CFD helps cut tractor-trailer drag coefficient by 10%

  • 14-May-2015 03:44 EDT
Eddie Stobart trial.jpg

Wirth Research has applied CFD to truck and trailer to reduce aerodynamic drag. (Click arrow at top right to view additional images.)

Air management for trucks is likely to be a focus for vehicle designers as manufacturers look to reduce fuel consumption and CO2 emissions to satisfy global regulations. That is the background for development of the WR AeroKit from Wirth Research, which has formed a strategic partnership with U.K. trucking company Eddie Stobart to deliver a truck and trailer aerodynamic kit.

Wirth Research has a background in motorsport, using computational fluid dynamics (CFD) and other tools to design racecars, notably in the American Le Mans Series and World Endurance Championships. Its work is focused on aerodynamics. The truck project began in 2010.

“Over the years, we realized that we were developing methods, from wind tunnel testing to CFD modeling, going through a very rapid validation process. Through constant feedback into our systems we developed our systems as quickly as we developed our racecars,” explained Rob Rowsell, Engineering Group Manager at Wirth Research.

“We realized that we had tools that other industries didn’t have access to, because they don’t have that rapid turnaround that we do,” he continued. “In 2010, we decided to start looking into applying these technologies to other industries. The first thing we went for was to see if we could make transport vehicles more efficient, partly because motorsport has a [reputation] for being energy inefficient, so if we could do something to carbon offset our presence in motorsport, that’s a good thing.”

The company’s focus is on reducing drag, which it can measure in CFD and back up with track and on-road testing. The target was a 10% drag reduction compared with a vehicle fitted with a standard aerodynamic package, Rowsell said. Compared with a standard truck and trailer with no aerodynamic aids, the WR AeroKit yields a claimed drag reduction of 33%.

While it may be possible to fit a tractor-trailer combination in a larger wind tunnel, aerodynamic testing would be restricted, “especially with a truck,” said Rowsell. “One of the key things we were developing in CFD was not just focusing on the truck straight-ahead, as you would be forced to do in a wind tunnel, but looking at cross wind effects as well. A lot of the kit works better straight-ahead and a lot of the kit works better in cross winds, so that doubling up means that it’s a robust solution that should work in all weather [conditions].”

Rowsell explained that the methodology is a very similar process to what would be done with a racecar: “We took a laser scan of the vehicle and turned that into a CAD model. We then take that into our CAD and CFD packages and unleash the imagination of our aerodynamicists, coming up with different shapes here and there to see what’s working.

“So we went through a number of phases of that to come up with what would be an ideal shape. From there you start looking more deeply into regulations, legislation, and cost-benefit analysis and start whittling some of the pieces away again to come to a solution.”

The vehicle has been track tested at the MIRA proving ground and also at Millbrook. These are the two best-equipped test facilities in the U.K.

“CFD was predicting a 14% drag reduction for an early prototype,” said Rowsell. “We took that to MIRA and they confirmed a 14% drag reduction. Then after a phase of production design and cost-benefit analysis, we came up with a trimmed down and refined kit that was more suitable for a production run. We took that to Millbrook and that was confirmed at 10% drag reduction.”

The production kit consists of a new roof spoiler on top of the truck cab, which replaced the existing spoiler.

“That’s nicely streamlined and lines up nicely with the 4.0-m high trailer and encourages better flow over the top, reducing the separation bubble over the top and around the sides,” explained Rowsell. “The next high-profile piece would probably be the fender turning vanes on the front lower corners. They encourage a better flow around the fender and spin a vortex along the side that helps keep the flow low down, tight in close to the vehicle.”

Wirth Research has just been granted a patent for these components.

“Next up is the chin spoiler. That helps by reducing the high pressure on some of the forward facing surfaces under the truck,” he continued. “Then the final piece is the big new trailer side panel,” which replaces a panel that would have run parallel with the side of the trailer, with a sculpted design carrying the EU mandatory side guard in front of the panel.

“We did a number of iterations, some with the aero profile on the outside of the regulatory side guard and some with it on the inside, and the inside profile worked best,” explained Rowsell. “Some of it is about stopping the low energy flow under the trailer from spilling out and mixing with the free stream that flows down the sides, and it shields the rear wheels from high pressure on the front face of those as well.”

The prototype trailer included shrouding over the rear wheels, but cost-benefit analysis led to them being left out of the final design because of the cost of material and practicality of design that has to work with EU mandatory mudguards. “The benefit is very dependent on the operating cycle of the vehicle,” said Rowsell.

Similarly, the rear of the trailer remains unchanged, partly to meet EU length restrictions and also because cost-benefit analysis showed that a modified rear panel did not yield significant drag reduction.

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