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 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 travelling along the sides of the car are at atmospheric pressure, a lower pressure compared to the molecules at the front of the car.

Just like an air tank, if the valve to the lower pressure atmosphere outside the tank is opened, the air molecules will naturally flow to the lower pressure area, eventually equalizing the pressure inside and outside the tank. The same rules apply to cars. 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. See the diagram below.

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. See the diagram below.

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 tires, it's suspension and protrusions (ie. mirrors, roll bars). If you have witnessed the Le Mans race cars, you will have seen how the tails of these cars tend to extend well back of the rear wheels, and narrow when viewed from the side or top. This extra bodywork allows the air molecules to converge back into the vaccum smoothly along the body into the hole left by the car's cockpit, and front area, instead of having to suddenly fill a large empty space.

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.

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 (within reason) to provide the least amount of turbulence at high speed. See diagram below (Light green indicates a vacuum-type area behind mirror):

Lift (or Downforce)
One term very often heard in race car circles is Downforce. Downforce is the same as the lift experienced by airplane wings, only it acts to press down, instead of lifting up. Every object travelling through air creates either a lifting or downforce 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.

How does a car generate this low pressure area? According to Bernoulli, the man who defined the basic rules of fluid dynamics, for a given volume of air, the higher the speed the air molecules are travelling, 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, we said 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) downforce. 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 accellerates, 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. This can be seen in old 1950's racing sedans, where the driver would feel the car becoming "light" in the rear when travelling at high speeds. See the diagram below.

Not to be forgotten, the underside of the car is also responsible for creating lift or downforce. 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 downforce. The lower front of the car effectively restricts the air flow under the car. See the diagram below.

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 downforce.

Drag Coefficient
The 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.
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.
Have a slightly raked underside, to create low pressure under the car, in concert with the fact that the minimal ground clearance mentioned above allows even less air flow under the car.
If it sounds like we've just described a sports car, you're right. In truth though, to be ideal, a car body would be shaped like a tear drop, as even the best sports cars experience some flow detachment. However, tear drop shapes are not condusive to the area where a car operates, and that is close to the ground. Airplanes don't have this limitation, and therefore teardrop shapes work.

What all these "ideal" attributes stack up to is called the Drag coefficient (Cd). The best road cars today manage a Cd of about 0.28. Formula 1 cars, with their wings and open wheels (a massive drag component) manage a minimum of about 0.75.

If we consider that a flat plate has a Cd of about 1.0, an F1 car really seems inefficient, but what an F1 car lacks in aerodynamic drag efficiency, it makes up for in downforce and horsepower.

Frontal Area
Drag coefficient, by itself is only useful in determining how "Slippery" a vehicle is. 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, you 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.

Aerodynamic Devices

Scoops
Scoops, or positive pressure intakes, are useful when high volume air flow is desireable 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 Ducts
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 accellerate or decellerate 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. Vorticies 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.

Typical uses for NACA ducts include engine air intakes and cooling.

Spoilers
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.

Wings
Probably the most popular form of aerodynamic aid is the wing. Wings perform very efficiently, generating lots of downforce 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.

As the car travels through the air, some of the airflow is directed around the nose and to the sides of the front fenders and wheelwells. If the sides of the nose are angled from a top view, the air will flow out and away from the wheelwells, creating a low-pressure area just outside the wheels. The air under the hood is mostly stationary, and some of that air rushes to the wheelwells to fill the void created by this suction. This creates a low-pressure area under the hood, and the pressure differential between the top and bottom creates the downforce needed to provide more grip in the front of the car.