## 1 Introduction  
Since the flow field around the bridge pylon is unevenly distributed [1], the vehicle is inevitably affected by the change of crosswind speed whilst traveling on the bridge deck. Under the action of strong dynamic disturbances, high-speed vehicles may have control instability problems such as side slip and rollover [2; 3; 4]. In order to evaluate the driving safety on a bridge under a crosswind, numerous studies have been carried out on the aerodynamic forces acting on vehicles on bridges. [PERSON] and [PERSON] [5] obtained the aerodynamic coefficients of automobiles and discussed the accuracy of the different types of wind tunnel experimental results. [PERSON] et al. [6] compared the aerodynamic coefficients of vehicles in open area and on bridge using wind tunnel tests. In addition, [PERSON] and [PERSON] [7; 8] found that the wind turbulence, vehicle interference, and vehicle distance from the windward edge significantly affected the aerodynamic coefficients of vehicles. These studies were focused on the aerodynamic coefficients of vehicles on bridge girders, while for cable-stayed bridges and suspension bridges, the flow separation around the pylon led to a sharp increase in wind speed, which is more unfavorable for vehicle stability. [PERSON] et al. [9] studied the aerodynamic performance of high-sided vehicles as well as the flow field around the pylon when the vehicle was passing through the wake of bridge pylon zone by the numerical method. The flow field near the bridge pylon zone is complicated, since it contains the flow around the girder and the pylon, which are dominated by different 2D planes. From far away to the center of bridge pylons, the wind speed changes from the normal state to the amplification state and then quickly to the low-velocity state. This sudden change of wind speed in a short time period poses a great threat to driving safety. Some studies have examined this problem. In [10; 11; 12], the effects of wind on stationary vehicles and vehicles moving adjacent to the bridge pylon were studied, and the influence of the wind barrier on the wind environment above the bridge deck was determined through a wind tunnel experiment and numerical simulation. [PERSON] et al. [13] measured the aerodynamic force of a stationary vehicle near the pylon zone with and without wind barriers through a wind tunnel experiment, and applied the smoke display method to observe the flow field near the pylon zone. Similar research was conducted by [PERSON] et al. [14], who measured the sudden changes of the aerodynamic forces of a train when it was passing through the bridge pylon. [PERSON] et al. [15] compared the aerodynamic coefficients of three different types of vehicles with different sizes in an open area and in the pylon zone, and analyzed the changing patterns of the aerodynamic coefficients under the influence of wind barriers. The vehicle driving stability near the pylon zone was also studied and a multiparameter evaluation method was proposed, based on the drivers' reactions [16]. Most studies are concerned with the aerodynamic force on the vehicles near the pylon zone [17], and there is little research revealing the characteristics of the wind environment near the pylon zone. However, understanding the distribution pattern of the flow field in the vicinity of the bridge pylon zone is the foundation for evaluating the driving safety of vehicles passing by the bridge pylon zone. A large number of studies have shown that a sudden change in wind speed near the bridge pylon is unfavorable for driving stability [18], and countermeasures have been explored for improving the wind environment at the bridge deck and ensuring driving safety [19; 20]. Wind barriers, as an effective countermeasure, have had increasing attention from scholars [21; 22; 23]. [PERSON] et al. [24] numerically studied and compared the aerodynamic coefficients of a truck on an embankment both with and without the wind fences installed. [PERSON] et al.