## 2 Experimental Setup and Methods  
### Wind-Tunnel Experiment  
All wind-tunnel experiments were carried out in a blow-down wind tunnel at Lanzhou University. The wind tunnel was 55 m long and included a powerful fan system, a rectification section, a working section, and a diffuser. The length of its working section was about 22 m, and the cross-sectional area of its working section was 1.3 m (width) \(\times\) 1.45 m (height). The wind tunnel was controlled by a computer, and the wind speed could be adjusted between 3 and 40 m/s as the normal wind speed in nature [20; 21].  
As shown in Figure 2, in the experimental setup, roughness elements were in the front of the work section to generate a turbulent boundary layer with a thickness of 0.3 to 0.4 m. A monitoring Pitot tube was used to measure the inlet wind speed. A measuring Pitot tube was fixed to a frame to measure wind speeds around three models at the 17 points of 2.0, 4.0, 7.0, 10.0, 15.0, 20.0, 30.0, and 45.0 cm above the surface. The obtained data were used to analyze the spatial and temporal distributions of the wind speed. Particle image velocimetry (PIV) was used to measure the speed of sand particle on the railway track.  
The PIV system had a laser and transmitter, a high-resolution charge coupled device (CCD) camera, a synchronizer, and a computer with an image acquisition card. The laser emitter was on the top of the wind tunnel, and its laser beam coincided with the axis section of the model. The frequency of the laser was 5 Hz and the pulse energy was 120 mJ. The high-resolution CCD camera and the pulsed laser were synchronized with a pulse delay generator. Two CCD cameras with a pixel resolution of 2048 \(\times\) 2048 and a sampling rate of 500 frames per second were at the side of the wind tunnel. The field of these two cameras was about 21 cm (height) \(\times\) 42 cm (width). The images of particle tracking velocimetry (PTV) were processed to obtain the sand particle velocity and number density.  
Although there are many similarity requirements in wind-tunnel experiments, such as geometric, motion, dynamic, and thermodynamic similarity [22; 23], with sand flow it is difficult to perfectly simulate the shrinkage ratio of the model size, boundary layer size, and particle size in the real environment. Therefore, in this study, some compromises must be adopted [24]. Only the geometric similarity of the model, the motion similarity and dynamic similarity of the wind field, and the corresponding wind-tunnel results were  
Figure 2: Configuration of the wind-tunnel experiment, in which downstream of the roughness-element is the test surface, the model (100 cm wide and 100 cm long) is 9 m away from the sand bed, the sand bed (130 cm wide, 300 cm long and 5 cm deep) is placed after the roughness elements, A is the convex wind-break wall with P1 and P2 as its two slopes, B is the flat wind-break wall, C is the concave wind-break wall with P3 and P4 as its two slopes, and the coordinate origin (0, 0) of the camera view is as shown in the figure. Turbulence intensity is 0.05.  
mainly used in the mechanical studies as a key problem to be solved in current wind-sand physics [23; 25; 26].