The perfectibility of the helmet aerodynamics
Mitigating the inconvenience of air turbulence around helmets is a major challenge in helmet aerodynamics for bikers and high-speed users such as skiers, professional cyclists, and single-seater drivers. Indeed, these disadvantages, materialized by areas of detachment and recirculation of air on the surface of the helmet, cause discomfort, instability, and noise, degrading performance by increasing air resistance and disrupting the mental and muscular readiness required for piloting.
What solutions to mitigate these undesirable phenomena? Understanding aerodynamic instabilities and innovation in the design of adapted shapes show that significant advances can be made in these areas.
Understanding aerodynamic instabilities on a spherical shape
Aerodynamic instability near a surface is often generated by the conjunction of an insufficiently energized flow and the presence of a strong divergence in geometry. The flow lines then lose their adhesion to the surface and disperse in a swirl. These observations are essential to understand three-dimensional flows, especially those around helmets.
Why spherical shapes lead to instabilities in helmet aerodynamics?
Spherical shapes, such as helmets, are particularly prone to aerodynamic instabilities. Due to their geometry, air tends to separate from the surface, usually at the back of the median area corresponding to the maximum frontal section of the sphere when seen in the direction of the flow.
The importance of this phenomenon, which represents almost all the drag force that opposes the motion, is related to the kinetic energy contained in the thin layers of the flow as well as to the texture or granularity of the surface itself.
There is a correlation with the speed and the surface details of the object. The best demonstration of this is the very important distance difference that one obtains between the swing of a golf ball versus the one of a sphere of similar size in the same conditions. The drag of the first is in fact largely reduced by the 330 dimples it has on its surface.
The helmet is a derivative of a spherical shape, this being imposed by the fact that this form must be able to roll on the ground in case of accident. It thus presents aerodynamic phenomena similar to those of the sphere.
The instabilities will be illustrated by the variation of the location where the flow detaches, leading to the center of the pressure forces seeing its application point moving chaotically on the surface of the object. It should also be noted that these areas of swirling recirculation are also largely contributing to the generation of unwanted noise.
Consequences of turbulence and detachment on the helmet head assembly
Air turbulence around a helmet creates pressure fluctuations that result in noise. These fluctuations are caused by rapid variations in the speed and direction of the air flow, generating sound waves that penetrate the helmet.
They can also significantly increase the muscular effort required to hold the head in position. When the turbulent air shakes and destabilizes the helmet, it generates varying forces that require constant adjustments by the neck and shoulder muscles.
These noise and muscle strain stresses lead to increased fatigue, which can reduce overall performance and responsiveness of the user. In activities such as speed skiing and motorcycling, turbulence can reach high levels, also affecting concentration. For cyclists, turbulence increases air resistance, requiring more effort to maintain speed. For extreme sports like wingsuit, turbulence can compromise safety by disrupting flight stability.
Solutions to reduce turbulence: development of specific forms
The effects of turbulence vary on the type of helmet and activity. Thus, improvement solutions will focus on distinct objectives. For professional cycling helmets this will often be a drag reduction goal of the rider and bike assembly. For high speed helmets, stability, side wind tolerance and noise attenuation will also be part of the targets.
Importance of aerodynamic design in helmet aerodynamics
Some specific forms have proven to reduce turbulence. For example, helmets with a profiled shape and rounded edges ease the airflow to get around. The time trial bike helmets, with their elongated design, are also a good example of this optimization which allows a reduction in helmet drag of more than 10% compared to a conventional helmet. Similarly, motorcycle helmets with advanced spoilers help to effectively stabilize the area of detachments while also improving drag. Their generalization in motorcycle competition, such as the motoGP or the SuperBike, demonstrates this.
There are also turbulators, which are aerodynamic devices designed to transform the laminar flow of air into a turbulent flow in the thin layer of flow lines that follow closely the surface of the helmet. This boundary layer, which then becomes turbulent, will show a point of separation moving backward. This will lead to the reduction of the recirculation zone and will giv ea gain in drag, noise reduction and a better control of the instabilities. These turbulators can be integrated as small ridges or adhesive strips strategically placed on the surface of the helmet.
Development, design and optimization and by CFD simulation.
Development, design and optimization by calculation use simulation tools such as CFD (computational fluid dynamics) to model and analyze the airflow around the helmet. These simulations allow the shape of the helmet to be optimized in many details, as well as its integration with the user and the vehicle. The air flow inside the helmet is also taken into account. The CFD provides visualization of flow paths and recirculation zones. Finally, it allows to test and develop multiple configurations without having to build a physical prototype, giving then an economic advantage for the helmet manufacturer.
Finally, it may be possible to innovate by introducing variable aerodynamic devices depending on the speed. This would further contribute to improved performance and comfort, whether through reduced drag, noise or increased stability.
