## 3 Results
### Horizontal Speeds, Number Density, and Fluxes of Sand Particles  
Figure 9 shows the changes in horizontal speeds of sand particles along the x-direction on the leeward side with different terrains when the incoming wind speed \(u_{*}\) is 0.9701 m/s. The results show that the horizontal speeds of sand particles were increased first and then decreased negatively with x distance on the leeward side. The speeds reached their peaks within 0.4-0.5 m and were gradually decreased after 0.5 m. The increase of horizontal speed of sand particles was the largest in the case of C, which were \(-11.2\) m/s at 0.05 m and \(-10.2\) m/s at 0.1 m, respectively. Compared with that in case of C, in the cases of A and B the horizontal speeds of sand particles were respectively reduced by 33% and 13% at 0.05 m, and by 22% and 8% at 0.10 m, because the horizontal speeds of sand particles within 0.4-0.5 m were affected by the vortex zone, and the negative feedback effect on sand particles was significant. Moreover, the horizontal speeds of sand particles were reduced due to the obstruction of convex slope P2 and increased due to the concave slope P4. Therefore, the horizontal speed of sand particles in the case of C was higher than that in A or B.  
Figure 10 shows the change in sand particle number density along the x-direction at different heights. The results show that the sand particle number density of the three wind-break walls was first increased and then decreased. The sand particle number density of case C was the highest. Compared with that in case C, the peaks of case B were decreased by 21% and 47% at heights 0.05 and 0.10 m, respectively; and those of case A were decreased by 50% and 65% at heights 0.05 and 0.10 m, respectively. With the increase in height, the sand particle number density was gradually decreased and the amplitude of these changes was increased, indicating that the sand particle concentration was larger at a lower height and smaller at a higher location. Additionally, the sand particle number density was smaller on the side than that in the middle because the laser intensity on both sides was weaker, and the corresponding sand particle information captured by PIV was reduced.  
As shown in Figure 11, the horizontal sand flux in case C on the railway track was the largest, that in case A was the smallest, and that in case B was between the two.  
Figure 10: Number density of sand particles (\(u_{*}=0.9701\) m/s). (**a**) H = 0.05 m (above the track). (**b**) H = 0.10 m (above the track).  
Figure 9: Sand particle’s horizontal speed along the leeward side (\(u_{*}=0.9701\) m/s). (**a**) H = 0.05 m (above the track). (**b**) H = 0.10 m (above the track).  
The horizontal sand flux at different heights showed a wave-like change. At the height of 0.05 m, the maximum horizontal sand flux in cases of A, B, and C all appeared at 0.39 m along the x-direction. The horizontal sand flux in case C was the largest, and those in cases of A and B were lower by 70% and 47%, respectively, than that in case C. At the height of 0.10 m, the horizontal sand flux in cases of A and B were lower by 76% and 52%, respectively, than that in case C.