## 2 Methods for Drag Reduction in Drafting Formations in Race Walking
### Numerical Simulation Method Based on Computational Fluid Dynamics
#### 2.2.3 Computational Domains and Meshes  
This paper's main objective is to investigate the drag reduction mechanisms in drafting formations. Thus, ensuring that the computational domain aligns more closely with real-world conditions than the wind tunnel experiment becomes imperative. By adopting the core race walker's length as the reference (\(L\)), the dimensions of the entire computational domain are defined as \(35L\times 20L\times 10L\), as depicted in Figure 3. The left and right boundaries are positioned at a distance of \(10L\) from the central position of the core athlete, while the gap between the upper boundary and the core athlete measures \(15L\). To determine the boundary conditions for the athlete's upper, lower, left, and right sides in the CFD simulation, the actual situation serves as a guideline, and either the Symmetry boundary or Wall boundary is chosen. Symmetry boundary reduces the impact of boundary effects by setting the normal velocity and tangential force in the motion equation to zero and applying zero heat flux in the energy equation to decrease the temperature gradient along the symmetry plane. Wall boundaries assigned to surfaces of a fluid region prevent flow through those surfaces. When opting for Wall boundaries, it is necessary to account for the force conditions acting on the wall and furnish details concerning slip conditions, wall velocity, slip model, friction coefficient, and other pertinent parameters. In the actual open race walking environment, there are no upper, left, or right boundaries. Therefore, opting for the Symmetry boundary is recommended to minimize its influence on the overall flow field calculation. In real competitions, as athletes constantly move forward, there exists a relative speed difference with respect to the ground. Hence, it is advisable to adopt moving boundary conditions similar to those of a treadmill. For instance, [PERSON] (2016) [PERSON] (2016) conducted an analysis of the impact of moving ground on running. His findings revealed that employing moving ground conditions can mitigate the development of an unrealistic ground boundary layer, making the simulation more realistic. Nevertheless, due to hardware and technical constraints in wind tunnel experiments, the cited experiments in this article did not employ moving wall conditions, potentially introducing some errors. Therefore, this study includes the moving ground boundary in CFD simulation research and adopts settings consistent with [PERSON]'s moving ground boundary conditions. Furthermore, this paper compares the disparities in calculating the drag coefficient of race walking alone on moving ground versus stationary ground. The results indicate that employing moving ground conditions yields a drag coefficient of 0.70, whereas the use of stationary ground conditions results in a drag coefficient of 0.68, representing a decrease of 2.8%. The inlet velocity shown in Figure 3 is set to the typical race-walking velocity of 6 m/s, which is the combination of the race-walking velocity of 4 m/s in the absence of ambient wind and the headwind velocity of 2 m/s. The selected speed of 4 m/s is based on the core race walker's average speed of 1:22:03 in the 20 km of the 2021 Tokyo Olympics ([PERSON], 2016). The headwind velocity is selected based on the super wind speed of 2 m/s in track and field rules ([PERSON], 2016). The turbulence intensity is set to 0.5%, consistent with the value used in the wind tunnel experiment. At the outlet, a pressure boundary condition is applied with an ambient static pressure of 1 atm. The ambient temperature was set to 303 K, while the body surface temperature was set to 303 K.  
The 3D mesh was generated by a stable growth factor of 1.2 and an expansion layer thickness of 10 layers. The average mesh size near the surface is 5 \(\times\) 10\({}^{-4}\) m, while the average cell size at the outer boundary is 0.2 m. To ensure accurate results, all solid boundaries were directly inserted, resulting in a y+ value of approximately 1 at the surface of the human model.