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Business and innovation

Business and innovation

Jets of air reduce drag on a model car

04 Apr 2019
Car aerodynamics
Going with the flow: air jets on cars could someday reduce fuel consumption: (Courtesy: iStock/kloromanam)

Blasting jets of air many times a second from the back of a car is an energy-efficient way of reducing air drag – according to a team of academic and industrial researchers. The team is now doing further studies of the effect to see if it could be used to create vehicles that are more energy efficient.

There are about one billion cars in use worldwide – and most have roughly the same shape. In part, this is because air drag is responsible for a significant amount of fuel consumption at moderate speeds – and to minimize drag, car manufacturers have used advanced aerodynamics to converge on an optimal car shape.

Today there is little to gain from further changes to body shape and therefore car designers are looking for new ways to reduce drag. Now researchers in France, Germany and China — including some supported by the carmaker PSA Group – have shown that drag can be reduced by 7% at 90 km/h by introducing air jets blasting pulses of air from the back of the vehicle.

Crosswind yaw

The effect of the jets was studied in a scenario that mimicked a vehicle travelling in a moderate crosswind such that air flowed past the vehicle at a yaw of 5°. Yaw is a rotation in the horizontal plane relative to the vehicle’s direction of motion.  Such a crosswind upsets the symmetry of air flow around a vehicle and increases drag – which the team was keen to counteract.

The researchers installed four nozzles at the back of a simplified square-backed car model that was about 89 cm long. The nozzles were connected to a bottle of compressed air and the model was placed in a wind tunnel at a 5° yaw.

The team fired pulsed jets of air through the nozzles at two different frequencies – the low frequency being in the tens of hertz and the high frequency in the hundreds of hertz. The effect of the air jets was to reshape the vehicle’s wake. The team found that the high-frequency jets act like a virtual flap by subtly shifting the airflow, which increases the pressure on the back surface of the car. They also found that the low-frequency jets restored the symmetry of the wake, which was disrupted by the yaw. They found that jet pulses at both the high and low frequencies reduced the overall drag.

Minimizing turbulence

“The wake symmetrization equally increases the pressure on the base surface. In addition, the obtained balance between the windward and leeward shear layer has an important role in minimizing the rate of kinetic energy transfer from the mean flow to turbulence, which is also beneficial to drag reduction,” says team member Ruiying Li at the Pprime Institute  in Poitiers.

Despite the early success of this experiment, much more work must be done before jets could be implemented on vehicles to reduce drag. One important challenge is to reduce the noise created by the jets.

In terms of energy consumption, the energy used by the system was much smaller than the  energy savings that result from drag reduction. However, compressed air must be available for the system to work.

Reducing fuel consumption

Li told Physics World “With the jets, we are able to reduce 7% the aerodynamic drag at 90 km/h, thus reducing fuel consumption. In addition, the energy needed to supply the jets is negligible compared to the energy reduced by the drag reduction.”

More experiments are already underway, and the team is planning to optimize the drag-reduction process under changing wind conditions using active feedback control. This would make the design more appropriate for use in real-world conditions.

According to Christian Nayeri from TU Berlin, who was not involved in this study, “this work represents the state of the art in fundamental research in the field of active flow control for drag reduction of road vehicles including machine learning approaches.”

However, he believes that people trying to apply this technology to real vehicles will face many challenges. “On one hand, there is the need of reliable sensors and powerful actuators and their integration into the vehicle. On the other hand, the geometry of realistic road vehicles compared to the simplified geometry used in the study is much more complex. In fact, it deviates very much from passenger cars but rather corresponds to commercial vehicles such as trucks,” he adds.

This research was published in Physical Review Fluids.

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