Active Control of Coherent Structures in an Axisymmetric Jet

The primary objective of this work is to develop high-fidelity simulation model for jet noise control predictions and quantify the sound reduction when an external source frequency mode excitation is imposed on the jet flow. Whereas passive approaches using mixing devices, such as chevrons, have been shown to reduce low-frequency noise in jet engines, such approaches incur a performance penalty since they result in a reduced thrust. To avoid a performance penalty in reducing jet noise, the current work investigates a open-loop active noise control (ANC) system that utilizes a unsteady microjet actuator on the nozzle lip in the downstream direction to produce a desired effect on the jet flow-field dynamics thereby directly affecting the source source. In contrast to the passive approach, the proposed open-loop control design will utilize a local flow excitation device that can be turned off when not needed or adjusted according to the desired control signal. To make it feasible, the effectiveness of every forcing frequency mode has to be mapped for a certain jet velocity. This analysis considers an axisymmetric round jet at supersonic and subsonic speeds. Current studies are verified against previous low-order simulations conducted using Linearized Euler Equations (LEE), and compare qualitatively acheived noise reduction results against available experimental data. High-fidelity analysis, such as Detatched-Eddy Simulations (DES), was implemented using OpenFOAM, an open source CFD software. Results show that some excited frequency modes reduced the far-field jet noise by around 2 dB, supporting the use of unsteady microjet actuators as a jet noise reduction technology