## 2 Methods for Drag Reduction in Drafting Formations in Race Walking
### CFD Numerical Simulation Results and Verification  
#### 2.3.1 Results of Race Walking Alone  
Owing to disparities in human models and boundary conditions used in CFD simulation versus those in wind tunnel experiments, discrepancies may arise in their calculated results for race walking alone. Consequently, it is imperative to offer a comparison of the corrected drag coefficient to account for error sources in the wind tunnel experiment and CFD simulation. In this study, the discrepancies between the CFD simulation and wind tunnel experiment primarily pertain to the following aspects: (1) Model discrepancy: certain intricate structural details may be lost during the 3D scanning process for CFD analysis, resulting in delayed air separation and diminished drag. Loose clothing, for instance, causes earlier air separation and increases drag, resulting in an approximate increase of 4.2% [29]. Similarly, hair length contributes to a drag increase ranging from 4% to 6% [30]. Longer hair moves with more friction and interference with the surrounding air, leading to an increase in air-flow instability and separation phenomena. In addition, longer hair alters the flow field around the head, making it easier for the airflow to separate from the head surface, which further increases drag. (2) Posture discrepancies: prolonged standing during wind tunnel experiments can cause posture alterations and subsequent shaking. (3) System errors in both wind tunnel experiments and CFD simulations, including a higher blockage rate for race walkers, control and measurement errors in parameters such as wind velocity and direction, as well as discretization, mesh-related, turbulence models, boundary conditions, and numerical errors in CFD simulations. Error analysis has shown that wind tunnel experiments need to consider the impact of loose clothing and hair length on drag, which differs from CFD simulations. As a result, wind tunnel experiment drag is approximately 8.2% to 10% higher than that in CFD simulation. Additionally, the CFD simulation revealed that drag decreases by 2.8% when on moving ground compared to stationary ground. However, the wind tunnel experiment utilized a stationary ground, requiring an upward adjustment of the drag coefficient by 2.8%. Furthermore, it is essential to consider the systematic error in the wind tunnel experiment, which ranges from \(-\)3.9% to \(-\)4.2%. Accounting for these error factors, the wind tunnel experiment's drag coefficient will be adjusted from 0.77 to a range of 0.69-0.75. To validate the accuracy of drag coefficient calculations in CFD simulations, we conducted a comprehensive comparison that encompassed CFD simulations, corrected wind tunnel experiment outcomes, and corrected drag coefficients from similar studies. Specifically, the model employed by [PERSON] [17] does not account for the influence of  
Figure 4: Mesh convergence demonstrated by the final four refinement stages of single race walker. (**a**) Relationship between the number of cell sizes and the drag coefficient, (**b**) the mesh on and around the human body surface after mesh refinement. loose clothing and hair length on human body drag, and the boundary conditions precisely match those in this paper. Therefore, its drag coefficient [17] remains unaltered. However, [PERSON] [16], despite not accounting for loose clothing and hair length, utilized a stationary ground, necessitating an upward adjustment of its original drag coefficient of 0.81 by 2.8%. Other literature [31; 32] did not adjust the drag coefficient due to unknown wind tunnel experiment equipment and the applied theoretical analysis method. The detailed correction Cd and rationales are presented in Table 1.  
The CFD numerical simulation conditions employed in this paper closely resemble those in [PERSON]'s work. The sole difference lies in the positioning of the arms and legs in the human body model. However, the Cd values for the two are highly similar, with a deviation of only \(-2.8\%\).