## 4 Results of Pantographs with or without Platform Sinking
### Aerodynamic Results  
Figure 16 shows the flow field around the pantograph with or without the platform sinking. The difference in pressure between the windward surface and the leeward surface of the pantograph causes aerodynamic drag. The platform's sinking causes a significant change in the bottom flow field of the pantograph. Compared with the installation platform without sinking, the positive pressure amplitude of the windward surface of the base and insulator is significantly reduced. In addition, the downstream surface of the cavity is impacted by high-speed airflow to form local high pressure, which is one of the main sources of additional cavity resistance. The lower structure of the pantograph is shielded by the upstream surface of the cavity, and the strength of the airflow acting on the structure will be weakened, which reduces the aerodynamic drag. The flow of the airflow inside the optimized cavity is smooth, and there is no obvious large-scale vortex and recirculation zone. Compared with the improvement of the aerodynamic performance of the pantograph, the cavity flow-induced effect will become less important.  
Figure 14: Computational domain of pantograph.  
Figure 15: Computational grid for simulation.  
Figure 17 shows the distribution of turbulent kinetic energy around the pantograph with and without the platform sinking. Turbulent kinetic energy can effectively evaluate the intensity of highly unstable airflow around the pantograph, which is related to the generation of aerodynamic noise in the pantograph area. For the pantograph without platform sinking, the area with larger turbulent kinetic energy is concentrated around the bottom structure of the pantograph. After the platform sinks, the turbulent kinetic energy in the cavity decreases. The trailing edge of the optimized cavity reduces the flow separation. It improves the stability of the airflow, resulting in a decrease in the downstream high turbulent kinetic energy region, which indicates that the aerodynamic performance of the optimized cavity is better.  
The far-field receiver for evaluating aerodynamic radiated noise is 25 m away from the pantograph. As shown in Figure 18, a total of 19 acoustic receivers are arranged at an interval of 5 m from the ground 3.5 m. Figure 19 shows the far-field noise of the pantograph. Compared with the pantograph sinking without the platform, the pantograph cavity can significantly reduce the radiation noise of the pantograph. However, this does not change the sound propagation characteristics of the pantograph in the far field. The maximum reduction of the acoustic receiver is 1.99 dBA, and the average sound pressure level of all acoustic receivers is reduced by 1.31 dBA.  
Figure 16: Flow field around pantograph (colored by pressure coefficient). (a) Flat roof (b) Platform sinking.  
Figure 17: Distribution of turbulent kinetic energy around pantograph. (a) Flat roof (b) Platform sinking.  
The aerodynamic and acoustic results of the high-speed pantograph with or without platform sinking are shown in Table 5. The main sources of aerodynamic drag and noise of the pantograph without platform sinking are the panhead, base, and insulators. The aerodynamic drag of the base and insulators accounts for more than 50%, and the noise radiation energy accounts for 33.4%. After the installation platform sinks, the aerodynamic drag of the base and insulators decreases by 44.6% and 65.7%, respectively, the aerodynamic drag of the pantograph decreases by 28.7%, and the total drag (pantograph and cavity) decreases by 5.22%. In addition, the noise generated by each component of the pantograph is reduced. The noise of the pantograph at the \(y_{8}\) receiver is reduced by 1.99 dBA, and the total noise is reduced by 1.45 dBA.