PASSIVE NOISE CONTROL BY VANE LEAN AND SWEEP

B. Elhadidi

University of Notre Dame Notre Dame, IN 46556, USA

Abstract A model is developed for modelling the interaction of unsteady three-dimensional incident disturbances with an annular cascade of unloaded swept and leaned vanes. It is shown that the effectiveness of lean or sweep depends on the number of propagating modes and their radial distribution. Results show that vane lean and sweep can be used effectively for certain design configurations to reduce the radiated sound power. However, vane lean and sweep are not effective for all design configurations and for certain cases, they may result in increased radiated sound power.

Keywords: Turbomachinery noise, noise control, sweep, lean.

1. Introduction

Engine noise at approach or take-off, is one of the most critical components of total aircraft noise. For modern day large by-pass turbofan, the major noise contributor is that generated by the interaction of rotor wake with the stator vanes, known as fan noise. There are two major noise reduction techniques: active and passive noise reduction. Active suppression, such as boundary layer suction, has been successful, however, it implies delicate and expensive control systems. Passive suppression are relatively simple since they are incorporated in the fan design.

All passive noise reduction techniques rely on design experience. Classi­cally the number of rotor blades and stator vanes are selected such that the dominant propagating mode is cut-off for the first blade harmonic (Tyler and Sofrin, 1962). Also, duct liners can be used to reduce the noise, by attenuat­ing the propagating modes. However, duct splices used to connect the liners together, increases the number of attenuated propagating modes in the duct, re­ducing the liner efficiency considerably (McAlpine et al., 2003). Experimen­tal evidence suggests that increased rotor/stator spacing is an effective noise reduction mechanism (Gliebe, 1993, Groeneweg et al., 1991). However, the excess engine weight penalty, and extra external aerodynamic drag may af-

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K. C. Hall et al. (eds.),

Unsteady Aerodynamics, Aeroacoustics and Aeroelasticity of Turbomachines, 233-245. © 2006 Springer. Printed in the Netherlands.

fect the overall reduction gains. Furthermore, the authors have shown (to be published), that increasing the rotor/stator spacing will result in noise reduc­tion only if the swirling mean ft>w is hydro-dynamically stable. Vane lean and/or sweep are other effective techniques in noise reduction (Woodward R. and Berton, 1999, Envia and Nallasamy, 1999). More recently, rotor blade sweep has been experimentally examined as a means for noise reduction (Hei­delberg, 2003).

In this paper we investigate noise reduction by stator vane lean or sweep for the case of a constant uniform axial mean fbw. Previously, investigators calculated the unsteady vane response to incident vortical disturbances using a lifting surface formulation (Schulten, 1982, Schulten, 1997), or strip-theory coupled to duct propagation codes (Envia and Nallasamy, 1999). Schulten con­cluded that lean is not effective whereas Envia and Nallassamy showed results where lean is effective, if it is applied in the opposite direction to the rotor rotation. Also, Schulten predicted that vane sweep is effective for large sweep angles, whereas, Envia and Nallassamy concluded that any sweep angle will be effective. We attempt to examine the previous conclusions, by examining the modal composition of the propagating sound.

In the present paper we develop a three-dimensional numerical analysis to examine the effects of lean or sweep. We begin by formulating the problem. Results are then presented examining the efficiency of lean or sweep in reduc­ing the propagating sound power, and then conclude the paper.