The new Civic Type-R prototype didn't hold back on aero, with the addition of a Mitsubishi Evo-style spiked roof. So what is the function of these intricate fins?

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The new Honda Civic Type R prototype revealed at the Paris Motorshow was one of the most exciting cars to be presented, with brutish and aggressive styling we all hope makes it to production. Of all the crazy aero bits though, one particular element caught our attention: the little lumps at the top of the rear windscreen. They are called vortex generators, which were most famously used before on the Mitsubishi Lancer Evolution, and have become a niche modification for aero-obsessed petrolheads out there. To understand what benefits they bring to a car’s aerodynamics, we must first look at how airflow interacts with the moving body of a car. Due to friction between the solid surface and the air molecules passing over it, the air forms a fluid profile, with stationery air sitting at the meeting point between the fluid and the surface. This profile can also be called the boundary layer. The air speed then increases to what is called free stream velocity as the distance increases from the car’s body, as can be seen in the diagram below.

The air velocity increases as it gets further away from the bodywork

In terms of flow efficiency then, you want the air to be moving across the car as quickly as possible to avoid creating a large amount of friction, instead gliding smoothly over and away from the car. In terms of a car’s roof, the air is going to follow the curvature of the roof as much as possible, which is known as an ‘attached flow’. So, bringing a rear wing into the equation, the ideal situation would be a nice attached flow that follows the entire roofline of the car and acts upon the wing, both increasing downforce and the wing’s overall aerodynamic efficiency. Unfortunately, the airflow reaches a separation point somewhere around the end of the roof and diffuses off into free air space, leaving turbulent, slow-moving air to crash about the boot area meaning very little fluid flow ever makes it smoothly down to the wing. Factors like the rake of the windscreen and the curvature of the roofline have big implications on the characteristics of the airflow over the car and contribute to the placement of the point of separation.

The separation point can be seen clearly on the 300SL as the air stream leaves the roofline and flows proud of the rear window and boot

Although this separation point is inevitable, aerodynamic components like wings and diffusers can be optimised by controlling the distance at which the separation flow occurs. This can be achieved using - you guessed it - vortex generators. To keep an attached flow going for as long as possible, you need high energy air. So, looking at the fluid profile, you want to get the high energy air from the top of the profile as far down and therefore as near to the car’s body as possible.

Via YouTube channel KYLE.DRIVES

What the vortex generator does is disturb the airflow running over the bodywork, producing a swirl of air in-between the high energy and low energy streams. This draws in a stream of high energy air from free stream down into the boundary layer, increasing the boundary layer’s energy. High energy air sticks to the bodywork much more effectively and therefore increases the attached flow.

This is the reason that – on cars – vortex generators are found spaced across the end of the roofline, exploiting as much of the high energy air as possible. On road cars however, the design of them is not the most efficient. On airplanes and Formula One cars, vortex generators are sharp-edged and often triangular to create the most effective vortex possible at very high speeds. But on mass-produced road cars, safety regulations dictate that they need to be smoothed-off and rounded which decreases their effect on the air flow, resembling much more of a purely aesthetic component than people may presume.

The vortex generators found on Torro Rosso F1 cars