Low-dimensional techniques for active control of high-speed jet aeroacoustics

The present study is focused on the development of an empirical low-order dynamical system (LODS) of a Mach 0.6 high-speed axisymmetric jet to be ultimately used for closed-loop flow control. An identification method is implemented to solve for the coefficients of the ordinary differential equation (ODE) describing the evolution of the flow. This ODE is derived from a Galerkin projection of the Navier-Stokes equations onto a basis of proper orthogonal decomposition (POD) eigenfunctions. An extensive database of the velocity and acceleration fields is therefore needed to be used as training data for the dynamical system. A dual-time particle image velocimetry (DT-PIV) experiment is designed and carried out to measure velocity and Eulerian acceleration in cross-flow planes from 3 to 10 jet diameters downstream. The setup comprises two stereoscopic PIV systems that sample velocity at two consecutive instants, the time separation being carefully chosen to resolve the scales of interest in the flow. POD is applied and the resulting low order dynamical system is exposed and its dynamics are validated against data previously measured in this flow. A preliminary experiment measuring and correlating the near-field to the far-field pressure is carried out to identify the most sound productive region in the jet, where the DT-PIV experiment focuses, and to give insight into the nature of the propagative sound sources. The results of this experiment lead to the understanding that the axisymmetric mode (azimuthal Fourier mode 0) of the near pressure field is the best propagator to the far-field, which can guide flow control strategies for noise reduction. With this result in mind, synthetic-jet based actuators are designed to be able to provide hydrodynamic perturbations at the nozzle exit to exploit the non-propagative nature of the higher azimuthal modes. The three main aspects of this work (dynamical system development, sound source identification and flow control device design) applied to the high-speed axisymmetric jet lay the preliminary grounds towards the implementation of practical closed-loop flow control for far-field noise reduction