Towards Closed Loop Control of a High-Speed Jet for Noise Reduction
Date of Award
Doctor of Philosophy (PhD)
Mechanical and Aerospace Engineering
Mark N. Glauser
Aeroacoustics, Flow Control, Jet Noise, Jets, Proper Orthogonal Decomposition, Time resolved PIV
This body of work seeks to explore novel methods for reducing aeroacoustically generated noise through the introduction of localized perturbations around the periphery of a Mach 0.6 unheated 2-inch jet. Simultaneous measurements of the hydrodynamic pressure field are acquired at discrete axial locations downstream of the nozzle exit (3 and 6 jet diameters downstream), alongside simultaneous measurements of the far-field acoustic signature. We assess the effectiveness of both open loop and closed loop flow control studies as they compare to an uncontrolled jet, through the examination of resultant modifications to the low dimensional characteristics of the jet. Also assessed is the nature of the changes to the spectral content and subsequent cross-correlations between the near-field and far-field regions. The open loop control studies include a series of investigations geared towards characterizing the effectiveness of the chosen control device, a piezoelectrically driven synthetic jet actuation system, in exciting different instabilities. The test metric spans an array of cases in which the excitation frequency was varied at constant momentum flux and the momentum flux was varied at one constant excitation frequency. Also investigated was the effect of an amplitude modulated input signal, as well as forcing different asymmetric modes, in an open loop sense. Results suggest that the shear layer is very receptive to relatively low energy instabilities introduced at the lip. The magnitude of the spectral density plots are amplified in certain frequency bands, and in some cases the peak frequency location is shifted from its baseline location. The cross-correlations exhibit significant phase lag and strong periodicity as a result of a sinusoidal forcing of the developing shear layer. For all open loop control cases the far-field acoustic signature was increased. Phase two of the investigation employed the use of a simple proportional closed loop controller. The information used in the feedback loop was garnered from the spatial Fourier mode filtered pressure information acquired with the azimuthal array of pressure sensors. The experimental set-points were cycled through feeding back either Fourier mode 0 or 1 acquired at either of the two downstream pressure measurement locations. This information was used to dynamically amplitude modulate the driving signal sent to the synthetic jet actuators. It was found that the changes to the correlations between the various measurement locations were not as prominent, but there was a resultant reduction of ~1.22 dB in the OASPL at far-field regions closest to the jet axis.
The second wave of experiments incorporated the use of a time resolved particle image velocimetry system that allowed for measurement of the instantaneous radial and axial components of the velocity field along the jet centerline, from the exit of the nozzle to 7.5 diameters downstream, at a rate of 10 kHz. These measurements were taken in a simultaneous fashion with that of the near-field pressure and the far-field noise. These studies are limited to the analysis of one open loop control case against that of the uncontrolled jet. Cross correlations were performed exploring how both near-field Fourier filtered pressure and low dimensional Proper Orthogonal Decomposition (POD) modes relate to the far-field acoustics. Of interest are those signatures which exhibit the strongest correlation with the far-field, and subsequently how these structures can be controlled. It was found that almost none of the lower order time dependent POD modes of the velocity field correlate to the far-field, except at regions near the collapse of the potential core where POD modes a6(t) and a14(t) exhibit approximately a 10% correlation. The goal was to investigate how flow-induced perturbations of the developing shear layer might bring insight into how one may alter the flow such that the far-field acoustic signature is mitigated. The TR-PIV measurements will prove to be a powerful tool in being able to track the propagation of physical structures for both the controlled and uncontrolled jet.
Low, Kerwin Richard, "Towards Closed Loop Control of a High-Speed Jet for Noise Reduction" (2012). Mechanical and Aerospace Engineering - Dissertations. Paper 70.
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