Experimental investigation of the near-field pressure fluctuations of a compressible round jet

The aircraft noise is one of the main problems that affect the diffusion of the air transport near the downtown. For this reason in the past 50 years even more studies were performed for the comprehension and reduction of the noise generation phenomena. An experimental study of the pressure field generated by a subsonic, single stream, round jet is presented. This work is focalized on the near-field characterization; in fact the near-field is not well described as the far-field but it is important to obtain information on the noise sources. Near-field measurements are affected by some pressure fluctuations that seems sound; these fluctuations are usually defined as pseudo-sound. The pseudo-sound was defined in the past as undistinguishable by the sound and it hides the similarities between near-field and far-field jet noise characteristics. The main aim of this work is the understanding of the near-field noise properties separating the sound from the pseudo-sound contribution for a compressible round jet. The filtering algorithm proposed is based on wavelets and it is a novel approach in aero-acoustics studies. The wavelet filtering capture the intermittent events of a signal, these events are, in the pressure fluctuations, related to the coherent structures convected by the fluid.