# Investigation on Aerodynamic Noise Characteristics of Coaxial Rotor in Hover
## 1 Introduction  
Nowadays, high-speed capability is an important requirement in the design of helicopters. Several novel configurations have been put forward to meet this requirement. Compared with the conventional configuration of a single rotor, a rigid coaxial rotor compound helicopter has substantial advantages in aerodynamic performance [1]. Sikorsky Aircraft developed the X2 TechnologyTM Demonstrator aircraft based on the advancing blade concept (ABC) [2]. Its cruise speed is considerably higher than a conventional single-rotor helicopter [3]. However, a rigid coaxial rotor meets severe aerodynamic interaction, as a small rotor space is usually adopted to reduce the drag in forward flight. Correspondingly, this will cause complex interaction loads and special aerodynamic noise characteristics. Current research studies of coaxial rotors are mostly focused on the aerodynamic interaction problems [4, 5]. These provide important references for the acoustic analysis. However, studies about the aerodynamic noise of coaxial rotors are still limited. Fowcs Williams-Hawkings (FW-H) equations have been widely used in rotor aeroacoustics research [6]. In 2006, [PERSON] et al. [7] established the calculation method for the linear noise of a coaxial rotor based on an acoustic prediction program, WOPWOP, and studied the influence of configuration parameters on aerodynamic noise. The vortex line method was used to calculate the blade surface load. [PERSON] et al. [8, 9] built an acoustics prediction model based on the vorticity transport model (VTM), and investigated the blade-vortex interaction noise of a coaxial rotor in inclined descent. The wake characteristics of a coaxial rotor can be computed with reasonable accuracy by vortex methods. However, vortex methods are usually dependent on empirical formulas and the detailed unsteady interaction loads cannot be simulated well [10, 11]. By comparison, the CFD method has manymore advantages in the flow field simulation of a coaxial rotor, which can simulate the actual blade motions and detailed flow field of interaction. In 2016, [PERSON] et al. [12] analyzed the acoustics of a coaxial rotor based on the Rotor-craft Comprehensive Analysis System (RCAS), and found a new kind of impulsive sound pressure pulse at high-speed forward flight. [PERSON] and [PERSON] et al. [13; 14] analyzed the noise characteristics of a coaxial rotor in hover based on Farassat 1A equations and Reynolds Averaged Navier-Stokes (RANS) equations. However, the detailed causes of noise characteristics had not been studied. In recent years, [PERSON] et al. [15; 16; 17] used the CFD/CSD loose coupling method and PSU-WOPWOP program and conducted a series of studies of lift-offset coaxial rotor acoustics. The impulsive loading noise was found to be caused by \"blade-crossover\" interaction events. Based on the previous research, there are two main differences in coaxial rotor noise in hover, compared with a single rotor. At one aspect, the noise is formed by the superposition of two noise sources, contra-rotating rotors. Meanwhile, there are special interaction phenomena for a rigid coaxial rotor, which would cause new loading noise characteristics. At hover state, there is no aerodynamic interaction for a single rotor, meaning that the rotor load is quasi-steady. Thus, interactions of the coaxial rotor can be clearly explained through comparison. Meanwhile, hovering is the typical interaction state of a coaxial rotor, especially for the blade-meeting interaction. In forward flight, there are complex interactions, which makes it difficult to reveal the formation mechanism of noise. Thus, for the hover state, it is necessary to carry out a further analysis on the aerodynamic noise of a coaxial rotor, referring to the mechanism of interaction. This paper is further research based on the previous research [18; 19]. The previous CFD solver based on the RANS solver and moving overset mesh are adopted for flow field simulation, which provides sound source information.