By Ryan Lee Price

 BMW Aero 05 1200
Have you ever pushed your hand through water and noticed the divot you created (and noticed how the water swells up in front of your hand as it moves)? The water attempts to fill in behind your hand, but your hand is always one step ahead. As a result, a continuous vacuum sucks in the opposite direction of your hand, pulling water with it. This is the essence of Bernoulli’s Principle and the basics of aerodynamics as we know it today.
In 1738, scientist Daniel Bernoulli discovered that for a given volume of air, the higher the speed  the faster the air molecules are traveling, the lower the pressure becomes. For nearly three hundred years after Bernoulli, many talented mathematicians added to the principles of aerodynamics: Leonhard Euler’s inviscid flow (1750s), Navier and Stokes’ motion of fluid (1840s), 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).
If you were to place any modern “three-box-design” car (meaning it has a hood, a cabin and a trunk, all roughly box shaped) inside a wind tunnel, air would flow up the hood, over the windshield and across the roof. The majority of the airflow leaves the car at the end of the roof line. The slope of the rear window and deck lid 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.
Mercedes sedan in wind tunnel
To know what happens at the rear of your car at speed, you have to start at the front. To help answer this, let’s turn to one of the world’s least aerodynamic cars, an old Volkswagen Bus. 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 six sides of the box. This force is called “frontal pressure,” because air slows down as it approaches the front of the Bus, causing more molecules to be packed into a smaller space. At speed, the space directly behind the Bus is nearly devoid of air. This empty space is caused by the air molecules not being able to fill the hole as quickly as the Bus can make it. It is called flow detachment.
SUV in wind tunnel
Now, back to the three-box car. As the air flows over the hood of the car, it loses pressure and creates a small lifting force (like trying to suck the hood off the car), but when it reaches the windshield, it again comes up against a barrier and briefly reaches a higher pressure. The higher pressure area in front of the windscreen creates a down force. This is like pressing down on the windshield and slowing the car, while the front end is lifted upward. As the higher pressure air in front of the windshield travels over the glass, it accelerates, causing the pressure to drop. This pressure reduction lifts the car's roof as air passes over it, while air passing underneath the car adds additional lift. These forces create a tight-wire act, balancing between too much lift and too much drag.
Colin Chapman invented a new concept to provide down force without altering drag, called ground effect. He incorporated an air channel into the bottom of his Lotus 72 racer, narrow in front and wide in the rear. Since the car’s bottom was nearly touching the ground, the combination of channel and ground formed a closed tunnel. When the car moved forward, air entered the tunnel in the nose and expanded outward toward the tail. Air pressure was reduced at the tail, creating down force.
Colin Chapman's 72 Lotus Racer
The ultimate example of the down force concept was the Brabham Alfa BT46B, designed by Gordon Murray for the 1978 Swedish Grand Prix, which actually used a cooling fan to extract air from the skirted area under the car, creating enormous down force and hence amazing handling capabilities. After technical challenges from other teams, it was withdrawn after a single race and the practice was banned.
Brabham Alfa BT46B
On the three-box example, once air makes its way to the rear window, the drop created by the window and the flat trunk of the car leaves a sizable vacuum, a low pressure space that air is not able to fill quickly. The airflow detaches and the resulting lower pressure creates turbulence, which always deteriorates the drag coefficient.
For perspective, the worst possible aerodynamic streamlining might be expected from a parachute, which is designed to maximize wind resistance. The Coefficient of drag (Cd) of a parachute is about 1.35. The least possible resistance might be from an airplane wing, which has a Cd of about 0.05. Automobile Cd figures lie between these two extremes. In the past 80 years, automakers have managed to cut Cd figures for production models nearly in half, from about 0.70 to about 0.30. In a practical sense, gas mileage increases by five percent for every 10 percent improvement in aerodynamics, and in times of increased fuel costs, driving a car with a high Cd can be, quite literally, a drag.

Not only is Ryan Lee Price a freelance writer specializing in automotive journalism and a former long-time magazine editor, he is part of the technical editorial team that provides content for most all of the ChiltonPRO and ChiltonDIY products. He currently resides in Corona, California, with his wife Kara and their two children.