A. James Clark School of Engineering
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The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item GLOBAL ANALYSIS OF TRANSITIONAL HYPERSONIC FLOW OVER CONE AND CONE-FLARE GEOMETRIES(2024) Sousa, Cole Edward; Laurence, Stuart; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Accurately predicting the laminar-to-turbulent boundary-layer transition on hypersonic vehiclesremains one of the principal challenges in characterizing the expected heat loads and skin friction the vehicle will experience in flight. Ground facilities, while incapable of replicating the complete set of flow conditions found at hypersonic flight, play a critical role in providing physical measurements of the transition process. The experimental characterization of hypersonic boundary-layer disturbances, however, has traditionally faced limitations in its ability to provide spatiotemporally dense data sets comparable to those of computational fluid dynamics (CFD) investigations. The present work aims to provide global off-body measurements of hypersonic boundary-layer disturbances at frequencies much greater than that of the fundamental instability, enabling the exploration of nonlinear phenomena and more extensive comparisons between experimental and computational studies. The current methodology utilizes the fact that hypersonic-boundary layer disturbances havebeen observed to propagate at measurable and statistically predictable velocities. Particularly for the second-mode instability, the density gradient fields acquired by a calibrated schlieren system provide an avenue for resolving dense high-frequency spatiotemporal data. Disturbance propagation velocities extracted from the schlieren images are used to conduct a time-interpolation of the disturbances, which transforms spatially-available descriptions of the travelling waveforms into up-sampled temporal signals at specific pixel locations. When performed across the entire schlieren field of view, the resulting time-resolved signals have a new sampling frequency much greater than the original camera frame rate and a spatial density equal to the camera resolution. This enables the spectral analysis of high-frequency disturbances, including superharmonics of the fundamental instability, which are not originally resolvable from raw time series of the video data. The methodology is employed here in three different experimental data sets, comprising a7° half-angle sharp cone at zero incidence in Mach 6 flow, a 7° half-angle sharp cone at variable incidence in Mach 14 flow, and a cone-flare geometry composed of a 5° frustum with compression angles of +5°, +10°, and +15° at zero incidence in Mach 14 flow. A comprehensive global analysis is conducted on the linear and nonlinear development of the second-mode instability waves in each case. Pointwise measures of the autobicoherence are used to identify specific triadic interactions and the locations of their highest levels of quadratic phase coupling. Significant resonance interactions between the second-mode fundamental and harmonic instabilities are found along with interactions between these and the mean flow. Bispectral mode decomposition is employed to educe the flow structures associated with these interactions. A similar analysis is performed for the power spectrum, with power spectral densities computed for each pixel’s timeseries and spectral proper orthogonal decomposition employed to derive the modal structure and energy of the flow at specific frequencies. The instability measurements taken on the cone-flare geometry are the first of their kind atMach 14. The analysis reveals that incoming second-mode waves undergo extended interactions with the shock waves present at the corner, consistently leading to amplification of the waves and accelerating their nonlinear activity. The disturbance energy is also found to strongly radiate along the shock waves, a behavior that appears to be intensified at high Mach numbers. In the case of separated flow at the corner, additional low-frequency disturbances arise along the shear layer. Self-resonance of these disturbances leads to the radiation of elongated structures upstream of reattachment, which extend outward from the shear layer and terminate at the separation shock. This shear-layer disturbance is determined to be dominantly unstable between separation and reattachment but is significantly damped after reattachment.Item Physics and Modelling of Compressible Turbulent Boundary Layer(2023) Lee, Hanju; Martin, Pino; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Key findings from a research study that focuses on understanding the effect of Mach number, Reynolds number and wall temperature on compressible turbulent boundary layers (CTBL) in the hypersonic regime are presented in this dissertation. The study utilizes a comprehensive CTBL database developed using an in-house direct numerical simulation (DNS) code at the CRoCCo laboratory. The database encompasses a range of semi-local Reynolds numbers (800 to 34,000) and Mach numbers up to 12, incorporating wall-cooling. The effects of density and viscosity fluctuations on the total stress balance are identified and used to create a new mean velocity transformation for compressible boundary layers. The role, significance and physical mechanisms connecting density and viscosity fluctuations to the momentum balance and to the viscous, turbulent and total stresses are presented, allowing the creation of generalized formulations. We identify the significant properties that thus-far have been neglected in the derivation of velocity transformations: (1) the Mach-invariance of the near-wall momentum balance for the generalized total stress, and (2) the Mach-invariance of the relative contributions from the generalized viscous and Reynolds stresses to the total stress. The proposed velocity transformation integrates both properties into a single transformation equation and successfully demonstrates a collapsing of all currently considered compressible cases onto the incompressible law of the wall, within the bounds of reported slope and intercept for incompressible data. Based on the physics embedded in the two scaling properties, the success of the newly proposed transformation is attributed to considering the effects of the viscous stress and turbulent stresses as well as mean and fluctuating density viscosity in a single transformation form. The Reynolds number trends of large turbulent structures in compressible turbulent boundary layers are investigated using the pre-multiplied energy spectra based on the density corrected fluctuating streamwise velocity signal. Results demonstrate the existence of friction as well as semi-local Reynolds number trend associated with large-scale structures, similar to trends observable in incompressible turbulent boundary layers (ITBL). In particular, the behavior of turbulence in the inner layer is seen to exhibit dependence based on both definitions of Reynolds numbers. On the contrary, the strength of large turbulent structures is seen to be only dependent on friction Reynolds number. This result directly contrasts with the observation of the near-wall turbulent intensity peak increasing with semi-local Reynolds number. The discrepancy is mended with a suggestion that the large turbulent scales in the log layer of which the strength increases with friction Reynolds number, are modified through the changes in local fluid properties such that the scale interaction near the wall increases as semi-local Reynolds number. In another words, closer to the wall, the CTBL flow behaves like a semi-local Reynolds number flow, while closer to the freestream, it behaves like a friction Reynolds number flow. Furthermore, the present study examines the Reynolds number dependence of the length scale between small and large turbulent scales. The analysis highlights the inadequacy of using a univariable wavelength based on viscous, semi-local or outer length scales to differentiate small and large scales. Based on this, the use of Reynolds number-dependent length scales is recommended. Overall, the study provides valuable insights into the Reynolds number trends of large turbulent structures in CTBL, emphasizing the influence of both semi-local Reynolds number and friction Reynolds number on turbulence characteristics.