Maastricht, The Netherlands The Detection of Radiated Modes From

The bypass duct of an aircraft engine is a low-pass lter allowing some spinning modes to radiate out-side the duct. The knowledge of the radiated modes can help in noise reduction, as well as the diagno-sis of noise generation mechanisms inside the duct. We propose a nonintrusive technique using a circular microphone array outside the engine measuring the complex noise spectrum on an arc of a circle. The ar-ray is placed at various axial distances from the inlet or the exhaust of the engine. Using a model of noise radiation from the duct, an overdetermined system of linear equations is constructed for the complex amplitudes of the radial modes for a xed circum-ferential mode. This system of linear equations is generally singular, indicating that the problem is ill-posed. Tikhonov regularization is employed to solve this system of equations for the unknown ampli-tudes of the radiated modes. An application of our mode detection technique using measured acoustic data from a circular microphone array is presented. We show that this technique can reliably detect ra-diated modes with the possible exception of modes very close to cut-o