“The blue one?”, my husband asked, with a look on his face normally associated with a man in the diary aisle on a cell phone, except that I was standing right there.

“Not the blue label,” I said, “The blue milk”.

Yep. You can get blue milk. Also pink, as well as the standard white and chocolate. Lots of things come in unexpected colors these days – even air. Last week, ESPN introduced Draft Track, a computer enhancement that lets you “see” air going around the racecars.

Air is made of molecules and, when a car gets in the way, the air molecules have to go around. The car exerts a force on the air molecules and, by Newton’s third law, the air molecules exert equal and opposite forces on the car. This is the origin of downforce and drag, both of which play huge roles in how fast the car can go.

You can’t ‘see’ how the air flows over a car without some help. The first visualization technique was developed and used in wind tunnels. A wand held in front of the car releases smoke that has the same density as air. The smoke travels over the car and lets you see streamlines. Streamlines are lines that are tangent to the velocity of the airflow, which means they show the path the air molecules are taking.

The smoke technique is a little limited because is a little tough to use on the track. As computers got more powerful, it became possible to simulate the motion of air over a racecar using a technique called computational fluid dynamics (or, as EPSN likes to call it, ‘computative fluid dynamics’).

Describing the motion of a solid object, like a car, is fairly simple. If you know the initial position and velocity of a car, along with its acceleration, you can calculate where the car is at any time. It’s pretty easy to do in one dimension, a more complicated in 2D or 3D, but it’s still fairly straightforward.

The motion of most objects can be described by Newton’s Second Law (F = ma). It’s pretty easy to do for solids, because the individual atoms that make up a solid are connected to each other well and move as a single unit. The problem is that the molecules in a fluid (like air or water) aren’t as strongly attached to each other. Compare throwing a baseball with throwing a handful of water. The motion of fluids requires solving a much more complicated set of equations called the Navier-Stokes equations. The Navier-Stokes equations are differential equations, which are harder to solve than algebraic equations like F=ma.

Computational Fluid Dynamics (CFD) programs use computers to solve the Navier-Stokes equations for specific situations. A race engineer has to create a mesh model of the car in the computer, and – depending on how accurate the results need to be – also has to account for things like the effect of the track and the rotation of the wheels. It takes millions and millions of calculations just to get simple information like how fast air moves across a particular part of the car. Even with as much computer power as we have today, the computers can’t take every single detail into account, so the solutions are usually approximate.

A particularly good example of the power of CFD can be found in an article from GM Racing. The lines in the pictures below are called streamlines, which show how air flows over the car. The streamlines are colored and the color code follows the rainbow. Red areas indicate high pressure and blue or violet indicate lower pressure.

The two simulations show how air moves over the COT (top) and the old car (bottom), and really illustrate how the air provides different amounts of force on different parts of the car. For example, the high-pressure area on the decklid (a.k.a. the trunk) that has high pressure is larger on the old car than on the COT.

Airflow can be laminar (meaning in lines) or turbulent, which means that the airflow is chaotic and rotational. You can see the transition in smoke from a cigarette: The smoke starts off near the cigarette in straight, distinct lines, and then becomes turbulent, forming curlicues as it moves away from its source. The GM Racing article also shows a simulation that predicts significantly less turbulence behind the COT than behind the car we’re running this weekend at Pocono.

I say predicts because, despite the enormous amount of computer power available, the CFD simulations can’t include everything. (The initial feedback from most drivers isn't in total agreement with the CFD predictions.) The aerodynamicists creating and using these programs have to decide what factors they think are most important and what they think can be safely ignored. Using CFD in your race program isn’t like loading up Microsoft Word on your PC. It takes a lot of expertise to create a CFD simulation, and it takes expertise to interpret the results correctly. Like most technology, it can be used or mis-used, which brings us back to the blue air.

From what I can surmise given ESPN’s very limited explanation – is that yellow air represents laminar flow and blue indicates turbulence. At Talladega, they introduced something called "Draft Lock", which was supposed to indicate when the two cars were close enough that they were drafting each other. 'Air usually flows smoothly over the car until it starts to separate from the car’s surface. Separation usually occurs toward the rear of the car, which explains why the air is mostly yellow in front and blue in back. (Note: They switched the meaning of the colors after the first race.) I'm not sure what they are showing, but they don't appear to be streamlines.

It's an impressive technology, developed by a professor from the University of Washington, Zoran Popović, who researches ways to speed up computations for things like virtual reality and computer games. You can read more about the technique in his group's paper Model Reducations of Complex Dynamics. Their technique uses pre-computation to speed up the calculations by up to 100,000 times -- the only reason it is possible to do this calculation in real time. They have some other interesting simulations on their website. They use as inputs the GPS information derived from the transponders the cars carry. This information also is used by Racef/x^TM, a development of SportsVision, Inc., which is how the little tags follow the cars around the track on television, and how features like PitCommand and RaceView on NASCAR.com's TrackPass work.

It remains to be seen how illuminating Draft Track is in terms of helping race fans understand the very complex field of aerodynamics. Draft Track has gotten mixed reviews from NASCAR pundits. If you look at the pictures from the GM Racing articles, you probably noticed that they are a) much more detailed; b) provide a lot more information and c) take a little time to digest. TV doesn’t have that much time, so I assume that the blue/yellow is a compromise attempt to provide just enough information that can be digested in the thirty seconds the blue and yellow air are on the screen. ESPN seems to have decreased their use of it as the season went on.

Incidentally, we couldn't bring ourselves to buy the blue milk. Rainbow colored air doesn't bother me, but blue milk was just a little too much to stomach.