Downstream Vane Research Articles

Discrete frequency noise reduction modeling for application to fanjet engines

A model for the generation of discrete frequency noise due to rotor–stator interactions in a fanjet engine is examined to ascertain its relevance to noise reduction studies. Predictions are obtained for the noise reduction between a base fan stage and one redesigned to achieve reduced discrete frequency noise utilizing response functions corresponding to incompressible flow isolated airfoil transverse and longitudinal gust analyses as well as incompressible and compressible flat plate airfoil cascade transverse gust analyses. The correlation of the various predictions with the actual fan test results demonstrated that the viscous wake-generated noise model utilizing the compressible flow cascade response function was the one most appropriate for noise reduction studies of fanjet engines. The significant parameters and their relative importance were then identified by means of a parametric study of the base fan based on the compressible flow cascade response analysis. This study showed that the dominant parameter relevant to discrete frequency noise generation is the interblade phase angle, equal to the ratio of the number of upstream rotor blades to downstream stator vanes. Other parameters of importance included the Mach number, reduced frequency, and vane solidity.

Reducing Blade Tonal Noise Generated by Vane-Axial Fans

An experimental investigation was made to determine the effectiveness of unequally spaced blades and blade/vane interaction techniques in reducing the pure-tone aerodynamic noise generated by vane-axial fans. Balanced mirror-image and other modulated blade patterns were investigated for 13, 19, 20, and 21 blades in combination with nine equally spaced downstream stator vanes. In-duct discrete-frequency noise analyses were obtained. Blade-passing frequency and its harmonics were modulated and reduced 10–20 dB, using unequally spaced blades. Large spacing deviation had the effect of spreading the sound energy to more harmonics of rotational frequency. In some instances, significant downstream stator vane and upstream strut frequencies were observed. Predictable pure-tone multiples of rotational frequency were also detected. Elimination of inlet strut frequency and large reductions at the harmonics of blade-passing frequency were obtained upon replacement of the inlet bell and stationary nose cone and eight support struts with an inlet bell and spinning nose cone. Unequally spacing the blades of the fan had little effect on the ability of the fan to move air.

Experimental Noise Studies of Inlet Guide Vane-Rotor-Stator Interactions for a Single-Stage Axial-Flow Compressor

Farfield noise-radiation patterns for the rotor alone, the inlet guide vane and rotor, the rotor and downstream stator vanes, and the inlet guide vane-rotor-stator combination are compared for a constant rotor speed and mass flow. For the configurations studied, the radiated-noise levels due to the interactions of rotor and downstream stator vanes are appreciably less than those clue to inlet guide vane and rotor interactions. It has been possible to correlate variations in the noise levels, with a knowledge of local conditions in the vicinity of the blades.