Experimental investigation of the near-field noise generated by a compressible round jet

An experimental study of the pressure field generated by a subsonic, single stream, round jet is presented. The investigation is conducted in the near-field region at subsonic Mach numbers (up to 0.95). The main task of the present work is the analysis of the near-field acoustic pressure and the characterization of its spectral properties. To this aim, a novel postprocessing technique based on the application of wavelet transforms is presented. The method accomplishes the separation of nearly Gaussian background fluctuations, interpreted as acoustic pressure, from intermittent pressure peaks induced by the hydrodynamic components. With respect to more standard approaches based on Fourier filtering, the new technique permits one to recover the whole frequency content of both the acoustic and the hydrodynamic contributions and to reconstruct them as independent signals in the time domain. The validity of the method is verified through several tests on measured signals and comparisons against literature data. The near field acoustic pressure is analyzed in terms of spectral content, Sound Pressure Level and directivity and the results are compared to published far-field observations. Simultaneous velocity/pressure measurements have been also performed using a hot wire probe located in the region close to the nozzle exit. The analysis of the velocity/pressure correlations clearly shows that the main noise sources are located close to the end of the potential core while the largest hydrodynamic pressure peaks are generated further downstream.