Aerodynamics analysis of a Peugeot 406

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Aerodynamics analysis of a Peugeot 406 (7/18/08)

Michael Bo Hansen was kind enough to publish some computational fluid dynamics (CFD) models of a Peugeot 406 car (4th row). The styling of the car is similar to many cars so I decided to do a little analysis on the images before doing some small scale wind tunnel testing of my own. The images have been lightened and areas of interest left at normal lighting with a red border. Click on the images to get to Michael's originals. Wind speed is show via color progression (Roy G Biv), with violet being slowest and red being fastest.

The grill of the car is generally one of the first contact points the car will have with the air ahead so some slowing is expected. That air is used for two reasons, cooling and combustion. It isn't necessary for the cooling to encounter a huge block; heat will be carried away if the hot area runs parallel with the air flow just as well as if it was perpendicular. If the then hot air can be carried away efficiently we'll decrease drag. For combustion, it's important to have fairly clean air that isn't piping hot, which reduces the amount of oxygen per square meter.

Some wiper blades work on drag, which are generally smaller than their spring bound cousins. The drag causes them to adhere to the window and wipe away the water rather than the springs. Either can easily sit in a "bay" when not in use to reduce drag.

Wheel wells also induce drag but provide for circulation of the air around the brakes and tires. If it gets too hot components can fail.

Underbody drag is visible in this image along the sides because that air is moving slower than the side air. The two interact and cause slowing on the sides. If that air can be smoothed out it'll increase in speed and create a low pressure zone, which F1 cars famously use to create downforce. The Chaparall 2J took this idea to a new level, quickly banned by racing bodies, by building a car like a hovercraft and reversing the hover engines so they pull the car down to the ground rather than floating.

Picture a fish swimming in water. The water near the fish flows along its body, while away from the fish the water goes straight, as if the fish isn't even there. The water near the fish is what's called the boundary layer, demarking the line between the regular water and the fish, and is extremely low drag, so we want to keep it rather than having to form it all over again.

The roof to rear window transition contains a surprise... no boundary flow separation! The window to trunk transition shows some slowing (drag), and then after the trunk things get very complicated. Most obvious is the green air from the trunk just heading off on it's own. Behind each wheel we find vortexes, just like the contrails jets form. Why?

Remember, the car is shaped like an airplane's wing and works like one too: low pressure above, normal pressure below and to the sides. When the side air towards the top gets to the back it'll go to where the low pressure is, the middle. The lower side air ambles along where it is, but there's just enough of a twist to it that the vortexes form, and create drag.

This shot provides a more nuanced perspective. We see slowing caused by the side mirrors, hubcaps, wheels themselves, and even the little spot where they put the fog lights in the sport and luxury versions of these cars. We also see a general slowing of the air on the hood. Since it's angled new air is joining with the boundry layer. Oddly this most likely cushions the impact vs. what happens at the very front of the car.