Dissertation Defense: Jet noise source localization and identification

Committee Members

• Dr. Datta Gaitonde, Chair
• Dr. Mo Samimy
• Dr. Jen-Ping Chen
• Dr. Sandip Mazumder
• Dr. Brenda Henderson

Abstract

The exact mechanism by which jet turbulence is filtered into acoustic motion remains unexplained. The current work aims at localizing acoustic sources in a Mach 1.3 turbulent cold jet and identifying its causal dynamics using Implicit Large-Eddy Simulations. To understand the role of turbulent fluctuations at specific locations in generating the nearfield of the jet, a novel technique termed Synchronous Large-Eddy Simulations (SLES) is developed. This method tracks the non-linear evolution of small perturbations from desired regions (windows) in a time-varying base flow, providing superior insights into the generation of intermittency and directivity, compared to traditional linear stability analyses based on steady basic states. SLES performs two simulations in a lock-step manner, and at each step, native fluctuations are generated in a chosen spatial window from the first (or baseline simulation), scaled to small values and then injected into the second (or twin simulation) to provide a forcing in the targeted region. At subsequent times, the difference between the two simulations provides a snapshot of the evolution of the perturbation field associated with the forcing at the chosen spatial window. The perturbation field, which is equivalent to the solution of the forced Navier-Stokes equations linearized about the time-evolving base flow is then statistically analyzed to identify its filtering and modulation by the turbulent core of the jet. Results are described for the supersonic jet with forcing at lipline and centerline locations. The end of the potential core is found to be a sensitive zone where perturbations are amplified leading to secondary sources. Perturbations within the shear layer are initially channeled towards the core and undergoes higher amplification, before propagating outward. Statistical analyses quantify intermittent events which have a major role in creating the nearfield sound signature and yields polar variation of the most significant frequency band. To identify the acoustic sources and the propagated field of the jet, the flowfield is decomposed into its acoustic, hydrodynamic and thermal modes (which are referred to as the Fluid-Thermodynamic (FT) modes) using the Momentum Potential Theory. The hydrodynamic mode represents the shear layer roll up and turbulent mixing, while the acoustic and thermal modes exhibit a wavepacket nature in the core. The acoustic wavepacket also generates the nearfield radiation of the jet and its spatio-temporal amplifications in the core due to the present of vorticity results in nearfield intermittent events. The acoustic and hydrodynamic modes closely follow the theoretical fall rates and the former possess the features of experimentally observed model sound spectra along the downstream and sideline polar angles. Inter-modal energy transfers in the non-linear flow are analyzed using the transport equation for the universal acoustic variable, Total Fluctuating Enthalpy (TFE). TFE sources identifies intruding vortices in the potential core as the principal mechanism by which acoustic sources are generated in the jet. The acoustic wavepacket and its nearfield fluctuations play a central role in transporting TFE outward from the core, resulting in the perceived sound signature of the jet.