Aerodynamics in a nutshell
In 1738, scientist Daniel Bernoulli discovered that an increase in airflow velocity (speed) in relation to the surrounding free air stream causes a decrease in pressure where the faster flow occurs. What that means is that, 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. Proof of his force can easily be created when you put your hand out of the window of a moving car. John H. Lienhard observes, "When I was a kid, I entertained myself on long auto trips by putting my hand out the window and turning it at various angles to the wind. The forces, even on a child's small hand, were quite strong. And small changes in the shape and orientation of my hand made huge differences."
From here, the concept of aerodynamics took a disconcertingly complicated turn for the worse. One of Bernoulli's students, Leonhard Euler penned the Euler equations over a 25-year period around 1750. The equations accurately represent the flow of any fluid, including air. The unfortunate part about his equations was that nobody could solve them, so they remained theories. The key issue missing from Euler's description of fluid motion was the problem of friction, or what modern aerodynamicists call skin drag. During the early 19th century, two mathematicians, Frenchman Louis Navier and Englishman George Stokes, independently arrived at a set of equations that were similar to Euler's but included friction's effects. Known as the Navier-Stokes equations, these were by far the most powerful equations of fluid motion, but they too were unsolvable until the mid-20th century. Of course, one person's theory leads to others, equally complicated: Hermann von Helmholtz's concept of vortex filaments (1858), Frederick Lanchester's concept of circulatory flow (1894), and to the Kutta-Joukowski circulation theory of lift (1906).
The funny thing to note here is that none of these concepts were used to create the mother of all aerodynamics: flight. The Wright Brothers used trial and error, since they were bicycle repairmen not mathematicians, and they hadn't heard of a wind tunnel until afterward. This is all fine, well, and good for those in the know, but concepts need to be practical and applicable to the automobile. So, let's break it down.
Examples of vehicles designed primarily on car aerodynamics are those that lie low to the ground, are sleek in design, and have rounded lines with reclining windshields that allow the air to easily flow over and around, rather than "butt up" against flat or vertical surfaces. Two words: sports cars. Specifically those that come to mind are Lotus, Lamborghini, Ferrari F355, McLaren F1, and so on. But, what allows them to cut through the air better than, say, a bus or a blunt-nosed SUV? How does air move about a typical car under speed?
Most cars on the road are aerodynamically sound enough on their own to not really need any help from fancy aerodynamic accessories-that is, at legal speeds, of course. Any modern car, for example, fits this category, and under normal driving conditions it maintains its integrity throughout the trip. However, for those on the track, the benefits and effects of the forces created by the air are severe and evident. When you aim your tires toward the timing lights at the end of the quarter-mile or through the paces on a road course, you can easily reach speeds where the forces of physics take over and have a drastic effect on the car's performance (not to mention the outcome of the race).
If you were to place any modern car inside a wind tunnel, such as a Mercedes sedan, a type of car known as a three-box design-meaning it has a hood, a cabin and a trunk, all roughly box shaped-air would flow up the hood, over the windshield and across the roof. The majority of the airflow leaves the car straightly at the end of roof line. The dramatic drop of the rear window and decklid creates a low pressure area around the back of the car. This low pressure acts as a vacuum that sucks some air back toward the car, thus creating turbulence. In addition, the rear window's roughly 45-degree angle causes the airflow to be particularly unstable on a high-speed Mercedes. Turbulence always deteriorates drag coefficient, in effect adding weight to the car.
But, to know what exactly happens at the rear of your car at speed, you have to first start at the front. To help answer this, let's turn to an unlikely source, one of the industry's most beloved but least aerodynamically sound cars, an old Volkswagen Bus. "If a box on wheels is what they want," said VW head Heinz Nordhoff in 1946, "then a box on wheels is what they'll get." As the blocky shape of the Bus drives down the road, it literally punches a hole in the air, which is forced out of the way via the four sides of the box. This force is called Frontal Pressure, which creates high pressure as the air rams into the front of the Bus and packs in together. The usual measure of aerodynamic efficiency is the drag coefficient, Cd. It compares the drag force, at any speed, with the force it'd take to stop all the air in front of the car. Drag coefficients for the first boxy autos with large frontal surface areas were up over 0.70. Instead of letting the air slip past, they brought most of it to a halt right in front of the car (think of the flat grille and headlights of an older Volvo).