UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
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Item Effects of Vibrational Nonequilibrium on the Acoustic Noise Radiated by a Compressible Boundary Layer(2023) Gillespie, Graeme Ivry; Laurence, Stuart J; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Turbulence-generated acoustic noise is of critical concern in the nozzle flows of conventional high-speed wind tunnels, where the disturbance environment encountered by models in the freestream is substantially stronger than that experienced in atmospheric flight and leads to much reduced transition Reynolds numbers. To obtain more accurate comparisons of experimental, computational, and free-flight data, a new control mechanism is needed to reduce freestream disturbance levels. Therefore, the aim of the present work is to investigate the ability of vibrational nonequilibrium processes to attenuate acoustic radiation emitted by turbulent boundary layers in high-speed facilities. Predicting the attenuation from vibrational nonequilibrium processes remains a challenge, and there exist limited experimental data for model validation, particularly at elevated temperatures. To better understand the absorption properties of various gas mixtures, a heated acoustic chamber is developed to measure the attenuation of CO2, N2O, and mixtures of CO2/He, CO2/N2,and N2O/He at temperatures up to 529 K. In mixtures of CO2/He at room temperature, an increase in helium is found to decrease the peak attenuation modestly, but increase the peak attenuation frequency. At higher temperatures, the peak attenuation increased substantially, but as the helium fraction increased, the rate of increase in peak attenuation drops and the values asymptote at lower temperatures. These results illustrate that varying the fraction of helium in mixtures of CO2/He can shift the attenuation to a desired frequency range, providing a method to control acoustic radiation. The effects of vibrational nonequilibrium processes on turbulence-generated acoustic noise are investigated in a Mach-2.8 shock-tunnel facility at the University of Maryland. CO2, N2, He, and He/CO2 mixtures are injected into the lower boundary layer of the flow through a porous plate located in the upstream region of the test section. A four-point Focused Laser Differential Interferometer (FLDI) positioned above the turbulent boundary layer is used to obtain freestream fluctuation measurements assumed to be representative of entropic fluctuations propagating along streamlines and acoustic disturbances along Mach lines. Compared to a boundary layer of pure air, the injection of 30%, 35%, and 40% He/CO2 mixtures resulted in reduced fluctuation powers correlated along a Mach line in the frequency range of 200−800 kHz. Minimal reductions in fluctuation power were measured along corresponding streamlines; therefore, it could be concluded that the vibrationally active gas species in the boundary layer primarily affected acoustic radiation and not entropic disturbances. As measurements are affected by noise radiated from the boundary layers on all four walls of the facility, a mathematical disturbance model is created to examine the sensitivity of the measured attenuation to acoustic disturbances propagating from the lower boundary layer only. Disturbances are modeled as Gaussian wave packets propagating along Mach lines from the four test section walls and along streamlines. Modeling the acoustic disturbances from the lower boundary layer with a 15−30% amplitude reduction resulted in amplitude spectral densities and cross power spectral densities that agreed well with the FLDI measurements. Thus, the injection of a vibrationally active gas into a turbulent boundary layer has the potential to significantly reduce acoustic-disturbance amplitudes in the freestream, greatly expanding the utility of conventional high-speed facilities to study flows in which transition plays an important role.Item Effect of Cooling on Hypersonic Boundary-Layer Stability(2022) Paquin, Laura; Laurence, Stuart J; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The prediction of boundary-layer transition on hypersonic vehicles has long been considered a primary design concern due to extreme levels of heating and dynamic pressure loading this transition induces. While it has been predicted that the temperature gradient between the vehicle and the local freestream can drastically alter boundary-layer stability, experimental research on the topic over the past fifty years has provided conflicting results. This study investigates the relationship between the wall-to-edge temperature ratio and boundary-layer stability on a slender cone. Campaigns in two wind-tunnel facilities were conducted: one set within the HyperTERP reflected-shock tunnel at the University of Maryland, and one set at the high-enthalpy T5 reflected-shock tunnel at the California Institute of Technology. Both sets of campaigns employed non-intrusive, optical diagnostics to analyze the structures and spectral content within the boundary layer. In the first part of the study, performed in HyperTERP, an experimental methodology was developed to vary the wall temperature of the model using active cooling and passive thermal management. This allowed the wall temperature ratio to be varied at the same nominal test condition (and thus freestream disturbance environment), and three thermal conditions were established for analysis. Simultaneous schlieren and temperature-sensitive-paint (TSP) imaging were performed. Calibrated schlieren images quantified the unsteady density gradients associated with second-mode instabilities, and TSP contours provided insight into the thermal footprint of mean boundary-layer structures. It was found that, overall, cooling shrunk the boundary-layer thickness, increased second-mode disturbance