# Wind Tunnel Test on Windblown Sand Two-Phase Flow Characteristics in Arid Desert Regions
## 3 Experimental Results and Discussion
### Sand Concentration and Sand Transport Rate in the Windblowon Sand Flow Field  
In Aeolian Physics, sand transport rate is one of the main parameters to describe the transport characteristics of windblowon sand two-phase flow, which refers to the sand mass passing through unit bed width in unit time. While in structural wind engineering, sand concentration and velocity are important parameters for evaluating the effect of windblowon sand load on the engineering structures. Sand concentration refers to the sand mass per unit volume in the windblowon sand flow field. The sand transport rate and sand concentration profiles below 1 m can be presented by analyzing the collected sand at 50 different heights of the sand sampler in the windblowon sand flow field. The equation can be expressed:  
\[\left\{ \begin{array}{c}q=Q/(Lt)\\ \rho_{s}=Q/(AtU_{z})\end{array} \right. \tag{2}\]  
where \(q\) represents the sand transport rate in the windblowon sand flow field, in g/cm/s, \(\rho_{s}\) represents the sand concentration, in g/m\({}^{3}\), \(Q\) represents the collected sand mass, in g, \(U_{z}\) represents the mean wind speed, in m/s, \(t\) represents the sand sampling time, in s, \(A\) represents the inlet area of the sand sampler, in m\({}^{2}\), \(L\) represents the width of the inlet of the sand sampler, in cm [33; 34]. Since the sand concentration and the sand transport rate can be linearly converted, and both of them share the same curve trends, only the plot of sand concentration with height is presented in Figure 9. The results show that three kinds of sand concentrations decrease with increasing heights. Below 0.3 m, the sand concentration varies widely at different heights; above 0.3 m, the sand concentration is minor and varies little at different heights. Results further indicates that sand particles mainly move within the 0.3 m layer under the action of wind force. The concentration of the three kinds of sand particles increases when the indicated wind speed increases. At the same indicated wind speed, the concentration of the coarse sand, fine sand and mixed sand increases in turn, which is because the particle size of fine sand is smaller than that of the coarse sand, and they are more likely to sulfate under the action of wind force. As a result, more sand particles are lifted by the wind, resulting in a larger concentration. The concentration of mixed sand is higher than that of fine sand due to the following reasons: Compared with the other two kinds of sand beds, the mixed sand bed is a non-uniform sand bed. Sand particles with different particle sizes could interact and affect each other when they jump off the bed and move in the air stream, making the fine sand particles more likely to saltate [34; 51]. The least-square method is used to fit the sand concentration at different heights. Regression analyses indicate that the relationship between sand concentration and height can be described by the exponential function. Similarly, the relationship between sand transport rate and height can also be described by the exponential function. The formula is as follows:  
\[\left\{ \begin{array}{c}q=A\exp(-z/B)\\ \rho_{s}=a\exp(-z/b)\end{array} \right. \tag{3}\]  
where \(a\), \(b\), \(A\) and \(B\) are regression coefficients. Table 2 indicates that the correlation between the sand concentration (or sand transport rate) and height is reasonably good. The correlation coefficient \(R^{2}\) is over 0.94 for all particle sizes and indicated wind speeds. Formula (3) is in the same form as the decay function of the sand transport rate proposed by [PERSON] for the 0.16 m near-surface layer based on wind tunnel tests [55]. [PERSON] et al. also came up with the notion that changes in the sand transport rate below 0.6 m with height should be expressed by the exponential function [34; 40]. The qualitative understanding in this study is basically consistent with the findings in previous studies.