Development and application of carbon dioxide enhanced filtered Rayleigh scattering for high speed low density flows

This dissertation presents the development of a laser-based flow diagnostic technique called carbon dioxide enhanced Filtered Rayleigh Scattering and demonstrates its utility for instantaneous planar imaging of cold, high-speed low-density wind tunnel flows. Gaseous carbon dioxide is introduced into the facility air supply and condenses into nanometer sized polymolecular clusters during the nozzle expansion. These clusters have a much larger scattering cross section than the constitutive molecules in air, yet still fall in the Rayleigh size range. In the boundary layer region the high temperature produced by viscous dissipation causes the carbon dioxide clusters to sublimate producing strong gradients in scattering at the interface between the hot boundary layer and the cold free stream gas. Filtered Rayleigh Scattering from the condensed phase allows visualization of both free stream features and near-wall boundary layer structure. The resulting images provide qualitative insight into the bulk flow behavior and are suitable for quantitative characterization of important turbulent boundary layer properties.; This dissertation discusses; the design and characterization of the carbon dioxide seeding system; experimental testing and numerical modeling to quantify the thermodynamic and fluid mechanic perturbations due to condensation-induced heat release; numerical modeling to predict the mean cluster characteristics; work to characterize the dynamic and thermal response of the condensed phase clusters; and the application of this technique for quantifying turbulent structure convection in a hypersonic boundary layer.; The research demonstrated that a carbon dioxide seeding mass fraction as low as 0.7% provided excellent image signal to noise. The heat release associated with this seeding level produced a ≈ 3.6% decrease in the nozzle exit Mach number but no significant change in the test section flow profile. Analysis and experiments indicate the clusters have a very narrow size distribution with a mean radius of ≈3–5 nm.; This technique was used to examine the convection and evolution of structures in a zero-pressure-gradient turbulent boundary layer produced in a Mach 8 flow. Structures at the outer edge of the boundary layer were found, on average, to move with the free stream velocity while deeper in the layer the structures convect at 0.95–0.98u_∞. The broadband convection velocity profile exhibited behavior similar to that observed by other investigators at lower Mach numbers.