# Wind Tunnel Test on Windblown Sand Two-Phase Flow Characteristics in Arid Desert Regions
## 3 Experimental Results and Discussion
### Impact Pressure of Sand Particles in the Windblown Sand Flow Field  
The loads acting on infrastructures and civil structures include wind loads and impact loads of sand particles in the windblown sand environment. However, the existing codes only consider the impact of wind load, which is not conducive to the efficient maintenance of infrastructures and civil structures in the windblown sand environment. To analyze the variation of impact pressure with wind speed, particle size, and height, a simplified impact pressure model of sand particles in a windblown sand flow field is established based on the assumption that the impact of sand particles on structures is an elastic collision. Based on the momentum theorem, the impulse of sand particles in the windblown sand flow is obtained:  
\[I=Ft=2 mU_{s}\approx 2 mU_{a} \tag{7}\]  
where \(I\) represents the impulse of sand particles, in N-s, \(F\) represents the impact force of sand particles, in N, \(t\) represents the impact time of sand particles colliding infrastructures and civil structures, in s.  
Sand particles mass within an impact time:  
\[m=\rho_{s}A_{s}U_{s}t\approx\rho_{s}A_{s}U_{a}t \tag{8}\]  
where \(A_{s}\) represents the impact area of sand particles colliding infrastructures and civil structures, in m\({}^{2}\). Substituting Formula (8) into Formula (7), the impact pressure of sand particles on infrastructures and civil structures is obtained:  
\[P_{s}=2\rho_{s}U_{s}^{2}\approx 2\rho_{s}U_{a}^{2} \tag{9}\]  
where \(P_{s}\) represents the impact pressure of sand particles on infrastructures and civil structures, in Pa.  
We could obtain the impact pressure profiles of structures in the windblown sand environment through Formula (9), as shown in Figure 15. Similar to the energy distribution law, the impact pressure of sand particles is related to wind speed, height and particle size. For sands with all particle sizes and wind speeds, the impact pressure exponentially decays with increasing heights. However, the research conducted by [PERSON] et al. showed that the critical height of the impact pressure profile was approximately 20 cm [33]. In other words, the impact pressure increases above 20 cm and decreases below 20 cm with increasing height. It is mainly due to the different simulation methods adopted for the sand concentration profile in the wind tunnel. In this study, we simulate the common windblown sand environment by spreading sand on the floor of the wind tunnel. However, [PERSON] et al. simulated the sandstorm environment by falling sand from the top of the wind tunnel [33]. Due to different research purposes, objectives, and test methods, the obtained sand concentration profiles, and impact pressure profiles, in the above two studies are also different. As can be seen from Figure 15, the impact pressure above 0.6 m is quite small and could be ignored, which is mainly because the sand concentration above 0.6 m is quite small. At the same indicated wind speed, the impact pressure in the mixed sand, fine sand, and coarse sand are successively reduced, as their impact pressures are positively correlated with their mass. For the same sand particles, because the impact pressure is proportional to the square of the speed, the impact pressure increases with the increasing wind speed. For sands with all particle sizes and wind speeds, the impact pressure can be fitted with the least-square method. Figure 15 shows that the relationship between impact pressure and height can be expressed as the exponential function (\(P_{s}=\alpha\exp(-\beta z)\), where \(\alpha\) and \(\beta\) are regression coefficients obtained by the least-square method). Table 4 indicates that the correlation between the impact pressure and height is reasonably good. The correlation coefficient \(R^{2}\) is over 0.94 in any situation; the values of coefficients \(\alpha\) and \(\beta\)at different conditions indicate that the decay curve of the impact pressure is affected by the wind speed and the particle size at the same time.