## 1 Introduction  
With the progress of aircraft engine noise reduction technology, airframe noise has been identified as a new challenge to the aeronautical industry [1]. The main source of the airframe is the landing gear [2], followed by the high lift devices. Therefore, the landing gear has been widely studied [1, 2, 3, 4, 5]. In the early stage, in order to identify the most dominant sources of the landing gear, the European research program RAIN carried out full-scale aeroacoustics tests in the German Dutch wind tunnel (DNW) on the landing gear of A340 aircraft [3, 6, 7]. Based on experimental research, add-on noise reduction technology was developed to realize the overall noise reduction up to 3 dB [8]. However, this noise reduction method cannot be applied to current aircraft in the short term. In order to take into account the noise impact in the design stage, the European research project SILENCER (\"Significantly Lower Community Exposure to Aircraft Noise\") was committed to how to design low-noise landing gear in the development stage [4]. Then a European Co-financed project called TIMPAN (\"Technology to IMprove Airframe Noise\") was launched [2]. It mainly studied some more advanced low-noise designs for aircraft landing gear and the high lift wing. At present, there are still many projects to study airframe noise [9; 10; 11; 12], fairing [13] technology has been widely studied, and some novel noise reduction approach such as air curtain [14; 15] and wire mesh screen model [16] have been proposed. For further information, please refer to a review paper by [PERSON] et al. [17]. As a fundamental scenario aeroacoustics simulation, tandem cylinders have been studied by many researchers. For instance, different turbulence models were utilized to conduct to numerical simulate [18; 19], and the results were compared with the experimental data of NASA achieved from Basic Aerodynamic Research Tunnel (BART) and Quiet Flow Facility (QFF) [20; 21; 22]. In addition, [PERSON] [23] used numerical approach to study the acoustic performance of two square cylinders. A large number of landing gear experiments provide a comprehensive data set for numerical simulation [7]. At the beginning, the flow field simulation of landing gear mainly adopts RANS (Reynolds-Averaged Navier-Stokes) method [5]. The hybrid RANS/CAA approach was used for the simulation of broadband sound generation [24]. Then, owing to the DES [25; 26; 27] (Detached Eddy Simulation) and LES (Large Eddy Simulation) [28; 29; 30] ability to capture the small-scale turbulence characteristics related to high-frequency noise, they have been widely used to solve the landing gear flow field. Many researchers have developed different new methods on the basis of LES and DES [31; 32; 33; 34]. [PERSON] [35] used the LBM (Lattice Boltzmann Method) and FW-H (Ffowcs Williams-Hawkings) methods to study the effect of the grid on the aeroacoustics characteristics of the landing gear. [PERSON] [26] compared CFD simulation with experiments in detail to verify the flow field, thereby evaluating the accuracy of CFD results for noise prediction. At present, the main noise radiation simulation methods are FW-H, APE (Acoustic Pertubation Equations) [36], and LEE (Linearized Euler Equation) [37; 38]. The Linear Euler method has been used to solve the sound propagation problems in the near and intermediate sound fields [39]. Some researchers have derived the convection form of the source term and verified the correctness of the source term representation method [40], [PERSON] [37] used linear N-S equations to calculate the noise problem of jet engine blades. [PERSON] [38] used linear Euler equations to study the numerical solutions of monopole, dipole, and quadrupole noise and verified them through analytical solutions. At present, there is relatively little research on the application of LES/LEE to landing gear. Analyzing the flow characteristics of landing gear is the key to solving the problem of aerodynamic noise of landing gear [3].