New aero technology to help automakers achieve 2012 CO2 limits

Automakers can increase the efficiency of their aerodynamic development process & design aerodynamic cars that produce less CO2, thanks to a new flow visualisation and measurement technique.

Wind tunnel time becomes more productive as aerodynamicists can ‘see' the entire airflow & directly correlate modelling predictions with test results in a common analysis environment. By bringing wind tunnel development and the virtual world together, aerodynamics has taken a huge technological step forward.

Automotive development specialists MIRA have pioneered the approach and are particularly keen to apply the technique to complex areas such as the vehicle wake, which contributes significantly to drag. Drag reduction is a key enabler in the delivery of vehicles that meet the new EU 2012 fleet average CO2 legislation.

MIRA use Hollywood derived motion capture cameras and helium filled bubbles to provide large volume flow field mapping in their Full Scale Automotive Wind Tunnel. The MIRA tunnel remains the only commercially available full scale wind tunnel in the UK. The novel visualization technique measures the airflow in real time and post-processes the data in the same virtual environment used for Computational Fluid Dynamics (CFD) simulation, leading to a much deeper understanding of the underlying flow structures responsible for a vehicle's aerodynamic performance.

Angus Lock, MIRA's Head of Aerodynamics explains: "One of the challenges for aerodynamicists is the fact that air is invisible, so you can't easily ‘see' what's going on. Wind tunnels detect forces like downforce and drag, but never explain how those forces are generated. To reveal the flow structures at play you need to use flow visualisation. This revolutionary technique allows us to get a really good view of the flow field, enhancing both CFD studies and wind tunnel development."

The key advantage of the new technique is the large measurement volume. Existing techniques such as: wool tufts, fluorescence paint, smoke wands, Laser Doppler Anemometry (LDA) or Particle Image Velocimetry (PIV), only detect what's happening on a surface or in a small slice of the flow. MIRA's system captures the entire test section of the wind tunnel in 3-D.

MIRA's system works by seeding the wind tunnel airflow with the tiny helium bubbles that serve as targets to be tracked by a 12 camera array. Providing a minimum of two calibrated cameras see a bubble, a three-dimensional fix is obtained using stereo-photogrammetry. The neutrally buoyant 3mm bubbles are injected into the flow in areas under investigation. Each bubble is tracked, revealing where flows originate and how they interact with the bodywork and other structures within the flow field. The system is capable of tracking hundreds of bubbles in real time, giving a much greater understanding of how a vehicle's aerodynamic properties are generated.

An in-house code then converts the motion capture data into the format required by commercial post processing software. This vital step unlocked the capability to compare virtual and experimental data sets simultaneously in the same software environment.

Improvements in wind tunnel measurement techniques can have a wider impact on aerodynamic development, as Lock explains: "Often CFD results are validated against the force data generated by a Wind Tunnel balance, which could be just a small contact point under each wheel. When there are discrepancies between the two sets of results, it is difficult to tell where the differences lie, as the CFD data may have 100 million discreet data points, and the wind tunnel only 4. Comparing only the global forces is a blunt tool, now we can really understand the wider flow field and superimpose this data on the CFD predictions to see how the two compare."

The recently announced EU 130g/Km fleet average CO2 legislation combined with the current global economic crisis places renewed pressure on automotive development budgets, so the announcement couldn't come at a better time for the industry. MIRA's technique allows automakers to extract maximum efficiency from their vehicle's styling & return maximum value for their development spend.