Experimental study of aerodynamic behaviour of stretchable sport fabrics
Creators: name: Moria, Hazim
Publisher: name: RMIT University
Year: 2024

description: In recent years, there has been great interest in the effects of different garments and their aerodynamic characteristics. It is widely believed that the reduction of aerodynamic drag can enhance athlete’s performance. There has been little understanding in the aerodynamic properties and behaviours of athlete’s garment as limited research is reported in the open literature. Therefore, the primary objective of this research is to understand the aerodynamic characteristics and gain a greater insight in order to establish relations between sports garments’ physical parameters and aerodynamic properties. A series of stretchable knitted and woven fabrics used in speed sports garments (e.g., sprint, cycling, speed skating, downhill skiing, ski jumping and swimming) have been studied for a wide range of Reynolds numbers (Re) and angles of attack (α). The findings revealed that the drag polar (C<sub>L</sub>/C<sub>D</sub> ratio) for 3D circular cylinder with smooth and rough surfaces (varied by knitted and woven fabrics) has been established for a range of Reynolds numbers (Re = 5.06 × 10<sup>4</sup> to 2.30 × 10<sup>5</sup>) and angles of attack (α = 0° to 90°). The drag polar allows determining the aerodynamic efficiency of sports fabrics (i.e., garments) and their optimal design. The aerodynamic behaviour of knitted fabrics is found to be quite different to that of woven fabrics. With an increase of stretch, the surface morphology of knitted fabrics becomes courser and thereby triggers an early airflow transition. In contrast, the stretch on woven fabrics makes the surface morphology smoother which delays the flow transition. The minimum drag coefficient (C<sub>Dmin</sub>) of stretchable knitted fabrics is directly proportional to relative roughness whereas the critical Reynolds numbers is inversely proportional to the relative roughness (ε = 1.39 × 10<sup>-4</sup> to 7.73 × 10<sup>-4</sup>) within Reynolds numbers investigated (Re<sub>crit</sub> = 1.83 × 10<sup>5</sup> to 1.00 × 10<sup>5</sup>). On the other hand, the minimum drag coefficient (C<sub>Dmin</sub>) of stretchable woven fabrics is proportional to the relative roughness, however the relationship of critical Reynolds numbers (Re<sub>crit</sub> = 1.17 × 10<sup>5</sup> to 2.34 × 10<sup>5</sup>) with the relative roughness (ε = 3.689 × 10<sup>-4</sup> to 1.319 × 10<sup>-4</sup>) is non-linear. Knitted fabrics with lower relative roughness, distance and gap area between yarns generate greater aerodynamic efficiency (C<sub>L</sub>/C<sub>D</sub>) at high Reynolds numbers. Similarly with the higher relative roughness, distance and gap area between yarns, the knitted fabrics offers an aerodynamic benefit at low Reynolds numbers. A notable reduction of aerodynamic benefit in woven fabrics was found under unstretched condition. However with increased stretches, the aerodynamic advantage (C<sub>L</sub>/C<sub>D</sub>) increases almost linearly. The stretched woven fabrics are found to be aerodynamically beneficial at high Reynolds numbers whereas the stretched knitted fabrics are at low Reynolds numbers. The practical implication of these research findings is multi-fold. The drag polar of smooth and rough cylindrical surfaces can be applied not only for the development of engineered sports garments but also for the optimal athlete’s body orientation. Additionally, the findings can be utilised in developments of various projectile shapes for military and sports applications.