  Citation: Meloni, S.; Jawahar, H.K. A Wavelet-Based Time-Frequency Analysis on the Supersonic Jet Noise Features with Chevrons. Fluids 2022, 7, 108. https://doi.org/10.3390/ fluids7030108 Academic Editor: Mehrdad Massoudi Received: 11 February 2022 Accepted: 11 March 2022 Published: 16 March 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). fluids Article A Wavelet-Based Time-Frequency Analysis on the Supersonic Jet Noise Features with Chevrons Stefano Meloni 1, * and Hasan Kamliya Jawahar 2 1 Department of Engineering, University of Roma Tre, 00146 Rome, Italy 2 Department of Aerospace Engineering, University of Bristol, Bristol BS8 1TR, UK; hasan.kj@bristol.ac.uk * Correspondence: stefano.meloni@uniroma3.it Abstract: A detailed investigation of the statistical properties of the near-field pressure fluctuations induced by an under-expanded jet, by varying the nozzle exit shapes has been presented. Experiments using different convergent Chevron nozzles were carried out in the anechoic chamber at the University of Bristol to assess the importance of the Chevron shape on the near pressure field emitted by a single stream under-expanded jet. Measurements were carried out through an axial microphone array traversed radially to various positions for jet in an under-expanded condition at Mach number M = 1.3. The intermittent behavior is investigated considering the standard statistical indicators and a wavelet-based conditional approach, including the phase angle. The intermittent degree of various features related to different scales, such as Screech tones and broadband shock associate noise were estimated. A series of recently developed wavelet-based tools were assessed as a viable approach to investigate the noise emitted by under-expanded jets. Keywords: aeroacoustic; jet noise; chevron nozzles; near-field 1. Introduction One of the key aircraft noise sources that should be taken into account during the design of modern aircraft is the jet noise which dominates the take-off and the cruise phase [1]. The generation of sound by a jet exiting into a fluid medium is of great interest for several aeronautical applications and it has long been recognized by several previous studies [2,3]. A large body of literature has clarified that the dominant noise source in the subsonic case is the turbulent mixing nose, which is generated by the large-scale turbulence structures/instability waves of the jet flow [4,5]. During takeoff, when a high level of thrust is required, jet exiting flow from the engine’s nozzle is under-expanded. This could also happen during the cruise phase due to the combined effect of low external static pressure and required thrust to maintain the flight Mach number in maneuvers or changing the lane [6,7]. An under-expanded jet plume is characterized by a shock cell train, which creates a series of compression and expansion into the flow, generating a further shock- associated noise [7–9]. This form of noise generated by non-ideally expanded supersonic jet comprises of two components: Screech tones and Broadband Shock Associated Noise (BBSAN). Screech tones are discrete tones that originate from an acoustic feedback loop between the shock cell train and the nozzle lip [10]. Screech is unusual among resonance phenomena, in that the resonance is entirely contained within the flow itself [11]. BBSAN, on the other hand, is generated from a weak interaction between downstream propagating large scale turbulent structures and the quasi-periodic shock cells in the jet plume. Contrary to Screech, BBSAN is present in both under and over-expanded jets. One of the characteristics of BBSAN (unlike Screech) is that the peak frequency varies as a function of observer position, a phenomenon observed in experimental data. The inclusion of these noise components in the analysis is essential to solve the problem and thus provide tolerable noise levels in the cabin to improve passenger comfort. Fluids 2022, 7, 108. https://doi.org/10.3390/fluids7030108 https://www.mdpi.com/journal/fluids