Influence of thermochemistry on Mach reflection in hypersonic flow

Abstract

Real gas thermochemistry can significantly impact the aerodynamics of hypersonic systems. For example, shock stand-off distance in front of a blunt body has been shown to depend on the degree of chemical dissociation. High temperature effects can also alter shock-shock interaction phenomena, but the degree of the modification and its consequences can be challenging to predict. Sanderson et al. experimentally investigated oblique shock impingment on a bow shock (Edney type IV configuration) in a flow with significant gas dissociation. Previous studies had suggested significant increase in heat transfer at jet impingement due to real gas effects, however, experiments showed no dependence of peak heat transfer rate on stagnation enthalpy. The influence of nonequilibrium gas chemistry on Mach and regular shock reflection has been investigated in a number of numerical studies. Burtschell et al. numerically investigated a wedge geometry located in a Mach 7 free stream, a setup similar to that used in the present experimental work. Mach stem height and hysteresis behavior was examined. Burtschell et al. found a strong dependence of transition angles, Mach stem height and location on the gas flow model. For a given wedge angle, the inclusion of real gas chemistry led to a significant decrease in Mach stem height. Chemical-vibration coupling, however, slightly increased the height of the Mach stem. Direct Monte-Carlo simulations of a shock reflection with and without real gas effects carried out by Gimelschein et al. also found a substantial effect on Mach stem height and transition angle. However, an experimental study in dissociating nitrogen and carbon dioxide, ionizing argon and frozen argon could detect no effect on the transition condition due to finite relaxation length at the conditions of the experiment. In the present work, we experimentally investigate a Mach reflection generated by two opposing wedges in a Mach 7.1 free stream. The main goal of this work is to determine directly what kinds of real gas effects occur behind a normal shock in a Mach reflection configuration for a previously selected run condition. Experiments are carried out in an expansion tube facility which is capable of simulating high enthalpy hypersonic flight conditions, and a significant degree of vibrational excitation and chemical dissociation are expected behind the normal shock. In high enthalpy gas flows, emission spectroscopy can be used to characterize the test gas composition and thermodynamic state. As impulse facilities, expansion tubes produce a challenging experimental environment for probe measurements with issues such as short test times, high temperatures and velocities, and diaphragm fragmentation. The non-intrusive nature of spectroscopy makes it an attractive technique for determining flow field properties in impulse facilities. Spectrally resolved studies have been previously used as a means towards characterizing high-enthalpy run conditions. Work completed at the X1 and X2 superorbital expansion tube facilities used emission spectroscopy to measure electron number density behind a bow shock and to identify sources of visible radiation. Time-resolved spectral methods were used in the JX1 expansion tube facility to determine the useful test time. Using the CARS technique, temperature profiles were determined for a hypervelocity blunt body flow field using the T3 shock tunnel facility. Using the free piston shock tube/tunnel facility TCM2, laser spectroscopy was used for species identification and shock front temperature profile diagnostics and spontaneous Raman spectroscopy was used to analyze the self-luminosity of nitrogen hypersonic flows for varying enthalpy conditions. In the current experiments, asymmetric wedges are used to generate a Mach stem, with a free shear layer at each triple point. Imaged spectroscopic measurements behind the Mach stem are presented. The spectra confirms flow dissociation and verifies the appropriateness of a run condition which in the future is to be used towards investigating high-temperature effects upon shear layer structure in hypersonic flow.

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