# Aerodynamics Analysis of Speed Skating Helmets: Investigation by CFD Simulations
## 6 Results and Discussions
### Helmet's Comparison  
Figure 11 visualizes the head/helmet wake characteristics of the simulations by means of streamwise velocity \(v_{x}/U_{\infty}\) at the median plane. There is a noticeable flow behaviour variance among the simulations, in particular in the wake's height. The trailing edge shape strongly influences the separation point and, so, the orientation of the wake. In fact, the larger the radius of curvature of the upper head/helmet surface, the later the boundary layer separation occurs, reducing the height of the wake. On the other hand, spherically shaped trailing edges promote earlier boundary layer separation, pushing the wake upwards, as the bare head and the design 4 cases. Design 6 shows a slightly shorter wake height compared to design 5. Indeed, the truncated gap is filled up with recirculating flow, which acts as an artificial trailing edge surface. If a comparative analysis is done together with the drag reduction values gathered in Table 2, one can observe that the lower the vertical deflection of the wake, the lower the total drag of the skater is. Figure 10: Drag area percentage of the skater without helmet for the individual body parts at \(U_{\infty}=15\) m/s. HHN indicates Head, Helmet (in this case not present) and Neck) and Neck. The skin friction \(C_{f}\) contour plots over the upper side of the head/helmets are shown in Figure 12. This parameter was used to determine precisely where the boundary layer separates. If \(C_{f}\) becomes zero, the wall-normal velocity derivative is null and, hence, separation takes place. Designs 5 and 6 are both capable of pushing the separation line the most downstream among all helmets, whereas the bare-head case exhibits the most upstream flow separation. The boundary layer separation is directly connected to the previously discussed wake height. The later the flow separated, the lower the wake height will be. This is also illustrated in Figure 13, where suction downstream of the head/helmets is shown. When no helmet is used (Figure 13a), a large low-pressure region is formed downstream of the head of the skater, which contributes significantly to the drag. The use of a round helmet (design 4, Figure 13c) increases the extent of the low-pressure region behind the head, but diminishes the low-pressure region behind the ears, thus causing a marginal improvement on the skater's drag. The use of the other helmet designs, instead, enables one to postpone the flow separation on the skater's head, and therefore suppresses the  
Figure 11: Non-dimensional streamwise velocity \(v_{x}/U_{\infty}\) on the median plane behind the model head and helmet. Figure 12: Top view skin friction coefficient. Black dotted lines indicate where boundary layer separation occurs at the upper side of the head/helmet. low-pressure region behind the skater's head, yielding lower aerodynamic drag. The highest benefits are achieved with designs 5 and 6 (Figure 13,e), where the low-pressure region is shifted away from the head and merges with the suction over the skater's back, thus annihilating its contribution to the drag. Every study case is compared to the bare-head case as percentage difference in Table 2. The use of all the helmets provided a performance enhancement compared to the bare head case, even if the frontal area increased. The baseline Omega and design 6 helmets showed a 4.3% and a 5.9% improvement respectively relative to bare head. Thus, design 6 upgraded by 1.6% the overall performance of the skater instead of wearing Omega. In contrast, the helmet design with ear covers (design number 4) presented the worst results among every helmet. The drag coefficient of Omega, design 1, design 2 and design 3 cases lied within the uncertainty of the numerical simulation results. So, ventilation holes at the already separated helmet trailing edge cause a negligible variation of the total drag force. Likewise, drag coefficients of design 5 and design 6 were within the simulation uncertainty. Therefore, they could be considered to have the same performance.