Abstract:  After a brief review of the status and relevance of jet noise, we discuss wave-packet structures that are observed in the near pressure field of turbulent jets and their relation with the radiated acoustic field.   These wavepackets have been observed for many years,  but their importance to the far-field noise is still debated, particularly in subsonic jets.  We discuss a theoretical framework for modeling the wave packets based on the mean velocity field, and, in particular, review results from linear Parabolized Stability Equations (PSE) models.  The models generally provide quantitative agreement with the spatial distribution of wave packet amplitude, wavelength and phase speed inferred from near-field microphone measurements and time-resolved particle image velocimetry.  We also consider new parabolizations of the Euler and Navier-Stokes equations that remedy some flaws in the PSE approach.  In particular, we present a novel framework for parabolization that is based on techniques previously developed for nonreflecting boundary conditions.  The new technique may prove valuable in multiple applications.  For turbulent jets, they are able to directly represent the radiated acoustic field associated with modal solutions of the linearized equations of motion, and may in future provide direct, computationally efficient models for the sound radiated by turbulent jets, and strategies for its control. Bio:  Tim Colonius is Professor of Mechanical Engineering at the California Institute of Technology.  He received his B.S. from the University of Michigan in 1987 and M.S and Ph.D. in Mechanical Engineering from Stanford University in 1988 and 1994, respectively.   He joined the Caltech faculty in 1994, where he and his group develop and use numerical simulations to study a range of problems in fluid mechanics, including aeroacoustics, flow control, instabilities, and bubble dynamics.   He is a Fellow of the American Physical Society and has held visiting positions at Cambridge University and the University of Poitiers.   Host: Francesco Bullo