Vehicle Aerodynamics

The word comes from two Greek words: aerios, concerning the air, and dynamis, meaning powerful

The word aerodynamics is derived from two Greek words aerios, relating to the air, and dynamis, meaning powerful. Aerodynamics is the branch of dynamics that deals with the motion of air and other gaseous fluids and with the forces acting on bodies in motion relative to such fluids.A body in motion is affected by aerodynamic forces. The aerodynamic force acts externally on the body of a vehicle. The component of the resultant aerodynamic force which opposes the forward motion is called the aerodynamic drag. The aerodynamic drag affects the performance of a car in both speed and fuel economy as it is the power required to overcome the opposing force. The other component, directed vertically, is called the aerodynamic lift. It reduces the frictional forces between the tyres and the road thus changing dramatically the handling characteristics of the vehicle. The aerodynamic force is the net result of all the changing distributed pressures which airstreams exert on the car surface. Therefore aerodynamic studies are very important as far as the car stability is concerned.The aerodynamics of a car includes different areas for consideration. Forces created by the relative motion of the vehicle through air (drag force, lift force, down force), noise produced by the air flowing around the car body, use of the air flowing within the cars body for other purposes such as cooling the engine or brakes.The main concerns of automotive aerodynamics are reducing drag, reducing wind noise, and preventing undesired lift forces at high speeds. For some classes of vehicles, it may also be important to produce desirable downwards aerodynamic forces, to improve cornering.

The road vehicle industry started taking into consideration the aerodynamics of the vehicles in the early 1900. In the first stages of the development of self-propelled vehicles, the shapes and designs of the vehicles was inherited from horse driven carriages. The first automobiles however were moving with low speeds on bad roads. There was no need in examining the aerodynamic nature of a vehicle. The driver and passenger could be protected from wind, rain and mud with the simple, traditional design of horse-drawn carriages. The increase of automobile speed resulted in the exposure of drivers and passengers to the airstreams. As a result the introduction of structural parts such as windscreens was developed to protect the occupants from airstreams effects. Gradually the development increased always according to the needs of both the aerodynamic effects and art properties (shape, style) around a vehicle.

The first automobile to be developed according to the aerodynamic principles was a torpedo-shaped vehicle that had given it a low drag coefficient but the exposed driver and out of body wheels certainly disturbed its good flow properties. In a car like this, the ground along with the free-standing wheels and the exposed undercarriage caused disturbed flow.

As the years passed the studies on aerodynamic effects on cars increased and the designs were being developed to accommodate for the increasing needs and for economic reasons. The wheels developed to be designed within the body, lowering as a result the aerodynamic drag and produce a more gentle flow. The tail was for many years long and oddly shaped to maintain attached streamline. The automobiles became developed even more with smooth bodies, integrated fenders and headlamps enclosed in the body. The designers had achieved a shape of a car that differed from the traditional horse-drawn carriages. They had certainly succeeded in building cars with low drag coefficient.

Road conditions have limited the width of automobiles. It is said this width was established by the width needed for two horses running comfortably side by side drawing a carriage. Length is not as much of a restriction but long bodies were not efficient enough for traffic use.

They hadnt yet been able to do something with the long tail. It offered so much in the aerodynamic effects that it was difficult to design a low-drag vehicle without a long tail. The designers tried to find solutions in reducing drag, producing attached and gentle flow with working with small details on the body and shape optimization. Details such as radii of edges and pillars, camber of panels, tapering, size and location of spoilers can be varied to give smoother flow and thus less drag. Later in addition to that the aerodynamists started with a basic body which is a one volume body having the dimensions of the final car. Many improvements in car design made it able to achieve a body with very low drag coefficient from the 1980s but some of them were followed by complaints. For example there were many complaints for the windows of cars being inclined more and more, allowing sunrays to enter through even larger glass panels and heat up the car interior. Indeed, it does not seem rational to reduce drag coefficient in order to save fuel, but require an air-conditioning system that uses up all the fuel just saved.The designers nowadays are trying to develop, the concept of the car as a mean of comfortable transportation which could offer to the occupants safety and most of all style.

Aerodynamic drag

In order to explain the Aerodynamic drag the two forces - the frontal pressure and the rear vacuum have to be analyzed.

