All fleet engineers and truck operators know that good aerodynamics will save fuel, but in the search for every last drop of efficiency, the wind tunnel is being replaced by computing power. Welcome to computational fluid dynamics, and what it is doing for you.
Mr 10%
It’s hard to imagine any heavy truck these days that doesn’t have some form of aero kit fitted. A 10% fuel saving between a naked tractor and one fitted with even simple roof and side deflectors is an accepted figure in the industry. And, of course, it’s a job that trailer makers need to attend to as well. In 2011, Daimler put a shot across the trailer makers’ bows by introducing its ‘Aero Trailer’ at the Kortrijk CV show. It was in part a call for length regulations to be relaxed, as they had proven significant gains from a rear diffuser – as in F1 – and a boat-tail design of just 40cm which was reckoned to reduce the drag of the entire vehicle by a further 10%.
Designs like that, and its predecessors, were the product of time-consuming wind tunnel testing; first with scale models, and ultimately the full-size version. The laborious and expensive process of constructing clay models, and the many junked versions of the glass fibre result, were the physical way of dealing with an issue in the physical world. Aerospace, and the exotic world of Formula One design, now uses a new way to manage airflow to best effect: Computational Fluid Dynamics, CFD.
A virtual success
The IRTE’s annual conference in September had a view into the scope and possibilities that CFD can bring to commercial vehicle design. But first, what is it?
CFD is a branch of fluid mechanics that uses numerical methods and algorithms to solve and analyse problems that involve fluid flows. Computers are used to perform the calculations required to simulate the interaction of liquids and gases with any variety of shapes and surfaces. As the speed of computing increases – and it’s a ceaseless forward trend – so better solutions can be achieved. Software research is constantly improving the accuracy and speed of these highly complex simulations.
Rob Lewis, Managing Director of TotalSim, a CFD consultancy based in Brackley (deepest F1 territory) said that the applications for its use are almost limitless. He says, “The air or water flow over, or through an object, can be replicated, measured and manipulated in a way just not possible in a wind tunnel, or by any other method. It makes the designing of anything from wind turbine blades, to oil rig legs easier, faster, cheaper and more accurate.” With a range of the most diverse applications, the IRTE’s delegates got the point. As non-veterinarians they were not directly interested in scanning the nasal cavity of a horse, but they were attracted to the idea of redesigning a gearbox case to improve lubricant movement.
Detail
In looking at the detail that CFD can provide in comparison to the traditional wind tunnel, the advances are spectacular. It would be easy to think that those smoke trails gave a perfectly accurate representation of how air behaves flowing over a truck or bus. But a CFD simulation shows it to be far more complex.
As Lewis commented, “Drag does not go in straight lines, it’s chaos.” A truck and trailer combination has a far more ragged airflow over it than those traditional smoke trails would lead us to believe. In F1, where the fine detail of the front wing on Lewis Hamilton’s car looks like a bizarre artwork, it is the finely honed result of a CFD study. In the same way, alterations to a truck’s mirrors, or the cab’s frontal dirt deflectors, can now be much more accurately profiled, and with a deeper understanding of the implications of just a small tweak here or there. And it’s all done so quickly.
A much larger percentage of a truck and trailer’s drag is now open to scrutiny and adjustment. CFD can map the airflow through the truck chassis, around the engine and gearbox and away to the rear. Here the ‘dirty’ air (aerodynamics, not emissions) is passed to the trailer’s underside which, compared to the polished sides of the smooth box bodywork above, is a mess of angular metal structures and axles that look as if they have been specifically designed to be as un-aerodynamic as possible. It’s been very much a case of out of sight out of mind in these hidden recesses. Wind tunnels with smoke and paper strips could not investigate here, CFD can.
As smooth airflow is essentially a double act between truck and trailer, Daimler was seemingly working ahead of the curve in giving trailer makes food for thought at Kortrijk three years ago. With less research and development cash in the trailer industry, this could have been a concern a few years ago. But CFD will open the door to hitherto unaffordable levels of development refinement, for companies with more modest budgets.
The vanguard
But many commercial vehicle designers are already on the case. DAF’s Aerobody, designed at the Leyland plant, used CFD as part of the process, Gray & Adams is designing vortex segments on the rear sections of its trailer roofs, and the Don-Bur teardrop trailers owe their distinctive profile to CFD.
TotalSim’s Lewis made a plea for collaboration and joined-up-thinking from truck and trailer makers, but he warned that CFD was not a fix-all solution that can jump out of a computer and on to the highway. “There is still an element of experimentation needed to see what works in a range of operating conditions,” he said. But CFD holds the promise of slashing the time taken to get to a working design, and cutting the wasted resources and labour previously lost in junked prototypes. As the use of CFD balloons, it will help push the commercial vehicle industry forward in its search for efficiency.
