# Study on Influencing Factors of Helicopter Brownout Evolution Based on CFD-DEM
## 4 Coupling Simulation Analysis
### Result of Hovering Height 0.52R  
The movement and distribution of simulated sand particles are studied. Figures 8-10 show the distribution of sand around the fuselage at different times when the hovering height of the UH60 is 0.52R, the particle radius is 10 um, and the number of increased particles is 2500 per second.  
In the brownout phenomenon, the height of sand particles is related to the probability of dust collision with the fuselage or blade, and to the degree of wear and erosion of the blade surface. The height is also related to the visual field of the pilot in the cockpit. The height of particles from 0 mm to 4000 mm is counted; after this, every 200 mm is the interval. The particles leaving the ground (d > 200 mm) are analyzed separately, and the average height is counted. The results are shown in Figures 11 and 12.  
From Figure 11, it can be seen that the height of particles from the ground increases gradually within 3 s, but more than 50% of the particles are still concentrated near the ground (0-0.1R) and are not entrained into the air by the airflow. The height of the other particles increases gradually in 3 s, and the vertical distribution is more uniform.  
From Figure 12 it can be seen that, when t = 3 s, the number of suspended particles starts from 200 mm above the ground, and the number of suspended particles is negatively correlated with the height above the ground, and gradually decreases. The higher part has exceeded the height of 1R, while the particles near the position of 0.5R (3800 mm-4600 mm) account for about 2%. This part of particles may cause erosion to the blades. Particles near the cockpit height of 0.3R (2200 mm-3000 mm) account for about 5%, which will affect the pilot's vision. The average height of particles shows a rapid upward trend between 1 s to 2 s, and the average height gradually flattens after 2 s.  
In addition to the height, the movement speed of sand particles is also the basis for judging the brownout phenomenon, and the movement speed is related to the severity of sand erosion on the fuselage and blades. The distribution of particle velocity and the relationship between the average velocity and time are shown in Figures 13 and 14.  
Figure 11: The number of particles at 1 s and 3 s.  
Figure 12: The number of suspended particles at 3 s and the average height of particles in 3 s.  
Just after the particles are entrained off the ground (t = 1 s), the velocity is mainly concentrated between 10 m/s and 14 m/s. The number of particles moving at a high speed is less, and the overall average velocity is not fast. With the passage of time, the average velocity of particles gradually increases. After 2 s, the number of high-speed particles gradually increases, and the number of particles with a velocity of about 25 m/s also increases, while the average velocity of the particles with the largest proportion begins to decrease. When t = 3 s, the trend is more obvious; some particles have a longer time to experience the acceleration of air flow, the proportion of particles with high-speed motion increases significantly, and the other particles will be affected by the air flow. The velocity begins to decrease after the particles suspend or sink to the bottom.