Increased streamlining for lower emissions.

Mr. Woll, you’ve been the head of Aerodynamics at Mercedes-Benz since 1999. Many of the future scenarios created back then have already been realized today. What has changed?

First of all, our vehicles have become much more streamlined, and in many segments they have taken the lead. Two values are crucial here: the Cd value as the measure of the vehicle’s drag coefficient and the vehicle’s frontal area in square meters.

The new A-Class sedan has a Cd value of 0.22 and a frontal area of 2.19 m². That makes it the world’s most streamlined vehicle. It comes very close to the “aerodynamic sound barrier” of 0.2. In the 1970s, Cd values between 0.4 and 0.5 were still the norm.

What about SUVs?

In recent years we’ve made real quantum leaps in the SUV segment as well. The new GLE has broken the SUV sound barrier of 0.3 with a Cd value of 0.29. Of course that’s also having positive effects on the acoustics. By comparison to 1999, when we really started to work on optimizing the wind-noise factor, our cars have become much quieter. In the GLE we’ve been able to reduce the wind noise by about two thirds compared to the original model. And for the A-Class that figure is at least 60 percent.

To what extent do aerodynamics influence how CO₂ targets can be reached today?

Their influence is significant. Today both the individual and fleet-wide targets in Europe are very ambitious. In recent years our vehicles have not become more lightweight, because standards have risen regarding space, safety, and comfort. This rise must be overcompensated for by more efficient drive technology — and consequently by better aerodynamics. In addition, under everyday conditions low air resistance reduces fuel consumption.

Is there a rule of thumb for that?

Of course! If we reduce the Cd value by 0.01, the vehicle will save almost 1 gram of CO₂ per kilometer, according to the NEDC. According to the new WLTP, the vehicle will save as much as 1.5 grams of CO₂ per kilometer, and in highway driving it will save up to eight grams of CO₂ per kilometer, depending on the speed.

What will change for you as a result of the switch to the WLTP test cycle, which is now valid worldwide?

For us aerodynamics engineers, a great deal will change. The average speed of the WLTP test cycle will be 35 percent higher (NEDC 34, now 46 km/h), but that’s only the smaller part of the change. The biggest change for us will be the fact that the WLTP takes all of the optional equipment into account for the very first time. We have to indicate the changes in the Cd value for every conceivable combination of optional equipment with an accuracy of two percent. As you can imagine, that will require a detail-oriented approach because there are so many aerodynamically relevant combinations of optional equipment.

For a long time, people said that if you want to break world records with your Cd value, the rear of your vehicle has to be as long as possible. Today SUVs also get top marks for their aerodynamics. What has changed?

A long and tapering rear is still good for the vehicle’s aerodynamics. That simply depends on the negative pressure that is generated there. However, today we know that the perfect alignment of the rear is just as important as its length, because we also have to prevent horizontal vortices that absorb energy.

You may have occasionally noticed something similar in airplanes. Nowadays all airplanes have small winglets at the tips of their wings in order to reduce horizontal vortices. It’s the same thing that birds of prey have had on their wings for thousands of years.

How do you make these horizontal vortices visible?

In the past we did that in the wind tunnel, initially with the help of woolen threads (“Incidentally, I didn’t invent them!” he laughs) and later on with smoke. Today we use computational fluid dynamics (CFD), which helps us not only to optimally identify vortex structures but also to trace them back to their point of origin and sometimes even eliminate them. That helps us aerodynamics engineers a lot — especially when the trend in design is to have a wide and sporty rear with a wide track and wide shoulders.

You mentioned birds of prey just now. What role are bionics playing in aerodynamics today?

Today we understand many of the solutions that nature has created. But we also know that there are no wheels in nature, and wheels play a major role for us in terms of air resistance. That’s why bionics are not very relevant to aerodynamics.

As requirements grow, demand for a holistic and sustainable development process is also increasing. At which point is your expertise called on?

From the first proportion models to the model update. For example, if you miss something when you’re working on the basic design of a new vehicle, you can do everything right from then on but you still won’t be able to get really good values any more. In addition, because of the new legislation we’re affected by every small change that alters the airflow conditions, whether it’s in the outer shell, the engine compartment or the underbody.

Today we’re already calculating many of these changes on the computer. In this age of digitalization and full connectivity, are wind tunnels becoming obsolete?

No, computers and wind tunnels are outstanding tools that complement each other perfectly.

What does this complementarity look like in practice?

In the early phase — that is, until the first model decisions have to be made in the design process — we really do rely completely today on CFD calculations. That’s because these calculations don’t require any complicated models made of clay or foam, and they show the relationships between the various form parameters very precisely. In the late phase as well, we develop some vehicles purely digitally, and when we get to the optimization of the air resistance we don’t need to do any work whatsoever in the wind tunnel. However, these are vehicles that are not expected to set any Cd value records. Instead, for these cars design is a very high priority, and they have only a very small effect on fleet-wide fuel consumption.

So for all the other vehicles, the wind tunnel is an absolute must?

That’s right. Especially if we can’t come to an agreement with the designers, the wind tunnel is an indispensable tool for finding the “perfect” solution. The wind tunnel is irreplaceable when it comes to a detailed optimization down to a half-thousandth of a unit, because it’s very precise and very fast. If you’re well prepared when you put your vehicle in the wind tunnel, you can get all the measurements for up to 40 variants in eight hours. Within five minutes we can quickly add some cardboard to the rear of the vehicle and conduct a measuring procedure for which the computer would still need about a day.

Does this mean that the WLTP is making the wind tunnel even more important?

Yes, you could put it that way. As I’ve already briefly mentioned, ever since the introduction of the WLTP the air resistance of the vehicle including all the optional equipment is relevant to certification. This means that, in this area, we have to rely on the wind tunnel 100 percent too.

To what extent can you already digitally simulate wind noise today?

The development of wind noise is so complex that we won’t be able to calculate it on the computer for years to come. In addition to the flow, we also have to correctly simulate the generation of the sound and, above all, the propagation of the sound. Because of the different levels and length scales of turbulence and sound, that’s not very easy to do. It’s true that we’ve already gotten very far in terms of sound generation, but it will take us a long time to correctly simulate the sound transmission paths through a variety of materials such as glass, steel, rubber, and plastics. The situation today is this: Our aeroacoustics wind tunnel, which is the quietest in the world, is absolutely essential to our daily work.

Mr. Woll, thank you for these suspenseful insights into your daily work. We are looking forward to the continuation of our conversation focusing aerodynamics of electric vehicles.