EXPERIMENTAL INVESTIGATION OF COMPRESSIBLE AND INCOMPRESSIBLE JET AEROACOUSTICS IN FREE AND INSTALLED CONFIGURATIONS THROUGH ADVANCED TIME-FREQUENCY ANALYSIS

Abstract

In the present work advanced time-frequency analysis techniques have been applied to experimental jet aeroacoustic data. The study in the time and frequency domains has been carried out by standard Fourier analysis and innovative wavelet-based procedures. The objective of the thesis project was to exploit the potentialities of time-frequency analysis approach to get a better understanding of the jet physics and lay foundations for jet noise modelling. Two experimental applications were taken into account. The first one concerns the decomposition of the near pressure field of a free jet into its hydrodynamic and acoustic components. Simultaneous near-field and far-field pressure measurements on a single-stream compressible jet installed in a fully anechoic chamber were exploited to derive three innovative wavelet-based techniques for the separation of the hydrodynamic and acoustic pressures. The statistical and spectral features of the two components were characterized addressing the effect of the jet Mach number and the spatial location of the near-field microphone. For the first time, a direct link between the separated acoustic pressure in the near field and the actual noise measured in the far field was established, highlighting the different physical nature of the sound and pseudo-sound components. The second application concerns the interaction between an incompressible jet and a surface. Experimental tests were carried out on a simplified laboratory-scale model where a rigid flatplate was installed tangentially to the nozzle axis for different radial distances of the plate from the jet. Simultaneous velocity and wall pressure measurements were performed in order to assess the effect of the plate on the aerodynamic field and to characterize the wall pressure statistics and spectral content. Velocity/pressure cross-statistics was provided in the time and frequency domains as well. Furthermore, the jet flow acceleration was computed starting from the measured velocity field. The acceleration field was characterized in both free and installed jet conditions. Cross-correlations and cross-spectra between acceleration and wall pressure signals were investigated as well. Finally, a conditional sampling procedure based on wavelet transform was applied to the database in order to characterize the coherent flow signatures related to the velocity/acceleration and wall pressure fluctuations underlying the jet-plate interaction phenomena.