Low Pressure Compression System Effects on Fan Assembly Forced Response

This paper describes a numerical modelling methodology for fan blade forced response calculations by considering the low-pressure compression system (LPCS) as a whole in order to include flow distortions caused by the asymmetric flight intake upstream, and the pylon downstream. Emphasis also is placed on blade mistuning or mis-placement which may be due to inherent manufacturing and assembly tolerances, or to small inservice displacements. Several levels of geometric complexity were used in the analysis, ranging from an isolated fan bladerow to a complete LPCS of a large-diameter aero-engine, consisting of the intake duct, the fan assembly, the outflow guide vanes, the pylon and a downstream nozzle. The aerodynamic model was coupled to a finite element model of the fan assembly for computing the blade vibration levels. The study revealed two major findings. The first is the unsteady forcing under one engine-order (1EO) excitation is found to be linked to the mean shock position on the fan blade, the highest forcing occurring when the shock is just swallowed since this position is particularly sensitive to pressure fluctuations. The second finding is that the 1EO fan assembly forcing resulting from an asymmetric intake and the pylon are of comparable magnitude but their relative phasing is the key parameter in determining the overall fan forced response levels.