For decades noise-induced hearing loss has been a concern of the Department of Defense (DoD). My research investigates noise generation and dispersion in supersonic jets and focuses on the fluid-dynamic regime typical of high-performance turbojet and turbofan engines. The goal of my research is to understand how dispersion and propagation of wavepackets can be modified by noise reduction strategies based on secondary injections of fluid with a different temperature from the main jet. The research is organized into three studies that focus on instability, large eddy simulations, and resolvent modes.
The first study is a computational investigation of the role of thermal non-uniformity on the development of instability modes in the shear-layer of a supersonic $M= 1.5$, $Re=850,000$ jet. Cold fluid is injected at the axis of a heated jet to introduce radial non-uniformity and control the spatial development of the shear layer. The mean flow is analyzed with an efficient 2D and 3D Reynolds-averaged Navier-Stokes (RANS) approach using the SU2 code platform for 3 different cases -baseline, centered, and offset injection. Different turbulence models are tested and compared with the experiments. The coherent perturbation is analyzed using linear parallel and parabolized stability equations (PSEs).
The second study investigates novel formulations of large eddy simulation models using an arbitrary high order discontinuous Galerkin scheme. The LES analysis focuses on both numerical issues (such as convergence against the polynomial order of the mesh), modeling issues (such as the choice of subgrid model), and underlying physics (such as vortex stretching and noise generation). Wall models are used to capture the viscous sublayer at the nozzle. The Ffowcs Williams-Hawkings (FW-H) method is used for far-field noise predictions for all cases. Three-dimensionality is studied to investigate how injection in the shear layer acts to create a rotational inviscid core and affects the mixing of the cold fluid and noise dispersion.
The third study extends the (first) instability study by considering (global) resolvent modes. Such optimally forced modes of the turbulent mean flow field will identify the turbulent coherent structures (wavepackets) for different turbulence models at $M=1.5$. The LES simulations performed in the second study will be used to extract the mean flow and the dynamic modes for comparison. My research plan is to perform the resolvent analysis of the axisymmetric mean flow fields for the thermally activated case (i.e., the centered injection) and compare it to the baseline jet case. Different turbulence models will be investigated to determine the correct alignment of dynamic and resolvent modes. Finally, I will consider the three-dimensional, non-axisymmetric mean flow created by offset injection described in the second study, which requires evaluating the convolution products of resolvent modes and base flow. Such three-dimensional resolvent compressible modes have never been identified in the context of supersonic jets. / Doctor of Philosophy / For decades noise-induced hearing loss has been a concern of the Department of Defense (DoD). Research in this area is critical to US national security and valued by both the aircraft industry and government. The noise generated during take-off and landing is hazardous to the crew personnel who work around this vicinity. A reduction of noise can significantly decrease medical expenditure and allow the aircraft industry to meet the stringent community noise requirements. Among the various techniques of noise reduction analyzed over the years, thermal non-uniformity stands out for its simple implementation and cost-effectiveness, especially in after-burner turbojets. Thermal non-uniformity with a cold secondary stream introduces low-velocity fluid in a supersonic jet by locally increasing the density while matching the mass flow rate. Changes to the velocity profile are localized; different regions of the jet emit sound at different frequencies and radiation angles, thus the link between injection location and noise control is not well understood. Using different computational tools this research investigates the link connecting thermal non-uniformity, turbulent production, and sound generation. Injection at different radial locations affects the two mechanisms of sound radiation in different ways. The first mechanism, the Kelvin Helmholtz instability, can be studied as an eigenvalue problem that represents the spatial growth of normal modes. De-coherence of these modal fluctuations can be obtained by injecting secondary fluid directly into the shear layer. This injection mode is called offset injection. The present research shows that the thickening of the shear layer due to low-velocity fluid delays the formation of Kelvin-Helmholtz modes in the offset case. Thus, the outskirts of the jet produce pressure fluctuations with a lower spectral energy density. The second mechanism, the Orr instability, can be analyzed as non-modal growth of acoustic perturbation forced by the breakdown of the core of the jet. LES and stability analysis shows that centered injection is highly effective in reducing the Orr radiation. Resolvent modes explain that the rationale is the delay and reduction of a secondary resonant peak between spatial eddies and forcing caused by changes in the mean profile responsive to secondary injection. Our analysis also explains why the offset injection is more effective at a low polar angle, while centered injection reduces acoustic radiation towards high polar angles. Parametric studies of different injection strategies, i.e., location and number of injection ports are performed to demonstrate the best strategy for noise level reductions.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/106659 |
| Date | 16 November 2021 |
| Creators | Chauhan, Monika |
| Contributors | Aerospace and Ocean Engineering, Massa, Luca, Adams, Colin, Lowe, Kevin T., Meadows, Joseph |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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