Short Pulsed Laser Methods for Velocimetry and Thermometry in High Enthalpy Facilities

A suite of laser-based diagnostics is proposed to measure velocity and temperature simultaneously using unseeded techniques in high enthalpy flows relevant to reentry flight. The two main types of regions that are found in a typical hypersonic flow field around a vehicle are addressed by developing separate diagnostics for each. In regions far from the body where the flow is mostly non-dissociated, femtosecond laser electronic excitation tagging (FLEET) is proposed for velocity combined with planar Rayleigh scattering to measure temperature via the imaging of an acoustic wave triggered by the FLEET pulse. In the highly dissociated region near the stagnation point of the reentry body, either backward air lasing or radar REMPI will be applied to spectrally resolve a transition of atomic oxygen. The latter two techniques use the same two-photon excitation scheme, but backward air lasing relies on a population inversion induced in the measurement volume, and radar REMPI relies on the microwave interrogation of an induced plasma. The goal of the Phase I study will be to determine which of these two techniques provides the best signal-to-noise ratios in the dissociated regions, and to establish the performance of the combined FLEET/Rayleigh scattering method in the non-dissociated regions.

The diagnostics using the proposed techniques would be a welcome addition to aid ground tests at NASA arc jet facilities, hypersonic wind tunnels, and shock tunnels. Non-intrusive measurements of velocity and temperature are required for validating computational fluid dynamic modeling and simulation codes that incorporate real-gas kinetic and transport models that are used to predict aerothermodynamics associated with planetary reentry flights. The diagnostics can be used as tools in experiments that focus on the understanding of high temperature gas physics and chemistry. Also, simultaneous acquisition of velocity and temperature is needed in understanding dissociating flow environment such as those occurring in hypersonic inlets, scramjets, and over heat shields and other thermal protection systems.

The unique capability to measure temperature and velocity in high enthalpy flows will be attractive to the private space industry, including companies developing space launch systems for low Earth orbit and planetary exploration. These include contractors to NASA for delivery of cargo and astronauts to the International Space Station, as well as companies, such as SpaceX, that have ambitions of visiting Mars. In addition, the techniques developed herein can be used for combustion velocimetry and thermometry, e.g., measurements in premixed and diffusion flames.

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