# Wind Tunnel Test on Windblown Sand Two-Phase Flow Characteristics in Arid Desert Regions
## 2 Experimental Apparatus, Methods and Design  
### Wind Tunnel Test  
Experiments were carried out in the blow-down wind tunnel at the Key Laboratory of Solar Thermal Energy and Photovoltaic System, Chinese Academy of Sciences (Figure 1). The test section of the wind tunnel is 20 m long, 3 m wide and 2.5 m high. The testing wind speed is continuously adjustable from 1.5 m/s to 30 m/s. The results for the flow and turbulence uniformity along and across the test section are as follows: the non-uniformity coefficient of the velocity field, \(\mu\leq 1.0\%\), the non-uniformity coefficient of the directional field, \(\Delta\alpha\leq 1.0\%\), \(\Delta\beta\leq 1.0\%\), the turbulence intensity of the uniform flow field, \(\varepsilon\leq 1.0\%\), the axial static pressure gradient, \(\left|dC_{p}/dx\right|\leq 0.003\) m\({}^{-1}\). The bed surface device supplying sand is installed in the test section, including a number of sand trays with the bottom area of 2 m \(\times\) 1 m. They can be flexibly assembled and used to conduct the windblown sand test. All these sand trays can slide and be fixed in the test section. Therefore, the amount and height of the windblown sand can be controlled through adjusting wind speed and positions of sand trays, so that the sand concentration distribution at the test location conforms to that in the windblown sand environment. The sand sampler (Figure 2) was used to collect the blown sand at different heights to determine the vertical distribution characteristics of sand particles. The sand sampler is 1-m-high and there are 50 sand inlets with a cross-section of 20 mm \(\times\) 20 mm at 50 different heights. Each sand inlet is connected to a detachable sand chamber. The sand concentration in the range of 1 m can be gained by weighing the collected sand amount with the help of an electronic balance with an accuracy of 0.001 g. The spacer between the sand inlets is made very thin to reduce the measurement error. At the same time, in order to reduce the interference of the sampler with the airflow, the front end of sampler is wedge-shaped and each sand chamber is connected to a vertical vent with a screen mesh to maximize the collection efficiency by reducing the air pressure in the sand chamber. The evaluation of the sand sampler in the wind tunnel shows that the device is highly efficient, especially for sand particles with low inter-particle cohesion (with diameters of over 0.1 mm) and the overall efficiency reaches up to 90% [34]. The wind speed profile in the windblown sand flow field is obtained by the wind profiler (Figure 3) placed behind the sand trays, which is specially used for windblown sand tunnel tests [33; 43; 44; 45]. The wind profiler with a deviation of 0.15% of the measured value has 9 probes in the range of 1 m (mounted at 5 mm, 10 mm, 15 mm, 50 mm, 100 mm, 250 mm, 500 mm, 750 mm and 1000 mm) [33]. To further ensure the measurement accuracy, all the probes in the wind profiler are cleaned after each windblown sand test. Figure 1: Windblown sand tunnel test section and wind field layout. Figure 2: The sand sampler. Sand samples are taken from Tengger Desert with a particle size of no more than 1.0 mm [46]. The particle size of the fine sand, coarse sand and mixed sand obtained through the screening instrument is 0-0.5 mm, 0.5-1.0 mm and 0-1.0 mm, respectively. The volume ratio of coarse sand to fine sand in mixed sand is approximately 1:3. The sand sampler is about 15 m away from the entrance to the test section and 5 cm away from the upwind sand tray. The bottom plate of the lowest sand inlet is flat with the wind tunnel floor. The sand bed device assembled by the sand trays is 4 m in length, 3 m in width and 2.5 cm in depth, which could ensure the full development of the saltation movement [34; 40]. Before testing, put the sand samples into the sand trays and make their surface flat; and cover a piece of geo-textile on the sand trays before the preset wind speed is reached, so that the sand samples would not be blown away.