Aero-Optical Interactions Along Laser Beam Propagation Paths in Compressible Turbulence

Aero-optical interactions along laser beam propagation paths in turbulent compressible separated shear layers are examined on the basis of experiments on direct imaging of the refractive-index field. Imaging of the index-of-refraction field is performed using digital photography techniques with laser-induced fluorescence. A pulsed ultraviolet laser sheet is directed through the shear layer to generate the fluorescence by exciting acetone vapor molecularly mixed with air in the freestream, resulting in light in the visible spectrum that is recorded with a high-resolution digital camera system. We introduce the idea of the interaction optical path difference and its associated rms value (IOPD rms ), and we investigate these quantities as functions of propagation distance along the laser beam direction as well as functions of the laser aperture size. Evidence of nonmonotonic behavior of the IOPD rms as a function of propagation distance in the flow is observed for individual realizations. The relative magnitude of this nonmonotonic behavior depends on the laser aperture size in relation to the large scales of the flow. These observations of nonmonotonic behavior suggest the presence of a turbulence-induced mechanism for partial reductions in the beam aberrations, which is related to physical and geometrical properties of high-gradient refractive-index turbulent interfaces.