Mechanisms of sound generation in a rounded impinging jet: tonal vs broad-band noise

Abstract: It is well known that impinging jets, under particular conditions, may radiate intense tonal noise [1]. In some cases,the sound generation is underpinned by the presence of a self-excited instability originated by a feedback loop between two kind of waves: a downstream-travelling KH wave, which is excited at the nozzle lip and propagates around the jet core position, and an upstream-travelling wave generated by the impingement of disturbances on the plate and propagates backward inside/outside the jet core. The non-localconstructive interaction of such waves gives rise to a series ofself-sustained global in time instabilities whose interactions,in some circumstances, are able to radiate an intense tonalacoustic field. In the presentation we will review the characteristics of the self-sustained mechanism and its noise radiation by using both a global and a local stability analysisthat will precisely identify the components of the feedbackloop. In order to better understand the fundamental physical mechanism leading to tonal (or broadband) sound emission, we propose a novel acoustic-hydrodynamic decomposition of the structural sensitivity tensor (originally developed for spatially localized instabilities) [2], which serves to precisely localize the active regions of the long-range feedback loop (i.e. where the conversion between the two kind of wavestakes place) and which is also suited to study other kind oflong-range instabilities such as thermo-acoustic, hydro-acoustic or thermo-diffusive instabilities. As an example, figures below show the results obtained by the proposed decomposition for a high subsonic case. Motivated by the results of such analysis, we then propose a minimal reduced model that faithfully represents the fundamental properties of the sound emission. More precisely, by applying equivariantbifurcation theory [3], we derive a reduced order model based on a resonant interaction of a series of unstable Hopfbifurcations which qualitatively predicts the characteristics of the sound emission. Possible applications to other problems (cavity flow, flow through apertures and flow past airfoils)and extension to turbulent regime will be briefly discussed.

Bio: Ph.D. in Applied Mathematics and Theoretical Physics, University of Cambridge, U.K.(2003). Professor in Fluid Dynamics, Department of Industrial Engineering (DIIN), University of Salerno, Italy (2023). Research interests: 1) the analysis of the linear and nonlinear development of global and self- excited instabilities with the aid of adjoint equations in wakes and jet flows 2) Aero-acoustics: generation of sound waves by fluid-dynamic instabilities 3) Aerodynamic efficiency of turboprop wings (in the context of European project ESTRO) 4) the study of elliptic and hyperbolic instabilities in the context of bluff-bodies wakes. 5) the study of convective instabilities and their receptivity to different kinds of disturbances. 6) the development of efficient numerical methods for the solution of the Navier-Stokes equations and the solution of large eigenvalue problems. 7) Non-Newtonian Flows.