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Title:The role of aerothermochemistry in double cone and double wedge flows
Author(s):Swantek, Andrew
Director of Research:Austin, Joanna M.
Doctoral Committee Chair(s):Austin, Joanna M.
Doctoral Committee Member(s):Dutton, J. Craig; Elliott, Gregory S.; Freund, Jonathan B.
Department / Program:Aerospace Engineering
Discipline:Aerospace Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Double Cone
Double Wedge
High-Speed Imaging
Heat Transter
Shock Boundary-Layer Interactions
Expansion Tube
High Enthalpy
Abstract:In this work, hypervelocity flows over double cone and double wedge geometries are studied. The flow configurations established over the double cone/double wedge models are extremely sensitive to thermochemistry, and thus serve as ideal benchmarks for validating chemical models. The goals of this research are: i) to investigate the coupling between the fluid mechanics and thermochemistry in these flow fields by varying freestream flow composition and enthalpy, ii) to implement a diagnostic suite for time-resolved surface and freestream measurements, iii) to investigate the nature of flow field unsteadiness across various test conditions, and lastly iv) to extend the experimental database for shock wave boundary/layer interactions. An expansion tube is used to generate flows with enthalpies ranging from 2.2-8.0 MJ/kg (2-4 km/s) and Mach numbers from 4-7. The expansion tube is a novel impulse facility for accelerating a test gas to these velocities, while maintaining a minimally dissociated freestream. Additionally, the facility allows variation of the freestream composition (between nitrogen and air), while maintaining freestream test parameters (Mach number, density, enthalpy) to within 0.5$\%$. Two models are used: a 25-55 degree double cone model and a 30-55 degree double wedge. There are four diagnostic components to this research which aim to enable a better understanding of these canonical flow fields. Single frame, high resolution schlieren photography is used to visualize various flow features including: the separation zone formed in the corner, the triple point interaction, and a supersonic shear layer. From these images, a separation zone length scaling parameter is determined. This parameter, derived for wedge geometries, is successfully applied to conical geometries by using a judicious choice of flow properties for scaling. In the wedge image series, nitrogen test conditions exhibit a distinct increase in bow shock standoff distance. Additionally, aft wedge shock impingement in nitrogen occurs upstream, compared to air. The second portion consists of heat transfer profiles taken over the double wedge model. Fast response (~1 microsecond), coaxial thermocouples are used to measure average heat transfer values through the established test time. Differences in heat transfer profiles between air and nitrogen are seen at flow enthalpies as low as 3.6 MJ/kg. In all test conditions where a difference is seen, air exhibits augmented heating compared to nitrogen. This is limited to the region surrounding peak heating. Fluctuations in the established profile are quantified via the standard deviation of the signal. Fluctuations normalized by the mean are seen to be highest in regions of shock boundary layer interaction and separation. The third part of the research consists of high speed schlieren imaging. High speed data (75-100 kHz framing rates) has been taken which visualizes the establishment process of the shock interactions, as well as of the separation zone. Distinct differences between nitrogen and air are observed, including: increased triple point establishment time in nitrogen, and the transient nature of shock waves. Establishment times of the shock configurations are compared with establishment times from the heat transfer traces, and experimental correlations from the literature. Normalized establishment times of 2-8 are observed, in agreement with historical data (5.5-11). Shock tracking algorithms are employed to trace and plot the profiles of the transient shock configurations for further analysis. Fast Fourier Transforms of shock location are computed and the frequencies are compared to frequency predictions for an acoustic wave traveling between the bow shock and shear layer. The fourth and final part of this work investigates the nitric oxide (NO) emission spectrum in the ultraviolet band. Spectra are obtained at four locations behind the bow shock (0, 2, 4, 6 mm) in the highest enthalpy test condition. Simulated NO vibrational spectra are used to make estimates of the vibrational temperature at these four locations. The temperature is seen to peak at the 0 mm location, being similar in magnitude to the predicted frozen post shock temperature (~7700 K). A decrease in temperature is seen when traversing downstream, however temperatures do not approach the equilibrium temperature (~3900 K), indicating this region of the flowfield is in non-equilibrium. An increase of temperature is seen in the furthest downstream point (6 mm), and may be a result of viscous heating in the shear layer, which this interrogation point falls near.
Issue Date:2012-12
Rights Information:Copyright 2012 Andrew B. Swantek
Date Available in IDEALS:2013-02-03
Date Deposited:2012-12

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