## 4 Results and Discussion  
### Flow Characteristics of Pantograph Area  
In this section, the flow characteristics around the pantograph and the installation bases are discussed. As shown in Figures 13 and 14, the vorticity and the instantaneous iso-surface vorticity as shown by the normalized Q-criterion, and the velocity streamlines around the pantograph are presented, respectively. In Figure 13, the flow separates over the pantograph surface. A series of vortices detaches from the leeward side of each rod and they interact with each other downstream. The hairpin vortices of different size are observed. In addition to the common characteristics of the flow field mentioned above, Figure 13a,b also shows that the flow field characteristics of the pantograph are different due to the opening directions of the pantograph. For the down-pantograph with flush installation base, the incoming flow first impacts on the corner of the upper and lower arm rods, and then strikes the rear of the carbon skateboard and the base in the downstream area. The turbulent upstream flow field hits downstream complex structure like the carbon skateboard and base again, which makes the flow field in the wake region more complex. Because the contact area between the pantograph and the incoming flow is concentrated, the eddy size under the larger contact area is also larger. For the up-pantograph, the distance between different structures on the pantograph is larger, and the inflow flows directly to the downstream area after impact with the carbon skateboard. Compared with the down-pantograph, the rear eddy scale is smaller. For Figure 13c, when the pantograph is placed in the cavity, the flow field above the cavity shielding part is almost the same as when it is installed on the flush surface. However, for the case that the pantograph is installed in the sunken base, the flow field characteristics have something in common with the general flow field in a cavity, as previously studied in [28; 29; 30]. However, the presence of the pantograph affects the pressure fluctuation on the cavity surface, and then it further affects the noise induced by the pantograph. For a pantograph that is installed in the sunken cavity, the base and insulator are basically located in the cavity, avoiding direct collision with the incoming flow. Generally, the flow past a cavity exhibits a strong relationship with the cavity length (L) to height (D) ratio as studied by [PERSON] and [PERSON] [19]. For the present study, the ratio of L/D of the cavity with pantograph is about 4.7. Therefore, according to the criterion of [PERSON] and [PERSON] [19], it is considered as the open cavity. In addition, the pressure field in the cavity is coupled with the shear layer shed from the cavity upstream edge, and the pressure fluctuation intensity in the cavity depends on the characteristics of the shear layer and on the evolution of momentum and vorticity during the impact of the shed shear layer with the cavity rear bulkhead as studied by [PERSON] et al. [31]. Because the presence of the pantograph, the typical open cavity flow field structure is destroyed. As shown in Figure 14, one can observe that the air-flow acts on the pantograph first, and then vortices shed by the pantograph impinge on the surface downstream. A part of the air flow returns to the front of the cavity and continues to act on the pantograph. This differs from an empty cavity flow in which the shear layer shed at the cavity upstream edge impacts the cavity downstream edge, unobstructed. The pressure wave (feedback pressure wave) propagating upstream after acting on the surface of the cavity collides with the pressure wave reflected from the front wall when it arrives upstream at the last time. The air circulation inside and outside the cavity exchanges, resulting in pressure fluctuation in the cavity. The pressure fluctuation on the base and insulator surface even as well as the overall pressure fluctuation on the surface of the pantograph is affected by the flow in the cavity.