Bubble tracking

Written by: Tom Shelley | Published:

Tom Shelley reports on a development in wind tunnel monitoring with significant results

The combination of small bubbles with film making techniques is allowing aerodynamic behaviour inside wind tunnels to be seen by engineers in unprecedented detail.

The result is the verification and refinement of computational fluid dynamics (cfd) models. The technique is proving to be of great benefit to both motorsport, which want to corner faster, and to the mainstream where aerodynamic improvement can improve the fuel economy of heavy goods vehicles and SUVs or reduce the cabin noise inside cars.

US company, Sage Action, supplies the proprietary equipment to make the bubbles. It fills them with helium until they are 3mm in diameter, so the weight of the fluid film is equal to the lifting power of the helium making the bubbles neutrally buoyant.

The wind tunnel in question is at Mira and uses a motion capture camera system devised by Vicon, which is headquartered in Oxford. It uses multiple cameras to capture motion in 3D normally for filmmaking and computer game development but has branched out in to applications in medicine, sport and engineering.

The technology is usually used by tracking reflective markers attached to the bodies of actors or athletes to capture movement. This can then be applied to computer animation or to analyse and improve sporting performance. Many sports institutions in preparation for the 2012 Olympics are currently using motion capture technology.
In the wind tunnel, the object of the exercise is to closely track the bubbles and, hence, the airflow in three dimensions. It captures the behaviour of every eddy in the entire volume of the wind tunnel in which a vehicle is placed, including effects occurring well away from the vehicle surface. Vortices cause drag, and turbulence causes noise and buffeting. The ultimate aim is to eliminate vortices, or cause them to break up as soon as they start to form to reduce unnecessary turbulence.

Traditional techniques such as wool tufts and fluorescent paint are limited to showing what is happening immediately adjacent to the vehicle surface while smoke wands or rakes only give a crude, subjective visualisation. Laser techniques such as Laser Doppler Anemometry and Particle Image Velocimetry are limited to point or plane measurement over small areas, though they do offer high spatial and temporal resolution.

The motion of helium bubbles has been studied by observing the illuminated tracks on photographs, either as they pass through a light sheet, or are caught by a strobe light. But this requires interpreting static 2D images, and does not show detailed real-time movement in 3D.

Simple flows are not a problem but as Mira principal engineer, Angus Lock explained at January's Autosport Engineering event: "Cfd has reached the point where modelling can predict time averaged drag to within 1-2% of what is actually measured. Wind tunnels detect forces like down force and drag, but never explain how these forces are generated. 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 100million discreet data points, and the wind tunnel, only has four."

The present development began as a collaboration between what was, Vicon Motion, MIRA and the Motion Analysis Research and Rehabilitation Centre (MARRC) at the University of Worcester. Areas of research at the latter establishment cover topics that include health and safety, occupational health, clinical assessments, sports performance, injury and rehabilitation and equine assessment. MARRC then dropped out of the wind tunnel project, leaving Vicon and Mira to take it forward with the assistance of a grant from the Technology Strategy Board.

The facility at Mira includes 12 MX-T40 cameras, monitoring a volume that is 3m wide by 3m high by 8m long. A larger volume could be monitored, but it would require more cameras. Up to 244 cameras can be connected to a single Vicon system, feeding data to a single PC.

Calibration is achieved by moving a calibrated marker wand around in front of the cameras. The system can then automatically ascertain the cameras' position, orientation and focal length. Using this information it builds up a picture of the measurement volume in 3D. Provided a bubble is in sight of at least two calibrated cameras, its position and motion can then be accurately tracked. The system is capable of tracking hundreds of bubbles in real time.
An in-house developed code is used to convert the motion capture data into the format required by the post processing software used in the cfd package. This allows it to be directly compared with cfd predictions.

Using the new process, Lock says that it can identify individual vortices. "It gives us far more information than we have ever had before," he says. "It allows a detailed comparison of real world, measured behaviour with cfd predictions." He added that large scale structures tend to correlate very well, but said nothing about small scale structures, such as winglets for Formula 1 cars.

Mira is talking to two Formula 1 teams but has already successfully applied the technology to evaluating the aerodynamics of Land Rovers, trucks and motorcycles. It says its work on trucks has resulted in a 16% reduction in drag that equates to an 8% fuel saving. The motorcycle work was not so much aimed to enhancing speed, as improving the experience of riders, particularly reducing buffeting. Purchasers of expensive motorcycles are often middle aged, and demand a certain degree of comfort along with the thrill of going fast.


* Technique allows the capture of detailed motion of all air movements around a body in a wind tunnel.

* The data obtained by tracking buoyancy neutral bubbles can be post processed and directly compared with cfd software.

* Benefits include the ability to reduce vehicle drag and improve fuel economy, reduce cabin noise, improve the comfort of motorcycle riders and help Formula 1 cars corner even faster.

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