Steady Jet Technology Cuts Drag on Fastback MIRA Model
In a groundbreaking study that could reshape the future of automotive aerodynamics, researchers from Tongji University have demonstrated how targeted air jets can significantly reduce drag on modern fastback vehicles. The team, led by Chen Yu, Wang Zhijun, Wang Kewei, and Yang Zhigang, focused their efforts on the MIRA fastback model—a design increasingly common among today’s passenger cars due to its sleek profile and fuel efficiency potential. Their findings, published in the Journal of Tongji University (Natural Science), reveal that precise application of steady blowing at the vehicle’s rear can cut aerodynamic resistance by up to 2%, with meaningful energy savings even after accounting for the power required to generate the airflow.
As global automakers race toward carbon neutrality by mid-century, every percentage point in fuel or energy efficiency counts. Aerodynamic drag remains one of the largest contributors to energy consumption at highway speeds, particularly for electric vehicles where extended range is a key selling point. Industry estimates suggest that a 10% reduction in drag can lead to a 5% improvement in fuel economy. With this in mind, passive design improvements have reached a point of diminishing returns, prompting engineers to explore active flow control methods that dynamically alter airflow during operation.
Among these emerging technologies, active jet control has gained attention for its ability to manipulate boundary layers and wake structures without altering the vehicle’s physical shape. Unlike traditional approaches such as boat-tailing or underbody diffusers, jet-based systems offer adaptability across driving conditions and can be toggled on only when needed, minimizing unnecessary energy expenditure. However, most prior research has centered on bluff-body models like the square-back Ahmed body, which exhibit large, unstable wakes amenable to control. The effectiveness of such techniques on more aerodynamically refined fastback designs—where flow separation is already delayed and wake turbulence is reduced—has remained uncertain.
The Tongji team sought to fill this knowledge gap by applying steady jet actuation to the MIRA fastback model, a benchmark geometry in automotive aerodynamics known for its realistic proportions and complex flow field. Using high-fidelity computational fluid dynamics (CFD), they simulated various jet configurations, measuring both drag reduction and net energy savings—the latter being a critical metric that balances aerodynamic gains against the power cost of operating the jets.
The study examined five potential jet locations: J1 at the top of the sloping rear window, J2 on the left and right edges of the same surface, and J3, J4, and J5 positioned at the upper, side, and lower edges of the vertical tailgate, respectively. Each jet was modeled as a continuous slot blowing air tangentially into the surrounding flow. The key parameters under investigation included jet momentum coefficient—a dimensionless measure of the energy input relative to the vehicle’s kinetic energy—and jet angle, defined as the deviation from perpendicular to the local surface.
Initial results revealed a stark contrast between jet placements. When jets were activated on the sloped rear surface (J1 and J2), the outcome was counterproductive: instead of smoothing the flow, the injected air triggered premature separation, increasing overall drag by as much as 17.3% in the case of J1. This adverse effect stemmed from the disruption of an already optimized flow path. The MIRA fastback’s sloped roof allows airflow to remain attached well into the rear section, creating a relatively small separation zone. Introducing jets at this stage interfered with the natural pressure recovery process, effectively enlarging the low-pressure wake and worsening drag.
In contrast, jets applied to the vertical tail section—specifically J3, J4, and J5—produced consistent drag reductions. Among them, J3 (top edge) delivered the best performance, reducing drag by 1.2% when operated at a momentum coefficient of 1% and a 0° angle (i.e., blowing straight out from the surface). J4 (side edges) followed closely with a 0.8% reduction, while J5 (bottom edge) contributed a more modest 0.4%. These improvements were attributed to localized pressure recovery on the rear face of the vehicle. By injecting air at the trailing edges, the jets energized the boundary layer, delaying separation and increasing static pressure across the vertical tail. This elevated base pressure directly countered the dominant form of drag in fastback vehicles: pressure drag caused by the low-pressure wake behind the car.
The researchers then explored how varying the momentum coefficient affected performance. Momentum coefficient values of 1%, 3%, 5%, and 7% were tested with all three rear jets (J3–J5) active. At lower coefficients (1% and 3%), drag reduction remained stable at approximately 1.2%. However, as the momentum increased to 5% and beyond, the benefits diminished and eventually reversed. At 7%, the system actually increased drag, turning what was meant to be a fuel-saving technology into a net energy drain.
This nonlinear response was traced to the formation of localized low-pressure zones near the jet orifices themselves. At high blowing intensities, the rapid ejection of air created suction effects that offset the gains from base pressure recovery. Given the relatively small surface area of the vertical tail on a fastback design, even minor pressure drops in this region had an outsized impact on total drag. The findings underscore a crucial insight: for fastback vehicles, less is more. Low-energy jets are not only sufficient to achieve meaningful drag reduction but also avoid the pitfalls of over-actuation.
