When objects move through air, forces are generated by the relative motion between air and surfaces of the body, study of these forces generated by air is called aerodynamics. Based on the flow environment it can be classified in to external aerodynamics and internal aerodynamics; external aerodynamics is the flow around solid objects of various shapes, where as 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, in the field of civil engineering as in the design of bridges and other buildings, where they help to calculate wind loads in design of large buildings.
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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 where traveling at very slow speeds there were not any aerodynamic problems but with 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.
Although this was a good thing for families, it didn’t take long before the issue of aerodynamics came back into the picture in the form of fuel economy. During the 1970’s there was a fuel crisis and so the demand for more economical cars became greater, which led to changes in car aerodynamics. During the 1970’s there was a fuel crisis and so the demand for more economical cars became greater, which led to changes in car aerodynamics. If a car has poor aerodynamics then the engine has to do more work to go the same distance as a car with better aerodynamics, so if the engine is working harder it is going to need more fuel to allow the engine to do the work, and therefore the car with the better aerodynamics uses less fuel than the other car. This quickly led to a public demand for cars with a lower aerodynamic drag in order to be more economical for the family.
Only about 15% of the energy from the fuel you put in your tank gets used to move your car down the road or run useful accessories, such as air conditioning. The rest of the energy is lost to engine and driveline inefficiencies and idling. Therefore, the potential to improve fuel efficiency with advanced technologies is enormous.
Now a days almost all cars are manufactured aerodynamically , one misconception that everyone has is aerodynamics is all about going faster, in a way it is true but it is not all about speed, by designing the car aerodynamically we can reduce the friction that it encounters and there by power needed to overcome would be less thus fuel can be saved; In the modern era where our fuel resources are fast depleting all the efforts are to find alternate sources of energy or to save our current resources or minimize the use of current resources like fuels, so now a days aerodynamics are given very much importance as everyone like to have a good looking , stylish and fuel efficient car.
STUDY OF AERODYNAMICS OF CARS
In order to improve the aerodynamics we must first know how the flow of air past a car, if we visualize a car moving through the air. As we all know, it takes some energy to move the car through the air, and this energy is used to overcome a force called Drag.
DRAG
A simple definition of aerodynamics is the study of the flow of air around and through a vehicle, primarily if it is in motion. To understand this flow, you can visualize a car moving through the air. As we all know, it takes some energy to move the car through the air, and this energy is used to overcome a force called Drag.
Drag, in vehicle aerodynamics, is comprised primarily of two forces. Frontal pressure and rear vaccum.
DRAG FORCE AT LOW SPEEDS
The total drag force decreases, meaning that a car with a low drag force will be able to accelerate and travel faster than one with a high drag force. This means a smaller engine is required to drive such a car, which means less consumption of fuel.
CAR WEIGHT
As with the parts inside the engine, when the entire car is made lighter, through the use of lighter materials or better designs, less force is required to move the car. This is based on F=MA or more accurately, A=F/M, so as mass of the car decreases, the acceleration increases, or less force is required to accelerate the lighter car.
FRONT END
Frontal pressure is caused by the air attempting to flow around the front of the car. As millions of air molecules approach the front grill of the car, they begin to compress, and in doing so raise the air pressure in front of the car. At the same time, the air molecules traveling along the sides of the car are at atmospheric pressure, a lower pressure compared to the molecules at the front of the car. The compressed molecules of air naturally seek a way out of the high pressure zone in front of the car, and they find it around the sides, top and bottom of the car. Improvements at the front can be made by ensuring the ‘front end is made as a smooth, continuous curve originating from the line of the front bumper’. Making the screen more raked (ie. not as upright) ‘tends to reduce the pressure at the base of the screen, and to lower the drag’. However, much of this improvement arrives because a more sloped screen means a softer angle at the top where it meets the roof, keeping flow attached. Similar results can be achieved through a suitably curved roofs.
REAR END
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.’
Flow detachment applies only to the "rear vacuum" portion of the drag equation, and it is really about giving the air molecules time to follow the contours of a car's bodywork, and to fill the hole left by the vehicle, The reason keeping flow attachment is so important is that the force created by the vacuum far exceeds that created by frontal pressure, and this can be attributed to the Turbulence created by the detachment.
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's 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.
Where most road cars get into trouble is the fact that there is a large surface area on top of the car's roof. As the higher pressure air in front of the wind screen travels over the windscreen, it accelerates, causing the pressure to drop. This lower pressure literally lifts on the car's roof as the air passes over it. Worse still, once the air makes it's way to the rear window, the notch created by the window dropping down to the trunk leaves a vacuum, or low pressure space that the air is not able to fill properly. The flow is said to detach and the resulting lower pressure creates lift that then acts upon the surface area of the trunk.
Not to be forgotten, the underside of the car is also responsible for creating lift or down force. If a car's front end is lower than the rear end, then the widening gap between the underside and the road creates a vacuum, or low pressure area, and therefore "suction" that equates to down force. The lower front of the car effectively restricts the air flow under the car. So, as you can see, the airflow over a car is filled with high and low pressure areas, the sum of which indicate that the car body either naturally creates lift or down force.
WINGS & SPOILERS
What this wings or spoilers does is it prevents the separation of flow and there by preventing the formation of vortices or helps to fill the vaccum 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, and hence the car it's mounted to.
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. .
But how is the difference in pressure generated? Well, if you look closely at the drawings, you'll see that the upper side of the wing is relatively straight, but the bottom side is curved. This means that the air that goes above the wing travels a relatively straight path, which is short. The air under the wing has to follow the curve, and hence travel a greater distance. Now there's Bernoulli's law, which basically states that the total amount of energy in a volume of fluid has to remain constant. (Unless you heat it or expose an enclosed volume of it to some form of mechanical work) If you assume the air doesn't move up and down too much, it boils down to this: if air (or any fluid, for that matter) speeds up, its pressure drops. From an energetic point of view, this makes sense:
if more energy is needed to maintain the speed of the particles, there's less energy left do do work by applying pressure to the surfaces.
In short: on the underside, air has to travel further in the same amount of time, which means it has to speed up, which means its pressure drops. More pressure on top of the wing and less on the underside results in a net downward force called downforce.
CONCLUSION
Earlier cars were poorly designed with heavy engines , protruding parts and rectangular Shapes due to which they consumed large quantities of fuel and and became unaffordable all theses factors lead to the development and need of aerodynamics in the design of cars now it would be fair to say that all most all cars are tested for getting the optimum aerodynamic configuration.
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