Investigation of Acoustic Modes Generated by Rotor-Stator Interaction

This paper describes characteristics of the unsteady loadings on the stator surface generated by fan rotor-stator interaction and acoustic modes in a fan duct. Three-dimensional unsteady CFD analysis and three-dimensional linear theory method were performed to predict the unsteady loadings, and the both results agree well. The remarkable point is that the high amplitude areas arrayed in the span-wise direction on the mid-chord were observed. It can be shown that three-dimensional linear theory, which is useful for parametric study due to an advantage of less calculation time, has a potential to predict the characteristics of the unsteady loadings, although it includes some assumptions. Through parametric study, it is found that the high amplitude areas arrayed on the mid-chord are amplified suddenly as parameters satisfy a condition. The parameters were identified, and it comes to be clear that the characteristics of the unsteady loadings are much influenced by the high amplitude areas and their amplification. It is also found that as the rotor-stator spacing is stretched, the amplitude of the unsteady loadings does not attenuate monotonously, but attenuates with oscillating. Not only the amplitude of the unsteady loadings but also the array of them on the mid-chord affects the acoustic power levels, because it makes the specific radial mode dominant. Up to 10dB difference can be found whether the dominant mode is “cut-on” or “cut-off”. It can be said that to understand the characteristics of the unsteady loadings on the mid-chord is very important to reduce and control the fan noise generated by rotor-stator interaction.