Detective work in the wind tunnel

Dr. Kentaro Zens, the development engineer responsible for the aerodynamics and aeroacoustics of the Audi RS e-tron GT, says: “On the road, the vehicle moves through the air. Here in the wind tunnel, it’s the exact opposite: The vehicle is stationary and we channel the air around it as evenly as possible. We go to great lengths. Only when the airflow interacts accurately with the vehicle are we able to obtain accurate measurement results that we can rely on.”

Zens sits at his workstation next to the control panels with which the operators regulate the wind tunnel. He can read all the relevant data on screens: What is the drag coefficient, how high is the front-axle lift, how high is the rear-axle lift, at what wind speed and what belt speed?

Standing next to him is Thomas Redenbach, Head of Aerodynamics & Aeroacoustics Development – Vehicle Projects: “When the Wind-Tunnel Center went into operation, it was the first automotive wind tunnel worldwide to combine ground simulation of real-world road conditions for aerodynamics with such extremely quiet aeroacoustic functionality.”

Today, the wind tunnel runs up to six days a week from 7:00 a.m. to 10:30 p.m. in two shifts. And when the Worldwide Harmonized Light-Duty Vehicles Test Procedure (WLTP) certification was introduced by legislators, its capacity was utilized to the full. Moni Islam says: “The complexity of this wind tunnel called for the full commitment and technical expertise of our sister department, which has been operating it for us every day for many years. At the time, our colleagues from wind-tunnel operations provided us developers with 23 hours of testing time per day, as we are required to submit certified wind-tunnel data to legislators as proof of the WLTP values.”

Nevertheless, computer simulations are also playing an increasingly important role in aerodynamic development. Computational Fluid Dynamics (CFD) simulation reproduces airflow on the computer to enable analysis and visualization of flow patterns. So why the time-consuming and expensive work in the wind tunnel? Thomas Redenbach: “The wind tunnel is our everyday tool and also enables us to validate the results from the simulation. We want to keep developing the simulations and, in order to ensure they are valid and accurate, we have to check the calculations against the test results.”

Yet computer simulations are getting better and better and becoming more and more important. Kentaro Zens says: “With the Audi RS e-tron GT, we did an exceptionally large amount of simulation work—over nine million CPU-hours. I spent 150 hours in the wind tunnel with the vehicle, which isn’t very much at all. By way of comparison, it was 600 hours for the Audi R8.” This indicates not only the quality of the Audi RS e-tron GT design but also that the development process was significantly shorter—a path Audi is aiming to take also with future models.

Moni Islam adds: “The wind tunnel and CFD are two complementary tools for the aerodynamicist. The wind tunnel is very accurate and quick, enabling us to work highly efficiently in the dynamic development process. Simulation provides us with an incredible amount of information but requires effort in terms of preparation of the models and analysis of the results. With only one of these two tools, state-of-the-art aerodynamic development would not be possible.”

For electric vehicles like the Audi RS e-tron GT, the full package offers benefits in terms of aerodynamics (the closed underbody being just one example of where this applies). However the challenges facing the 31-strong aerodynamic vehicle development staff in Moni Islam’s department are growing. He defines their aim as follows: “Every count by which we can improve the drag coefficient increases the driving range.”

Aerodynamicists identify this potential in the vehicle through simulation results that indicate sensitivities: If I change the geometry slightly at point X of the shape, how much does that affect the airflow? And then begins what Islam describes thus: “Aerodynamics is also meticulous detective work because you can’t see the air. You have to try to narrow down the problem using an analytical approach based on the values delivered by the balance in the wind tunnel.”

To achieve this, the engineers also work with various add-on parts generated with rapid-prototyping technology. Initially, CAD designs are created to define the geometries of the components—for example, an air intake on the front apron. The colleagues from model management then convert the desired variants, of which there may be several, into a test component using this advanced technology. The different variants of the components are subsequently tested in sequence on the vehicle model. The measurements provide drag and lift coefficients. These results are then selectively compared with the CFD simulations of exactly the same configuration to ensure valid simulation results.

“You can develop 80 percent of a vehicle’s aerodynamics in 20 percent of the time. But we invest an enormous amount of time in the last 20 percent of the aerodynamics—teasing out the drag counts in a host of tiny optimisation steps,” says Thomas Redenbach, describing the detective work in the wind tunnel. “It takes this high level of dedication and attention to detail to produce top-quality results.”

So what was the most difficult detail in terms of airflow in this Gran Turismo for the aerodynamics experts responsible for the Audi RS e-tron GT? Kentaro Zens thinks for a while. “The front apron with its four interconnected ducting components. The air flows into the intakes, the shutter inside closes—and that’s when the problem starts. The air can escape all over the place and you don’t want that. Keeping the airflow under control here and fine-tuning it precisely is critical. It’s a huge team effort as the colleagues from vehicle safety, parts engineering, production and assembly all have to work with me.”

Zens also makes specific reference to the design of the so-called air curtains in interaction with the wheel arch: “We had close coordination with the Audi designers on a weekly basis. This resulted in an optimal aerodynamic transition from the front end to the side around the air curtain that also fits seamlessly into the overall design as a coherent theme. Everything about the Audi RS e-tron GT has a function and a purpose. That’s authentic functionality, which is something I really like about the vehicle.”