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When objects move through air, forces are generated by the relative motion between the air and surfaces of the object. Aerodynamics is the study of these forces, generated by the motion of air, usually aerodynamics are categorized according to the type of flow as subsonic, hypersonic, supersonic etc.
It is essential that aerodynamics be taken in to 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 softwareâ„¢s are also available now days to ensure the optimal aerodynamic design.
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.
AERODYNAMIC FORCES ON A BODY
It is the sum of all fluid dynamic forces on a body normal to the direction of external flow around the body. Lift is caused by Bernoulliâ„¢s effect which states that air must flow over a long path in order to cover the same displacement in the same amount of time. This creates a low pressure area over the long edge of object as a result a low pressure region is formed over the aerofoil and a high pressure region is formed below the aerofoil, it is this difference in pressure that creates the object to rise
CL= Coefficient of Lift, dependent on the specific geometry of the object, determined experimentally
d= Density of air
V=Velocity of object relative to air, A=Cross-sectional area of object, parallel to wind
It is the sum of all external forces in the direction of fluid flow, so it acts opposite to the direction of the object. In other words drag can be explained as the force caused by turbulent airflow around an object that opposes the forward motion of the object through a gas or fluid.
where: CD= Coefficient of Drag, dependent on the specific geometry of the object, determined experimentally.
d= Density of air.
V=Velocity of object relative to air.
A= cross section of frontal area.
Since drag is dependent on square of velocity it is most predominant when object is traveling at very high speeds. It is the most important aerodynamic force to study because it limits both fuel economy of a vehicle and the maximum speed at which a vehicle can travel.
It is actually just the weight of the object that is in motion.i.e. the mass of the object multiplied by the magnitude of gravitational field.This weight has a significant effect on the acceleration of the object.
When a body is in motion a drag force is created which opposes the motion of the object so thrust can be the force produce in opposite direction to drag that is higher than that of drag so that the body can move through the fluid. Thrust is a reaction force explained by Newtonâ„¢s second and third laws, The total force experienced by a system accelerating in mass m is equal and opposite to mass m times the acceleration experienced by that mass.
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 1950â„¢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 didnâ„¢t take long before the issue of aerodynamics came back into the picture in the form of fuel economy. During the 1970â„¢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 1970â„¢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.
This diagram below shows the typical use of cars energy that it gets,
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.
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.
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.
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 Ëœfront end is made as a smooth, continuous curve originating from the line of the front bumperâ„¢. Making the screen more raked (ie. not as upright) Ëœtends to reduce the pressure at the base of the screen, and to lower the dragâ„¢. 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.
This graph clearly shows that drag force is directly proportional to frontal area.(results of wind tunnel tests)
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 Ëœraising and/or lengthening the boot generally reduces the drag. In plan view, rounding corners and Ëœall
forward facing elementsâ„¢ will reduce drag. Increases in curvature of the entire vehicle in plan will usually decrease drag provided that frontal area is not increased. ËœTapering the rear in plan viewâ„¢, usually from the rear wheel arch backwards, Ëœcan produce a significant reduction in dragâ„¢. Under the vehicle, a smooth surface is desirable as it can reduce both vehicle drag and surface friction drag. ËœFor a body in moderate proximity to the ground, the ideal shape would have some curvature on the underside.â„¢
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.
Scoops, or positive pressure intakes, are useful when high volume air flow is desirable and almost every type of race car makes use of these devices. They work on the principle that the air flow compresses inside an "air box", when subjected to a constant flow of air. The air box has an opening that permits an adequate volume of air to enter, and the expanding air box itself slows the air flow to increase the pressure inside the box. See the diagram below:
NACA stands for "National Advisory Committee for Aeronautics". NACA is one of the predecessors of NASA. In the early days of aircraft design, NACA would mathematically define airfoils (example: NACA 071) .
The purpose of a NACA duct is to increase the flowrate of air through it while not disturbing the boundary layer. When the cross-sectional flow area of the duct is increased, you decrease the static pressure and make the duct into a vacuum cleaner, but without the drag effects of a plain scoop. The reason why the duct is narrow, then suddenly widens in a graceful arc is to increase the cross-sectional area slowly so that airflow does separate and cause turbulence (and drag).
NACA ducts are useful when air needs to be drawn into an area which isn't exposed to the direct air flow the scoop has access to. Quite often you will see NACA ducts along the sides of a car. The NACA duct takes advantage of the Boundary layer, a layer of slow moving air that "clings" to the bodywork of the car, especially where the bodywork flattens, or does not accelerate or decelerate the air flow. Areas like the roof and side body panels are good examples. The longer the roof or body panels, the thicker the layer becomes (a source of drag that grows as the layer thickens too). Anyway, the NACA duct scavenges this slower moving area by means of a specially shaped intake. The intake shape, shown below, drops in toward the inside of the bodywork, and this draws the slow moving air into the opening at the end of the NACA duct. Vortices are also generated by the "walls" of the duct shape, aiding in the
scavenging. The shape and depth change of the duct are critical for proper operation.
Spoilers are used primarily on sedan-type race cars. They act like barriers to air flow, in order to build up higher air pressure in front of the spoiler. This is useful, because as mentioned previously, a sedan car tends to become "Light" in the rear end as the low pressure area above the trunk lifts the rear end of the car. See the diagram below:
Front air dams are also a form of spoiler, only their purpose is to restrict the air flow from going under the car.
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. Spoiler 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. See the diagram below:
Wings, by their design require that there be no obstruction between the bottom of the wing and the road surface, for them to be most effective. So mounting a wing above a trunk lid limits the effectiveness.
