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

Abstract

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. The jet noise has well-known characteristics; many acoustic models exist and well predict the jet far-field noise but actually not all the noise causes are known. Some theories assign the noise production to the large scale structures, other to the fine scales, to the pressure fluctuations or to the turbulent interactions; the main topic of the aeroacoustics research is the identification of the main phenomena that produce noise. 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 studying the near-field is important to obtain information on the noise sources. Near-field measurements are affected by some pressure fluctuations that, measuring with a microphone, 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. This approach gives also the capability of splitting the pressure signal in two components and to correlate them with velocity information to locate phenomena that contribute to the sound generation. In this work a nozzle is designed to generate a standard compressible jet with the capability to work between Mach 0.3 and Mach 1. The nozzle is installed in a semi-anechoic chamber and experimental studies were performed for qualifying the flow characteristics and the far-field noise. Near-field pressure fluctuations are studied using two synchronized microphones and the signals are filtered using the wavelet approach. The sound and pseudo-sound are studied to compare the near-field and the far-field jet characteristics. Hot-wire measurements are performed in synch with sound measurements to locate the jet zones where the maximum noise production is located. Acoustic hot-wire perturbations are evaluated as negligible and the velocity-noise correlations give interesting results. The analysis shows that there are common characteristics between near and far-field noise. The sound in the near-field has clear directivity that depends on the wave-lengths magnitude. The shape of the sound spectra has also strong directional dependence and has similarities with the far-field; pseudo-sound instead has no directional dependence of the spectra that only depend on the distance from the jet. Both sound and pseudo-sound source zones are identified and match with previous works.