Forced response analysis has become commonplace for predicting the vibration amplitude of turbomachinery blading. These analyses are usually limited because they rely on predicting a well defined source of flow distortion, such as blade wakes and shocks etc. However, the sources of excitation of civil fans are not well defined and yet are able to produce high levels of force. The objective of the work described in this paper is to investigate the forced response of a large civil fan assembly using CFD. An unsteady, time accurate, 3D CFD model of the complete low pressure compression system has been used to calculate the modal response of a large civil fan. The mesh consists of the ground plane, intake, fan, OGV, bypass duct and compressor inlet stator, with every aerofoil passage modelled. The analysis tool allows calculation of a time history of modal response for a range of modes simultaneously to provide a description of the overall vibration behaviour. The results of the analyses have been used to investigate the modal contributions to the off-resonant first engine order response at a range of operating conditions to assess the contribution of various geometric features. The response is shown to compare well with measured strain gauge data for both ground and altitude conditions. The response of the majority of resonances was found to be heavily influenced by the presence of the ground plane, which is consistent with the available experimental data.
Modal response analysis of non-uniform and asymmetric DL under plane-wave illumination
