This is Part 4 of a series of articles on aerodynamic features of performance cars. This article will explore the theory and purpose of vortex generators in automotive applications.
A vortex generator is a small fin-shaped device. This fin is at an angle to the airflow, causing an increase in pressure on one side and a reduction in pressure on the other side. When these high-pressure and low-pressure flows interact at the edge of the fin, it creates a vortex – a region where the flow spins around an axis.
To understand how vortex generators work, it is helpful to understand how the airflow behaves close to the surface of the vehicle. At the surface, the velocity of the fluid is zero. There is a transition between this stationary airflow and the free moving flow above the surface. This transition is called the boundary layer. This exists because there is friction between the air and the surface it is flowing around.
As the air in the boundary layer moves along a curved surface, it loses kinetic energy and slows down as it changes direction to stay attached to this surface. This deceleration eventually reverses the direction of the flow, which causes it to detach from the surface. This effect is known as flow separation. It creates a swirling, turbulent wake in the airstream.
On some cars with a highly sloped rear roofline, the flow may separate from the top of the car when the roof turns downward, which causes drag and reduces the effectiveness of the rear wing. A set of vortex generators along the rear edge of the roof will curb this effect. They create vortices which mix the slow airflow of the boundary layer with the much faster overall airflow around the car. This mixing is called “energizing” the boundary layer. The boundary layer will thus maintain its kinetic energy and stay attached to the surface. Now faster and smoother air will interact with the rear wing, producing more downforce.
Summary
Vortex generators delay or prevent flow separation at the rear of the car to improve the effectiveness of the rear wing.
References
Siorai 