# Wind Tunnel Test on Windblown Sand Two-Phase Flow Characteristics in Arid Desert Regions
## 3 Experimental Results and Discussion
### Movement Form of Sand Particles in the Windblown Sand Flow Field
The cumulative mass curves of fine sand, coarse sand and mixed sand at three indicated wind speeds are shown in Figure 7, and some characteristic parameters of particle movement are shown in Table 1. The results show that no matter what size particles are, and how the wind speed is, the mass percentage in the maximum height is below 0.1%, which means the thickness of the entire windblown sand flow field is about 0.6 m. For sand particles with three different sizes, the accumulative mass percentage within the height of 0-0.3 m has exceeded 90%, which means the sand particles mainly move in this layer. The accumulative mass curve at the height of 0.3-0.6 m is close to the vertical line, indicating that the sand is in a low concentration and suspended. The mass of the suspended particles to total mass in percentage ratio is defined as the suspension percentage \(F_{s}\). For these three kinds of particles, \(F_{s}\) are quite small, and their mean value is between 1.0% and 1.7%. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Particle Size** & **Wind Speed (m/s)** & \({}^{1}\)\(Z_{90}\) **(cm)** & \({}^{2}\)\(F_{s}\) **(\%)** & **Mean \(F_{s}\) (\%)** \\ \hline \multirow{4}{*}{Coarse sand} & 14.74 & 12.2 & 0.8 & \multirow{4}{*}{1.0} \\  & 16.68 & 14.8 & 0.7 & \\  & 18.61 & 16.2 & 1.6 & \\ \hline \multirow{4}{*}{Mixed sand} & 14.74 & 11.9 & 0.7 & \multirow{4}{*}{1.4} \\  & 16.68 & 14.5 & 1.6 & \\ \cline{1-1}  & 18.61 & 15.4 & 2.0 & \\ \hline \multirow{4}{*}{Fine sand} & 14.74 & 11.0 & 0.9 & \multirow{4}{*}{1.7} \\  & 16.68 & 13.2 & 1.8 & \\ \cline{1-1}  & 18.61 & 14.4 & 2.3 & \\ \hline \hline \end{tabular} \({}^{1}\)\(Z_{90}\) is the height corresponding to the cumulative mass percentage of 90%; \({}^{2}\)\(F_{s}\) is the suspension percentage within the height of 0.3–0.6 m.  
\end{table} 
Table 1: Some characteristic parameters of sand particle movement. Figure 6: Movement form of sand particles in the windblown sand flow field. The sand particles on the ground (at the height z = 0) are defined as creeping particles, which are very tricky to measure by the sand sampler [54]. Given this, further quantitative analysis on the creep percentage is not performed in this study. Figure 7: Cont. In the windblown sand flow field, saltation is the most important movement form because the amount of saltating particles accounts for about 3/4. Moreover, the saltation has an effect on the creep and suspension in the windblown sand flow field, for example: (1) the momentum of the creep comes from the saltation; (2) the suspending small particles on the bed surface are difficult to be directly lifted by the wind because of the concealing effect of the underlying layer of the viscous flow and the adhesion between the particles. Only if the impact of the saltating particles removes them from the ground, the vortices in the air flow easily drive them to suspend and move in the air flow [52]. When the cumulative mass percentage of sand particles is 90%, the corresponding height is defined as the characteristic saltation height \(Z_{90}\). As can be seen from Figure 8, \(Z_{90}\) increases when the wind speed and particle size increase, which means the characteristic saltation height is the largest for coarse sand, followed by mixed sand, and the smallest for fine sand. Figure 8: The relationship between \(Z_{90}\) and wind speed. Figure 7: Cumulative mass percentage: (**a**) Coarse sand; (**b**) Fine sand; (**c**) Mixed sand.