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

This paper continues an earlier companion paper in which the results of act ive control computer simulations aimed at assessing the potential for reduc ing the fan tones radiated from the inlet of turbofan engines were presente d. The earlier study found that good levels of reductions in the sound powe r transmitted and sound pressure radiated towards the sidelines could be ob tained by using error sensors located in the radiated far field. The presen t paper is also concerned with these control objectives, but with the impor tant difference that the error sensors are located on the duct wall. The er ror 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 sin gle axial array and summing, an estimate of the mode amplitudes versus axia l propagation angle can be obtained. This in-duct pressure measurement can be related to the far-field radiation pattern. Practical issues are also ad dressed such as the influence on the array's performance of the in-duct bou ndary layer and interference from cut-off modes. The basis for the techniqu e is the close relationship between the in-duct ray-mode angles and those o f the far-field peak pressure lobe of the radiated field. (C) 1999 Acoustic al Society of America. [S0001-4966(99)03907-7].