Modelling Fan Noise Produced by Downstream Mounting Structural Blockage

摘要

Mounting structures located downstream of turbofan engine vanes create flow blockage with circumferential and radial non-uniformities, which extend upstream and interact with the fan blades. A model is developed to calculate the noise, dubbed blockage noise, resulting from this interaction mechanism. The flow upstream of the structural blockage and the guide vanes is numerically calculated. It is shown that upstream of the vanes, flow non-uniformities are dominated by the pressure and hence can be decomposed in terms of the duct modes. These modes are seen as rotating but decaying modes in the rotor frame of reference. The acoustic response is calculated using the CAAT-R code. Simple fan configurations with unloaded fans are studied first to validate and create bench mark results for the acoustic power. Results are extended to more realistic fan geometry with loaded blades and nonuniform flow with swirl. The acoustic power level is compared with that of the popular wake defects impinging on guide vanes. Although the level of the blockage non-uniformities is much smaller than those resulting from fan wake defects, acoustic power levels are comparable, indicating that blockage noise is an efficient noise source because of their small radial phase variation and low circumferential modal number.