# 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 test wind speed at which the particles on the sand bed start to move under the action of airflow is called the threshold wind speed, and the larger the particle size, the greater the threshold wind speed. According to existing research on the threshold wind speed [34; 46; 50], three test speeds of 14.74 m/s, 16.68 m/s and 18.61 m/s at centerline height of 1 m under the impurity-free wind condition, above the threshold wind speed, are selected for windblown sand testing in this study. The bed surface device supplying sand is used to simulate the windblown sand flow fields with fine sand, coarse sand and mixed sand. The coarse sand or fine sand bed surface can be approximately regarded as the bed surface composed of uniform sand. However, the mixed sand bed surface belongs to the non-uniform sand bed surface because it contains coarse sand and fine sand [34; 51]. Therefore, the results could indicate the influence of the particle size and bed surface properties on the characteristics of windblown sand flow field. In the following analysis and discussion, all distances refer to reduced-scale wind tunnel values. Understanding the basic movement form of sand particles is the basis of studying the windblown sand two-phase flow. As shown in Figure 6, [PERSON] concluded that there are three basic movement forms of sand particles under the wind force, which are creep, saltation and suspension through long-term field measurements and wind tunnel tests [17; 18; 19; 20]. The form of sand particles rolling or sliding along the bed surface is called creep; the form of sand particles jumping and leaving the bed surface is called saltation; and the form of sand particles suspending in the air flow for a certain time without touching the bed surface is called suspension. Sand saltation is triggered by sand particle bouncing on the ground. As such, both wind flow and particle impact shall contribute to the total amount of transported sand. The movement form of sand particles is affected by wind speed, particle size, and random nature of sand particle movement. The threshold wind speed is directly proportional to the square root of the particle size. Only when the wind speed reaches the threshold value, the sand particles would start to move [52]. At the same wind speed, smaller sand particles are prone to suspend or saltate, while larger particles creep. However, for mixed sand, fine particles are more likely to saltate [53]. The ratio of the cumulative mass of particles below a certain height to the total mass of particles in the entire windblown sand flow field is determined as the cumulative mass percentage of sand particles. In this study, we collect the data of sand mass at each height of the sand sampler to analyze the cumulative mass percentage of sand particles, which shows the ratio of the cumulative mass of particles below a height \(Z_{m}\) to the total sand mass collected at all heights in the sand sampler. The formula is as follows:\[CMP_{Z_{m}}=\frac{\sum\limits_{i=1}^{m}Q_{Z_{i}}}{\sum\limits_{i=1}^{n}Q_{Z_{i}}} \times 100\%,\ i=1,2,3,\cdots m\cdots n,\ 1\leq m\leq n \tag{1}\]  
where \(CMP_{Z_{m}}\) represents the cumulative mass percentage of sand particles below a height \(Z_{m}\), in %, \(Q_{Z_{i}}\) represents the sand mass at a height \(Z_{i}\), in g, \(n\) represents the number of sand collection heights of the sand sampler. When \(m\) equals \(n\), \(CMP_{Z_{m}}\) reaches 100%.