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A meshless method for predicting cavity noise is developed by using the discrete vortex method and the vortex theory. Firstly, the discrete vortex sound method is used to calculate the flow field of the cavity under uniform grazing flow, and the key parameters such as the strength, position and velocity of the point vortices are obtained. Then, based on vortex sound theory, the sound pressure of point vortex in the free space is obtained, and the time-domain boundary element method is introduced to simulate the scattering effect of the cavity surface. In the end, the spectrum of the sound pressure is analysed. This meshless method has high computational efficiency, and can obtain the key characteristics of the flow field and sound field simultaneously. It can improve the basic understanding of the vortex sound mechanism of cavity noise. At the same time, it provides a reliable model method with satisfactory efficiency, which can be readily extended to the corresponding engineering applications. INTRODUCTION With the increase of the speed of high-speed trains, the noise pollution problem gets more and more attention from the public, which has become an important limiting factor for the development of high-speed trains. In general, the main noise sources of high-speed trains include traction noise, wheel-rail noise and aerodynamic noise, and the last component will become dominant when the vehicle speed reaches 300 km/h (Thompson et al., 2015). Among the various aerodynamic noise sources, the noise from the innercoach space (which can be regarded as a rectangular cavity) is very complicated. When the grazing flow passes the surface of the inner-coach space of the running train, selfexcited oscillation (acoustic resonance (Gloerfelt, 2009)) can occur, due to the interaction between the shear flow outside the cavity and the acoustic wave in the cavity. This is a complex coupling phenomenon between the flow and the sound field. It has attracted enough attention from designers in many engineering applications, such as inner-coach space of high-speed train automobile sunroof, military bomb bay and wheel well of civil craft, therefore researches are greatly motivated to explore the underlying mechanism of this phenomenon. It is generally believed that the physical mechanism of acoustic resonance is that separation occurs when the flow reaches the sharp leading edge of the cavity, forming a shear layer with inherent instability. Frequency lock-in will occur when the oscillation frequency of the shear layer is consistent with the natural acoustic mode frequency of the surrounding cavity. The oscillation frequency of the shear layer does not change with the flow velocity within a certain speed range. At the same time, this phenomenon exacerbates the energy exchange between vortex and sound, resulting in large unsteady pressure fluctuation. In short, the excitation of the shedding vortex is the main cause of the acoustic resonance of the cavity. There are four main stages in the process of cavity noise generation: (1) due to the instability of the shear layer, the induced vortex disturbance is continuously increased, forming a non-uniform eddy current motion; (2) the vortex and rigid wall interaction generates pressure disturbance, causing the pressure field to be uneven; (3) The pressure disturbance propagates forward in the low velocity region of the cavity, and radiates energy to the space in the form of noise outside the cavity; (4) under the influence of pressure disturbance, the vortex shedding at the leading edge of the cavity is enhanced, and the initial shear layer disturbance is further increased. The semi-empirical equation first proposed by Rossiter (1964) relies on the selection of empirical constants. Most of these methods (Bilanin et al., 2012; Heller et al., 1971; Tam et al., 1978) can only estimate the modal frequencies of all possible oscillations in a simple cavity. However, such models cannot predict the amplitude of the oscillating mode and exhibit the mechanism of the acoustic resonance of the cavity. Although, the numerical method,such as DNS and LES (Colonius et al., 1999; Rowley, et al., 2002; Shieh et al.,

A Meshless Model for Cavity Noise Prediction Based on Vortex Sound Method