Frontal pressure is caused by the air attempting to flow around the front of the car. As millions of air molecules approach the front part 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.

Rear vacuum or wake is caused by the "hole" left in the air as the car passes through it. This empty area is a result of the air molecules not being able to fill the hole as quickly as the car can make it. The air molecules attempt to fill in to this area, but the car is always one step ahead. As a result, a continuous vacuum in the rear of the car sucks in the opposite direction of the motion of the car. This inability to fill the hole left by the car is technically called Flow detachment.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, it's tyres, it's suspension and protrusions (i.e. mirrors, roll bars).

The flow attachment is very important because the drag created by the vacuum far exceeds that created by frontal pressure, and this can be attributed to the turbulence created by the detachment. That is why in the early years of automotive industry the cars used to be designed with long tail. This was done as to maintain the streamlines created by the flow, attached.

Turbulence generally affects the "rear vacuum" portion of the drag equation, but if we look at a protrusion from the race car such as a mirror, we see a compounding effect. For instance, the air flow detaches from the flat side of the mirror, which of course faces toward the back of the car. The turbulence created by this detachment can then affect the air flow to parts of the car which lie behind the mirror. Intake ducts, for instance, function best when the air entering them flows smoothly. Therefore, the entire length of the car really needs to be optimized to provide the least amount of turbulence at high speed.

Lift (or Down force)

One term very often heard in race cars is down force. Down force is the same as the lift experienced by airplane wings, only it acts in the opposite direction, 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.

According to Bernoullis principle, for a given volume of air, the higher the speed the air molecules are traveling, the lower the pressure becomes and 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, we said that the air pressure was high as the air rammed into the front area of the car. Actually, 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, to suck the hood off the car. The higher pressure area in front of the windscreen creates a small down force. This is similar 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 its 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. This results in the rear of the car to feel lighter.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.Concluding, the airflow over a car is filled with high and low pressure areas, the sum of which indicates that the car body either naturally creates lift or down force.Drag CoefficientThe shape of a car, as the aerodynamic theory above suggests, is largely responsible for how much drag the car has. Ideally, the car body should:

Have a small grill, to minimize frontal pressure.

Have minimal ground clearance below the grill, to minimize air flow under the car. In combination to this, a raked underside with the rear of the car raised can create down force.

Have a steeply raked windshield to avoid pressure build up in front.

Have a "Fastback" style rear window and deck, to permit the air flow to stay attached.

Have a converging "Tail" to keep the air flow attached.To be ideal, a car body would be shaped like a tear drop, as even the best sports cars experience some flow detachment. What all these "ideal" features results to is called the Drag coefficient (Cd).

The best road cars today manage a Cd of about 0.28. Formula 1 car, with their wings and open wheels only manage a minimum drag coefficient of about 0.75. It really seems inefficient, but what an F1 car lacks in aerodynamic drag efficiency, it makes it up for in down force and horsepower. To understand the full picture, we need to take into account the frontal area of the vehicle. One of those new aerodynamic semi-trailer trucks may have a relatively low Cd, but when looked at directly from the front of the truck, we realize just how big the Frontal Area really is. It is by combining the Cd with the Frontal area that we arrive at the actual drag induced by the vehicle.

WIND TUNNELS

In automotive industry before a car design is sent for production, it is tested for aerodynamic efficiency. High performance vehicles are primarily characterized by high power-to-weight ratio. Powerful acceleration, massive deceleration, excellent control, combined with a high top speed and relatively low fuel consumption is things to consider in building a race or sports car.

There are three categories in which high performance cars can be divided. The sports cars which are designed for everyday use on public roads, the racing cars; built for competing with other cars in the same category on race circuits and the record cars which are specifically designed for high speed or maximum acceleration.

In all three categories the aerodynamics of such cars are of vital importance. They affect the cars stability and handling. They influence both performance and safety.

The main focus in race cars is on the down force and drag. The relationship between drag and down force is especially important. Aerodynamic improvements in wings are directed at generating down force on the race car with a minimum of drag. Down force is necessary for maintaining speed through the corners.

