You got a fast car: Reducing vehicle drag with active flow control

As a car moves, the layer of air closest to its surface experiences friction and loses momentum, struggling to follow the intricate outer contours of its body. Ultimately, this creates a turbulent wake behind the car and increases drag.

With cars getting faster and access to fuel becoming more limited, the need for more aerodynamic vehicle designs that reduce this air resistance is increasingly pressing. Ezhilarasan et al. modeled a method of adding energy to this slow-moving air to help it continue along the car’s shape, increasing its aerodynamic performance. Specifically, they simulated the effects of placing a rotating cylinder in the near wake to inject momentum into the airflow. By comparing cylinders of different sizes, they determined the cost-benefit tradeoff between the aerodynamic benefits of the cylinder and the energy needed to drive it.

“The key outcome is not simply that the largest cylinder performs best … While larger cylinders tend to offer greater drag reduction, they also demand higher input power,” said author Aakash Ezhilarasan. “Our analysis shows that the most effective configuration depends on balancing these competing effects, leading to an optimum value in the overall cost function at intermediate actuation conditions.”

Their method adds energy to the airflow just as it begins to slow down, effectively reducing the overall drag. This shows that even simple interventions can be impactful in controlling complex aerodynamics.

Though the researchers studied a rotating cylinder on a simplified car model, their technique can be applied to other momentum injection methods to energize the layer of air experiencing friction. With the current energy crisis, Patnaik said there is a need “to save every drop of petrol,” and the group hopes this work will inspire the vehicle aerodynamics community to integrate active flow techniques to balance aesthetics with drag reduction.

“Frankly, there is a long way to go,” said author Prasad Patnaik.

Source: “Momentum injection-based active flow control of a simplified automotive body,” by Aakash Ezhilarasan, Sridhar Muddada, Harish Pothukuchi, Sarma S. R. Akella, and B. S. V. Patnaik, Physics of Fluids (2026). The article can be accessed at https://doi.org/10.1063/5.0316152 .