Wavelet decomposition of hydrodynamic and acoustic pressures in the near field of the jet

Abstract

An experimental investigation of pressure fluctuations generated by a single-stream compressible jet is carried out in an anechoic wind tunnel. Measurements are performed using a linear array of microphones installed in the near region of the jet and a polar arc of microphones in the far field. The main focus of the paper is on the analysis of the pressure fluctuations in the near field. Three novel signal processing techniques are presented to provide the decomposition of the near-field pressure into hydrodynamic and acoustic components. The procedures are all based on the application of the wavelet transform to the measured pressure data and possess the distinctive property of requiring a very simple arrangement to obtain the desired results (one or two microphones at most). The hydrodynamic and acoustic pressures are characterized separately in terms of their spectral and statistical quantities and a direct link between the acoustic pressure extracted from the near field and the actual noise in the far field is established. The analysis of the separated pressure components sheds light on the nearly Gaussian nature/intermittent behaviour of the acoustic/hydrodynamic pressure. The higher sensitivity of the acoustic component to the Mach number variation has been highlighted as well as the different propagation velocities of the two pressure components. The achieved outcomes are validated through the application to the same data of existing separation procedures evidencing the advantages and limitations of the new methods

On the hydrodynamic and acoustic nature of pressure proper orthogonal decomposition modes in the near field of a compressible jet

“…As expected, the first POD modes, that are the most energetic ones, exhibit a hydrodynamic nature. It is interesting to underline that the number of the acoustic modes is larger for M j = 0.9, this result being related to the higher sensitiveness of the acoustic pressure to the jet Mach number variation (Mancinelli et al 2017b). FIGURE 6.…”

“…For a more detailed description of the experimental set-up and a qualification of the jet facility the reader may refer to Mancinelli et al (2017b).…”

“…Indeed, a microphone in the near field senses the fluctuations related to the non-radiating hydrodynamic pressure and the perturbations associated with the propagating acoustic pressure (Howes 1960). The necessity of separating the pseudo-sound and sound components led to the development of different approaches (see Arndt et al 1997;Tinney & Jordan 2008;Grizzi & Camussi 2012 and the recent papers by Mancinelli et al 2016Mancinelli et al , 2017b. In this work the hydrodynamic or acoustic nature of the POD modes is assessed through the computation of the wavenumber-frequency spectrum of the near pressure field re-constructed using each POD mode.…”

Free-Stream Effects on Jet-Installation Noise of a Dual-Stream Engine

“…In this section, a decomposition technique based on the wavelet transform is used to assess the phenomena behind the different noise characteristics with respect to the presence of free-stream. This technique has been proposed by Mancinelli et al [31] with the aim to separate the coherent part in a flow from that with a more chaotic characteristic. The coherent flow structures, in the form of instability waves, display strong hydrodynamic behavior, whereas the chaotic ones are usually linked to acoustic fluctuations.…”

“…The aim of this analysis is to remove the contribution of the scattered waves in the near-pressure field so that only the hydrodynamic content from the quadrupoles is assessed. The technique applied in this work is based on a recursive de-noising procedure (WT3), in which the acoustic pressure field is iteratively evaluated until a convergence criterion is satisfied, based on the wavelet coefficients being higher or lower than a threshold [31]. A default set of wavelet (Daubechies-12 type) and a unitary threshold coefficient are used for the decomposition of the near-field pressure around the dual-jet.…”