If someone asked you to mathematically describe the flow of air in this picture, you would probably just blink a lot and smile winningly - or run as fast as possible in the other direction. Even aerodynamicists are intimidated by the challenge of understanding what happens in the boundary layers around airfoils, cars, and golf balls. That's why George Haller and Thomas Peacock of MIT are feeling good this week: They've developed, and tested experimentally, a new mathematical theory that can predict exactly where the separation of flow occurs in unsteady, three-dimensional, real-life conditions.You can try all you want to hose dust off of a car, but it's not going to work. The reason for that is that the velocity of a fluid β like air or water β at a surface β like a car or a wing β is zero. At the exact point where fluid flow touches a solid, it's technically not moving. As you observe the flow as it moves away from the surface, however, the velocity steadily increases, until it matches the surrounding freestream velocity. What this means is that moving cars, planes, or golf balls all have boundary layers, areas around their surfaces where the velocity of the fluid flow has not quite reached that of the freestream. In the graphic below, the green arrows represent increasing velocity, and the blue-purple-red area is the boundary layer.