Go with the flow

A flow visualisation and measurement technique aimed at helping car makers increase the efficiency of their aerodynamic designs has been developed by Nuneaton-based automotive specialist, MIRA.

MIRA’s approach allows aerodynamicists to visualise the airflow and match this to their modelling predictions and test results. The system uses motion capture cameras (pictured below) and helium-filled bubbles to provide large-volume flowfield mapping in their Full Scale Automotive Wind Tunnel.

According to the company, this allows car makers to increase their efficiency and produce less CO₂ by reducing vehicle drag.

The visualisation technique measures the airflow in real time and processes the data in the same virtual environment used for Computational Fluid Dynamics (CFD) simulation.

Angus Lock, head of aerodynamics at MIRA, said: ‘One of the challenges for aerodynamicists is the fact that air is invisible, so you can't easily ‘see' what is 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.’

One of the main advantages of the new technique is the large measurement volume.

Existing methods, such as fluorescence paint, smoke wands, Laser Doppler Anemometry (LDA) or Particle Image Velocimetry (PIV), are only able to measure flowfield around the surface or in a small slice.

MIRA's system captures the entire test section of the wind tunnel in 3D.

Tiny helium bubbles are placed throughout the wind tunnel airflow and tracked by an array of 12 cameras.

When two calibrated cameras see a bubble, a 3D fix is obtained using stereo-photogrammetry.

Each bubble tracked reveals where the flow originates and how this can interact with the bodywork and other structures within the flow field.

Once the data is captured, an in-house code converts it into the format required by commercial post-processing software.

This provides aerodynamicists with the capability of comparing virtual and experimental data sets simultaneously in the same software environment.

Lock said: ‘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 four.

'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.’