aerodynamics in car
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21-02-2010, 06:39 PM
here i am be final year student, i am giving siminar on aerodynmics so please mail me aerodynamic study materials from basic...
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23-09-2010, 03:02 PM
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21-06-2013, 02:31 PM
Aerodynamics in Car
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Aerodynamics is the science of manipulating and making use of airflow.It deals with the flow of air and how it reacts with bodies in motion. When objects move through air, forces are generated by the relative motion between the air and surfaces of the object. The study of these forces which are generated by the motion of air is called Aerodynamics.Usually aerodynamics is categorized according to the type of flow such as subsonic, hypersonic, supersonic, etc.
Aerodynamics is important in a number of applications other than aerospace engineering. It is a significant factor in any type of vehicle design, including automobiles. It is important in the prediction of forces and moments in sailing. It is used in the design of mechanical components such as hard drive heads. Structural engineers also use aerodynamics, particularly aeroelasticity, to calculate wind loads in the design of large buildings and bridges. Urban aerodynamics seeks to help town planners and designers improve comfort in outdoor spaces, create urban microclimates and reduce the effects of urban pollution. The aerodynamics of internal passages is important in heating/ventilation, gas piping, and in automotive engines where detailed flow patterns strongly affect the performance of the engine.
It is essential that aerodynamics be taken into account during the design of cars as an improved aerodynamics in car would attain higher speeds and more fuel efficiency. For attaining this aerodynamic design, the cars are designed lower to the ground and are usually sleek in design and almost all corners are rounded off to ensure smooth passage of air through the body. In addition to it, a number of enhancements like spoilers, wings are also attached to the cars for improving aerodynamics. Wind tunnels are used for analyzing the aerodynamics of cars. Besides this, a number of softwares are also available now days to ensure the optimal aerodynamic design.
The word “Aerodynamics” comes from two Greek words: aerios, concerning the air, and dynamis, which means force. Aerodynamics is the study of forces and the resulting motion of objects through the air. Based on the flow environment it can be classified into external aerodynamics and internal aerodynamics; external aerodynamics is the flow around solid objects of various shapes, whereas internal aerodynamics is the flow through passages in solid objects, for e.g. the flow through jet engine air conditioning pipe etc. The behavior of air flow changes depends on the ratio of the flow to the speed of sound. This ratio is called Mach number. Based on this Mach number, the aerodynamic problems can be classified as subsonic if the speed of flow is less than that of sound, transonic if speeds both below and above speed of sound are present, supersonic if characteristics of flow is greater than that of sound and hypersonic if flow is very much greater than that of sound. Aerodynamics have wide range of applications mainly in aerospace engineering , then in the design of automobiles, prediction of forces and moments in ships and sails, the design of bridges and other buildings in the field of civil engineering where they help to calculate wind loads in design of large buildings
HISTORY & EVOLUTION OF AERODYNAMICS
Ever since the first car was manufactured in early 20th century the attempt has been to travel at faster speeds. In the earlier times, aerodynamics was not a factor as the cars used to travel at very slow speeds. As a result, there were not any aerodynamic problems but with the increase of speeds, the necessity for cars to become more streamlined resulted in structural invention such as the introduction of the windscreen, incorporation of wheels into the body and the insetting of the headlamps into the front of the car. This was probably the fastest developing time in automobiles history as the majority of the work was to try and reduce the aerodynamic drag. This happened up to the early 1950’s, where by this time the aerodynamic dray had been cut by about 45% from the early cars such as the Silver Ghost. However, after this the levels of drag found on cars began to slowly increase. This was due to the way that the designing was thought about. Before1950, designers were trying to make cars as streamlined as possible to make it easier for the engine, yet they were restricting the layout of the interior for the car. After 1950, the levels of aerodynamic drag went up because cars were becoming more family friendly and so as a consequence the shapes available to choose were more limited and so it was not possible to keep the low level of aerodynamic drag. The rectangular shape made cars more purposeful for the family and so it is fair to say that after 1950 the designing of cars was to aid the lifestyle of larger families.
Rear vacuum (a non-technical term, but very descriptive) is caused by the "hole" left in the air as the car passes through it. To visualize this, imagine a bus driving down a road. The blocky shape of the bus punches a big hole in the air, with the air rushing around the body, as mentioned above. At speeds above a crawl, the space directly behind the bus is "empty" or like a vacuum. This empty area is a result of the air molecules not being able to fill the hole as quickly as the bus can make it. The air molecules attempt to fill in to this area, but the bus is always one step ahead, and as a result, a continuous vacuum sucks in the opposite direction of the bus. This inability to fill the hole left by the bus is technically called Flow detachment .At the rear of vehicles, the ideal format is a long and gradual slope. As this is not practical, it has been found that ‘raising and/or lengthening the boot generally reduces the drag”. In plan view, rounding corners and ‘all forward facing elements’ will reduce drag. Increases in curvature of the entire vehicle in plan will usually decrease drag provided that frontal area is not increased. ‘Tapering the rear in plan view’, usually from the rear wheel arch backwards, ‘can produce a significant reduction in drag’. Under the vehicle, a smooth surface is desirable as it can reduce both vehicle drag and surface friction drag. ‘For a body in moderate proximity to the ground, the ideal shape would have some curvature on the underside.
