# Helicopter Blade-Vortex Interaction Airload and Noise Prediction Using Coupling CFD/VWM Method
## 1 Introduction  
It is well known that helicopter rotor impulsive noise originates from high-speed impulsive noise, which is due to a high tip Mach number on the rotor's advancing side, and the blade-vortex interaction noise on the advancing and retreating sides of the rotor disk. Blade-vortex interaction (BVI) [1] occurs when the strong tip vortices shed from the main rotor pass in close to the following blades during descending or maneuvering flight; this results in impulsive changes in blade loading, which radiates noise. Not only does BVI noise have significant effects on passenger comfort but the vibratory loads also increase pilot workload and maintenance costs. The calculation and analysis of BVI noise remain a challenging task in the field of helicopter aerodynamics and aerodynamic acoustics [2].  
The BVI aerodynamic load and noise strength, as well as the radiation direction, relate to many factors, such as the location of BVI events, the blade tip vortex structure, and the miss-distance. Within the past several decades, considerable effort has been devoted towards noise mechanisms and the alleviation of BVI [3; 4; 5; 6], but most of these studies have been focused on experiments. In numerical studies, the complexity of BVI renders simulation and analysis more difficult, consequently slowing its development. The rotor wake structure and the unsteady aerodynamic load distribution on the blade surface have been recognized as the most important parameters in order to better predict BVI noise. In 1990s, the computational fluid dynamics (CFD) method was gradually applied to rotoraerodynamic research [7; 8; 9; 10]. Some scholars further utilized this method to study BVI aerodynamics and noise [11; 12; 13]. However, the numerical dissipation inherent in the difference schemes and the interpolation errors make it hard to accurately capture the rotor tip vortices. Despite grid refinement, high order-accuracy schemes and adaptive grids can be used to reduce numerical dissipation, but these methods are still insufficient at capturing the wake structure entirely; furthermore, vast amounts of computation time and resources limit their applications in researching the rotor BVI problem. In recent years, a coupled method combining the wake model with the Euler solver was developed [14; 15; 16; 17] and several remarkable studies on BVI noise have been reported in [18; 19] demonstrating the capability of the coupled method to predict BVI noise; however, the wake solutions rely on empirical formulations due to the assumption of inviscid flow, resulting in some errors regarding BVI airload prediction.  
To overcome the deficiencies of the existing coupled methods in BVI noise prediction, a high-resolution method coupling CFD and the viscous wake method (VWM), developed by [PERSON] et al. in [20; 21], is employed for rotor wake simulation and impulsive airload calculation, and the generalized Farassat formulation based on the Ffowcs Williams-Hawkings (FW-H) Equation [22] is applied for noise prediction. The details of the coupled method are described in this paper. To validate this method, the rotor wake structure, blade airload, and noise characteristics are performed for the AH-1/OLS (Operational Load Survey) rotor under the BVI condition, and we present a comparison between it and the CFD/FW-H method. In addition, important parameters for governing BVI noise have been identified: the tip-path-plane angle, the thrust coefficient, the tip Mach numbers and the advance ratio. This work further examines the mechanisms for the effects of these four parameters on BVI noise amplitude and propagation direction by analyzing the effects of those on the characteristic parameters under different flight conditions.