Optical Detection of Transitional Phenomena on Slender Bodies in Hypervelocity Flow

Measurement of the phenomena associated with hypervelocity boundary layer transition requires the development of high-speed detection techniques. Two reasons for the current search for a reliable non-intrusive optical technique include: 1) high-enthalpy facilities suffer from a harsh environment in the test section after the test time, 2) optical diagnostics tend to achieve higher sampling rates than direct mechanical measurement. Two approaches utilizing optical detection are described in this work: temperature measurement by spontaneous emission and laser differential interferometry. A model for the detection of spontaneous emission from a seed gas is formulated, which enables the measurement of spatially-averaged temperature in the boundary layer. This model is applied to data taken during three experiments in a free-piston reflected shock tunnel. The results indicate that the technique may be used to track turbulent bursts in a hypervelocity boundary layer; however, the spontaneous emission technique ultimately falls short of the goal of tracking linear wave packets. Furthermore, the uncertainty in the results from the spontaneous emission experiments is large. Full-field laser differential interferometry is implemented. Preliminary results indicate the ability to track turbulent precursors. 1.0 INTRODUCTION The instability mechanism in hypersonic flow over cold-wall slender bodies is acoustic in nature; acoustic waves become trapped in a wave guide comprised of the body wall and the sonic line in the boundary layer [1–4]. Numerous investigations of flow over a slender cone in high-enthalpy facilities have been performed [5–8]; yet, detailed measurements of the character of the disturbances formed as a result of this acoustic wave guide have not been made. Data from cold hypersonic facilities (reservoir enthalpy: hR < 2 MJ/kg) indicate that the most strongly amplified frequency of the acoustic instability is typically below 500 kHz [3, 9, 10]. In high-enthalpy facilities (mass specific reservoir enthalpy: hR = 5-20 MJ/kg), the velocity at the edge of the boundary layer is considerably higher, so that the most strongly amplified acoustic mode frequency on a slender cone is also higher, ∼1-3 MHz [6, 7, 11]. These time scales eliminate piezo-electric pressure transducers, surface heat transfer gauges, and hot wire anemometry techniques as candidates for measuring the acoustic instability at conditions typical of an experiment in a high-enthalpy facility, due to inadequate temporal resolution. RTO-MP-AVT-200 241 Proceedings of AVT-200 Specialists’ Meeting on Hypersonic Laminar-Turbulent Transition, San Diego, CA. April 16-19, 2012. Optical Detection of Transitional Phenomena on Slender Bodies in Hypervelocity Flow This work describes the development of two techniques for the optical measurement of transitional boundary layer phenomena. One method introduces trace amounts of a seed gas with a strong oscillator strength (namely, lithium for its D1 and D2 spectral lines) into the test gas and tracking the spontaneous emission of the seed gas from the boundary layer; the rate of spontaneous emission is related to the temperature of the gas and, thus, indicates temperature transients. A second method of optical detection of transitional phenomena is also presented, high speed differential interferometry.