# Study on Influencing Factors of Helicopter Brownout Evolution Based on CFD-DEM
## 1 Introduction  
Brownout is the name of the limited visual field of a helicopter pilot after environmental impact, which easily occurs in a sand-dust environment. The main influence area of brownout is the circle around the helicopter which is several times of the radius of the rotor. The hovering flow field will entrain many sandy particles from the ground into the air, thus reducing the pilot's visibility of the surrounding environment. The degree of visual limitation depends on the type of helicopter (single rotor or coaxial twin-rotor), and the operation condition (helicopter landing posture or flight trajectory). In this environment, the pilot can easily lose a sense of space and direction, and can become unable to judge the ground position, horizontal attitude and other information accurately. The pilot will then make mistakes in the control of the helicopter, which can cause the helicopter to crash and result in injury to the pilot. As early as the 1940s, [PERSON] [1] started to study the problem of wind-drift sand in the desert of Libya. Combined with wind tunnel experiments, he produced a series of formulas for the efficiency of wind-drift sand transportation. [PERSON] et al. [2] established a dusty entrainment model using [PERSON]'s method. In the 1960s, [PERSON] [3] studied the way that turbulence carries sand from the ground. An entrainment hypothesis that turbulent eddy currents destroy the viscous layer and directly impact on the sand surface was proposed. [PERSON] and others of the US Army [4] studied the entrainment of the downwash flow when the helicopter is near the ground, which is called the blast effect. Subsequently, [PERSON] [5] carried out several large-scale experimental studies using a twin-rotor helicopter which hovered near the ground with a dust collector. As a result, it was found that the entrainment particles are larger when the helicopter is closer to the ground. In the 21 st century, particle image velocimetry (PIV) have become the mainstream methods for researchers to analyze brownout. [PERSON] et al. [6] used PIV to study the two-phase flow environment caused by a helicopter hovering on sands. It was found that in the process of the viscous merging of an adjacent wake vortex, the velocity of the upward flow increases significantly. It was observed that the sandy particles with enough height will recycle into the rotor wake and spray to the ground, causing dusty entrainment through the process of bombardment. [PERSON] et al. [7] conducted experiments based on PIV and found that the viscous diffusion process, strain-induced vorticity enhancement process and the generations of turbulence are the factors that affect the flow field characteristics. [PERSON] and [PERSON] [8] used PIV to observe the flow structure in front of and under the helicopter rotor during forward flight and found that forward flight has a significant effect on the wake compared with hovering, and the rotor wake will be closer to the blade and smaller. [PERSON] and [PERSON] [9] studied the effects of vortex filament deformation and viscous diffusion. [PERSON] and [PERSON] [10] used the Lagrangian method to calculate the movement of particles in the flow field, and simulated the two-phase flow containing sand. [PERSON] et al. [11] used PIV to study the scaled model with four different tip shapes, and measured the vortex core size and the vortex sheet velocity at 800\({}^{\circ}\) azimuth. The results show that there are some common flow characteristics among the four different tip shapes, and the difference of flow may ultimately affect the evolution of brownout. The slotted tip is especially effective in diffusing the tip vortex. [PERSON] et al. [12] used a computational fluid dynamics (CFD) method to study the influence of a discrete rotor blade model in the dusty cloud caused by helicopter downwash. The results of the wake velocity profile, ground shear force profile and dusty cloud density at different heights of the independent rotor in hover are given and compared with the steady model. [PERSON] et al.