A track with low speed corners requires a car setup with a high down force package. A high down force package is necessary to maintain speeds in the corners. This setup includes large front and rear wings. The front wings have additional flaps which are adjustable. The rear wing is made up of more than one section that maximizes down force.

The setup for a fast circuit with long straights and not so many low speed corners looks much different. The front and rear wings are almost flat and are used as stabilizers. The major down force is found in the shape of the body and underbody as explained above. Drag reduction is more critical on the fast circuit than on other circuits. Effective use of down force is especially pronounced in high-speed corners.

Bernoullis principle

One of the fundamental laws governing the motion of fluids is Daniel Bernoulli's principle, which relates an increase in flow velocity to a decrease in pressure. For example, for the same volume of air at the entry to the venturi tube below to pass through the constriction in the middle, the air must speed up. Based on Newton's theory that energy cannot be created or destroyed, just transferred, this increased speed must have a corresponding decrease in pressure, if the same volume of air is to move through the tube. As the air exits the constriction, it slows and regains its original pressure.

Bernoulli's principle is used in aerodynamics to explain the lift of an airplane wing in flight. A wing is so designed that air flows more rapidly over its upper surface than its lower one, leading to a decrease in pressure on the top surface as compared to the bottom. The resulting pressure difference provides the lift that sustains the aircraft in flight. If the wing is turned upside-down, the resultant force is downwards. This explains how performance cars corner at such high speeds. The down force produced pushes the tires into the road giving more grips.

Aerodynamics is found in everyday life in automotive applications, in passenger and commercial cars. The aerodynamic flow of air around every body in motion, as mentioned above, is affected by the shape of the body. This however affects the stability, the maneuverability of the car. The flow of air around the vehicle also causes noise.

The passenger cars have to be designed in such a way to fulfill the needs of the driver and passengers. These are safety, comfort, reduced noise, ventilation of passenger area. Passenger cars are required to have style. The shape has to be acceptable by the people to buy the cars. The marketing comes into the equation and in order for an automotive industry to sell its products; it must design them to look nice. The people buying the cars do not know about the aerodynamic but still the designer need to make the car within acceptable limits basically for fuel economy.

Nowadays cars are changed by their owners (young) to make the look sportier. They somehow have the need to get more out of their car, downforce, stability, better handling, and more power. Having more power under the hood leads to higher speeds for which the aerodynamic properties of the car given by the designer are not enough to offer the required downforce and handling. Extra parts are added to the body like spoilers, lower front and rear bumpers as to direct the airflow in different way and offer greater handling to the car.

Spoilers for example 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.

Front air dams are also a form of spoiler, only their purpose is to restrict the air flow from going These components combine to produce huge amounts of downforce, helping to keep the car planted through corners at high speeds. They also improve braking performance and acceleration due to the added traction.There is a price for this amount of downforce i.e. Drag. Redirecting the energy of the airflow to hold a car down creates more resistance for the car to push against. Although drag reduces top speed somewhat, the increase in cornering speeds makes for faster cornering, improved braking and acceleration so some drag is acceptable. Many race cars have drag coefficients of over 1.1, while modern production cars around 0.35. The big difference is that race cars have enough horsepower to compensate.

The key is to produce just enough downforce to maximize the average speed around a corner. If we produce too much downforce, the increased drag will slow the car excessively, too little downforce will hurt cornering speeds. It usually takes some experimenting with wing settings and other components to find the sweet spot for optimal performance.Keeping in mind that the basic shape of a production car generates a lifting force when moving through the air. The lift characteristics increase as speed increases. As a result, several aero components are mounted at the front and rear of the car. For example, a wing is usually mounted on the trunk lid; an air dam is attached to the front bumper cover. The effect produced by the component will generally be felt around the area it is mounted. Consequently, the rear tyres will 'feel' more of the wing's downforce than the front tyres. Since a wing is usually mounted behind the rear wheels, there will be a decrease in the load acting on the front tyres due to the fulcrum effect. Downforce from the wing will actually lift the front of the car.

