Formula 1: what are correlation issues and why are they different from 2021?
Today at 02:58 PM
The month of September marked McLaren's definitive overtaking of world champions Red Bull, both in the Constructors’ standings and in the hierarchy and overall on-track performance. Red Bull Team Principal Christian Horner explained that the issues with the RB20 single-seater were due to an incorrect correlation between the design tools and the track, which made it difficult to analyze how the updates were affecting the car's drivability and balance. This is a classic case of correlation problems, but different from those before the ground-effect regulations.
From CFD to the track
Aerodynamic design in Formula 1 follows a process composed of three main phases. The first is virtual simulation on the computer through Computational Fluid Dynamics analysis, followed by testing in a scale model in the wind tunnel. The final step is the installation of the new parts on the car, where correlation is immediately checked, verifying that the data measured on the track matches the predictions from the Computational Fluid Dynamics and wind tunnel.
If the results don't match, it’s called a correlation problem, which is sometimes unavoidable given that design tools can’t account for absolutely every facet of the real world. Since the 2021 Formula 1 championship, the sporting regulations also limit the resources available for Computational Fluid Dynamics, both in terms of computing hours and the number of processors, encouraging teams to set up simplified simulation models, analyzing only the most relevant aerodynamic phenomena.
In the past, once on track, a team could discover discrepancies compared to factory predictions in terms of aerodynamic load and balance at specific configurations, speeds, ride heights, stances, or steering angles. Although these same episodes can still occur with the current cars, the correlation problems Christian Horner refers to are far more difficult to identify and fix, considering the complexity of ground-effect single-seaters.
An example of this issue was given by Ferrari team principal Frederic Vasseur in 2023 when explaining the Maranello team’s correlation problems at the time: “When we bring something to the track, we get the gains we expect. When the wind tunnel predicts an increase of X points of load, we see exactly that on the track.” Under the same conditions and configuration, teams are able to see the same numbers on track as in simulations. The discrepancies, however, concern all those aspects and dynamics that cannot be analyzed in Computational Fluid Dynamics or in the wind tunnel. “Sometimes it’s easier to fine-tune new developments on the track rather than in the wind tunnel. There are aspects of the car that cannot be replicated in the wind tunnel. This didn’t happen before, but now there’s ground effect,” Frederic Vasseur pointed out.
The reasons lie in the current regulations. The cars in use up to 2021 ran higher and farther from the ground, a condition that could be accurately analyzed in the wind tunnel. Ground-effect cars, however, run much closer to the ground, with the floor becoming far more powerful in terms of generating downforce. This introduces aerodynamic anomalies that cannot be replicated outside the track.
A particularly critical situation is that of porpoising. The closer the floor gets to the ground, the more downforce it generates, but when it touches the asphalt, downforce drops dramatically, causing the car to rise and start the cycle over again, oscillating up and down. Another difficulty is the car’s passage over curbs and bumps, which, due to the car’s proximity to the ground, “destroy” the aerodynamics of the floor.
This means it’s about a whole new physics, as the current correlation problems are not just about matching wind tunnel data with track data, but about predicting the evolution of aerodynamic numbers in all those circumstances that cannot be simulated in the factory. In addition to aerodynamic load and balance under various configurations, such as ride heights, stances, or yaw angles, the transient phases are also very important, that is, the movements between one condition and another. Something that was less relevant with the 2021 Formula 1 cars.
“With the old regulations, which we had a good understanding of, there wasn't a need to study the car in the same dynamic sense,” reflected Andrew Shovlin, Mercedes’ head of track engineering, back in 2023. “You determined how it behaved at a certain roll angle, steering angle, or ride height, and in doing so, you captured everything that happened. But now the flow structures under the car are much more complicated and transient.” – he pointed out.
With ground-effect Formula 1 cars, there are now aspects of the real world that Computational Fluid Dynamics and the wind tunnel simply cannot capture. This is where the driver comes in, who, according to engineers, is crucial for conveying to the team what is really happening with the car on track compared to the simplified design models. Thanks to the driver’s feedback, the team can reconstruct the phenomena at play, providing the missing piece to make sense of the incomplete correlation between simulation and reality.
Looking at the Red Bull case, team principal Christian Horner recently commented regarding the RB20’s characteristic flaws, explaining that the wind tunnel doesn’t show the problem, but the track indeed does. Based on what has been explained above, however, talking about correlation problems is an oversimplification. There is correspondence between the factory and reality when it comes to scenarios that can be simulated in Computational Fluid Dynamics and the wind tunnel, with Red Bull’s difficulty lying in interpreting everything that happens to the RB20 single-seater during transients and those other dynamics that can only be analyzed on the track.
Red Bull's case is not isolated of course, as all teams on the grid have, at times, had to deal with the unforeseen issues of development, which are inevitable with a complex physics like that of ground-effect Formula 1 cars. Ferrari also knows this very well, considering the issues encountered by the Maranello technicians and engineers following the introduction of the aerodynamic package of updates for the SF-24 car at the Circuit de Catalunya in the Spanish Grand Prix weekend, which affected the balance of the car and needed several weeks to be fixed, leading to many valuable points lost by Charles Leclerc and Carlos Sainz during that time. In Milton Keynes, there is now confidence that they understand the nature of the problems, but the question remains whether there is enough time and resources to fix them quickly and respond to McLaren in the battle for the 2024 Formula 1 Constructors’ championship.
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