Aero guards cars against turbulence tumbles

Four years ago, speeding down the Stuart Highway in far north South Australia, two cars barrelled off the road at high speed.

The only damage to humans was dented pride. The cars were incredibly light, small and aerodynamic, clad in photovoltaic solar arrays and, prior to their untimely demise, were mixing it with the frontrunners in the Bridgestone World Solar Challenge, ‘racing’ from Darwin to Adelaide.

The reason they wound up with broken steering (and hearts) was due to diabolical 70km/h westerly wind gusts blowing across the highway.

Down went the entry from Germany, Team Sonnenwagen Aachen. Then, just minutes before the day’s scheduled finish, so too did the entry from Netherlands-based Solar Team Twente.

Solar racing cars relentlessly push the limits of what’s possible for lightweight and energy-efficient vehicle design. With every watt of energy from the sun needed to drive the car, every extra gram of weight increases rolling resistance.

Aerodynamic design maximises wind assistance from whichever direction it comes. Nice, smooth laminar airflow keeps air resistance lower, requiring less energy to push the car forward. Turbulent airflow, however, challenges car designers – air resistance goes up, requiring more energy to move the car at speed, and it also risks destabilising the vehicle (think of aircraft turbulence).

When you’re pushing the limits of weight and aero for a solar-powered race car, turbulent airflow is unwelcome. To turn the tables on turbulence, frontrunning teams in this year’s challenge have chased designs that will harness wind at any speed and direction.

Regulations allow for three-wheeled cars, offering one fewer point of road contact and reducing rolling resistance.

While designers have converged on a monocoque or ‘bullet’ body, one team is betting otherwise.

Sailing on the wind

The Dutch Brunel Solar Team is the world’s most successful solar racing team.

Among the top teams, it stands alone sticking to a ‘catamaran’ design, which draws on inspiration from other sports – and nature – for its aero agenda.

In 2019, it worked with Austrian company Bionic Surfaces to develop ‘Sharkskin’ – a ribbed, adhesive surface that cuts aerodynamically through the world’s oceans and allows Brunel cars to significantly reduce drag through air.

Now adopted across the world’s leading solar teams, it reduces turbulence when strategically placed on the body of the vehicle.

“You don’t put sharkskin where you have laminar boundary layer flow, you only put it in the turbulent regions,” says Jort Simons, one of Brunel’s aerodynamicists.

But sharkskin is old news. So how does a team that’s so used to winning – it’s won more solar races than any manufacturer in history – keep its chasers at bay?

Enter computational fluid dynamics. Across production and race vehicle design, automotive designers simulate hundreds of fluid flow possibilities across just as many vehicle variations.

Brunel has used CFD to refine the shapes of its catamaran foils; its canopy is shorter, its drag-reducing roof fairings and solar array are bowed.

“They’re curved in a similar way that aeroplane wings are, so they’re like an airfoil, like wings,” says Simons.

This matters, because when a car encounters destabilising crosswinds, the foils are capable of harnessing them for a boost: “It’s a bit like a sailboat.”

“When we get certain conditions [like 80km/h crosswinds], the wheel fairings are shaped like an aeroplane wing, and so that creates a force – both a component on the side and a component aimed forwards – and that … enables us to reduce the aerodynamic drag.

“In that way, the car actually consumes less energy in sidewind conditions.”

A fin to the finish

Brunel made world headlines in 2019 when its car burned to ashes while leading the race due to a battery malfunction. Just metres behind them, inheriting the lead and eventual victory, was the Belgian Innoptus team.

Innoptus has recently developed a ‘fin’ to prevent turbulence and enhance stability, similarly inspired by marine animals.

The fin is a retractable and rotatable rudder that harnesses crosswinds to reduce energy consumption and improve stability.

The catamaran design has been phased out in favour of a ‘bullet’.

And what a bullet.

“It’s a little bit like a dart effect,” says Christophe Chenut, one of new fin’s designers.

“When you use the fin, you will pull back your centre of pressure. You’re trying to have a stabilisation effect. It’s the same as in an arrow: you have the mass on the point at the front, and you have your centre of pressure on the back.

“And even when the wind is not in the perfect condition to push the car, you can use the fin to stabilize the car.”

By a nose

In a technological case of convergent evolution, almost all the front-running teams – except for Brunel – have simulated their way to bullet designs.

Among them is Twente – the unfortunates who were blown off-road last time – who have gone hard on stability, without compromising efficiency.

“We have really looked into what happened, of course,” says Tim Woertman, Solar Team Twente’s manager.

“But if you design a tank, it’s a very stable vehicle, but of course you can’t win the race.”

Twente’s new vehicle is more ‘relaxed’ than some of the competitors. The nose, for example, is far less angular than Innoptus’. This, the team says, is designed to enhance stability and leverage the side winds, in lieu of a fin or sail foils.

“It also makes it easier to drive as well,” Woertmans says, “Because you can imagine it’s not easy driving a car like this.”

Even more curvaceous is the nose of Top Dutch,  from Groningen in the country’s north. It too has opted for a sleek monoshell and has the most rounded front-end of the lot.

“We did hundreds of iterations of noses, of tails, of canopies,” says Ian Soede, one of Top Dutch’s aerodynamicists.

“Longer and narrower is, in general, just good, better.

“We converged to this – it’s still fairly pointy, but bulbous enough that it’s also good in side winds and air doesn’t separate [lending itself to turbulence] if you encounter a side wind.”

Rather than pushing the limits of what’s possible for the car’s design, Top Dutch has leant on reliability. Their car, unlike its competitors, has four wheels, “to make sure we can actually finish the race”.

Aeroad training

The Sonnenwagen team has been itching to hit the track after being crowned European champions last year. While its car is very similar to its regional competitors, it has also considered another factor in aerodynamics – road trains.

Dozens of semi-trailers will be encountered by competitors as they make their way south. It’s the crosswinds belched by these diesel-powered behemoths as they travel past these tiny solar cars that will truly test both car and driver.

Driving behind a solar car as a semi-trailer passes by in the other direction, an observer might see the little electric vehicle get pulled towards the larger vehicle and spat back out – all within the space of seconds. This turbulent airflow risks sending cars off-road, which is why safer, crosswind buffering aerodynamics have been a focus of competitors.

Sonnenwagen has modified its centre of pressure such that these billowing crosswinds don’t affect the yaw moment – a phenomenon that alters the direction of the vehicle due to an external force.

That reduces the demand on the human element inside the car.

“It [road train crosswinds] doesn’t affect the yaw moment of the car as much, so this enables the driver to counteract the aerodynamic forces and makes all the gusts really controllable in this car,” says Sonnenwagen’s technical lead Felix Hruschka.

The German car has a reduced profile to better handle crosswinds, but perhaps most importantly, they’ve emphasised driver training.

During 2,000km of pre-event testing in South Australia, the team’s three drivers were drilled in handling windy conditions, passing vehicles and uneven terrain. A chase vehicle following the race car also uses a windmeter to measure cross winds and radio this information to the driver.

“And in testing we stepped up the speed we were going next to road trains to keep it safe and learn what to do and how to handle the push and pull effects,” says Hruschka.

The Bridgestone World Solar Challenge gets underway in Darwin on Sunday 22 October.