## 1 Introduction  
The helicopter has the characteristics of vertical take-off and landing, hovering, and near-ground maneuverability, which plays an important role in military and civilian fields [1, 2]. For conventional helicopters, the forward flight speed is restricted by the dynamic stall of the retreating blade and the compressibility of the advancing blade. The compound helicopter has become a solution to increase the forward speed of the helicopter. At present, coaxial rigid rotor compound helicopters and tilt-rotor aircraft have been developed rapidly [3, 4, 5, 6]. The compound helicopter with a rigid coaxial rotor is one of the main configurations in the development of high-speed helicopters, and as the most important feature of the helicopter, the Advanced Blade Concept (ABC) rotor is adopted [7, 8, 9, 10]. The coaxial rigid rotor high-speed helicopter adopts the coaxial main rotor with upper and lower rotorsreversed and tail thrust propellers. The dynamic stall on the retreating blade is weakened by the lift offset of the rigid rotor, and the fuselage drag is overcome by the thrust propeller to achieve high-speed forward flight [11]. Sikorsky X2 and S-97 [12] are typical of compound helicopters. The research on the aerodynamic performance of helicopter rotors mainly includes the experimental method and CFD method [13; 14; 15]. The advantages of the experimental method are direct and reliable, while the disadvantages are long cycles and high costs. The computational fluid dynamics (CFD) method is developing rapidly. Its advantages are low cost, short period, and high precision. At present, CFD has become an important method of helicopter aerodynamic performance research [16; 17; 18]. At present, CFD technology has been widely used in the aerodynamic simulation of helicopter rotors. Overset grid method is applicable to helicopter hovering, forward flight, vertical flight, near-ground flight, and other complex flights, which is one of the most widely used methods in this field. [PERSON] [19] used the overset grids to study the interaction flow field of coaxial rotors. Compared with the flow field calculation of a single rotor, the calculation model is more complex and the calculation amount is larger. [PERSON] et al. [20] conducted an experimental study of the rotor/fuselage flow field. [PERSON] et al. [21] used the overset mesh approach to study helicopter unsteady flow problems, which were tested on a generic helicopter configuration in terms of load balance, scalability, and memory usage. [PERSON] et al. [22] adopted an unsteady rotor-fuselage interactional aerodynamics model to calculate the flow field of ROBIN rotor fuselage. [PERSON] et al. [23; 24] calculated the rotor flow field characteristics under the influence of the ground with the structure grid, which took into account the unsteady influence of moving blades on the flow field. [PERSON] et al. [25; 26] used unstructured grids to solve the hovering flow field of rotors. [PERSON] & [PERSON] [27] used unstructured grids and Navier-Stokes (N-S) equations to simulate the rotor flow field in hover. [PERSON] et al. [28] presented anisotropic grid adaptation for dynamic overset simulations to study rotor/fuselage-interaction flow field. [PERSON] et al. [29] studied the three-dimensional effects on the dynamic-stall characteristics of a rotor blade, which filled the gap regarding the insufficient three-dimensional dynamic stall of a helicopter rotor. [PERSON] et al. [30] used CFD modeling to compute the unsteady flow around the main rotor-fuselage. [PERSON] et al. [31] applied a high-resolution air load with accurate rotor wake to study rotor BVI (Blade-vortex interaction) analysis. [PERSON] et al. [32] focused on aerodynamic characteristics of rigid coaxial rotor of a high-speed helicopter in hover and forward flights in the 8 m\(\times\)6 m low-speed straight flow wind tunnel of China Aerodynamics Research and Development Center. [PERSON] et al. [33] developed the polyhedral nested grid to study the interference flow field of the Robin rotor/fuselage.