Active control of fan tones radiated from turbofan engines.: II. In-duct error sensors

Joseph, P., Nelson, P. A. and Fisher, M. J. (1999) Active control of fan tones radiated from turbofan engines.: II. In-duct error sensors. Journal of the Acoustical Society of America, 106 (2), 779-786. (doi:10.1121/1.427608).

Abstract

This paper continues an earlier companion paper in which the results of active control computer simulations aimed at assessing the potential for reducing the fan tones radiated from the inlet of turbofan engines were presented. The earlier study found that good levels of reductions in the sound power transmitted and sound pressure radiated towards the sidelines could be obtained by using error sensors located in the radiated far field. The present paper is also concerned with these control objectives, but with the important difference that the error sensors are located on the duct wall. The error sensor array described in this paper is comprised of a number of axial line array of discrete sensors uniformly distributed around the duct wall. By applying the appropriate phase delay between the sensor signals in a single axial array and summing, an estimate of the mode amplitudes versus axial propagation angle can be obtained. This in-duct pressure measurement can be related to the far-field radiation pattern. Practical issues are also addressed such as the influence on the array's performance of the in-duct boundary layer and interference from cut-off modes. The basis for the technique is the close relationship between the in-duct ray-mode angles and those of the far-field peak pressure lobe of the radiated field.

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Contributors

Author: P. A. Nelson

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