Article Precursors to self-sustained oscillations in aeroacoustic systems Vineeth Nair 1 and RI Sujith 2 Abstract In this study, we attempt to provide a repertoire of measures to forewarn the onset of impending flow-induced mechanical oscillations via online health monitoring. To illustrate the principles, the flow of air through a pipe terminated by a circular orifice plate is investigated at various flow velocities using a suitably placed pressure transducer. It is observed that the regimes correspond- ing to the production of a tone is presaged by operating conditions that display temporarily intermittent bursts of periodic pressure oscillations that emerge from a background of lower- amplitude aperiodic fluctuations. The various model-free measures prescribed in this paper serve as efficient precursors by characterizing these intermittent states, which can potentially arise in aeroacoustic systems when the flow is highly unsteady or turbulent. Keywords Aeroacoutics, Nonlinear dynamics, Precursors Introduction Pressure fluctuations in unsteady flows are classified as sound or pseudo-sound depending on whether the underlying pressure field is propagating or non-propagating. 1 The pressure variations p 0 in a sound field (acoustic waves) are dependent on the local speed of sound c 0 via p 0  Oð 0 c 0 uÞ, where  0 is the mean flow density and u refers to the typical magnitude of the local flow velocity. On the other hand, the local variations in pressure due to a pseudo- sound field vary as p 0  Oð 0 u 2 Þ, thus independent of the sound speed. When an unsteady flow passes through a confinement, both forms of pressure fluctuations are induced and these fluctuations are characterized by a multiplicity of time scales associated with local unsteadi- ness and acoustic wave propagation. When one of the local hydrodynamic time scales 1 Department of Aerospace Engineering, Indian Institute of Technology Bombay, Mumbai, India 2 Department of Aerospace Engineering, Indian Institute of Technology Madras, Chennai, India Corresponding author: Vineeth Nair, Department of Aerospace Engineering, Indian Institute of Technology Bombay, 400076 - Mumbai, India. Email: vineeth@aero.iitb.ac.in International Journal of Aeroacoustics 0(0) 1–12 ! The Author(s) 2016 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/1475472X16630877 jae.sagepub.com