Date of Award

Summer 8-2024

Access Type

Dissertation - Open Access

Degree Name

Doctor of Philosophy in Aerospace Engineering

Department

Aerospace Engineering

Committee Chair

R.R. Mankbadi

Committee Advisor

Vladimir V. Golubev

First Committee Member

Anastasios S. Lyrintzis

Second Committee Member

William Engblom

Third Committee Member

William MacKunis

College Dean

James W. Gregory

Abstract

This work addresses the issue of supersonic rectangular jet noise via bi-modal excitation. A Mach 1.5 heated rectangular jet with a 2:1 aspect ratio is considered. Theoretical work is presented in which a Reduced-Order Model (ROM) is used in conjunction with the Linearized Euler Equations to predict the nonlinear growth and decay of various frequencies due to the interaction between harmonically related modes and the mean flow. Scope is limited to a symmetric disturbance. The case of no interaction is used to help identify a dominant coherent structure, referred to as “f”. For the symmetric disturbance, a Strouhal number of 0.15, based on the height of the jet, is found to be the most amplified frequency. The ROM is then used including interaction where either the subharmonic (f/2) or harmonic (2f) are added to reduce “f”. It was found that adding harmonics are effective at reducing the peak of the fundamental depending on the initial phase angle. By assuming “f” to be the dominant noise source, it is possible that this is an effective noise reduction mechanism.

To validate the ROM, Large-Eddy Simulations (LES) are conducted starting with a previously validated case. The unexcited case is first considered. Using FFT, Strouhal number 0.15 is identified as the dominant coherent structure in the near field, which is consistent with the ROM. However in the far field, StH = 0.25 appears as the dominant frequency at the peak emissivity angle, though StH = 0.15 still appeared as a secondary peak and dominated lower angles. The ROM shows the requirement of natural amplification, thus StH = 0.15 is taken as the fundamental. Based on guidance from the ROM, the jet is excited with the harmonic, StH = 0.30, assuming StH = 0.15 to be naturally amplified. A second set of cases is considered where both StH = 0.30 and StH = 0.15 are excited with varying initial phase lags. In both cases, excitation is imposed using a pressure fluctuation. For all excitation cases, the coherent structure at StH = 0.15 in the near field, and the amount of reduction correlates to the amount of amplification of StH = 0.30, supporting the proposal of energy exchange between the two modes. In the far field, peak noise in the minor plane is not reduced since StH = 0.25 was not reduced. However, considerable noise reduction is observed at lower emissivity angles up to 2dB. It is shown that this noise reduction comes from reductions in StH = 0.15, which was the intended effect of the excitation.

The final aspect of this work focuses on the use of a feed-forward controller to control the actuation. Excitation studies have traditionally used open-loop control where only a single frequency is excited with an analytic function. There are very few published studies that have used real time sensing to excite jet flows. In this work, 4 actuators are placed along the span of the upper and lower nozzle surfaces for a total of 8 actuators. Upstream, each actuator has its own sensor that read the instantaneous pressure disturbance. Each actuator then responds with the opposite of that pressure disturbance, but out of phase 180 degrees. In addition, the actuator response is scaled with a proportional gain constant, Kp. In the near field, all feed-forward cases with positive gain values reduced the RMS pressure fluctuations in the initial shear layer, whereas the single-mode excitation increased it. The reduction in downstream pressure fluctuations is shown to have effects with the best results coming from Kp = 1.0. For all feed-forward cases, the near field reductions occur for a broad range of Strouhal numbers in the range of the peak radiated noise. In the far field, the feed-forward cases successfully reduced the low-angle noise by up to 2dB for the case of Kp = 1.0. Analysis of spectra shows that the feed-forward cases reduce a broad range of Strouhal numbers. The feed-forward case with Kp = 1.0 ultimately reduced the noise by more and for a wider range of Strouhal numbers than the single-mode excitation case. An additional set of cases with negative gain values are considered to create additive waves. Near field reduction is considerably lower for these cases and the minor plane far field noise was amplified. Amplification occurred for a large range of Strouhal numbers. It is ultimately suggested that the feed-forward control with gain values close to 1.0 can effectively reduce the noise.

Additional Files
GS9_Acceptance_s6.pdf (417 kB)
Download

Share

COinS