Efficient prediction method for broadband acoustic mode radiation from engine bypass ducts

Abstract

High levels of broadband noise produced by modern high-bypass turbine engines have a significant impact on the environment. High performance computational numerical methods are now taking an active role in this research area. The research presented in this thesis explores a method for efficient prediction of broadband aeroacoustic radiation from a turbofan engine bypass duct and the effect of the multi-mode propagation in the near-field of an engine bypass duct with bifurcations installed on. An accurate and high-order Computational Aeroacoustics (CAA) numerical scheme is used in two and-half and three dimensional linearised Euler equations to determine the results. For far-field predictions, the Ffowcs Williams- Hawkings (FW-H) method and the Acoustic Intensity Based Method (AIBM) could be used to solve the single mode problem. However, the current FW-H method can not be used for multi-mode problems due to its required demand for computational resources. AIBM is an efficient tool to predict the pressure in far-field based on the near-field solution calculated by CAA, and has a potential for multi-mode prediction in the far-field. The performances of the prediction of the radiation of bypass duct acoustics with mean flow have been analysed, with particular attention to the ducts with bifurcations. For the single mode case of the duct acoustics, the AIBM has been implemented and compared against CAA results in the nearfield. Comparison between AIBM and FW-H directivity pattern in the far-field region show good agreement. The clean duct cases for multi-mode are solved with the linearised Euler equations (LEE) in two-and-half dimensions and the results are analysed. For multi-frequency cases, the SPL directivity contour almost matches the pattern obtained by summing the results computed by single frequencies. Lower radial modes contribute more to the overall SPL value than higher ones. For the circumferential modes, lower ones are more likely to cut-on to more discrete frequencies. Finally, the three dimensional solver is used to determine the near-field multi-mode radiation from a generic engine bypass with bifurcations. The bifurcations can cause the acoustic pressure waves to be redirected. Interference between the diffracted modes increased the acoustic pressures. Results show that lower radial modes are smaller in amplitude, and are more likely to cut-on when the radial modes are higher. More complex patterns have formed, because of the bifurcation interference, compared to single mode cases. For different circumferential mode cases, the radiation peak angle increases as the circumferential mode increases.

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