LIFT OR DOWNFORCE
One term very often heard in race car circles is Down force. Down force is the same as the lift experienced by airplane wings, only it acts to press down, instead of lifting up. Every object traveling through air creates either a lifting or down force situation. Race cars, of course use things like inverted wings to force the car down onto the track, increasing traction. The average street car however tends to create lift. This is because the car body shape itself generates a low pressure area above itself. For a given volume of air, the higher the speed the air molecules are traveling, the lower the pressure becomes. Likewise, for a given volume of air, the lower the speed of the air molecules, the higher the pressure becomes. This of course only applies to air in motion across a still body, or to a vehicle in motion, moving through still air. When we discussed Frontal Pressure, above that the air pressure was high as the air rammed into the front grill of the car. What is really happening is that the air slows down as it approaches the front of the car, and as a result more molecules are packed into a smaller space. Once the air stagnates at the point in front of the car, it seeks a lower pressure area, such as the sides, top and bottom of the car. Now, as the air flows over the hood of the car, it loses pressure, but when it reaches the windscreen, it again comes up against a barrier, and briefly reaches a higher pressure. The lower pressure area above the hood of the car creates a small lifting force that acts upon the area of the hood (Sort of like trying to suck the hood off the car). The higher pressure area in front of the windscreen creates a small (or not so small) down force. This is akin to pressing down on the windshield.
WINGS & SPOILERS
What this wings or spoilers does is it prevents the separation of flow and thereby preventing the formation of vortices or helps to fill the vacuum in the rear end more effectively thus reducing drag. So what actually this wings does is that, The wing works by differentiating pressure on the top and bottom surface of the wing. As mentioned previously, the higher the speed of a given volume of air, the lower the pressure of that air, and vice-versa. What a wing does is make the air passing under it travel a larger distance than the air passing over it (in race car applications). Because air molecules approaching the leading edge of the wing are forced to separate, some going over the top of the wing, and some going under the bottom, they are forced to travel differing distances in order to "Meet up" again at the trailing edge of the wing. This is part of Bernoulli's theory. What happens is that the lower pressure area under the wing allows the higher pressure area above the wing to "push" down on the wing.
The way a real, shaped wing works is essentially the same as an airplane wing, but it's inverted. An airplane wing produces lift, a car wing produces negative lift or in other words what we call us, downforce. That lift is generated by a difference in pressure on both sides of the wing. .
Probably the most popular form of aerodynamic aid is the wing. Wings perform very efficiently, generating lots of down force for a small penalty in drag. Spoilers are not nearly as efficient, but because of their practicality and simplicity, spoilers are used a lot on sedans. The wing works by differentiating pressure on the top and bottom surface of the wing. As mentioned previously, the higher the speed of a given volume of air, the lower the pressure of that air, and vice-versa. What a wing does is make the air passing under it travel a larger distance than the air passing over it (in race car applications). Because air molecules approaching the leading edge of the wing are forced to separate, some going over the top of the wing, and some going under the bottom, they are forced to travel differing distances in order to "Meet up" again at the trailing edge of the wing. This is part of Bernoulli's theory. What happens is that the lower pressure area under the wing allows the higher pressure area above the wing to "push" down on the wing, and hence the car it's mounted to.
METHODS FOR EVALUATING AERODYNAMICS IN CARS
A wind tunnel is a research tool developed to assist with studying the effects of air moving over or around solid objects. Air is blown or sucked through a duct equipped with a viewing port and instrumentation where models or geometrical shapes are mounted for study. Various techniques are then used to study the actual airflow around the geometry and compare it with theoretical results, which must also take into account the Reynolds number and Mach number for the regime of operation. Threads can be attached to the surface of study objects to detect flow direction and relative speed of air flow. Dye or smoke can be injected upstream into the airstream and the streamlines that dye particles follow photographed as the experiment proceeds.
Traditionally, wind tunnel testing was a sizeable trial and error process, ongoing throughout the development of a vehicle. Today, with the high level of CAD prediction and pre-production evaluation, coupled with a greater human understanding of aerodynamics, wind tunnel testing often comes into the design process later. The wind tunnel is the proving ground for the vehicle's form and allows engineers to obtain considerable amounts of advanced information within a controlled environment.
Nowadays a large number of software’s are developed for the analysis and optimization of aerodynamics in automobiles. Earlier times the cars were worked directly on wind tunnels where they prepared different shapes or cross sections and tested upon the cars, during those times it was not possible to test the for small areas that is for a small part of front area etc there testing were made for the entire cross sections, But with the introduction of computational fluid dynamics i.e. the use of computers to analyze fluid flows where the entire area is divided in to grids and each grid is analyzed and suitable algorithms are developed to solve the equations of motion.Based on CFD large number of software’s are developed for the design and analyzing aerodynamics the most commonly used software’s are ANSYS,CATIA.
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