“Synchronized Large-Eddy Simulations to track Native Perturbations in a Turbulent Jet”
Jet noise prediction and control are of great interest to civil and military aviation because of health hazards, associated higher medical expenses and stricter noise regulations at airports. Decades of research has greatly improved understanding of noise generation dynamics, but significant gaps remain. In this work, a novel method is proposed to analyze the non-linear processes by which turbulent fluctuations in the jet are processed to yield the near acoustic field. The approach effectively tags perturbations at desired regions (windows) in the jet, so they may be tracked as they are processed by the turbulent environment. For this, we adopt synchronized Large Eddy Simulations (LES) in which two simulations are performed in lock-step manner. At each step, native perturbations are generated in the chosen window from the first (or baseline simulation), scaled to small values and then injected into the second (or twin simulation) to provide a perturbation-boost in the targeted region. At subsequent times, the difference between the two simulations provides a snapshot of the evolution of the perturbation-boost field associated with the chosen window. This field is then statistically analyzed to identify the mechanisms by which turbulence filters and modulates the signal to create the near field acoustic signature. Results are described for a 1.3 Mach cold jet with perturbation-boosts at different locations in the jet plume. Turbulence is seen to be processed in ways that are characteristic of the location of the perturbation-boost. The end of the potential core is found to be a sensitive zone where perturbations are amplified. Sources farther from the nozzle exit have relatively more upstream effect. Perturbations within the initial lip line shear layer are stronger, and are initially channeled towards the core before propagating outward. Statistical analysis is presented to quantify the presence of intermittent events which have a major role in creating the near field sound signature using forward correlation techniques starting from the location of the perturbation-boost.
53rd AIAA Aerospace Sciences Meeting, AIAA SciTech 2015