Study of High-speed Subsonic Jets using Proper Orthogonal Decomposition
The primary objective of this thesis is to advance Proper Orthogonal Decomposition (POD) methods to quantify similarities and differences between the turbulent structures in the mixing layer of transonic jets issued through baseline axisymmetric conical nozzle and ones with chevrons. The analysis is done using flow velocity data obtained through Particle Image Velocimetry (PIV). Two chevron nozzles with penetration levels at 2% and 4% are used. The mean velocity and TKE results show that chevrons reduce the potential core length, increase the jet spread, increase TKE levels immediately after the nozzle exit, and reduce TKE levels downstream. POD is used to further quantify these results. First 35 converged POD modes are investigated since these modes comprise of the majority of the large scale structures. The similarities and differences between the POD modes from different data sets vary along the jet. The study of the POD modes along the jet showed the growth of the structures in both size and strength in the downstream direction. The growth rates of the modes are suppressed by the chevrons and high penetration chevrons are more effective. This trend is also noticed in the energy distribution study among the modes. Despite having both the axial and radial modes energized by the chevrons near the nozzle, the stabilization of the modes resulted in lower energy contents in the downstream regions. Projection of the PIV images from the chevron configurations onto the baseline POD modes showed that both near the nozzle and downstream the highest energy containing baseline modes are attenuated by the chevrons, but the radial modes in the near-nozzle region. A metric used in order to quantify the similarities among the mode shapes between different flow data sets showed that in the near nozzle region the correlation between the modes corresponding to the chevron nozzles and baseline nozzle is very low due to the increased mixing. Correlation increases as the flow moves downstream indicating the regaining of the baseline flow features. The correlation levels seem to be reduced in the transitional region of the shear layer profile; however increases again afterwards until the end of the PIV domain, again indicating the reorganization of the flow. This metric also identifies the changes in the energy distributions among the modes due to the flow and chevron interactions. The secondary objective of this research is to use POD to further enhance validation methods when comparing computational and experimental data. The experimental (PIV) data consists of the 2D visualization of the velocity field corresponding to the baseline nozzle. The computational results are obtained through the Large Eddy Simulation(LES). A significant match in the characteristics of the mode shapes has been observed, especially in the lower modes, however, there is noticeable difference in the energy distributions between the flows. The LES predicts higher energy content at the lowest modes and the energy drop off rate is also higher. Getting the energy distribution matching between the experimental and computational results is necessary to ensure complete agreement between the turbulence levels.
Aerospace Materials, Proper Orthogonal Decomposition, Jet Shear Layer, Turbulent Kinetic Energy, Coherent Structures, Transonic Jet, Jet Noise
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