10th December, 2016

Aerodynamic evolution of the Geec

According to the PAC-Car book, aerodynamic forces account for about a third of the forces that push back against the motion of a highly efficient car. In other words, air is not the most important thing sucking energy from the car, but it is important. Geec designs have evolved radically in an effort to reduce aerodynamic drag forces.

In the beginning

The front wheels must be set wide apart to prevent the car from rolling in corners. The Geec 1.0 had wheels out in the open air, which is not an option for a seriously competitive car. A lot of drag is caused by rotating wheels with all their associated support structures, brakes and other hardware.

The Geec 1.0 competing in Rotterdam, with results of an aerodynamics simulation

The Geec 1.0 competing in Rotterdam, with results of an aerodynamics simulation

Exploring concepts

There are two ways to deal with the front wheels. You can enclose the whole car, including wheels, in a single wide body, or you can keep a sleek central body with faired (covered) external wheels. From 2014 to 2016, our aerodynamic designers, Paul Mannion and Sean Scally, carried out a series of computational fluid dynamics (CFD) simulations to find out which is better. Their tool of choice was ANSYS-CFX, industry-standard software kindly provided by our partners ANSYS and CADFEM UK & Ireland.

An aerodynamic concept with external faired wheels.

An aerodynamic concept with external faired wheels.

The faired-wheel design allows a narrow body with low drag. However, the wheels themselves and their support structures create large wakes which are always associated with drag. This is a complex design to implement – the fairings must have space for the brakes, and they must steer. Ideally, no gaps in the fairing should open up when the wheels make steering movement. We didn’t know it at the time, but this concept was very similar to TUfast-Eco, which went on to win our prototype battery-electric category in Rotterdam. So it must be good.

But we also looked at a range of wide, fully enclosing body designs. It turns out that a carefully designed wide body is slightly better than a narrow body with outrigger wheels as above. These wide bodies have a lot of extra bulk, but if the shape is good, they generate less drag than exposed outlier wheels – no matter how carefully those wheels are covered and streamlined. In addition, this shape is much simpler to build. The decision was made.

An early design (left) and final the version (right) of the body shape, with visualisation of disturbed wake flow.

An early design (left) and final version (right) of the body shape, with visualisation of disturbed wake flow.

Optimisation

Aerodynamics were computed for many iterations of the fully enclosed design, while keeping the body low enough and wheels wide enough to ensure good cornering. The images above show the final design tweaks. (The strange coloured surfaces in these pictures show disturbed rotating wake flow, like the wingtip vortices of an aeroplane. If you want to know more: they are isosurfaces of the λ2 criterion.) The earlier design (left) created strong wakes from the two front pods which housed the wheels. After that we made many rounds of changes – some were made specifically to improve the aerodynamics, and others were about making space inside the car for chassis, drivetrain etc (oh, and the driver) as the internal design evolved. The difference between the shapes is hard to see, but the aerodynamic effect is huge. The wakes are almost eliminated. That’s the shape that’s going to be realised as the carbon-epoxy skin of the Geec 2.0!

Josefine Kristy Web Developer

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Website: http://www.spikeincor.eu