## 1 Introduction  
With the rapid development of civil aviation, aeroacoustic problems were of wide concern among aviation manufacturing enterprises and researchers the world over. Strict noise airworthiness regulations are formulated by the International Civil Aviation Organization (ICAO) to limit the noise of civil aircraft. The research projects to reduce the noise level of civil aircraft were developed by the European Advisory Committee on Aeronautical Research (ACARE) [1] and the National Aeronautics and Space Administration (NASA) [2]. During takeoff and landing of civil aircraft, the flap lifting device is one of the main components that produce noise [3]. In the noise generated by flap lifting device, the side edge vortex shedding of the flap is the main noise source [4]. The research on the flap side edge noise source can be traced back to 1979, when [PERSON] [5] and [PERSON] [6] found that the flap side edge was an important aircraft noise source. [PERSON] et al. [7] confirmed the existence of severe surface pressure fluctuation in the flap tip region and demonstrated that the formation of pressure fluctuation is directly related to the coherent structures in the shear layer. [PERSON] et al. [8] analyzed and showed that the unstable wave in the lateral shear layer is the main cause of noise by using the instability theory. With the continuous progress of measurement technology, the understanding of flap side edge noise was deepened. [PERSON] [9] and [PERSON] [10] et al. respectively carried out several experiments on the side flow field of trailing edge flap. The flow field at different deflection angles were measured experimentally. They found that there is a vortex pair structure at the side edge of the flap. The result showed that the vortex breaking mechanism can induce noise under the condition of a large deflection angle. Moreover, a series of aeroacoustic experiments on the far-field noise of flap was carried out by [PERSON] et al. [11]. They found that reasonable geometric position and structural deformation can reduce the noise radiation of slat to a certain extent and maintain high aerodynamic performance. In recent years, the Computational Fluid Dynamics (CFD) and Computational Aero-Acoustics (CAA) were widely employed in the flow features and noise of wing-flap configuration [12]. The numerical investigation of unsteady flow field around high-lift device was presented by [PERSON] [13] using the Lattice Boltzmann method (LBM). The pressure fluctuation and aeroacoustics analysis of the high-lift airfoil was obtained. The numerical results showed that predictions of noise sources and their relative strengths were in good and reasonable agreement with the experimental data from microphone array measurements. The interference between the shear layer unstable wave and the upper and lower walls of the flap by using the CFD/CAA hybrid method was carried out by [PERSON] et al. [14] and [PERSON] et al. [15] also performed a series of unsteady turbulent fluctuations in the acoustic source surface of high-lift device. The transient flow field was obtained by solving the Unsteady Reynolds Averaged Navier-Stokes (URANS) and Large Eddy Simulation (LES) equations. And the acoustics were analyzed by solving Ffovcs-William and Hawkings (FW-H) equations to capture acoustic signals. [PERSON] et al. [16] adopted the LES method and the FW-H integral method to simulate the flow field of rotating blades and corresponding far-field noise. The numerical method was verified by the classic Knight and Hefner hover case, and the mechanism of noise generated by the two-bladed rotor in hover was analyzed. The results showed that unsteady flow phenomena such as vortex shedding and flow separation can be accurately captured by LES method, and the acoustic analogy method based on FW-H can accurately and efficiently solve the far-field aerodynamic noise.