The data gathered from PIV is helpful on its own, but it really becomes useful when we compare it with data gathered in the first lab. We can overlay the graph of pressure distribution that we obtained with the pressure taps on the velocity and streamline image taken from PIV (right). It now becomes apparent the lowest pressure is found when the velocity is highest: on the tip of the hood, and on the roof. We also now see the true reason that the pressure at tap 18 does not match that of tap 1. Behind the back edge of the car and to some extent on the back windscreen, there is a huge amount of turbulence, as shown by the circular streamlines. This random movement of air reduces the pressure behind the car. Of course, the air isn't simply moving in loops with axes parallel to the back of the car. The fluid particles are moving in three-space, but the nature of PIV allows us to only capture two dimesions. The turbulent flow is also constantly changing directions, but because the camera only took positions at two instants, we just have the velocity at this moment.
The low pressure caused the by turbulence creates a pressure difference between the front and back of the Charger. This is the reason for pressure drag, which in the case of automobiles is far larger than the surface drag. The huge regions of turbulence are created the boundry layer peels off from the surface of the vehicle. 
This happens when a sharp corner creates an steep adverse pressure gradient (that is, the fluid particles must move from areas of low to high pressure in a small distance). The boxy form of the General Lee causes separation in many places. This is a sharp contrast to the Nascar (left), which has almost no separation at all, up until the spoiler. However, the spoiler does create a gigantic turbulence zone behind the Nascar, larger than even the combined effects of all sharp edges on the Charger. C_D for the racecar is about .75, as opposed to 0.6 of the General Lee. But this drag is not for nothing. Nascar's downforce is eight times as great as the Charger's negative lift.
The degree of turbulence can also be seen in the CFD diagrams of the two cars. The velocity magnitude image (below, left) is similar to the PIV results (below, right), which mean that StarWorks did a good job predicting the flow. In the lab, we could not create turbulent energy diagrams. Since the velocity images matched so closely, it is likely that the turbulence results are accurate as well. They confirm what we saw with the streamlines, that there is some turbulence by the back windscreen of the Charger, quite a bit past the trailing edge, and a huge amount behind the Nascar.