To calculate the aerodynamic drag force on an object, the following formula can be used:
F = Ã‚Â½ CDAVÃ‚Â² Where:
F - Aerodynamic drag force
C - Coefficient of drag
D - Density of air
A - Frontal area
V - Velocity of object
In this system, D as air density is expressed in kg/mÃ‚Â³. The frontal area is the surface of
the object viewed from a point that object is going to. It's expressed in mÃ‚Â³. The better(lower) the number is the easier it is for air to pass around a car
It is the measure of the aerodynamic efficiency of the car .
METHODS FOR EVALUATING AERODYNAMCIS OF 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.
Now a days the aerodynamic studies are not constrained to the flow of air past cars but also a number of other factors like new methods are developed to provide a greater level of detailed information. Special pressure sensitive paint is now used in the wind tunnel to graphically show levels of air pressure on a vehicle how it is done is that ,Two different images are obtained, one at normal room air pressure (wind-off) and a second in which the wind tunnel is running (wind-on) at a desired test speed. These differences in color, from wind-off to wind-on, are used to calculate surface pressure.
A bank of blue lights illuminate the car to be tested that has pressure-sensitive paint applied on the driver's side window. The car and lights are in a wind tunnel at Ford Motor Company's Dearborn Proving Ground. Ford researchers have developed a computerized, pressure-sensitive paint technique that measures airflow over cars, shaving weeks off current testing methods. A digital camera near the blue lights captures this information and feeds it into a computer, which displays the varying pressure as dramatically different colors on a monitor.
The images obtained from tests in the wind tunnel are captured on computer. They can then be used to study air flow patterns across a vehicle, highlighting areas of possible refinement or improvement. Additionally, actual data from a production ready model can be compared with pre-production computer predictions which can in turn help improve the accuracy of the early design stages.
Now a days a large number of softwareâ„¢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 softwareâ„¢s are developed for the design and analyzing aerodynamics the
most commonly used softwareâ„¢s are ANSYS,CATIA.
Here are some of the features of commonly used software Alias surface studio
ALIAS SURFACE AND AUTO STUDIO
Alias Surface Studio is a technical surfacing product designed for the development surfaces. It offers advanced modeling and reverse engineering tools, real-time diagnostics and scan data processing technology. Surface Studio is comprised of a complete suite of tools for creating surface models to meet the high levels of quality, accuracy and precision required in automotive styling.
This software performs all the basics of design right from the sketching to evaluation.
A user interface that enables creativity and efficiency
A complete set of tools for 2D design work tightly integrated into a 3D modeling environment
3)2D / 3D Integration
Take advantage of your sketching skills throughout the design process. Add details and explore ideas quickly by sketching over 3D forms before taking the time to model them.
Industry-leading, NURBS-based surface modeler.
5) Advanced Automotive Surfacing Tools
Surface creation tools that maintain positional, tangent or curvature continuity between surfaces - for high quality, manufacturability results.
6) Reverse Engineering
Tools for importing and configuring cloud data sets from scanners for visualizing, as well as extracting feature lines and building surfaces based on cloud data.
7) Evaluation Tools
Tools to analyze and evaluate the styling and physical properties of curves and surfaces interactively, while creating and editing geometry.
Create photorealistic images using textures, colours, highlights, shadows, reflections and backgrounds.
Animations can be used for high quality design presentations, design analysis of mechanisms, motion and ergonomic studies, manufacturing or assembly simulation.
Support for industry-standard data formats and a wide range of peripheral devices.These softwareâ„¢s are now commonly in use as wind tunnel testing is an expensive process as compared to this softwareâ„¢s where we get more accurate and easily the test results.
AERODYNAMIC DESIGN TIPS
.) Keep the vehicle low to the ground, with a low nose, and pay attention to
the angle of wind shield.
.) Cover the wheel wells, Open wheels create a great deal of drag and air flow turbulence
.) Enclose the under carriage (avoid open areas-convertibles, etc.)
.) Make corners round instead of sharp
.) The underbody should be as smooth and continuous as possible, and should sweep out slightly at rear.
.) There should be no sharp angles (except where it is necessary to avoid crosswind instability ).
.) The front end should start at a low stagnation line, and curve up in a continuous line.
.) The front screen should be raked as much as is practical.
. ) All body panels should have a minimal gap.
.) Glazing should be flush with the surface as much as possible.
.) All details such as door handles should be smoothly integrated within the contours.
.) Minor items such as wheel trims and wing mirrors should be optimized using wind tunnel testing.
.) Using spoilers or wings.
FOR A VEHICLE YOU ALREADY OWN
Â¢ Keep your vehicle washed and waxed. This reduces skin friction.
Â¢ Remove mud flaps from behind the wheels.
Â¢ Add a spoiler to the front fender or the rear of the car. Having it on the front fender reduces air flow beneath the car, while having it behind will decrease the low pressure behind the car and reduce drag.
Â¢ Close your windows, put your top up, and close your sun roof. All at once!
Â¢ Avoid having roof-racks and carriers on your car.
Â¢ For pickups: cover the back, take the gate off, or at least leave the gate open. Air gets trapped in the bed and causes major drag.
Â¢ Place your license plate out of the air flow
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.
1) Road Vehicle Aerodynamic Design , Barnard R.H.
2) Introduction to Aerodynamics by Anderson.
List of figures
2. Aerodynamic forces on a body
3. History and evolution of aerodynamics
4. Study of Aerodynamic forces on cars
b) Lift or Downforce
5. Aerodynamic devices
6. Drag Coefficiant
7. Methods for evaluating Aerodynamics in cars
a) Wind tunnels
8. Aerodynamic Design tips