## 2 Experimental Set-Up
### Slipstream Measurements  
To explore the slipstream and flow feature of the present tested model, velocities were measured at a set of monitoring points. For both sides' measurements, a rake of probes located on the leading-edge side of the bridge and the other one on the leeward side of the train. Measurements were taken at full-scale equivalent distances of \(y\) = 1.68, 2.18, and 2.69 m from both train lateral surfaces. From there, measuring points would follow the lateral positions mentioned before for the side measurements. Equivalently, considering that \(z\) = 0 corresponds to the top of the rail, side measurements along the \(z\)-axis were taken at full-scale values of \(z\) = 1.21 and 1.55 m. In this way, the slipstream flow property sampling positions are shown in Figure 2 and are used on either side of the train for the windward and leeward samples. The \(x\)-axis was defined along the track, the \(y\)-axis was the lateral direction, and the \(z\)-axis was the vertical direction. \(x\) = 0 corresponds to the tip of the train nose. Note that the measurements were conducted at both sides of the train model, to highlight the effects of the crosswind effect. The measurement plane was located at the center of the bridge model along the \(x\) direction. The slipstream velocities during train running on the bridge were obtained and recorded by Cobra probes (Figure 3) and the corresponding signal processing system. The Cobra probe has proved useful in measuring the flow velocity and is frequently used in the studies of the train's slipstreams [(39; 40)]. In the present work, instantaneous velocities within the train-induced slipstream or/and under the influence of crosswind were measured using the Cobra Probe (Turbulent Flow Instrumentation), a four-pressure-hole probe with high-frequency response capable of measuring velocities in three directions [(41)]. Measurements were conducted at these points one by one. For each point, the sampling frequency of the Cobra probe was 2000 Hz and synchronized with the signal from photoelectric gates, thus the measured velocity could coordinate with the location of the train model. Figure 1: Moving train rig in CSU. Considering the unstable characteristics of the slipstream, the measured time history of the velocity at a specific position can vary significantly from each run [42]. In the present experiments, following the TSI regulations [43], the measurement at each monitor point was repeated 20 times to get a reliable ensemble-averaged result [44]. Considering some of the unique testing challenges [13] and identifying the crosswind effect on the slipstream, the Cobra probe points to the x-direction during the measurement due to the velocity of the moving train-induced flow being much larger than the crosswind. The measured \(u\), \(v\), and \(w\) velocity components correspond to velocities in x-, y-, and z-directions, respectively. Four light gates were used to determine vehicle velocity and acceleration. The vehicle's speed and acceleration were used to convert the measurements from the time domain to the spatial domain, in which all results are presented. Due to the aerodynamics and flow of the head train being particularly sensitive to the influences of crosswind [19; 45; 46], the main discussion concentrates on the head train. As shown in Table 1, the model section size was 0.227 m \(\times\) 0.2 m (height \(\times\) width). The model length was 1.616 m (corresponding to 27.15 m full-scale) with a height ratio L/H of 7.12. The spatial coordinates were also converted into full-scale quantities for easy comparison. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Dimensions of Train** & **Measurement Locations 1** & **Scale Ratio** \\ \hline Length = 27.15 m & & \\ Width = 3.36 m & \(y\) = 1.21 m and 1.55 m & \\ Height = 3.81 m & \(z\) = 1.68 m, 2.18 m, and 2.69 m & \\ \hline \hline \end{tabular} \({}^{1}\) y is the distance from the train center, \(z\) is the distance from the bridge deck.  
\end{table} 
Table 1: Test parameters.