousands of parts and takes up nearly halfthe vehicles weight. In contrast the braking system of a car has onlyapproximately 200 parts weighing less than 40 kilos and has to beable 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 acar?How does it multiply the force enough to stop something as big as acar?The basic idea behind any hydraulic system is very simple. The forceapplied at one point is transmitted to another point (as stated byPascals principle) using an incompressible fluid, generally oil. Mostbrake systems multiply the force in the process.The advantages of hydraulic systems are the pipe connecting the twocylinders can be of any length and shape allowing to choose any pathseparating the two pistons and the force applied is multiplied. 12. Here you can see the hydraulicbrake system of a car.It consists of a pipeline containing fluid.One end of which is connectedto the master cylinder fitted witha piston attached to the foot pedal.The other end of the pipeline isconnected to the wheel cylinderwhich has two steel caliper pistonson either side of it. Attached to thepistons is the brake drum and withinthe brake drum is the brake shoes.The area of cross-section ofthe wheel cylinder is greaterthan the area of the cross-sectionof the master cylinder 13. Let us see whathappens when brakes are applied.When the brakes are appliedthe foot pedal is pushed exertingpressure on the fluid inthe master cylinder. 14. This pressure is transmitted equally andundiminished throughout the fluid and tothe pistons of the wheel cylinder.This pushes the pistons outwards forcingthe brake shoes to press against therim of the wheel due to which the motionretards. On releasing the pressureon the pedal the return springforces the pistons of thewheel cylinder back and thefluid 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 byrailways all over the world It is based on the simple physical properties of compressedair A moving train has kinetic energy which needs to be removedin order for it to stop. The majority of trains still use the compressed air brakingsystem. These systems are known as air brakes or pneumatic brakes 16. Air Brake System used in Trains (Pneumatic System) cont.dThe force of the air pushes blocksor pads onto the train wheels.The compressed air is fed throughthe train by a brake pipe.Varying the level of air pressurein the pipe causes change in thestate of the brake on each vehicle.The driver can apply the brake,release it or hold it on aftera partial application. 17. Air Brake System used in Trains (Pneumatic System) cont.d When the driver places the brake valve in the application positionthis 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 ofthe valve has fallen causing the auxiliary reservoir pressure to pushthe valve towards the right so that the feed groove over the valvecloses. This in turn causes the connection between the brake cylinder andthe exhaust to be closed The connection between the auxiliary reservoir and the brakecylinder has become open. Auxiliary air feeds through into the brake cylinder This air forces the piston to move against the spring puttingpressure on the brake blocks which then are applied to the wheels. Air will still pass through the reservoir to the brake cylinder until thepressure in both equalises. 18. Case Study: Innovation in Braking SystemsAnti-Lock Braking system (ABS) Anti- lock braking(ABS) systems first came about around the 1920swhen it was applied to the concept of an automatic override system foraircraft brakes. ABS was primarily used up until the 1950s for aircraft brakingtechnology Car manufactures started to experiment with ABS in the 1960showever it became an expensive project which was soon abandoned In the 70s saw the addition of computer-controlled sensors which ledto the revival of ABS for safety purposes.Advantages Effective way to prevent crashing due to the sensors detecting lockupthus reducing hydraulic pressure at the wheelDisadvantages Debate on whether the driver should have full control of the car and notrely on a braking system that could fail Drivers tend to drive aggressively knowing they have the ABS to rely on 19. Innovation in Braking SystemsAnti-Lock Braking system (ABS)The existing hydraulic brakingsystem which consists of themaster cylinder, calipers,wheel cylinders, pads, shoesand associated connecting valves,line and hoses has the ABS systemincorporated into the car as well.The computer receives a signal fromthe individual sensors which arelocated at each wheelIt compares the speed of eachwheel with the other wheelsIf the comparison indicates wheellock up is present signals are sentto valves and actuators which raise orlower the hydraulic pressure to eachwheel which corrects the skid. 20. Innovation in Braking Systems cont...dAnti-Lock Braking system (ABS)This process is produced thousands of times per second enablingmaximum stopping ability under any conditionAll of these actions go unnoticed by the driver unless warnings lightsare shown signalling failure of the braking system.When the driver applies the brakes and ABS kicks the driver will feel ashudder or vibration. This is normal, however the driver tends to easeof the brakes. The driver should carry on applying the brakes whichwill eventually stop the car skidding. 21. Case Study: Fluid Mechanics in Lifting DevicesPrior to the introduction of the hydraulic jack in 1851 by RichardDudgeon, screw jacks were being used. Screw jacks took more timeand effort to raise the desired object.Scissor screw jacks are usually used to lift a car to change a flat tyreThe bottom of the jack rests on the ground while the top fits under thebody of a car. A screw is inserted in the center of the scissor systemand is turned to the right to raise the jack and lif