Aerodynamic noise of high-speed train bogies

Abstract

For high-speed trains, aerodynamic noise becomes significant when their speeds exceed 300 km/h and can become predominant at higher speeds. Since the environmental requirements for railway operations will become tighter in the future, it is necessary to understand the aerodynamic noise generation and radiation mechanism from high-speed trains by studying the flow-induced noise characteristics to reduce such environmental impacts. The aim of this thesis is to investigate the flow behaviour and the corresponding aeroacoustic mechanisms from high-speed trains, especially around the bogie regions. Since the prediction of the flow-induced noise in an industrial context is difficult to achieve, this study focuses on scale models with increasing complexity. The aerodynamic and aeroacoustic behaviour of the flow past an isolated wheelset, two tandem wheelsets, a simplified bogie and the bogie inside the cavity with and without the fairing as well as considering the influence of the ground are investigated at a scale 1:10. A two-stage hybrid method is used consisting of computational fluid dynamics and acoustic analogy. The near-field unsteady flow is obtained by solving the Navier-Stokes equations numerically through the delayed detached-eddy simulation and the source data are applied to predict the far-field noise signals using the Ffowcs Williams-Hawkings acoustic analogy. All simulations were run with fully structured meshes generated according to the guidelines based on a grid independence study on a circular cylinder case. Far-field noise radiated from the scale models was measured in an open-jet anechoic wind tunnel.

Good agreement is achieved between numerical simulations and experimental measurements for the dominant frequency of tonal noise and the shape of the spectra. Numerical results show that turbulent flow past the isolated wheelset is dominated by three-dimensional vortices. Vortex shedding around the axle is the main reason for the tonal noise generation with the dominant peak related to the vortex shedding frequency. The noise directivity shows a typical dipole pattern. Moreover, for both the tandem-wheelset and the simplified bogie cases, the unsteady flow developed around them is characterized by the turbulent eddies with various scales and orientations including the coherently alternating shedding vortices generated from the upstream axles. The vortices formed from the upstream geometries are convected downstream and impinge on the downstream bodies, generating a turbulent wake behind the objects. Vortex shedding and flow separation as well as interaction around the bodies are the key factors for the aerodynamic noise generation. The radiated tonal noise corresponds to the dominant frequencies of the oscillating lift and drag forces from the geometries. The directivity exhibits a distinct dipole shape for the noise radiated from the upstream wheelset whereas the noise directivity pattern from the downstream wheelset is multi-directional.

Compared to the wheelsets, the noise contribution from the bogie frame is relatively small. Furthermore, when the bogie is located inside the bogie cavity, the shear layer developed from the cavity leading edge has a strong interaction with the flow separated from the upstream bogie and cavity walls. Thus a highly irregular and unsteady flow is generated inside the bogie cavity due to the considerably strong flow impingement and interaction occurring there. Unlike the isolated bogie case, noise spectra from the bogie inside the cavity are broadband and a lateral dipole pattern of noise radiation is generated. The noise prediction based on the permeable surface source is formulated and programmed using the convective Ffowcs Williams-Hawkings method. Results show that the bogie fairing is effective in reducing the noise levels in most of the frequency range by mounting a fairing in the bogie area; and for the bogie inside the bogie cavity with the ground underneath, the far-field noise level is increased due to more flow interactions around the geometries and the ground reflection effect.

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