On-track testing - AeroTags : testing
Track testing is a very limited commodity for Formula One teams. In 2013, they are limited to just four pre-season tests – the three free practice sessions before a Grand Prix and a single young driver test. The recent furore surrounding Mercedes’ test session with Pirelli highlighted the importance of track time and the potential impact it can have on car development. Ultimately, there is no substitute for real-world running and teams need to maximise the volume and quality of data gained from track sessions.
With the current ban on full-scale wind tunnel testing, and the potential for even tighter restrictions in 2014, teams have been forced to gather more and more aerodynamic data from track testing in order to validate their scale model and CFD programmes. The bulk of this data is obtained by two methods – measuring air pressure at points on the bodywork, and measuring the aerodynamic loads on the aerodynamic surfaces. However, teams will also use non-data based analysis in the form of flow visualisation, more on which later.
The most common method for collecting pressure data is through the use of pitot tubes, sometimes arranged in arrays to analyse flow in a specific area. The latest generation of sensors feature built-in processors to provide individual pressure readings, usually to gauge airspeed, and can often be seen on cars in test configuration mounted high up above the airbox in clean airflow. The processor built into the base of the pitot compares the dynamic and static pressures to provide an accurate airspeed reading.
Arrays of pitot tubes without built-in processors are used to measure pressure differentials across an area. Often, and particularly when a team is struggling to correlate simulated and real-world performance, cars can be seen sporting these complex pressure sensing arrays next to key aerodynamic appendages. These are linked to a differential pressure sensor that processes the pressure provided by each tube. The data is then transmitted through the car’s CANbus network, greatly reducing the complexity of the wiring loom. Teams have used these very dense arrays of pitots in a host of placements, looking at everything from wheel wakes to diffuser flow.
In addition to pressure measurements, bespoke load cells are often incorporated into highly stressed downforce-generating components in order to ascertain load distribution across the surfaces. For example, sensors can be incorporated into the front and rear wing supports, encased inside the composite structure. The sensors use load cells to gauge the forces on the wing, and each cell produces outputs for lift, drag and pitching moments, and from these outputs the centre of pressure for the wing can be calculated.
While the data gathered using both of these methods can be invaluable, sometimes it is useful to be able to visualise the flow around a component. This is one of the oldest methods of analysing the aerodynamic performance of a car. In the past, wool tufts would be attached to the bodywork, and when the vehicle moved they indicated the direction of airflow over surfaces. The modern approach is to use ‘flow-vis’, a high-visibility fluid sprayed onto the surfaces of the car. As the car is driven, the fluid is pushed over the surface, leaving trace lines indicating the direction of flow. The traces can show up factors such as stalled flow or areas where the flow has reversed or is not heading in the desired direction.
Using a combination of these methods, engineers will gather data with which they can improve their off-track simulation models. It is this model validation that is the most valuable product of on-track testing. While it is useful to see if a new component has delivered tangible benefits on-track, it remains a fact that, for the foreseeable future, most development will occur in the virtual domain. Therefore, the more real-world data that can be fed to the simulators, the more effective developments will be when they hit the track.
Written by Lawrence Butcher