# Wind Speed Spectrum of a Moving Vehicle under Turbulent Crosswinds
## 1 Introduction  
The influence of crosswinds on a moving vehicle deserves constant attention. Severe wind disasters may influence the moving-speed limit of a vehicle, affect its transportation capacity, and even cause vehicle shutdown or accidents [1; 2; 3; 4; 5]. For example, China is one of the few countries in the world that is prone to severe wind disasters. In 2007, when the Lanzhou-Xinjiang Line crossed the Gobi gaze area in Xinjiang, China, a train vehicle was detailed by strong winds. There were 11 cars overturned and the line was interrupted for nine hours. On 24 February 2008, 48 Japanese trains were stopped, and 183 trains were delayed due to a serious wind disaster which affected about 140,000 people. Meanwhile, the wind has caused train vehicles to be late in some conditions. Wind loads have become one of the key influencing factors on the coupling vibration of wind-vehicle-bridge systems, which could help to evaluate the running safety and comfort effectively. Mostly, the vehicles run on girders and the wind load's influence on aerodynamic characteristic should be studied. The wind speed spectrum of a moving train plays a crucial role in examining the wind load characteristics of the train vehicle. The investigation into moving trains carries considerable engineering implications. The aerodynamic characteristics of static vehicles under crosswinds has been experimentally investigated considering the aerodynamic coupling effect between the train vehicle and a bridge [6; 7; 8]. Investigation of the wind speed spectrum is relatively rare. Originally, the total wind speed of a moving vehicle was obtained based on that of the static point from the traditional wind field simulation method. The wind flow field is artificially divided into several relevant wind speed intervals along the vehicle's movingdirection. As the wind field mutational effect would occur and leads to deviation in the calculation results which is inconsistent with the actual situation [9; 10], investigation of the wind speed spectrum of a moving point that is consistent with the actual situation is of great significance. Many scholars have studied the characteristics of the wind speed spectra of moving train vehicles. [PERSON] [11] theoretically derived the longitudinal wind speed spectrum of a moving vehicle. However, the derivation only targets the working conditions in which the angle between the wind speed and the train vehicle's moving direction is 90\({}^{\circ}\), which could not be widely applied. Obviously the wind yaw angle is varied, so the corresponding working conditions should be studied. The on-road turbulent wind environment has been measured utilizing a vehicle instrumented with mast-mounted cross wire and propeller-vane anemometers [12; 13]. In the conditions of the vehicle's moving speed at 27.8 m/s and the wind speed at 1-9 m/s, the measured longitudinal and lateral turbulence intensities of the moving point were 2.5\(\sim\)5% and 2\(\sim\)10%, respectively. This could also be obtained numerically, and the study of the wind speed spectrum of a moving point can provide the basis for analyzing wind speed time series and turbulence intensity. [PERSON] [14; 15] developed another analytical model of the wind speed spectrum. Based on the wind speed time series, [PERSON] [16] computed the unsteady aerodynamic load on a moving vehicle. However, the difference between the wind spectra of the static and moving points was not compared comprehensively. For example, the influence of wind yaw angles from 0\({}^{\circ}\)\(\sim\)180\({}^{\circ}\) was not investigated widely. [PERSON] [17] numerically studied the correlation characteristics of a moving vehicle preliminarily. This was only studied for a smaller vehicle's moving speed and the influence of the vehicle's moving speed was not calculated roughly. [PERSON] and [PERSON] [18] experimentally investigated the wind spectrum of a moving point under crosswinds in a wind tunnel.