frequencies, and increased the amplification rate of these instabilities. At nonzero angles of attack, cooling appeared to increase the azimuthal extent of flow separation on the leeward side of the cone. In the second part of the study, performed in T5, the disturbance structures and spectral content of laminar and transitional boundary layers were characterized under high-enthalpy conditions. Schlieren images indicated that, at these extremely low wall-to-edge temperature ratios, second-mode waves were confined very close to the wall in the laminar case. During the breakdown to turbulence, structures radiating out of the boundary layer and into the freestream were discovered. A texture-based methodology was used to characterize the Mach angles associated with these structures, and a wall-normal spectral analysis indicated a potential mechanism by which energy was transferred from the near-wall region to the freestream. The study presents some of the first simultaneous imaging of the flow structures and associated thermal footprint of boundary-layer transition within an impulse facility. The work also presents the first time-resolved, full-field visualizations of the second-mode dominated breakdown to turbulence at high enthalpy. Thus, the study imparts significant insight into the mechanics of boundary-layer transition at conditions representative of true hypervelocity flight.Item Fluid dynamics of boundary layer combustion(2017) Miller, Colin; Gollner, Michael J; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Reactive flows within a boundary layer, representing a marriage of thermal, fluid, and combustion sciences, have been studied for decades by the scientific community. However, the role of coherent structures within the three-dimensional flow field is largely untouched. In particular, little knowledge exists regarding streamwise streaks, which are consistently observed in wildland fires, at the base of pool fires, and in other heated flows within a boundary layer. The following study examines both the origin of these structures and their role in influencing some of the macroscopic properties of the flow. Streaks were reproduced and characterized via experiments on stationary heat sources in laminar boundary layer flows, providing a framework to develop theory based on both observed and measured physical phenomena. This first experiment, performed at the University of Maryland, examined a stationary gas burner located in a laminar boundary layer with stationary streaks which could be probed with point measurements. The gas temperature within streaks increased downstream; however, the gas temperature of the regions between streaks decreased. Additionally, the heat flux to the surface increased between the streaks while decreasing beneath the streaks. The troughs are located in a downwash region, where counter-rotating vortices force the flame sheet towards the surface, increasing the surface heat flux. This spanwise redistribution of surface heat flux confirmed that streaks can, at least instantaneously, modify important heat transfer properties of the flow. Additionally, the incoming boundary layer was established as the controlling mechanism in forming streaks, which are generated by pre-existing coherent structures. Finally, the amplification of streaks was determined to be compatible with quadratic growth of Rayleigh-Taylor Instabilities, providing credence to the idea that the downstream growth of streaks is strongly tied to buoyancy. The next phase of the project was performed at the Missoula Fire Sciences Laboratory, where a hot plate in a laminar boundary layer was examined. In addition to manipulating the wind speed, the local buoyant force was controlled via the surface temperature of the hot copper plate. Infrared thermography was employed to detect streaks by means of local surface temperature fluctuations, and a novel and consistent method for tracking streaks and quantifying important properties was developed. Streak spacing was seen to be lognormally distributed, and the initial spacing, which was consistently between 60-70 dimensionless wall units, was shown to be governed by the incoming boundary layer. Streak spacing increased downstream of the plate, with higher plate temperatures resulting in larger magnitudes of spanwise fluctuations in surface temperature. Finally, streak behavior became more chaotic downstream, as streaks would meander rapidly and persist for shorter durations. The final phase of the study, performed at the Missoula Fire Sciences Laboratory, examined a saturated fuel wick in the same experimental configuration as the hot plate. Streaks were detected in the flame via high speed video, and tracked using the previous developed algorithm. Streak spacing was lognormally distributed, with the initial spacing (60-75 wall units) again being controlled by the incoming boundary layer. Spacing between coherent structures increased downstream, likely due to buoyant amplification. The width of streaks grew to an apparent assymptote, indicating a settling of length scale controlled by the time and rate of growth. Further downstream, coherent structures no longer resembled well-ordered streaks but more complex structures resulting from streak aggregation. Overall, trends for streaks are consistent in both the hot plate and the flame, indicating that the behavior of streaks is governed by similar mechanisms in both scenarios. Although the initial instabilities are governed by the incoming wind, buoyant forces cause the growth and aggregation of these structures. These local instabilities are capable of affecting macroscopic properties of the flow, including heat transfer to the surface, indicating that a two-dimensional assumption may fail to adequately describe heat and mass transfer during flame spread and other reacting boundary layer flows.