Sound Generation and Propagation in a System Consisting of Two Periodic Rows of Channels

The aim of this work is to improve the design of the cooling fan system integrated in the traction motors of trains in order to reduce the aerodynamic noise. The latter is the dominant noise contribution at high rotation speeds. The use of numerical simulations to predict the noise remains very expensive, especially when several geometrical parameters must be tested. The analytical methods are better suited at the early design stage due to the very low computational time compared to numerical simulations. The methodology is to split the system into generic components addressed separately. The present work is focussed on the system consisting of guide vanes and cooling channels. This system can be considered as two periodic rows of channels separated by a small distance. The first mechanism investigated in this study is the sound generation by the impingement of the wakes of an upstream impeller with circumferentially unsymmetrical blade-spacing on the guide vanes. The second one is the transmission of the acoustic waves generated by the latter through a row of thick-walled channels. These waves are partially reflected by the channels, generating an upstream field. Back-and-forth acoustic waves develop between the channels and guide vanes. The sound propagation in this complex geometry is achieved by the use of two transmission models which take into account the wall thickness. A two-dimensional analytical approach is used, based on the mode-matching method. The modal expressions are written in each domain and matched according to conservation laws of fluid dynamics. An iterative method is used in order to take into account the multiple reflections of the acoustic waves between the guide vanes and the channels. The influence of various parameters on sound generation and transmission, such as the frequency, the modulation angles of the impeller blades, the dimensions of the guide vanes, those of the cooling channels and the distance between the guide vanes and channels is discussed in this work