Design selection and analysis of a swept and leaned stator concept

This paper describes a two-part analytical study of the benefits of stator sweep and lean for reducing rotor-stater interaction tone noise. The first part of the paper describes a design study aimed at selecting a sweep and lean configuration that maximizes noise reductions for a candidate low-noise stator. In the second part, the predicted noise reductions that can be achieved by such a stator are compared with its measured benefits. The results from the first part indicate that the kinematic relationship between the rotor wakes and stator vanes is the principal factor in determining the achievable noise reductions. It is shown that properly chosen sweep and lean enhance wake tilting as seen by the stator vanes. This, in turn, increases the number of wake intersections per vane leading to reduced noise levels. This argument suggests that to reduce noise, sweep and lean must be chosen in such a way that the number of wake intersections per vane is increased. Thus, a simple design rule is proposed for implementing sweep and lean. To achieve significant noise reductions, the rule calls for a sweep configuration for which vane tip is downstream of its root and vane lean that is in the direction of the rotor rotation. In the second part of the study, a detailed assessment of the acoustic performance of the swept and leaned stator is carried out by comparing its predicted and measured noise reductions. Overall, these comparisons show that the predicted benefits of the swept and leaned stator are in good agreement, qualitative as well as quantitative, with the measured reductions for the fan speeds that are relevant to the standard noise certification procedures. Furthermore, the results demonstrate the validity of the design criterion as well as the theoretical tools used in the design and analysis phases of this study. (C) 1999 Academic Press.