Turbulent Refractive Fluid Interfaces and Aero-Optical Wavefront Distortions: Experiments and Computations

A combined experimental/computational technique useful for aero-optics is developed with emphasis on the interfacial-fluid-thickness approach. The proposed technique is useful both for low-energy and high-energy laser applications. The experiments enable flow imaging and beam measurements at large Reynolds numbers and the computations, combined with the experiments, permit the study of aero-optical interactions at both low and high laser energies. Experimentally, the method consists of using laser-induced fluorescence for the refractive-field imaging and Hartmann sensing for the optical-wavefront measurements. In the UC Irvine pressurized single-stream wind tunnel, acetone vapor can be seeded in the ambient air or in the high-speed air, or in both. This is useful to compare density-effects due to compressibility alone and mixing-effects due to dissimilar gases. A custom-built high-resolution Hartmann wavefront sensor is developed that is useful to measure the propagated wavefronts simultaneously the refractive interfaces. Computationally, an optical beam solver is developed based on the eikonal equation with a marching algorithm in order to simulate the propagation of the optical wavefronts through the refractive-index field. The computations are combined with the experiments by employing the flow images as the refractive field, for the low-energy-laser case. For high-energy laser pulsed beam propagation, the eikonal equation is coupled with an absorption law to modify both the refractive field and the wavefront profile. This approach, which combines flow imaging with geometrical-optics computations, is especially useful for the study of near-field aero-optical distortions in separated flows, as would be generated by high-maneuverability aircraft, whereas far-field beam propagation requires Fourier optics.