What is a vortex generator?
Take a look at this photo of a Mitsubishi Lancer Evolution 9 MR FQ-400.
Just above the rear glass, there is a row of what appears to be spikes.
Those are vortex generators, whose job is to ultimately increase downforce on the car by preventing separation of the airflow over the top of the car.
They were most famously used on the Mitsubishi Lancer Evolution 9, with some nicknaming it “the Bart Simpson haircut”.
What do they do?
To understand how vortex generators got their name, we first must look at the airflow over the top of a car.
Ideally, airflow should follow the curvature of the car as much as possible, in what’s called “attached flow”.
That attached flow meets the surface of the car in an area called the boundary layer.
However, when the air stream reaches the back of the car, it doesn’t follow the car’s curvature down towards the boot.
Instead, the airflow reaches a separation point just after the roofline and before the boot, where it flows off into turbulent air.
To keep the high-speed attached airflow continuing down over the boot (and the rear wing), vortex generators are used.
As per their name, these little spikes generate vortices to “disturb” the air flowing over the rear of the car.
Those vortices swirl, or mix, the high-speed air with the boundary layer to keep the air attached to the car’s surface.
Do vortex generators really work?
Generally speaking, the faster airflow, the more effective an aerodynamic aid will be.
That’s why the bodywork on Formula 1 cars works so effectively at keeping the car pinned to the track as they regularly reach speeds in excess of 200km/h.
Vortex generators on aircraft also help to smoothen airflow over the wing, as seen here:
On a road car, the airflow simply isn’t fast enough to generate sufficient effect.
In fact, vortex generators need to be large and tall enough to “intercept” the high-speed airflow over the rear of the car, but can’t be made so because of regulations.
So essentially, vortex generators on cars are mostly for aesthetic effect, but at least you’ll look cool going fast!