## 4 Simulation Analysis Results and Discussion
### Temperature Distribution Pattern during Braking
#### 4.1.2 Analysis of Internal Gap Structure of Ventilated Brake Discs  
During the braking process of a high-speed train, the high-speed rotating ventilated brake disc introduces the surrounding air into its internal gap structure. Due to the high fluid-flow velocity, the irregular flow along the gap structure forms turbulence. At the same time, the air is thrown out by the centrifugal force generated by the rotating disc, and the heat of the ventilated disc is carried away. The internal gap structure of the ventilated disc dissipates the heat generated by the braking process in the form of convective heat transfer. The highest temperature of the braking process occurs at the friction contact surface at 41 s of braking, and the temperature field cloud is shown in Figure 10. The axial section temperature cloud of the gap structure at 41 s and its section amplification temperature cloud are shown in Figure 11.  
Figure 11: Temperature clouds in the axial section of the interstitial structure. Figure 9: Three-dimensional cloud of time-temperature along the radial direction of a ventilated brake disc. It can be seen from Figure 8 that the high-temperature zone is mainly distributed in the contact friction zone between the brake disc and the brake pad, with the highest temperature reaching 268 \({}^{\circ}\)C. At the same time, under the action of heat conduction, the temperature generated in the friction contact zone is continuously transferred to the gap structure along the axial direction, reaching 100 \({}^{\circ}\)C in the middle of the gap structure. It can be concluded from Figure 11 that the heat generated on the outer side of the friction surface makes the temperature transfer along the axial side of the gap structure more evident than the gradient on the inner side. During the braking process, the ventilated disc is subjected to a pair of symmetrical brake pads, resulting in a balanced temperature distribution along the axial direction. To make it easier to observe the temperature changes in the gap structure of the ventilated disc, a three-dimensional position-time-temperature cloud was fitted along the axial direction at the highest temperature point of the disc surface during the entire braking process, as shown in Figure 12. In Figure 12, the \(x\)-axis represents the axial position of the ventilated disc clearance structure, the \(y\)-axis represents the braking time, and the \(z\)-axis represents the temperature. The temperature of the part of the ventilated disc clearance structure closer to the disc surface rises first and then falls. In the initial stage of braking, the amount of the gap structure is far from the disc surface, and the heat generated by the contact friction contact area has not yet been transferred to the centre of the gap structure. In contrast, the air introduced inside the gap structure forms an irregular flow on its surface, taking away its surface heat. The heat dissipated by the gap structure in convective heat transfer is higher than the heat transferred to the gap structure. The temperature of the gap structure part drops continuously during the initial braking stage. To more intuitively study the temperature variation law of the gap structure with time, the temperature variation with time of the nodes selected at equal spacing along the axial direction at the radius of the highest temperature point of the disc surface during the whole braking process, according to Figure 11, is shown in Figure 13. It can be seen from Figure 13 that the temperature of the nodes near the friction contact surface changes significantly with time. In the initial braking stage, the heat generated by the friction between the ventilated disc and the brake pad is higher than the heat dissipated by the ventilated brake in convective heat dissipation. Hence, its temperature rises continuously with time.