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

A suite of pulsed laser diagnostics is proposed for studying aspects of planetary entry and Earth atmospheric reentry in arc jets. For example, dissociation of molecules impacts the flow-field physics, including surface heat flux and catalytic surface reactions. Results obtained during the Phase I effort point to three promising diagnostic techniques: Rayleigh Scattering Polarimetry (RSP) for dissociation fraction, Thermal Acoustic Wave (TAW) thermometry for gas temperature, and Radar Resonance Enhanced Multi-photon Ionization (Radar REMPI) for gas temperature and velocity. The RSP technique is based on the differences in the polarization of Rayleigh-scattered light between atoms and molecules. The TAW technique is based on the determination of wave speed from the propagation of an acoustic wave generated by a laser spark from the focused beam of a pulsed laser. In the case of Radar REMPI, temperature and velocity are obtained through the spectral broadening and frequency shift associated with two-photon resonance interactions in atomic oxygen and nitrogen.

The proposed diagnostics are needed in test programs for atmospheric reentry and planetary entry at NASA's arc jet facilities, hypersonic wind tunnels, and shock tunnels. Non-intrusive unseeded measurements of temperature, velocity, and dissociation are required for validating fluid/chemistry models that incorporate real-gas kinetics, including those used to predict planetary entry aerothermodynamics. The diagnostics can serve as tools in very high enthalpy flow experiments that focus on testing the integrity of thermal protection systems.

The unique capability to measure temperature, velocity, and dissociation fraction in high enthalpy flows will be attractive to the private space industry, including low Earth orbit and planetary exploration. Programs include delivery of cargo and astronauts to the ISS, and deep space exploration, i.e. involving Mars. Other applications are combustion diagnostics in premixed and diffusion flames.

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