Evolution of aerodynamic testing in F1 - Measurements

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This article follows Part 1 - Windtunnels, focusing on wind tunnel design and operation.

Instrumentation and measurements

Contrary to popular belief aerodynamicists spend very little time wafting smoke wands around in the wind tunnel. Like the full-size cars, they represent, wind tunnels models require and contain an array of sensors and instrumentation, albeit for the sole purpose of measuring aerodynamic data.

Measuring forces

Aerodynamic forces and moments are measured using force balances, which are constructed of an arrangement of flexures designed for compliance in the x, y and z directions. Compression and extension of these flexures is measured by strain gauges which measure the change in electrical resistance over a long piece of thin wire. Force balances can sit internal (about the size of a tissue box) or external (about one or two cubic meters) to the model, with each team/facility having a preferred method as discussed above.

On top of measuring the total aerodynamic forces generated by the car, the loads produced by some individual components of the car are also measured. The front and rear wings are equipped with load cells (single axis balances) for the measurement of downforce, while the lift produced by the wheels is measured using contact patch load cells, similar to an electronic kitchen scale, mounted under each tyre under the rolling road. An advantage of mounting the wheels off the body with horizontal spars as shown above in the 90’s wind tunnel, as was the case up to the mid-2000’s, is the capability to measure the drag force produced by the exposed wheels; however, mounting the wheels on the body makes measuring wheel drag more difficult.

Measuring surface pressures

Surface pressures are measured using small holes, about 0.5mm in diameter, in the surface of the car called pressure taps. As well as being used to detect aerodynamic phenomenon, such as flow separation, measuring pressures allows the force on a particular surface to be determined by integrating the pressures collected over the area in question,

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more surface pressure taps will give a more accurate picture of the force on a surface. This is the same way teams can tell when diffuser performance is lost on the full-size car; for example on Stoffel Vandoorne’s car in Abu Dhabi 2017 where the team could see one side of the diffuser was stalling, which was later found to be caused by a length of tape that had become wrapped around one of the diffusers vanes.

Marussia 50% scale wind tunnel rear diffuser, surface pressure tap locations (41 total) marked with a circle, from @andylaurence.

Each surface pressure tap is connected by a long plastic tube to a pressure transducer, shown below, which uses strain gauges to measure the deflections on miniscule diaphragms caused by the pressure at the surface tap. This is another difference between wind tunnel testing in the 90’s compared to now; in the past the pressure tubing would have to pass outside the tunnel to a manometer (typically a U shaped tube of glass filled with a relatively dense liquid which changes height in the tube when the pressure at the tap changes, the change in fluid height had to be marked and accurately measured with a ruler after the testing concluded). Each tap required its own manometer so only a few surface pressures could be measured at a once, depending on the number of manometers available and because logging more than a few surface pressures would require stopping the tunnel, changing the tube on the manometer from one pressure tap to another and then repeating the experiment. Modern pressure transducers can log hundreds of pressure taps simultaneously, and are small enough to fit multiple transducers inside the model, each of which is electronically controlled. Measurements can then automatically be output and superimposed onto the CAD for comparison between CFD and track data.

There will also be a pressure measurement taken within the sidepod cooling and other assorted heat exchangers.

Example of a 64-channel electronic pressure transducer (length ~115mm), from scanivalve.com.

Surface flow visualisation

Visualising the surface flow is an important technique for determining the direction of flow as well as flow state (i.e. laminar or turbulent) and regions of high vorticity. A common method of flow visualisation would be wool tufts (or any light, thin chord) about 15-20mm long which are adhered to the surface. The wool tufts sit flat to the surface in regions of laminar flow, with their tails indicating the local direction of flow. After the boundary layer transitions to a turbulent one, the tufts flap around a bit, but will still indicate the average flow direction. In separated flow the tufts can drop under their own weight or even reverse, while in regions of vorticity the tufts will spiral. Placement of the tufts is important in order to capture all of these flow conditions as flow features will be missed where tufts are not present.

Flow-viz paint is a method which is familiar to all F1 fans. It is made from a mixture of a powdered dye (often UV reactive) and paraffin oil, to create a low viscosity liquid which easily flows over surfaces. Teams will have their own recipes and preferred colours, Marussia/Manor favouring pink (above), sometimes using multiple colours to show how the airflow mixes along the car. Unlike the wool tufts the flow-viz will flow around all the surfaces (assuming a sufficient application) and the aerodynamicists learn to read the distinct patterns created and how they indicate the various flow features mentioned above.

Measuring the flow field

Particle Image Velocimetry (PIV) was a technique which became more readily available to teams in the 2000’s; previous to that measurement of the flow field was possible using a pressure probe on a traverse system, similar to the probes on aero-rakes for track testing. Where these pressure probes would intrude on the airflow, and thus slightly alter the flow around the car, PIV is unobtrusive and uses strobed lasers and synchronized cameras to track the progress of a particulate injected into the airflow. The cameras take hundreds of photographs and specialist software then determines numeric values for the direction and speed of the airflow (vectors). Using PIV is also quicker when collecting large datasets all around the car, which can then be compared to CFD and aero-rake data collected from the track.

Schematic of PIV set up in F1 wind tunnel, left from Nakagawa et al (2016), right from Ogawa et al (2009).

Other onboard instrumentation

  • Front and rear laser distance sensors for accurate ride height measurement.
  • Pneumatic systems for recreating exhaust flows.
  • Closed circuit systems for monitoring car condition.

References

Federation Internationale de l'Automobile. Formula One Sporting Regulations 2018; 2017 Dec [Accessed: 7/5/18];

Nakagawa M, Kallweit S, Michaux F, Hojo T. Typical velocity fields and vortical structures around a formula one car, based on experimental investigations using particle image velocimetry. SAE International Journal of Passenger Cars-Mechanical Systems. 2016 Apr 5;9(2016-01-1611):754-71.

Ogawa A, Yano S, Mashio S, Takiguchi t, Nakamura S, Shingai M. Development methodologies for formula one aerodynamics. Development Methodologies for Formula One Aerodynamics. Honda R&D Technical Review 2009, F1 Special (The Third Era Activities), pages 142{151, 2009.

Toet W. What did I do in Formula 1; 2015 Oct [Accessed: 7/5/18]

Other useful sources

Newey A. How to build a car;
Barlow J, Rae W, Pope A. Low-Speed Wind Tunnel Testing;
James Allen on F1. Insight: F1 wind tunnel technology reaches amazing new levels;
Mercedes AMG Petronas Motorsport. F1 wind tunnel model explained;
Racecar-Engineering. Manor MRT06: http://www.racecar-engineering.com/cars/manor-mrt06/ ;

Text by Vyssion and jjn9128

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