Go with the flow

An airflow simulation technique involving helium bubbles inside a wind tunnel could help in the design of environmentally friendly cars. Siobhan Wagner reports.

A new wind tunnel technology that allows designers to ‘see’ airflow could help automakers design cars that are more aerodynamic and produce less carbon emissions.

Engineers at the British automotive test company MIRA developed the visualisation technique, which uses hundreds of tiny helium-filled bubbles and an array of 12 Hollywood-style motion-capture cameras inside a Full Scale Automotive Wind Tunnel. The MIRA tunnel is currently the only commercially available full-scale wind tunnel in the UK.

The neutrally buoyant 3mm bubbles are injected into the flow in areas under investigation and the cameras track each bubble and reveal more information about flow than was previously possible. ‘With this we can tell where flows originate, what the flow is doing at any one moment in time and how it evolves in time,’ explained Angus Lock, MIRA’s head of aerodynamics.

The technique is a significant improvement on current wind tunnel tests, which detect forces such as downforce and drag on a car but cannot explain how those forces generate.

The MIRA method maps a large volume of flow with specially calibrated cameras that use triangulation to interpolate 3D positions of all the bubbles. The flowing bubbles can be watched live and the MIRA team has developed an in-house code to convert this motion capture data into the format required by commercial computational fluid dynamics (CFD) post processing software.

Lock said this new flow visualisation technique can help redesign many different areas of the car that contribute to drag. It is a particularly ‘hot topic’, he said, with current EU legislation requiring that new cars in 2012 reduce CO2 emissions to 130g/km.

A lot of work on drag reduction focuses on managing the wake at the rear of the vehicle, but Lock said traditional flow visualisation techniques have trouble detecting specific areas of energy loss there.

Existing methods such as wool tufts and fluorescence paint only detect what is happening on a surface. Techniques such as smoke wands, Laser Doppler Anemometry (LDA) and Particle Image Velocimetry (PIV) only detect what is happening in a small slice of the flow.

Lock added that organisations such as NASA have used helium-filled bubbles as ‘a subjective flow visualisation technique’, but MIRA’s method for tracking the bubbles provides much more information.

‘What we’re doing is measuring the position of those helium bubbles, hundreds if not thousands of them, in any one moment in time,’ he said. ‘With the ability to capture the positions of those bubbles, we can then infer velocity information from them and build up a picture of what the flow is doing behind the car.’

Lock added this will give a clearer understanding of where regions of high-energy loss are in the wake and will lead to vast improvements in the design of the car.

MIRA has not yet used the wind tunnel in a commercial project. The technology was developed under a British government project that finished last month.

Jaguar Land Rover, which is supporting the project, has already expressed interest in using the wind tunnel to design some of its cars in the future. Lock said MIRA is also currently talking to two Formula 1 teams that are keen to try the technology.

‘MIRA has a very strong presence in the automotive world so I’m sure there may well be other partners that we work with globally,’ he added.

Lock said the first commercial application could be close.