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Detective work in the wind tunnel

Detective work in the wind tunnel

In the Audi aeroacoustic wind tunnel, aerodynamics specialists are perfecting the shape of the Audi RS e-tron GT.

Copy: Bernd Zerelles − Photo: Robert Fischer Film: graupause Reading Time: 9 min

Close-up of light shining through the turning vanes in the Audi aeroacoustic wind tunnel.Close-up of light shining through the turning vanes in the Audi aeroacoustic wind tunnel.

The first thing you notice when looking at the fan in the Audi aeroacoustic wind tunnel is that gap between each of the tips of the 20 blades on the wind tunnel rotor and the concrete surround. Several perplexing centimetres. Is this an energy-wasting lack of precision? Dr. Moni Islam, Head of Development Aerodynamics & Aeroacoustics at Audi, provides reassurance: “When the turbine runs at the maximum power output of 2,720 kW, the centrifugal force stretches the aluminum-coated blades, almost completely closing this gap. After all, we’re generating the same force here as a wind speed of up to 300 km/h acting on the test vehicle.”

Then, everyone has to leave the wind tunnel. The 20 blades of the five-metre-wide fan slowly start to turn. The rotated airflow is initially stabilized by the 27 guide vanes of the stator behind. Two corners follow where the air is distributed evenly through specially designed turning vanes. Grids downwind from the vanes break up the worst of the turbulence, which is unavoidable in the vicinity of the corners and the fan. The air then passes through a honeycomb layer to straighten the flow and into a large stilling chamber downstream. It is then accelerated through the nozzle at a factor of 5.5 before reaching the Audi RS e-tron GT in the plenum, the main chamber, at exactly the desired speed.

Light coming through the vertical turning vanes in the Audi aeroacoustic wind tunnel. Light coming through the vertical turning vanes in the Audi aeroacoustic wind tunnel.
Thomas Redenbach, Head of Development Aerodynamics & Aeroacoustics at Audi, runs his hand along the side of the Audi aeroacoustic wind tunnel. Thomas Redenbach, Head of Development Aerodynamics & Aeroacoustics at Audi, runs his hand along the side of the Audi aeroacoustic wind tunnel.
Side profile of a column of air flowing over an RS Audi e-tron GT in the wind tunnel.Side profile of a column of air flowing over an RS Audi e-tron GT in the wind tunnel.

The vehicle stands on a precision scale which measures the aerodynamic forces on the vehicle. Its wheels stand on four small belts which ensure that they turn at the wind speed. A wide belt under the car simulates the movement of the carriageway relative to the vehicle at all running speeds. In addition, precision-adjustable perforated plates in the floor in front of the vehicle extract part of the airflow—the so-called boundary layer—before it reaches the car. Aerodynamicists call this design “full ground simulation”: This helps guarantee realistic air circulation around the vehicle.

Once the air has passed the Audi RS e-tron GT, the widened stream from the plenum is captured by the downwind collector and directed back into the wind tunnel circuit and to the rotor turbine. This completes the cycle of the air through the Audi aeroacoustic wind tunnel—and if you think it sounds complicated, that’s because it truly is.


Dr. Kentaro Zens, the aerodynamicist responsible for the Audi RS e-tron GT, standing under the underbody of an RS e-tron GT.Dr. Kentaro Zens, the aerodynamicist responsible for the Audi RS e-tron GT, standing under the underbody of an RS e-tron GT.
Dr. Kentaro Zens and Thomas Redenbach discuss the RS e-tron GT in front of a computer in the observation room at the Audi aerodynamics testing facility.Dr. Kentaro Zens and Thomas Redenbach discuss the RS e-tron GT in front of a computer in the observation room at the Audi aerodynamics testing facility.

Going to great lengths to achieve the perfect airflow

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.”

Every thousandth by which we can improve the drag coefficient leverages potential in terms of range

Dr. Moni Islam

Portrait of Dr. Moni Islam, Head of Development Aerodynamics & Aeroacoustics at Audi, standing in front of the wind tunnel's active silencer. Portrait of Dr. Moni Islam, Head of Development Aerodynamics & Aeroacoustics at Audi, standing in front of the wind tunnel's active silencer.
Close-up of how airflow behaves after passing by the exterior mirror of the Audi RS e-tron GT.Close-up of how airflow behaves after passing by the exterior mirror of the Audi RS e-tron GT.
Close-up of the computer and workstation of the aerodynamicists at Audi.Close-up of the computer and workstation of the aerodynamicists at Audi.

Simulation cannot replace the wind tunnel

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.”

We invest an enormous amount of time in the last 20 percent of the aerodynamics

Thomas Redenbach

Leveraging potential in terms of range

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.

Two Audi aerodynamicists inspect the underbody of an Audi RS e-tron GT on a raised platform. Two Audi aerodynamicists inspect the underbody of an Audi RS e-tron GT on a raised platform.
Close-up of a hand pointing out the plastic lip on the underbody of the Audi RS e-tron GT that directs airflow efficiently. Close-up of a hand pointing out the plastic lip on the underbody of the Audi RS e-tron GT that directs airflow efficiently.

Detective work for every thousandth

“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.”

Side profile of the wheel of an Audi RS e-tron GT, a smoke lance demonstrating how the air flows past the wheel arch. Side profile of the wheel of an Audi RS e-tron GT, a smoke lance demonstrating how the air flows past the wheel arch.

The aim of aerodynamics is to facilitate design.

Dr. Kentaro Zens

Zens also tested the positioning of the rear spoiler millimetre by millimetre in the wind tunnel to determine the best option. And another example is also close to his heart: The angle integrated into the taillight. “There is a lot of turbulence at the rear of the Audi RS e-tron GT, in particular due to its pronounced three-dimensional shape. Guiding the airflow cleanly around this is a challenge. In the simulation, we saw that there was still room for improvement around the taillight.”

Fortunately, César Muntada, Head of Light Design at Audi, was also present during this wind tunnel measurement. He quickly modeled a slight outward curve into an indentation in the taillight on the clay model, which now appears in exactly the same form on the production vehicle. This modification enabled designers and aerodynamicists to ensure that the airflow breaks away at the rear in a controlled manner instead of turning inward and creating turbulence (which would significantly impact the drag coefficient). “In aerodynamics, we aim to facilitate design,” says Kentaro Zens, describing this collaboration. And that includes meticulous detective work in the wind tunnel.

Side profile of the rear spoiler of an Audi RS e-tron GT, a smoke lance demonstrating how the air flows over it. Side profile of the rear spoiler of an Audi RS e-tron GT, a smoke lance demonstrating how the air flows over it.
Close-up of the rotor turbine inside the Audi aeroacoustic wind tunnel.Close-up of the rotor turbine inside the Audi aeroacoustic wind tunnel.
 

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