## 2 Experimental  
### Experimental Setup and Wind Tunnel  
PIV is adopted to capture the velocity fields mainly in the leeward surface of the train. The wind tunnel walls are equipped with optical glass windows on the sides and top for laser shining and camera imaging. The origin of the coordinate system was defined at x = 3.125\(H\) (see Figure 1a). The front and back edges of the floor were processed into streamlined lines to reduce the interference with the airflow, see Figure 1, and the test desk was set at 200 mm. Tests were conducted at an oncoming velocity of 30 m/s, corresponding to a Reynolds number (\(Re=\frac{\rho V_{\infty}H}{\mu}\), \(3.27\times 10^{5}\), where \(\mu\) is the air viscosity coefficient, 1.8 \(\times\) 10\({}^{-5}\) Pa\(\cdot\)s) based on the train height \(H\).  
The experiments were executed in an opened-loop, low-speed wind tunnel located at the Central South University, with a length, width and height of 10 m, 1000 mm and 800 mm of the test section. A 1:25 scaled model was chosen, and incoming wind at 15\({}^{\circ}\) yaw made that blockage 3.3%. After the grid was installed, the wind speed ranged from 0 to 40 m/s in the test section.  
For obtaining the uniform turbulent velocity field, a grid was arranged at the air inlet of the wind tunnel experimental section, with the width of the grille strip \(b\) = 60 mm and the net distance between adjacent grilles \(M-b\) = 220 mm. The nose point of the train model is 6 m downstream from the grid at a 0\({}^{\circ}\) yaw angle. The camera was fixed on the coordinate frame and arranged inside the wind tunnel along the \(x\)-direction; the turbulence intensity was 5%.  
Specifically, PIV was composed of a synchronous controller, CCD camera, YAG laser, particle generator and computer. The thinnest part of the laser sheet was about 0.5 mm. Air was seeded with tracer particles of approximately 15 \(\upmu\)m in diameter. A Nikon Len (80-200 mm lens) was adopted to capture the images. The maximum resolution of the camera was 6600 \(\times\) 3300 pixels. In the present study, a thin pulsed laser sheet with 500 mJ pulsed energy at wavelength \(\lambda\) = 532 nm and 6 ns pulse duration were used to illuminate the flow region of interest and stimulate the particle scattering light, which was received by the CCD camera at last. Image spatial resolution was 34.82 \(\upmu\)m/pixel. The cross-frame time \(\Delta t\) was 25 ms. The area of each PIV image captured was 250 mm \(\times\) 150 mm. The displacement error of the tracer particle in both \(y\) and \(z\) directions was lower than 0.1 pixel. Since each pixel represents 0.063 mm, the uncertainties of the velocity and vorticity for this experiment were about 1.5% and 2.5%, respectively.  
The original images were handled by the commercial software Tecplot 360 EX 2017. Before image correlation, a high-pass filter was implemented to emphasize the tracer particle information and to eliminate any inhomogeneous light in the background. The wind speed value determined the interrogation window size to 64 \(\times\) 64 pixels window size. Overall, less than 2% of the vectors were removed. In order to obtain a quantitative analysis of train leeward vortex evolution by flow field test system.  
An experiment was carried out with a cobra probe, which can measure the wind speed below 100 m/s, with an accuracy error of less than 1 m/s, but it cannot measure the reversed flow [15; 16; 21], see Figure 1b.  
Figure 1: Schematic of experimental arrangement: (**a**) PIV; (**b**) cobra probe.  
The sampling frequency of the cobra probe is 1250 Hz in the wind test, which is recommended a sampling frequency of above 800 Hz by the manufacturer.