Control of aerodynamic noise generated by high-performance jet engines continues to remain a serious problem for the aviation
community. Intense low frequency noise produced by large-scale coherent structures is known to dominate acoustic radiation in the aft angles.
A tremendous amount of research effort has been dedicated towards the investigation of many passive and active flow control strategies to
attenuate jet noise, while keeping performance penalties to a minimum. Unsteady excitation, an active control technique, seeks to modify
acoustic sources in the jet by leveraging the naturally-occurring flow instabilities in the shear layer. While excitation at a lower range of
frequencies that scale with the dynamics of large-scale structures, has been attempted by a number of studies, effects at higher excitation
frequencies remain severely unexplored. One of the major limitations stems from the lack of appropriate flow control devices that have
sufficient dynamic response and/or control authority to be useful in turbulent flows, especially at higher speeds. To this end, the current
study seeks to fulfill two main objectives. First, the design and characterization of two high-frequency fluidic actuators ($25$ and $60$
kHz) are undertaken, where the target frequencies are guided by the dynamics of high-speed free jets. Second, the influence of high-frequency
forcing on the aeroacoustics of high-speed jets is explored in some detail by implementing the nominally 25 kHz actuator on a Mach 0.9 ($Re_D
= 5\times10^5$) free jet flow field. Subsequently, these findings are directly compared to the results of steady microjet injection
experiments performed in the same rig and to prior jet noise control studies, where available. Finally, limited acoustic measurements were
also performed by implementing the nominally 25 kHz actuators on jets at higher Mach numbers, including shock containing jets, and elevated
temperatures. Using lumped element modeling as an initial guide, the current work expands on the previous development of low-frequency (2-8
kHz) Resonance Enhanced Micro-actuators (REM) to design actuators that are capable of producing high amplitude pulses at much higher
frequencies. Extensive benchtop characterization, using acoustic measurements as well as optical diagnostics using a high resolution
micro-schlieren setup, is employed to characterize the flow properties and dynamic response of these actuators. The actuators produced
high-amplitude output a range of frequencies, $20.3-27.8$ kHz and $54.8-78.2$ kHz, respectively. In addition to providing information on the
actuator flow physics and performances at various operating conditions, the benchtop study serves to develop relatively easy-to-integrate,
high-frequency actuators for active control of high-speed jets for noise reduction. Following actuator characterization studies, the
nominally 25 kHz ($St_{DF} \approx 2.2$) actuators are implemented on a Mach 0.9 free jet flow field. Eight actuators are azimuthally
distributed at the nozzle exit to excite the initial shear layer at frequencies that are approximately an order of magnitude higher compared
to the \textit{jet preferred frequency}, $St_P \approx 0.2-0.3$. The influence of control on the mean and turbulent characteristics of the
jet, especially the developing shear layer, is examined in great detail using planar and stereoscopic Particle Image Velocimetry (PIV).
Examination of cross-stream velocity profiles revealed that actuation leads to strong, spatially coherent streamwise vortex pairs which in
turn significantly modify the mean flow field, resulting in a prominently undulated shear layer. These vortices grow as they convect
downstream, enhancing local entrainment and significantly thickening the initial shear layer. Azimuthal inhomogeneity introduced in the jet
shear layer is also evident in the simultaneous redistribution and reduction of peak turbulent fluctuations in the cross-plane near the
nozzle exit. Further downstream, control results in a global suppression of turbulence intensities for all axial locations, also evidenced by
a longer potential core and overall reduced jet spreading. The resulting impact on the noise signature is estimated via far-field acoustic
measurements. Noise reduction was observed at low to moderate frequencies for all observation angles. Direct comparison of these results with
that of steady microjet injection revealed some notable differences in the initial development of streamwise vorticity and the redistribution
of peak turbulence in the azimuthal direction. However, despite significant differences in the near nozzle aerodynamics, the downstream
evolution of the jet appeared to approach near similar conditions with both high-frequency and steady microjet injection. Moreover, the
impact on far-field noise was also comparable between the two injection methods as well as with others reported in the literature. Finally,
for jets at higher Mach numbers and elevated temperatures, the effect of control was observed to vary with jet conditions. While the impact
of the two control mechanisms were fairly comparable on non-shock containing jets, high-frequency forcing was observed to produce
significantly larger reductions in screech and broadband shock-associated noise (BBSN) at select under-expanded jet conditions. The observed
variations in control effects at different jet conditions call for further investigation. / A Dissertation submitted to the Department of Mechanical Engineering in partial fulfillment of the
requirements for the degree of Doctor of Philosophy. / Fall Semester 2017. / September 19, 2017. / Active Flow Control, Actuators, High-frequency excitation, High-speed Jets, Jet Noise Control, Particle Image
Velocimetry / Includes bibliographical references. / Farrukh Alvi, Professor Directing Dissertation; M. Yousu Hussaini, University Representative; Rajan
Kumar, Committee Member; Jonathan Clark, Committee Member; Jonas P. R. Gustavsson, Committee Member.
| Identifer | oai:union.ndltd.org:fsu.edu/oai:fsu.digital.flvc.org:fsu_605027 |
| Contributors | Upadhyay, Puja (author), Alvi, Farrukh S. (professor directing dissertation), Hussaini, M. Yousuff (university representative), Kumar, Rajan (committee member), Clark, Jonathan E. (committee member), Gustavsson, Jonas (committee member), Florida State University (degree granting institution), College of Engineering (degree granting college), Department of Mechanical Engineering (degree granting departmentdgg) |
| Publisher | Florida State University |
| Source Sets | Florida State University |
| Language | English, English |
| Detected Language | English |
| Type | Text, text, doctoral thesis |
| Format | 1 online resource (332 pages), computer, application/pdf |
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