###### Abstract  
The interior noise of a high-speed train due to the external flow disturbance is more than ever a major problem for product developers to consider during a design state. Since the external surface pressure field induces wall panel vibration of a high-speed train, which in turn generates the interior sound, the first step for low interior noise design is to characterize the surface pressure fluctuations due to external disturbance. In this study, the external flow field of a high-speed train cruising at a speed of 300 km/h in open-field and tunnel are numerically investigated using high-resolution compressible LES (large eddy simulation) techniques, with a focus on characterizing fluctuating surface pressure field according to surrounding conditions of the cruising train, i.e., open-field and tunnel. First, compressible LES schemes with high-resolution grids were employed to accurately predict the exterior flow and acoustic fields around a high-speed train simultaneously. Then, the predicted fluctuating pressure field on the wall panel surface of a train was decomposed into incompressible and compressible ones using the wavenumber-frequency transform, given that the incompressible pressure wave induced by the turbulent eddies within the boundary layer is transported approximately at the mean flow and the compressible pressure wave propagated at the vector sum of the sound speed and the mean flow velocity. Lastly, the power levels due to each pressure field were computed and compared between open-field and tunnel. It was found that there is no significant difference in the power levels of incompressible surface pressure fluctuations between the two cases. However, the decomposed compressible one in the tunnel case is higher by about 2-10 dB than in the open-field case. This result reveals that the increased interior sound of the high-speed train running in a tunnel is due to the compressible surface pressure field. a +
Footnote †: journal: Applied Sciences  
techniques can be applied either in scaled models in wind tunnels or full-scale running trains [5; 6; 7; 8]. However, experimental results obtained from scaled models have difficulty in accurately representing real trains or actual operating conditions. By the way, full-scale testing under actual conditions can provide accurate information, but is unlikely to be possible in the design process. Furthermore, relying entirely on this late and expensive approach can add significant development costs and time to dealing with noise issues. For this reason, accurate numerical prediction of aerodynamic noise is very valuable in that it allows for noise assessment and improvement of full-scale trains at an early stage, sufficient to influence the design. Recently, numerical studies [9; 10; 11; 12; 13] were carried out on the aerodynamic noise of high-speed trains. In general, it has been reported that the first bogie, the pantograph, and the inter-coach space, are the primary aerodynamic noise sources of high-speed trains. However, these studies used a simplified body model or only a single part. [PERSON] et al. [14] performed hybrid computational aeroacoustic simulations on a full-scale geometry of a high-speed train to estimate the flow-induced noise contribution radiated to the far-field. However, this study focused only on externally radiated noise, which is relevant to rail transportation community noise. The interior noise of a high-speed train due to external flow disturbance is also one of the critical issues for product developers to consider during a design state. The external surface pressure field induces the vibration of the wall panels of a high-speed train, and this vibration generates the interior sound. Therefore, it is the first step for low interior noise design to characterize the surface pressure fluctuations. The fluctuating surface pressure field consists of two distinct components: incompressible (or hydrodynamic) and compressible (or acoustic) pressure fields.