See those little bumps above the rear windscreen on a Mitsubishi Evo? They're called vortex generators, and help improve the performance of the rear wing by limiting airflow separation off the back. Before the vortex generator was brought to the mainstream car community, it had a long history on aircraft and Formula One cars, but how does it actually work?
Underside of an F1 car, showing the vortex generators at the start of the diffuser kick line. Taken by Craig Scarborough.
Ever heard of wing stall? It's a condition in which the dominant airflow no longer remains attached to the wing, and thus doesn't follow the direction of the body. Generally, this is bad (there are a few cases where you want it, but that is for another day). Stall is caused because we end up with something called an "adverse pressure gradient". Basically, the air has a low pressure peak near the front of the wing (where it accelerates to maximum velocity), then the pressure slowly increases as you move further back (as it returns more towards freestream velocity). In our car example, the low pressure peak is near the roofline and the high pressure area more towards the boot.
A good diagram of the flow characteristics over a wing, courtesy of http://aerospaceengineeringblog.com
So why is this adverse pressure bad? Well, when a fluid passes over a surface, the particle nearest to the surface is effectively attached to the surface, and thus stationary. The flow then slowly increases in velocity as we get to the freestream. This area of low velocity near the surface is called a "boundary layer". The problem is that as we encounter adverse pressure gradients, it slows the boundary layer down, as the high pressure in front of it pushes it backwards. This is no problem for the freestream air, which has heaps of momentum to overcome this, but for the boundary layer it can lead to reversal of the flow direction, which ultimately leads to flow separation.
How do we get past this? Add more momentum and velocity in to the boundary layer! To do this, we can take air from the freestream flow and mix it with the boundary layer, and one of the easiest ways to do this is by creating a vortex. Vortices of the correct size constantly mix air from the freestream and boundary layer, keeping the boundary layer energized and the flow attached. Creating these vortices makes drag at the vortex generator, however by reducing separation it is possible to reduce overall vehicle drag. More importantly though, reduced separation means we can push aero devices harder, getting more downforce.
F1 cars are covered in vortex generators, perhaps the most obvious ones are those mounted to the sidepods.
There are also places where using vortex generators is either incorrect or just plain stupid. For example, I've been seeing a lot of GT86's running around with vortex generators lately. Not only do most coupes lack the steep roofline required to need vortex generators, but these stick-on kits often have a raised lip at the front, which would be tripping the flow, kicking it up and thickening the boundary layer downstream. The less steep rear windscreen also means less cross flow, so the near straight vortex generators aren't going to be making much of a vortex. And even if these were generating vortices, the low pressure core would mean you would be making lift, while also increasing the drag on the car. And let's not even start on guys that put vortex generators at the back of spoilers on hatchbacks...
For a more in-depth explanation, check out the video below! It's quite old so you'll have to forgive the sound quality, but it's full of useful information.
Kyle Forster is a qualified Aerodynamicist, race car engineer, and the man behind JKF Aero, a firm that offers a variety of aerodynamic consultancy services for racing purposes. If you have any questions for Kyle or have any suggestions for future videos, drop them in the comments below!