Jeonglae Kim

Adjoint-Based Optimal Control of a Mach 1.3 Turbulent Jet for Noise Reduction

It has been shown that adjoint-based optimization can be used for an effective control of a turbulent flow via circumventing the complexity in turbulence and directly optimizing the controller 1,2 In this study, the adjoint-based optimization is applied to reduce the sound generated by a Mach 1.3 jet and find a corresponding optimal control. A baseline uncontrolled Mach 1.3 jet is accurately simulated by a high-fidelity numerical scheme and compared well with measurements. An acoustically quieter state of the Mach 1.3 jet is found and analyzed in detail. The optimization is ongoing for more noise reduction. A constrained optimization algorithm is formulated to optimize a controller with constraints in space and time, which can be used to satisfy a specific hardware requirement. I. Introduction Turbulent jet noise has defied a simple mechanistic description making noise reduction a trial-and-error effort. The subtlety of the noise source and the complexity of the flow turbulence that generates the sound is the root cause of this difficulty. We have designed means of using adjoint-based optimization to circumvent this complexity and reduce noise directly. Its application to drag reduction in incompressible boundary layers 2 and improved aerodynamic design 3 has recently been extended to the noise reduction of compressible mixing layers 1,4 and a supersonic jet flow. 5 Our most recent effort focused on developing the adjoint-based optimization algorithm using a high-fidelity numerical scheme based upon an overset grid approach. 5 In this study, the uncontrolled Mach 1.3 jet and its noise mitigation using the adjoint-based optimization are discussed in more detail. A database of the simulated jet, including its near-field turbulence and far-field sound, is established and compared with measurements. The initial noise reduction is modest compared to our previous test, 5 but the optimization is ongoing. In our initial formulation, no constraints were included with the result that the optimal control identified is hard or impossible to realize in hardware, and so serves primarily as a means of investigating noise mechanisms and modeling issues. In the present study, we therefore extend the optimization formulation so that the actuator is constrained so that its space and time character corresponds to specific hardware.

Aeroacoustics Control of a Turbulent Mach 1.3 Jet Using Adjoint-Based Optimization

Past work has shown that adjoint-based optimization can identify perturbations that make unsteady free shear flows substantially quieter. These perturbations are interesting because they offer the opportunity to study noise mechanisms and because they themselves constitute successful noise-reducing controls. Here we apply these techniques to an experimentally validated simulation of a Mach 1.3 pressure-matched turbulent jet. The noise is successfully suppressed. Changes in both the turbulence and acoustic fields are analyzed, including with a proper orthogonal decomposition. The principal effect of the control is the suppression of particularly loud intermittent events. Despite the significant reduction, only small changes in the jet structure are observed.

Effect of Large-Eddy Simulation Fidelity on Predicted Mechanisms of Jet Noise Reduction

The aeroacoustic control of aMach 1.3 turbulent jet demonstrates an unexpectedly subtle importance of accurate numerics for predicting and reducing turbulent jet noise. We observe that two nominally high-wave-number filters (as used to stabilize large-eddy simulations), one explicit that is optimized based upon an integral criterion and the other implicit with a tunable parameter, lead to significant changes in the near-field turbulence with concomitant changes in the acoustic field on the order of 2–4 dB. An apparent consequence is that an adjoint-based optimization procedure identifies different controls for the two simulated jets despite similar reductions of radiated sound. This suggests that sound-generating mechanisms of the Mach 1.3 turbulent jet may be more fundamentally modified by subtle changes in numerics than previously expected.

Mechanisms of Jet Noise Reduction and Their Impact on Large-Eddy Simulations (invited)

Recent work by the authors on turbulent jet control of a Mach 1.3 turbulent jet provides evidence of the importance of accurate and well understood numerics and their effects on reducing turbulent jet noise. We observe that two nominally high-wavenumber filters, one exp licit that is based on an optimized dispersionrelation-preserving expression and the other implicit with a tunable parameter, lead to significant changes in the near-field turbulence without showing concomitant ch ange in the acoustic field on a decibel scale. An apparent consequence is that an adjoint-based controller acts differently on the two simulated jets.

Super-Resonances in AEDC Altitude Test Cells

We report on simulation and analysis efforts to understand and predict the high amplitude aeroacoustic resonances (& 160dB) that have been observed in Arnold Engineering Development Center (AEDC) jet engine test cells. The basic configuration of these cells is a jet exhausting through a cylindrical duct. We summarize progress in developing analytical tools to diagnose the energetics of the resonating acoustic modes, direct numerical simulations to investigate the phenomenology of the resonance in model geometries, and full-scale simulation tools to anticipate resonances.