In the eye of the hurricane
Transport Magazine

In the eye of the hurricane

Aerodynamics is measurable: the skills of the developers at Mercedes-Benz are clear in the wind tunnel.

The Actros consumes up to five per cent less fuel than its predecessor. This is in part due to its improved aerodynamics. This was achieved through testing on the computer, on the road and, indeed, in the wind tunnel.

The enormous blower could easily be the backdrop for a new Hollywood production, serving as the engine of a gigantic spaceship, for example. The axial blower has a diameter of 8.5 metres, while the nine red-painted wings are each 2.5 metres long. Pictures are actually being taken at the facility on the day we are there. There was no movement involved though, rather they were portrait photos of Michael Hilgers, Head of CAE Vehicle Functions in the Commercial Vehicle Development at Mercedes-Benz. 

Here, in the Daimler AG wind tunnel in Untertürkheim near Stuttgart, Hilgers and his colleagues have been instrumental in ensuring that the Actros is more aerodynamic and thus more fuel-efficient than all of its predecessors.

The following figure shows the importance of aerodynamics. In a current truck in long-haul operation in Europe, around one third of the available mechanical energy is used to overcome air resistance. The lower this resistance, and thus the more aerodynamic the truck, the lower its consumption. The Actros enables fuel savings of up to five per cent compared to its predecessor. As much as 1.3 per cent can be attributed to the aerodynamically optimised MirrorCam alone, which replaces the conventional external mirrors.

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Michael Hilgers, Head of CAE Vehicle Functions in Commercial Vehicle Development at Mercedes‑Benz, presents the power of the blower in the wind tunnel in Untertürkheim.

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250

kilometres per hour wind speed can be generated by the wind tunnel’s fan.

But how do the engineers actually carry out the testing? The wind tunnel blower can produce storms with speeds of up to 250 kilometres if required. The vehicle is positioned on a hub with an integrated scales for the tests. This enables a wide range of flow conditions to be simulated.

“We perform random tests to confirm the aerodynamic improvement of component concepts.”

Michael Hilgers, Head of CAE Vehicle Functions

The goal of these simulations is to optimise the cd-value, or the drag coefficient, of a truck. “We carry out random testing here to verify the aerodynamic improvement of concept components,” says Michael Hilgers, explaining the basic approach. “The computational flow calculation always runs in parallel. This digital simulation is based on computational fluid dynamics, or CFD.” The aerodynamic measures are also validated in road use.

With the Actros, the wind tunnel work provided valuable insight into the design of the MirrorCam, as well as for the positioning of its camera arms to the right and left of the driver's cab. “The top and bottom parts of the A-pillar and the top part of the B-pillar were discussed,” Michael Hilgers explains. 

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View of Detail I: there is practically no air turbulence around the MirrorCam in the Actros.

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Frontal air flow: in the wind tunnel work, the developers’ focus was on the camera arms of the MirrorCam and the end flaps of the driver’s cab.

9000

cubic metres of air move horizontally around a 125-metre-long, ring-shaped channel.

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View of Detail II: there is also no turbulence at the end flaps – the right side of the driver’s cab in the picture.

A real Actros was used for the tests, where the wing mirrors were replaced by prototypes of the camera arms – installed one after another in the three positioned being tested. The truck was positioned on the scales and the blower started up. The scales enabled the engineers to measure the aerodynamic force acting on the vehicle as the air flows around it. Result: the optimal position for the camera arms is in the A-pillar in the edge of the roof.

A solution was also sought to prevent scattered light coming in from above from impacting the performance of the cameras. During these tests, the small roof with which the MirrorCam arms are now equipped proved successful. The engineers also played a major role in the development of the new, concave-shaped end flaps of the driver’s cab. The optimised end flaps also ensure that the Actros requires less fuel than any of its predecessors.

In addition to reducing fuel consumption, the engineers also look at dirt retention during the tests in the wind tunnel and the CFD analyses. “This mainly involves safety-related areas such as the front and side windows and the camera arms lenses,” Hilgers explains. “Aerodynamics influences the amount of dirt from your own vehicle and from the vehicles ahead that will stick.”

It is not only the aerodynamics specialists’ own work that is so important, but also coordination with colleagues from other key disciplines, particularly with the designers and constructors. After all, not everything that is useful for aerodynamics is necessarily desirable from a design perspective or feasible for constructors. 

Conversely, the aerodynamics specialists need to veto some of the ideas from their colleagues. “Ultimately, everybody involved is aware of one thing,” Michael Hilgers adds. “The goal is always to develop the best-possible solution together.”

The wind tunnel in Untertürkheim.

The wind tunnel in Untertürkheim has been used by Mercedes-Benz developers for eighty years. Thanks to relevant modernisation, it is still state of the art. Two DC motors, each with a power of 2,500 kW, set the axial blower in motion – so powerful that they can even deliver wind force 17. During the process, around 9,000 cubic metres of air are moved horizontally in a 125-metre-long, ring-shaped channel. The test vehicle is positioned on a hub with a diameter of twelve metres, so it can be exposed to the wind both frontally and laterally at any desired angle. In addition to a dynamometer, also integrated into the hub is a six-component scale. It is used for highly accurate calculation of various different forces, including air force. The forces are transmitted to load cells via levers and linkages and can then be evaluated.

“It’s always a matter of coming together to develop the best solution.”

Michael Hilgers

Photos: Daimler, Lars Kruse