# Aerodynamic Drag Reduction on Speed Skating Helmet by Surface Structures
## 4 Results and Discussion
### Fluid Field Profile
It was believed that this flow fluctuation was caused by air hitting the surface of the helmet, and this turbulence intensity dissipated rapidly in the region outside of the boundary layer. At the position of \(\theta=90^{\circ}\), \(\sqrt{\overline{u^{\prime 2}}}/U\) for the original helmet was much smaller compared with others, and the turbulence far from the surface had dissipated completely. For the riblet helmet model, the peak of \(\sqrt{\overline{u^{\prime 2}}}/U\) was significantly higher than the bump model at \(\theta=90^{\circ}\), which was related to the riblet height. The turbulence had been suppressed at the valley between two adjacent riblets, and the vortex had been lifted to the top of the riblets. In addition, the riblets also interacted with airflow at their top region, thereby generating a strong velocity fluctuation. For the bump helmet, however, \(\sqrt{\overline{u^{\prime 2}}}/U\) had a significant increase near the surface at the position of \(\theta=90^{\circ}\). The airflow interacted with the staggered bumps and became unstable. Therefore, the boundary layer transformed from laminar to turbulent rapidly. Kinetic energy was transferred from outside of the boundary layer to the inside; thus, the flow could overcome the adverse pressure gradient, and the separation line moved downstream. Figure 13: Pressure coefficient distribution on the midline of the CFD helmet surface. Figure 15a-c illustrated the vortex iso-surfaces based on Q criterion (\(Q=1\times 10^{5}\)). For the original helmet, the vortex generated in the wake region where the flow had separated from the surface, and the airflow was stable before the separation. However, the situation was completely different for the helmet model with surface structure. Many vortices were generated on two sides of the structured helmet, where the riblet structures were situated. Figure 15d-f presents the streamlines on the CFD helmet surfaces. It was found from the streamlines at the two sides of the helmet that there was an obvious angle between the riblet and the velocity direction, and as a result, the vortex was generated from the top of the riblet structures. The boundary layer of airflow could easily transform from laminar to turbulence under this condition. As shown in Figure 15d-f, there was a large difference in surface streamline between the original and riblet/bumped models. The velocity direction also changed in the valley of the riblets compared with the original model, which elucidated the delayed separate position on the side of the helmet (Figure 12). However, there were very few vortices at the middle region of the helmet, where the riblet structures were parallel to the streamlines that had almost no effect on the boundary layer. Therefore, the separation positions were almost the same for the type 1 helmet and the original model. Bumps in the middle region had a different effect compared with the riblets. A vortex was generated from the bumps and promoted the transition of the boundary layer. This had an agreement with the separation position and the turbulence intensity, as shown in Figures 12 and 14. To assess the contribution of the drag reduction by helmets to the overall average drag of the skater, a full-scale complete mannequin was tested in the wind tunnel. The drag ratio of the simplified mannequin to the complete one is shown in Figure 16. It was found that the ratio of the drag changed by approximately 12% with the increase in velocity. Referring to the experiment results shown in Figure 11, the total drag reduction rate was approximately 0.84% at the velocity range of 12-16 m/s, which is just as important when every second counts. Figure 16: The drag ratio of the simplified mannequin to the complete one. Figure 15: The vortex iso-surface based on the Q criterion at (Q = 10,000) of CFD helmet models: (**a**) original model, (**b**) riblet model, (**c**) bumped model; (**d–f**) surface streamline on the original and structured models.