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 to achieve the perfect airflow. Only when the airflow hits the vehicle at precisely the right point are we able to take exact measurements that we can rely on.”

Zens sits at his workstation, next to the control panel where 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 Development Aerodynamics/Aeroacoustics – Vehicle Projects: “When the wind tunnel center went into operation, it was the first car wind tunnel worldwide to combine ground simulation of real-world road conditions for aerodynamics with such extremely quiet aeroacoustic functionality.”

Nevertheless, computer simulations are also playing an increasingly important role in aerodynamic development. CFD (Computational Fluid Dynamics) 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 says: “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 representative, we have to check the calculations.

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 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. But the challenges facing the 31-strong aerodynamic vehicle development staff in Moni Islam’s department are growing. He defines their aim as follows: “Every thousandth by which we can improve the drag coefficient leverages potential in terms of 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 as this: “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 scale in the wind tunnel.”

To achieve this, the engineers also work with different add-on parts in a rapid prototyping process. Firstly, CAD designs are created to define the geometries of the components—for example, an air intake on the front spoiler. The colleagues from model management then convert the desired variants, of which there may be multiple, into a test component. The different variants of the components are then tested in sequence on the vehicle model. The measurements provide drag coefficient and lift values. These results are then selectively compared with the CFD simulations of exactly the same configuration to ensure reproducible 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 thousandths in a host of tiny optimizations,” 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 spoiler with its four interconnected components. The air flows into the intakes, the shutter inside closes—and that’s when the problem starts. The air flows 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, design, 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.”