Flow structure and optical beam propagation in high-Reynolds-number gas-phase shear layers and jets

We report on the structure of the scalar index-of-refraction field generated by turbulent, gas-phase, incompressible and compressible shear layers and incompressible jets, and on associated beam-propagation aero-optical phenomena. Using simultaneous imaging of the optical-beam distortion and the turbulent-flow index-of-refraction field, wavefront-phase functions were computed for optical beams emerging from the turbulent region in these free-shear flows, in an aero-optical regime producing weak wavefront distortions. Spatial wavefront-phase behaviour is found to be dominated by the large-scale structure of these flows. A simple level-set representation of the index-of-refraction field in high-Reynolds-number, incompressible shear layers is found to provide a good representation of observed wavefront-phase behaviour, indicating that the structure of the unsteady outer boundaries of the turbulent region provides the dominant contributions

Whole-Field Measurements of Turbulent Flows for the Study of Aero-Optical Effects

Planar laser-Rayleigh scattering has been employed to simultaneously image the index-of-refraction field of a turbulent jet of ethylene into nitrogen and the optical degradation of a laser sheet caused by this turbulent-flow field. The optical degradation occurs in t he turbulent-jet region and manifests itself as phase-front tilts that result in a measurable spatial amplitude modulation (streaks) in the emerging pulsed-laser sheet. The experiments were conducted at elevated pressure, increasing the index-of-refract ion gradient s and improving the signal-to-noise ratio over measurements conducted at atmospheric pressure. The high index-of-refraction gradients in these experiments placed the optical far field within the field of view and allowed us to capture caustic formation in the distorted emerging laser sheet, simulating the aero-optics effects expected at large distances from the smaller index-of-refract ion fluctuations one would more typically encounter

Flow Structure and Optical Beam Propagation in High Reynolds Number, Gas-Phase Shear Layers and Jets

We report on the structure of the scalar index-of-refraction field generated by turbulent, gas-phase, incompressible and compressible shear-layers and incompressible jets, and on associated beam-propagation aero-optical phenomena. Using simultaneous imaging of the optical-beam distortion and the turbulent-flow index-ofrefraction field, wavefront-phase functions were computed for optical beams emerging from the turbulent region in these free-shear flows, in an aero-optical regime producing weak wavefront distortions. Spatial wavefront-phase behavior is found to be dominated by the large-scale structure of these flows. A simple level-set representation of the index-of-refraction field in high Reynolds number, incompressible shear layers is found to provide a good representation of observed wavefront-phase behavior, indicating that the structure of the unsteady outer boundaries of the turbulent region provides the dominant contributions