# Aerodynamic Simulation of Helicopter Based on Polyhedron Nested Grid Technology
## 1 Introduction  
Experimental methods and computational fluid dynamics (CFD) simulation are the main approaches to study the aerodynamic characteristics of helicopters. The experimental approach is more convincing, and sometimes more effective, for classic problems such as airframe drag reduction [1, 2]. However, the experimental method has the disadvantages of long period and high investment. Meanwhile, the spatial resolution of the experimental method is low, so the detailed features inside the flow field are difficult to observe. Currently, with the improvement of CFD technology, its features of intelligence, convenience, and high precision are increasingly prominent. CFD has become an important method in the aerodynamic performance research of helicopters. At present, CFD technology has made great contributions to aerodynamic calculation of helicopters [3, 4]. In particular, the overlapping grid method [5] has attracted much attention in this field because of its unique advantages. It is suitable for almost all flight conditions of a variety of helicopters, and is currently one of the most widely used methods in this field. In addition, the sliding mesh method is often applied to helicopter hovering. It was proposed and validated by [PERSON] et al. [6]. For rotor CFD/CSD coupling simulation, various deformation mesh methods have been widely used, among which the spring mesh method is the most famous. [PERSON] et al. [7] improved the spring mesh method and solved its shortcoming of easily forming grid deformities, so that it could deal with the problem of large deformation movement. The adaptive mesh method is widely applied in capturing the wake of the rotor vortex. This method usually uses a Cartesian grid as the background grid to adapt the background mesh conveniently. The famous American Helios project [8, 9] successfully applied the adaptive mesh method to the rotor vortex wake capture and achieved remarkable effect. [PERSON] and [PERSON] [10] greatly improved the convergence speed of the simulation through the multigrid technology. [PERSON] et al. [11] developed a method to generate coarse grids of multiple grids, which can greatly reduce the convergence time of simulation. [PERSON] and [PERSON] [12] added multigrid technology to the structured overset mesh of the rotor, and the result shows that the multigridusing a structured grid can better guarantee the quality of the coarse grid, thereby improving the calculation accuracy. [PERSON] et al. [13] developed a coupled simulation technique combining viscous wake model (VWM), computational fluid dynamics (CFD), and computational structural dynamics (CSD) to predict unsteady aerodynamic loads of rotors. [PERSON] et al. [14] established the CLORNS code, a robust unsteady rotor solver, to perform the transient simulation of the helicopter flow field, and the results show that the code has high accuracy in calculating aerodynamic loads of helicopter rotors. [PERSON] et al. [15] developed a CFD trimming method of helicopters in forward flight and achieved a good trim effect. [PERSON] et al. [16] proposed an unsteady actuator disk method based on surface circulation distribution, and used it to simulate the flow field of rotors in hover and forward flight states. The results show that this method is effective in capturing the major vortex structures around the disk. [PERSON] et al. [17] proposed a method based on CFD simulation to predict the airborne spray drift and settlement under complex conditions, and applied it to the research on the downwash flow field and droplets movement law of unmanned agricultural helicopters. As one of the fastest developing mesh technologies in recent years, polyhedral mesh technology has the advantages of fewer meshes, faster convergence speed, and higher calculation accuracy. Therefore, its application in engineering is increasingly widespread. However, there is little research on the application of this technology in helicopter flow field simulation, and the relevant literature available for reference is scarce.