With optimal placement and momentum established, the team turned to jet angle as a final tuning parameter. Angles of 0°, 30°, 45°, and 60° were evaluated, with all three rear jets active and momentum coefficient fixed at 1%. The results showed a clear peak in performance at 45°. At this angle, drag reduction climbed to 2%—double the improvement seen at 0°—and net energy savings reached 129.7 watts, a figure representing real-world efficiency gains.
Flow field analysis revealed why 45° was optimal. At this angle, the injected air effectively deflected the shear layer—the thin region between the vehicle’s boundary layer and the free stream—toward the center of the wake. This inward deflection narrowed the wake width and pushed the vortex cores farther downstream, reducing turbulence intensity near the base and enhancing pressure recovery. In contrast, at 60°, the jets began to induce secondary vortices close to the surface, particularly near the J3 and J5 slots. These vortices created localized separation zones, lowering static pressure and eroding the aerodynamic benefit.
The 45° angle thus struck an ideal balance: it provided enough angular momentum to reconfigure the wake beneficially without introducing new instabilities. This finding aligns with broader principles in fluid dynamics, where moderate control inputs often yield the best trade-off between effectiveness and robustness.
Beyond the immediate performance metrics, the study contributes to a deeper understanding of active flow control in practical automotive contexts. One of the most persistent challenges in this field is the so-called “net energy paradox”: while a control method may reduce drag, the energy required to operate it can negate or even exceed the savings. The Tongji team addressed this directly by calculating net power savings—the difference between the power saved due to reduced drag and the power consumed by the jet system. Their results confirm that, under the right conditions, active jet control can indeed deliver positive net energy returns, making it a viable candidate for real-world implementation.
The implications for vehicle design are significant. As automakers continue to refine fastback and liftback silhouettes—now standard on sedans, SUVs, and electric vehicles—passive aerodynamic gains are becoming harder to achieve. Active systems like the one studied here offer a path forward. Modern vehicles already incorporate complex climate control, braking, and suspension systems that consume auxiliary power; adding a low-energy jet system for drag reduction would be a natural extension. Compressed air could potentially be sourced from existing HVAC or brake air systems, or small electric blowers could be integrated into the rear structure.
Moreover, the control logic could be adaptive. Since drag reduction is most beneficial at higher speeds, the system could remain dormant during city driving and activate only on highways. Sensors monitoring speed, wind conditions, and battery state (in EVs) could optimize jet operation in real time, maximizing efficiency without driver intervention. Such a system would embody the principles of smart, responsive engineering—precisely the direction the industry is moving.
The study also highlights the importance of tailored solutions. What works for a boxy truck or a race car may not translate to a consumer sedan. The failure of jets on the sloped rear surface underscores this point: active control must be applied with a deep understanding of the local flow physics. In fastback vehicles, the rear window is not a site of major separation, so disturbing it does more harm than good. The vertical tail, however, remains a critical zone for pressure recovery, making it the ideal target for intervention.
From a computational standpoint, the research demonstrates the maturity of CFD as a tool for aerodynamic development. The team used STAR-CCM+ to solve the Reynolds-averaged Navier-Stokes equations with a realizable k-ε turbulence model, a combination well-suited for complex, separated flows. Grid independence was verified through multiple mesh refinements, and results were validated against wind tunnel data from the Shanghai Automotive Wind Tunnel Center. The close agreement between simulation and experiment lends credibility to the findings and suggests that such studies can reliably guide design decisions before physical prototypes are built.
Looking ahead, several avenues for further research emerge. The current study focused on steady (continuous) blowing, but pulsed or oscillatory jets might offer even greater efficiency by exploiting flow instabilities at specific frequencies. Additionally, machine learning techniques could be employed to autonomously optimize jet parameters in real time, adapting to changing conditions more effectively than pre-programmed settings. Integration with other active systems—such as adaptive spoilers or grille shutters—could also yield synergistic benefits.
The work also raises questions about manufacturability and durability. While the concept is elegant in simulation, real-world implementation must contend with dust, ice, and mechanical wear. Jet slots would need to be protected from clogging, and actuators must operate reliably over tens of thousands of miles. Nevertheless, given the automotive industry’s track record in solving complex engineering challenges—from turbochargers to hybrid powertrains—these hurdles appear surmountable.
In conclusion, the Tongji University study provides compelling evidence that active jet control can enhance the aerodynamic performance of modern fastback vehicles. By focusing on the vertical tail and operating at low momentum with a 45° angle, the system achieves a 2% drag reduction with a net energy gain. This level of improvement, while seemingly modest, translates into tangible benefits for fuel economy and electric range. As the automotive world transitions to a more sustainable future, innovations like this—rooted in rigorous science and practical engineering—will play a vital role in reducing emissions and extending the capabilities of next-generation vehicles.
Chen Yu, Wang Zhijun, Wang Kewei, Yang Zhigang, School of Automotive Studies, Tongji University; Journal of Tongji University (Natural Science), DOI: 10.11908/j.issn.0253-374x.24747