If a wing is added to an already 'balanced' car, then the tendency will be to increase understeer because of the slight front-end lift. Since most production-based cars are designed to understeer, the addition of a wing will make this tendency even worse at higher speeds. To correct understeer, the easy fix is to add downforce to the front. A properly designed air dam - with or without a splitter - will add some much-needed downforce. To improve high-speed handling, we would normally add downforce in proportion to the car's lengthwise weight distribution. In many front-wheel-drive cars, which have about a 60/40 split, adding downforce in the same percentages to the front and rear retains a balanced handling feel.

The size and design of the wing and the size and type of the air dam and splitter will determine how the high-speed handling will be affected. Usually the air dam is a fixed size and shape, whereas the wing can be adjusted for more or less downforce, depending on the wing's angle of attack relative to the oncoming air stream. Increasing the nose-down attitude will result in more downforce - up to a point. Inverted wing

An inverted wing is a device that generates downforce by creating a pressure difference between the top and bottom wing surfaces. The oncoming air splits at the wing's leading edge, where some air goes over and the rest goes under the wing. Because of the wing's profile, the air going over the top is moving slower than the air on the bottom. In addition, Bernoulli's law states that slower-moving air possesses a higher static pressure. As a result, the higher-pressure air on top pushes down more than the lower pressure air on the bottom pushes up. This pressure difference creates downforce. The presence of the wing modifies airflow over the car, resulting in slight pressure differences that need to be considered for the generation of overall downforce.

Most wings have a constant cross section along the wingspan. Other more sophisticated wings change in both airfoil type and size in addition to a step in the wing's angle of attack (at approximately 20 to 25 per cent of the wingspan) towards the end of the span. These complex three dimensional '3D' wings can be more effective than the simple examples because the design takes into account the actual flow arriving at the wing. At both ends, the air coming off the rooftop-to-window juncture has a different angle of approach compared with the air going over the middle of the roof. By designing a wing to take into account this local airflow condition, more downforce and less drag can be achieved.DiffusersThe diffusers help to drive the low-pressure from beneath the car. The most common one is the upswept duct at the rear and below the bumper. The other type is located directly behind the splitter leading into the front wheel wells. Aerodynamically, both of these diffusers achieve the same thing i.e. minimising pressure under the carA rear diffuser helps drive the under-car flow by exposing it to the turbulent low-pressure wake region behind the car, using this low pressure to suck the flow out. In addition, the diffuser slows the air emerging from the underbody region by expanding it through a larger-area opening. They are effective in generating large amounts of downforce by increasing air speed underneath, thereby reducing pressure. Since this low-pressure region acts on a large surface area, plenty of downforce can be generated. Even if pressure below the diffuser is only half a psi lower than outside, over a 3x6-foot area, that equates to over 1000 pounds of downforce.

Vertical fences are installed within the diffuser channel to ensure that flow remains attached to the diffuser. Side skirts

Side skirts are used to reduce the amount of air that goes under the car from the sides. If an air dam is used, air under the car is at a low pressure, which causes the higher-pressure air on the outside of the car to come rushing in. The effectiveness of the skirts depends primarily on how close to the ground the lower edge can be maintained. That edge should be less than a half-inch from the ground, otherwise the skirts' effectiveness diminishes rapidly as the gap increases.Air Dams and Splitters

Air dam's job is to restrict the amount of air going under the car. By using a vertical barrier made from either a composite material or aluminium sheet, the air dam effectively reduces the opening leading to the underside of the car. By restricting flow under the car, more air is forced around the sides and over the top of the bodywork at higher pressure. The limited air forced underneath has to pass through faster and thus at a lower pressure which causes a suction effect. Air dams are more common in production cars with higher ride heights and bumpers.

Splitters, the horizontal plate extending forward and underneath the air dam, use the same principle but operate differently. Since the front of the car is a blunt shape, the oncoming air is slowed substantially, resulting in a high-pressure zone known as a stagnation point. By placing a horizontally protruding splitter plate right in the thick of this high-pressure zone, a large amount of efficient downforce can be generated. The splitter splits the high-pressure zone from the low-pressure high-speed flow moving under the car. Pressure varies with the car's speed squared, so downforce increases quickly as the speed increases. Generally, the effects are felt at speeds over 75mph. Downforce can be increased or decreased, depending on the amount of exposed splitter area, and an adjustable splitter area can be used to fine-tune the aerodynamic balance.