Identification of multipole noise sources in low Mach number jets near the peak frequency

A beam-forming algorithm is developed to identify noise source structures in low Mach number jets near the peak frequency. Pressure data from a conical phased microphone array are Fourier decomposed in the azimuthal direction, and the dominant eigenvectors of a cross-spectral matrix are extracted for each azimuthal mode; subsequently, a robust adaptive beam-forming technique is applied to a cross-spectral matrix reconstructed from the primary eigenmodes. The coherent sound signals are then decomposed into multipoles in spherical coordinates, and the primary mode and its axial position are identified for the axisymmetric and first two azimuthal modes. The proposed beam-forming algorithm is applied to an unheated round jet over a range of Mach numbers and frequencies. The result indicates that at low Mach numbers, the second zenithal mode (i.e., quadrupole) appears to be dominant for each azimuthal mode, and the source position slightly shifts downstream with increasing azimuthal mode number.