Adaptation of the Azimuthal Decomposition Technique to Jet Noise Measurements in Full-Scale Tests

To develop jet noise-reduction concepts, it is necessary to understand the physics underlying the noise-generation process. Decomposition of the sound field into azimuthal modes is one of the effective experimental methods that allows extracting subtle features of different types of noise sources. For turbulent flows, the interpretation of the noise analysis results using the azimuthal decomposition technique was developed by the authors in previous papers. In the present paper, a generalization of this method allowing the decomposition of the jet acoustic field into azimuthal modes by means of reduced number of microphones is developed. It is shown that jet noise measurements by only three microphones in each cross section allow reconstruction of three azimuthal mode directivities (axisymmetric, first, and second) for both low and moderate frequency bands. Furthermore, two-microphone measurements, provided the microphones are properly located, make it possible to reconstruct directivity of the axisymmetric mode and total directivity of the first and second modes. The latter method is shown to be suitable for using with an outdoor test bench. The modified methodology is first validated on a small-scale laboratory database. Next, it is applied to the jet of a real engine in ground tests. The information about the azimuthal modes properties may be used to investigate physical aspects of the large-scale jet noise sources.