The advent of the new technical regulations in 2022 emphasized the previous exploitation of the ground effect, reintroducing the lateral Venturi channels to generate downforce from the underbody with dynamics similar to those exploited with the cars of the 80s. The different aerodynamic philosophy of the new regulatory cycle has thus brought back the phenomenon of porpoising, the vertical oscillation of the car body at high speeds, the prevention of which forced the teams to make strong compromise choices on the set-up, which in the case of Ferrari and especially Mercedes have limited their maximum potential in the first half of the season.
The triggering of porpoising is connected to the progressive crushing of the car body to the ground under the thrust of the aerodynamic load as the speed increases. The reduction in the height from the ground induces a critical condition in the flow below the single-seater, resulting in a loss of load which causes the car to rise again, to the point where the load generation returns to its previous values. The cycle thus starts all over again, giving rise to a repeated oscillation. Fans have often wondered why team simulation toolssuch as wind tunnel analyzes or CFD studies, have not been able to predict the phenomenon, also considering how porpoising was known in Formula 1 having already been experimented in the 80s. From the outside, various hypotheses have been formulated, such as the elasticity of the treadmill or the limited speed of the air flow in the wind tunnel, without however ever being able to provide a complete explanation.
During a recent interview with FormulaPassion.itMcLaren’s Technical Director James Key explained the reasons behind the teams’ failure to predict porpoising: “Honestly I think many of us underestimated the chances of aerodynamic rebound occurring. However, the problem is that it is very difficult to replicate, because in a simulation the car is never recreated in its real conditions, whether in the wind tunnel, in the simulator or in other forms of simulation. The simulator with the pilot inside cannot predict it, because it reacts to the input data that are provided and if the imposed data does not induce oscillation, this will never happen in the virtual environment ”.
Key therefore entered into the merits of the limited representativeness of the analyzes conducted in the wind tunnel. In fact, although the wheels of the model are in contact with the ground, the weight is not placed on them, but on a vertical anchor hanging from the ceiling. It follows that the mechanical behavior of the car is completely neglected and disconnected from the aerodynamic one, as it cannot replicate the stiffness and damping of tires and suspensions. Particular emphasis is placed on the hysteresis mechanisms, i.e. in the non-linear response of tires and suspension elements which causes a time lag between the applied force and the oscillation, aspects not present in the model used in the tunnel: “In the wind tunnel you can have a certain level of rebound, but we must also consider that the model used has no suspension and does not unload the weight on the wheels, so the vertical stiffness is completely different, since it is rigidly anchored to a ‘ rod hanging from the ceiling. It is a drastically different scenario from that experienced by the car on the track, with completely different inertia and pitch frequencies“.
The result is that, while being able to induce in the wind tunnel those critical flow conditions such as to lead to the pressure drop from the bottom, it is not possible to observe the same mechanical response observed on the runway in the form of repeated vertical oscillations: “It is therefore possible to obtain the same aerodynamic phenomenon, but not the same reaction from the caror not encounter the phenomenon at all “James Key continued. “In the gallery you are not recreating how the car behaves on the track, but only a part of the aerodynamic forces and flow structures. Porpoising is not a purely aerodynamic phenomenon, but it is also a function of the stiffness of the tires, which includes a hysteresis that causes a delay, which in turn puts everything out of phase. Similarly, hysteresis and suspension characteristics are not replicated in the wind tunnel. Similarly, in the simulations you don’t have such a level of hysteresis or a time-varying behavior ”. As explained by James Key, therefore, the complete understanding of porpoising can only be based on an empirical approach, exploiting the data recorded on the car in motion: “We are learning how to better predict porpoising thanks to the data collected on the track, but trying to replicate it is terribly difficult. I think this is why we see some teams suffering from it a lot and others who haven’t experienced it at all, because we didn’t find out until we hit the track. “
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