# Aerodynamic Drag Reduction on Speed Skating Helmet by Surface Structures
## 1 Introduction  
Aerodynamic drag plays an important role in competitive games such as short-track speed skating. Many drag reduction methods have been adopted in recent years, such as optimized postures and drag reduction skating suits. In addition to these, researchers also concentrated on helmets that must be used in formal competitions as protection gear. However, not many reports about aerodynamic performance of skating helmets could be found. Taking cycling helmets as the example, [PERSON] [1, 2] used the wind tunnel method to study competitive time trial cycling helmets and provided an analytical procedure to test the drag of those helmets. [PERSON] [3] also studied the competitive cycling helmets with dimples and ribbed structures using wind tunnel tests, and the results showed that the frontal area played a critical role in pressure drag and that the dimple helmet might provide a marginal improvement to the aerodynamic performance. It was also found in the research by [PERSON] [4] that the frontal area influenced the wake region of the helmet. Computational fluid dynamics (CFD) are useful to understand the mechanisms of drag reduction and flow field, and has been used in much research regarding competition sports [5, 6]. It was found that the tail shapes of cycling helmets were important to the total drag by influencing the pressure behind the athletes [7, 8, 9, 10]. Drag reducing by surface structure as a passive control method without energy inputting has attracted many researchers. The drag reduction method with riblets is one of the few techniques that has been successfully applied in both the laboratory and aerodynamic configurations. Inspired by the shark skin, many researchers attempted to make replicas and optimize the geometry at different flow fields [11]. The drag reduction performance of the riblet surface with different cross-sectional shapes was studied through experiments. [PERSON] obtained a frictional drag reduction of 8% under turbulent flow conditions [12]. [PERSON] et al. tested the drag reduction characteristics of the riblet structured surfaces with various cross-sectional shapes, including sawtooth, semicircular, and rectangular riblets [13]. The results showed that the riblet surface with the sawtooth structure had a better drag reduction effect compared with others. The aforementioned riblet surfaces with various cross-sectional shapes were proposed by scaling on the placoid scales from the shark skin, and the direction of riblet structure was parallel to the velocity. The convergent and divergent riblet structures that were not parallel to the velocity direction had been studied by [PERSON] [14], and the velocity filed close to the wall was obtained by experiments. [PERSON] et al. [15] obtained a surface with a drag reduction rate of 10% by duplicating, scaling, and optimizing the ridged structure of the placoid scales from the shark skin. For curved surfaces where the flow is separated from the surface, the surface structure can also reduce the resistance by influencing the transformation of fluid from laminar to turbulent. The longitudinal grooves, which are parallel to velocity, can suppress the spanwise vortex, such as V-groove [16] and U-groove [17]. Meanwhile, spanwise surface structures on cylindrical surfaces, i.e., vertical to the velocity direction, could change the separation point because of the interaction of fluid and groove [2; 18], and still worked even if the depth of the groove was very shallow [19]. The circular dimple surface is another uniform surface structure that can create a disturbance in the boundary layer to accelerate the transition from laminar to turbulent. [PERSON] studied the possibility of drag reduction in baseball sports [20], and this structure attracted more attention to the drag reduction field afterward. Many researchers studied the dimple structures on the cylinder model and obtained the drag reduction effect [21; 22]. In addition, other types of dimples, such as hexagonal geometry showed similar drag reduction effects [23].